3rd Conference

code ISAP 3rd Conference — Titles & Abstracts
03000 3rd International Conference on the Design of Asphalt Pavements – Volume 1 – Preliminary pages and Table of Contents
03001 The Bearing Capacity of Pavements with Frost Retarding Layers. A Test Road Study
Olle Andersson, Bjorn Orbom, Goran Ringstrom

For an extensive study of frost penetration and bearing capacity of roads with frost retarding layers of different composition and different thickness and surrounded by different types of bases and sub-bases a test road was built in western Sweden in 1966-67. During the construction period and the following year the various layers were followed up by measurement of dynamic elastic modulus by the wave propagation method, and several sections were subjected to repeated plate loading applying up to 50000 loads at each point in order to study the fatigue properties. For investigation of the development of bearing capacity the test road under traffic is followed up by measurements each year implying plate bearing tests, levelling, cross profile and longitudinal profile by the CHLOE profilometer.

Most frost penetration retarding materials caused an increase in rebound deflection and did in some cases cause a disastrous permanent deflection, which necessitated rebuilding. Some materials had no detectable influence upon the permanent deflection. The serviceability index as measured by the CHLOE meter decreased faster on some sections having insulating layers but showed in some sections a slower decrease than on the reference sections. The dynamic elastic modulus of the base course increased during the first year by more than 30% also in those having insulating layers beneath.

The test road will be followed up for several years to come but has already given positive results for the frost protection aspect of road design.
03002 The Measurement of Traffic Axle Load Distributions for Pavement Design Purposes
J. E. B. Basson. G L. Dehlen, R. G. Phillips, P. J. Wyatt

One of the required inputs for the design of new pavements, or of overlays to existing pavements, is accurate data on the axle loads of the traffic they will carry. Estimation procedures based on visual traffic counts together with the results of earlier axle load measurements made with loadometers or static weighbridges are sometimes employed to obtain such data for the present traffic. As the variables associated with the conversion of traffic counts to axle loads at a particular location include the commodities carried, the type of vehicle, the season, and police control of illegal axles, such an estimation procedure cannot be expected to render acceptably accurate results at all locations.

By means of traffic surveys the effects of some of these variables have been quantified. These surveys have revealed that there is no estimation procedure which does not result in significant inaccuracies in the design thickness of a pavement. By contrast, the direct measurement of axle load distributions, using one of several dynamic weighing techniques, reduces the errors in the evaluation of present traffic to negligible proportions, and is considered to be the approach most suitable for pavement design purposes.
03003 Ste. Anne Test Road – Flexible Pavement Design to Resist Low Temperature Cracking
R. A. Burgess, O. Kopvillem, F. D. Young

Low temperature transverse cracking of bituminous pavements is a form of pavement distress prevalent in Canada and the northern United States. This distress induces a deterioration in pavement performance through spalling, heaving or settling at the cracks and reduces pavement service life. In 1967, a test road, designed and instrumented for the investigation of this problem, was constructed at Ste. Anne, Manitoba, This road incorporated twenty-nine test sections involving a number of different bituminous mix materials, pavement structures and subgrade types, believed to be potentially important in the study of transverse cracking. After five years of service, the following conclusions can be drawn with respect to the relative performance of the test sections:
(1) Pavements incorporating high viscosity type asphalts and softer grade asphalts exhibit a greater resistance to transverse cracking.
(2) Pavements with thick asphalt bound layers exhibit a lower frequency of transverse cracking.
(3) The type of subgrade influences the frequency of transverse cracking.
(4) The asphalt content of the mix, in the range of one percent below Marshall optimum to one-half percent above Marshall optimum does not appear to be significant in affecting pavement transverse cracking frequency.

A laboratory study was conducted in conjunction with the field programme to investigate the possibility of correlating laboratory predicted fracture temperatures with the actual field performance of the St. Anne test sections. Predicted fracture temperatures were determined by calculating that temperature at which accumulated thermal stresses exceed the tensile strength of the compacted mix. Additionally, since the tensile properties of an asphalt binder are proportional to the tensile properties of a mix made with that binder, the possibility of predicting pavement field performance by a knowledge of binder properties alone was investigated.

Accumulated thermal stresses of the binders and their mixes were calculated from their respective stiffness moduli at one-half hour loading time (cooling rate} over the appropriate temperature range. Accumulated thermal stress and breaking stress (tensile strength) were plotted as a function of temperature. The predicted fracture temperature is the intercept of these curves. Comparison of the laboratory and field results revealed that there is an excellent correlation between the laboratory predicted fracture temperatures of the binder and mix and the temperature of initial cracking of the asphaltic pavement in the field. For practical purposes, therefore, the tendency of an asphaltic pavement to crack can be predicted by a knowledge of the binder stiffness modulus at low temperatures and long loading time. Conversely, the binder, or mix, stiffness parameter may be used as a pavement design criterion to alleviate the transverse cracking problem.

These research findings would also imply that it is necessary to be more selective in the use of materials and pavement designs with due consideration being given to their performance under the prevailing climatic conditions.
03004 The Response of Asphalt Pavements to Low Temperature Climatic Environments
J. T. Christison, K. O. Anderson

The response of asphalt pavements to environmental conditions is well recognized, but not easily quantified. Variations in temperature , particularly freezing conditions, may result in non-traffic load associated distress. Analyses of several pavement systems in Western Canada where continuous temperature records were collected, has yielded specific information on temperature gradients and time variations at various depths throughout the pavement structures.

To extend this information to other climatic areas, a computer program has been developed that utilizes a finite difference method to predict the thermal regime within pavement systems. The necessary input parameters for the program are the thermal properties of the component layers and readily available meteorological data. Comparisons of predicted and recorded temperatures have shown excellent agreement.

Calculated temperatures throughout the asphalt concrete surface layer, together with laboratory evaluation of fracture strength have been used to predict the pavement susceptibility to thermally induced fracture. Results of field studies support the analysis technique described.
03005 A Study of Subgrade Moisture Conditions in Connection with the Design of Flexible Pavement Structures
T. Y. Chu, W. K. Humphries, S. N. Chen

In the structural design of flexible pavements, the supporting power of subgrade soils should be evaluated on the basis of anticipated moisture conditions. It is, therefore, necessary to predict subgrade moisture conditions and to simulate these conditions in performing laboratory tests for subgrade evaluation. In this study, field investigations were conducted at selected sites in South Carolina for determining subgrade moisture variations under existing pavements. Findings from these investigations were utilized In the development of laboratory procedures for treating subgrade soil specimens in order to simulate anticipated moisture conditions. While laboratory investigations reported in this paper were carried out in connection with a particular type of subgrade evaluation test, the procedures developed for specimen treatment may be adapted to other types of test for similar purposes.
03006 The Influence of Climatic Factors on the Structural Design of Flexible Pavements
D. Croney, J. N. Bulman

Current design standards for flexible pavements in Britain are based on the results of full-scale road experiments extending over 20 years. The influence of seasonal and climatic factors is taken into account by such a protracted period of observation. In extending the use of the standards to other climatic environments or in applying methods of structural analysis to them, the influence of climate on the performance of the various elements of the pavement needs to be appreciated.

The first part of the paper considers the influence of climate on the strength of soil foundations. Recent research in the field of soil mechanics is used to isolate three broad subgrade moisture conditions based on rainfall, evapo-transpiration and the position of the water-table. The role of vapour transfer in influencing subgrade strength is also discussed.

High temperatures reduce the modulus of the bituminous elements of the pavement and again the effects of this on the stress regime are discussed. Augmented vertical stresses in the subgrade, as well as a loss of stability in the bituminous material may lead to increased deformation under traffic during hot weather. However, the ability of the materials to accept higher levels of strain reduces the risk of cracking under these conditions.

Sub-zero temperatures normally lead to a much increased modulus both in bituminous materials and subgrades. The transfer of water into the subgrade during the downward progress of the zero isotherm will however reduce the elastic modulus of the subgrade drastically immediately ofter the thaw. The implications of this on pavement stresses must be considered.

Published data have been used to estimate for four soil types and for two granular sub-base and base materials the probable effect on strength of a wide range of climatic conditions. Strengths are expressed in terms of CBR or elastic modulus. Used in conjunction with structural design procedures these values will give some indication of the influence of climate on pavement thickness requirements.
03007 The Classification of Traffic for Pavement Design Purposes
E. W. H. Currer, P. D. Thompson

For the structural design of new pavements the information available about traffic is normally restricted to census data obtained from visual counts. Such counts are made in Britain at about 4-yearly intervals on a national basis, but for major projects supplementary studies will normally be required. It has been usual practice to differentiate between private cars, light commercial vehicles (delivery vans of estimated unladen weight not exceeding 1.5 tonnes), heavy commercial vehicles and public service vehicles. The total of heavy commercial and public service vehicles only has been used in the past for structural design purposes. More recently however it has been recognised that the axle-load spectrum of the commercial traffic is of major importance.

An electronic weighbridge suitable for installation in the carriageway surface was developed many years ago and effort is now directed towards the production of an alternative capacitor-type pad which can be placed on the carriageway surface. Equipment of the former type has been used to obtain the axle-load spectrum of the traffic on existing roads, but the cost precludes its use for routine survey purposes. The objective of research in this field in Britain has been to determine by limited surveys mean spectra for commercial axles appropriate to different classes of road, e.g. industrial motorways and major trunk roads, rural major roads, secondary rural roads and roads in residential areas. A summary of results to date is given in the paper. Typical spectra obtained in this manner are converted to an equivalent number of standard 8200 kg (18,000 lb) axles per 100 commercial axles, using the A.A. S. H.O. equivalence factors.

In using this approach, to evaluate from the commercial traffic flow the cumulative number of equivalent 8200 kg (18,000 lb} axles carried by a particular carriageway lane during the design life of a pavement, two other pieces of information are required:-
(1) the average number of axles per commercial vehicle
(2) the distribution of commercial traffic between the carriageway lanes.

Studies of these two factors have recently been made in Britain and the results are summarised in the paper.
03008 A Contribution to the Establishment of Design Loads for the Thickness Design of Flexible Road Pavements
H. Keller

Theoretical and empirical research is constantly producing new ideas on flexible pavement design, but these are not being incorporated in the principles used in actual practice for assessing useful design loads. This report is designed to draw attention to research that has been done on the specification of service load data for roads in the Federal Republic of Germany. It also outlines the work that remains to be carried out in the future.

It lists initially the requirements that are currently made for loading data for flexible pavement design and indicates possible ways of simplifying data presentation.

The report deals with surveys already carried out on service loads: starting with the pattern of stress distribution between truck tires and carriageway surface, it discusses vertical axle loads assessed in permanent surveys as well as sample and indirect surveys. It also deals with the data required on loading periods and loading points.

Explanatory measurement examples are cited for various characteristics of truck traffic, Finally, the report takes current loading data requirements and available data as a basis for indicating what still has to be done in flexible pavement design research to establish design criteria for the service loads of roads.
03009 The Transient and Long Term Performance of Pavements in Relation to Temperature
N. W. Lister

At the present stage of development of a structural method of design applicable to British conditions it is possible to model the transient stress-strain behaviour of several types of flexible pavement with some confidence whereas progress in solving the more complex problem of the long term behaviour of pavement materials and subgrades under repetitive loading is understandably slower. It is therefore of value to try to relate levels of transient stress and strain generated within the road directly to their observed long term behaviour.

Most British pavements tend to deteriorate by deformation of the road surface followed by cracking at a relatively late stage. Elastic analysis of a pavement with a rolled asphalt surfacing and base indicates the critical nature of stress-strain conditions at high temperature and these are related to the development of deformation in an experimental road section under traffic, nearly all of which took place under high temperature conditions.

Full-scale repeated loading tests carried out in a Road Machine under controlled conditions of wheel load and temperature on a pavement with a crushed stone base are described. The results again demonstrate the critical importance of temperature in determining deformation behaviour and indicate the existence of a critical value of vertical stress in the subgrade above which subgrade deformation occurs.
03010 A Model Utilizing Climatic Factors for Determining Stresses and Deflections in Flexible Pavement Systems
Charles R. Marek, Barry J. Dempsey

In this paper, a model for determining the stresses and deflections at various locations in a multi-layered flexible pavement system as a function of load, climatic exposure conditions, and material characteristics is described. Application of the model to a flexible pavement section at the AASHO Road Test and comparison of theoretical stresses and deflections with measured stresses and deflections at various times during the life of the section is made. The comparisons relate the applicability of the model for pavement analysis.

The model combines a heat-transfer model for evaluating frost conditions and temperature-related effects, with an elastic layer model for stress and deflection determination. The heat-transfer portion of the model was derived from one-dimensional, forward-finite-difference, heat-transfer theory. This portion of the model was used to generate temperature profiles and frost-line locations in the pavement system at a specified time, It was designed to include many input parameters such as short-wave radiation, long-wave radiation, convection, and air temperature. Other factors considered are physical properties and thermal properties of the pavement materials including unit weight, moisture content, material classification, thermal conductivity, heat capacity, and latent heat. The model was developed so that appropriate thermal properties of the pavement materials are used depending on whether or not an unfrozen or frozen state exists. In addition, it can be easily expanded to include newly developed parameters. The model has been programmed for computer solution.

The stresses and deflections at various locations in a multi-layered flexible pavement system were determined based on Burmister’s elastic layer theory. A computer routine, developed by Chevron Research Corporation and modified to work on the University of Illinois’ computer system, was employed for solution.

Modular values and layer thicknesses were established based on layer interface location, frost-line location, and layer condition (frozen or unfrozen). Stiffnesses of the asphaltic concrete surface and binder layers were determined by use of the procedure developed by Van der Poel and revised by Heukelom and Clomp. A correction for air content when in excess of three per cent, as suggested by Van Draat and Sommer, was also employed. Average layer stiffnesses for the surface and binder layers were determined and utilized for computation of stresses and deflections in the system at a specified time.

Using the model, theoretical stresses and deflections for Section 581 from the AASHO Road Test were computed for the period of July 1, 1959 to June 30, 1960. Comparisons were then made between theoretical and measured stress and deflection values to substantiate the applicability of the model for theoretical analysis of pavement systems.
03011 German Experiences with the Replacement of Granular Frost Blankets by Other Types of Construction
H. Proksch

During the years 1954 – 1970 numerous research projects and experiments were carried out in Germany in order to investigate by which means granular frost blankets customarily used for 40 years may be substituted by other types of construction. The studies being undertaken involved comparative experimental field projects using asphalt membrane envelopes, full-depth asphalt pavements and bituminous and plastic thermal insulating layers as well as extensive laboratory investigations,

The paper describes the experimental projects in detail and reports the results already available of measurements of subsoil moisture content, frost penetration depth, thermal properties of the bituminous and elastic materials used and of load bearing characteristics applying plate bearing tests, dynamic load tests and the Benkelman Beam.
03012 The Effect of Climatic Factors on Benkelman Beam Deflections in the Melbourne Area of Victoria, Australia
A. Ratnarajah

A programme of testing to evaluate the effects of seasonal climatic variations on pavement deflections as measured by the Benkelman Beam was initiated in March 1969 jointly by the Australian Road Research Board and the Country Roads Board, Victoria. The areas selected for testing were all in the vicinity of Melbourne. In order to minimize the effects of variations in subgrade type, groups of sites in geologically similar areas were selected. Five such areas were chosen. The frequency of testing was approximately monthly for the first twelve months and two monthly thereafter.

Results to date indicate that ambient temperature as such has no effect on the subgrade that is measurable in this investigation. Rainfall however has a measurable effect on the subgrade. Its effect on Benkelman Beam deflections is tied in with the effects of the permeability of the subgrade and with the surface drainage conditions at the site. The effects of evaporation of water from the ground surface would be to reduce the amount of water falling on the surface that would otherwise percolate into the pavement and subgrade. No attempt has been made to evaluate this effect.

The tests have given some basis on which a correction factor to the observed deflections may be applied in certain situations when the deflection testing is not carried out at the time the subgrade is at its weakest.
03013 Prediction and Observation of the Performance of a Flexible Pavement on an Expansive Clay Subgrade
B. G. Richards, R. Gordon

For an experimental section of road pavement in the Darling Downs, Queensland, investigations carried out earlier on a highly expansive clay subgrade suggested an ultimate equilibrium subgrade suction of the order of 350 p.s.i. or 6 per cent dry of optimum moisture content (OMC) for standard AASHO compaction. During and after construction, rapid drying in the vicinity of the edges of the sealed pavement caused serious longitudinal shrinkage cracking at the edge of the seal in less than 12 months. Subsequent seasonal rainfall entering these cracks led to serious deformations and loss in surface shape and, in many cases, to shear failure. Practical methods are suggested which may overcome these problems.

A laboratory programme was devised to investigate the volume change characteristics and repeated loading ‘resilient’ moduli as functions of subgrade suction and initial compaction. The results indicate that for an expansive clay subgrade, the volumetric and vertical strain can be linearly correlated with the logarithm of the suction over the suction range considered. The compacted material is also isotropic with the vertical strain being exactly one third of the volumetric strain.

The repeated loading ‘resilient’ moduli are very sensitive to suction and the logarithm of the moduli can also be linearly correlated with the suction logarithm. From the "Shell Rational Method" design charts for lightly trafficked roads, these correlations are found to have a variation of pavement thicknesses from 15 in. at the equilibrium suction to over 4 in. at OMC depending on compaction conditions.

Finite element analyses have been made for deformations due to volume changes under changing subgrade moisture conditions and the deformations due to an 18,000 lb axle loading with moisture and stress dependent material properties. These theoretical predictions have been compared with preliminary field observations obtained from the experimental sections. Both the predicted and observed behaviour are in general agreement and clearly indicate the important influence of subgrade moisture on the performance of a pavement built on expansive clay subgrades in a semi-arid environment.
03014 Effects of Environment on Pavement Temperatures
R. H. Williamson

Climatological considerations are of considerable importance in the design, construction and maintenance of both rigid and flexible pavements.

This paper describes some of the deleterious effects of warm environments on road pavements and presents ways in which both empirically obtained long-term temperature data and theoretically evaluated temperature variations in multilayered structures may he related to practical engineering problems.

The motivation for, and evaluation of, a simulation model which will predict pavement temperatures using finite difference techniques is presented, and the advantages of using such a model for the acquisition of relevant data on pavement temperatures enumerated.

Pertinent areas in which the mechanisms of heat transfer are not well understood are outlined and some existing concepts questioned.
03015 Laboratory Evaluation of Rutting in Base Course Materials
Richard D. Barksdale

A method is presented for evaluating the relative performance of unstabilized base course materials with respect to rutting and is then used in the evaluation of a number of materials. A general method is also proposed for calculating rut depth occurring in flexible pavements, The proposed methods make use of the plastic axial strains obtained from the repeated load triaxial test.

Cylindrical specimens 6 in. in diameter and 12 in. in height of crushed stone and soil-aggregate mixtures were placed in a conventional triaxial cell and subjected to 100,000 load repetitions using a constant confining pressure and a triangular stress pulse.

Stress-strain curves giving the relationship between deviator stress, confining pressure and plastic axial strain were constructed for each material studied using the repeated load test results. The concept of a rut index was proposed which can be calculated making use of the plastic stress-strain relationship, and is approximately proportional to the rut depth that will occur in the base after a desired number of load repetitions, The rut index appears to offer a practical laboratory method for evaluating the relative performance of base materials used in pavements having similar structural configurations.

An evaluation of the test results using the rut index approach indicates that under good conditions of drainage and proper maintenance of the pavement surface, carefully selected blends of 20 percent soil and 80 percent stone should perform satisfactorily. Soil aggregate blends having properties similar to the materials tested should probably not be used at all under poor drainage conditions, and 40-60 blends should not be used even under good conditions of drainage. The results further indicate that only a sufficient amount of fines should be used in a crushed stone base to permit proper compaction if the amount of rutting in the base is to be minimized. Furthermore, even though the specified gradation and density may be the same, bases constructed from aggregates obtained from different sources may exhibit different rutting characteristics.

A general engineering method for estimating the rut depth in a flexible pavement after a desired number of load repetitions was proposed which utilizes nonlinear Layered theory, the plastic stress-strain response of the component materials, and a hyperbolic, plastic stress- strain law. Field verification is now needed of both the proposed rut index and the general method for predicting rut depth.
03016 Stiffness of Pavements Characterized by the Dynamic Modulus Evd – Definition of Dynamic Plate Loading
G. Baum

The deformation properties of unbound pavement layers are studied by means of static plate loading tests. Attempts to use static plate loading on the surface of bituminous pavement layers failed on account of the properties of viscoelastic bituminous road materials,

This paper describes that static plate loading can be replaced by defining a dynamic plate loading test. This is done by substituting sinusoidal forces for the static step forces produced by the load plates.

The sinusoidal forces are generated by an unbalance vibration machine and set a road surface in vibrational motion (and/or, depending on the test to be made, any surface of a pavement layer). The generated vibrations are related to various parameters.

To characterize the vertical deformations at a point of measurement the theory of simple linear vibration systems is used.

The investigations have brought out that only one of the constants derived from the technique of vibration gives sufficient technical information to be a valid test data. This is a spring constant which is applicable to frequencies exceeding 20 to 40 Hz (higher frequency domain).

By means of the dynamic theory of the elastic half-space the dynamic deformation value, E(sub vd), is calculated from the spring constant.

Pavements are considered as one-layer systems. In addition, by measuring the velocity of surface waves at low frequencies the effective modulus, E(sub 2), of the subgrade of pavements is determined. And by the use of the Burmister theory the effective modulus of elasticity, E(sub 1,b), of the "upper layer" (pavement) with a thickness of h is calculated from E(sub vd) and E(sub 2).

The systematic procedure of the investigation is thoroughly described. For lack of space, however, references are used to complement the description. Finally, full particulars on the procedure can be found in the description of 26 test sections of the road experiment "Grunbach" (Germany) especially designed for base course testing.
03017 Fatigue Tests on Pavements by Pulse Generators
H. Behr

For the study of the Optimum thickness design Of pavements and the selection Of suitable road materials many approaches have been used ranging from purely theoretical calculations to observations of road experiments under heavy test traffic with special trucks. The paper describes the basic considerations which have led to finding still another way by the use of simulative fatigue testing roads in a laboratory.

The equipment (pulse generators) installed at the Bundesanstalt fur Strassenwesen (BAST) – Federal Road Research Institute – is explained in detail. The results of an analysis of the pattern of loading produced by truck tires and pulsed loading are compared to prove the similarity of the most important load criteria.

Seventeen fatigue tests have been made by means of the pulse generators described in the paper. Each comprised 600,000 to 1,000,000 passages per track (30 cm) on the test sections built. The simulative loadings correspond to moving wheel loads of 3 – 10 Mp (approx. 30 – 100 N). The results obtained hitherto are relative to 9 selected pavements with bases of dry-bound crushed stone and asphalt-coated gravel varying in thickness, as well as to special designs with heat insulating layers of plastic foam. The bearing properties of these structures under dynamic loading are defined and the behaviour of roads subjected to fatigue tests described.

Special attention is paid to permanent surface deformations, since, in a way, this characteristic is a very interesting criteria for the description of riding quality and rutting. An attempt is described to establish an approximative law for permanent deformations in relation to the magnitude of loading and the number of load repetitions. The dependency on the load is expressed by deformation coefficients.

With the reservations that the laws still have to be placed on more secure foundations by results of further fatigue tests, load equivalents have been calculated.

The load equivalents are a first step towards calculating the effect of mixed traffic, i.e., various load quantities in random sequence. In addition the real test acceleration produced by fatigue tests can be determined on the basis of load equivalents relative to a road under traffic.

Before more detailed data can be stated, results from further fatigue tests must be waited for. The future program of research is briefly outlined.
03018 Assessing the Properties of Materials for the Structural Design of Pavements
J. Bonnot

At the present time, knowledge concerning the mechanical properties of materials has not progressed as rapidly as pavement design methods, and the conventional tests used for selecting the composition of pavement mixes are not directly related to the mechanical characteristics affecting their performance in the pavement. The Laboratoires des Ponts et Chaussees have undertaken the development of a series of tests allowing the basic properties of pavement materials to be fully defined.

For the gravel treated with hydraulic binders used in the base course, the modulus of elasticity, which is required for calculating the distribution of stresses and strains in the pavement, is determined from the stress-strain curve obtained during direct tensile testing. The values obtained are characteristic of very rigid materials.

The risk of fatigue failure is examined by means of fatigue bending tests; the observed behaviour is very different from that of asphaltic materials and this has major consequences on pavement design. The risk of cracking by shrinkage is examined from strain failure under direct tension, or by means of special tests in which the prevented thermal shrinkage process is reproduced in the laboratory. The spacing between the cracks may be estimated from the breaking strength under direct tension.

For asphaltic materials, the moduli values necessary for calculating the distribution of strains and stresses in the pavement are given by the complex modulus test. Other modulus values may be obtained through the relaxation test or from stress-strain curves obtained during the direct tensile test. Master curves of these different moduli are compared. The risk of fatigue failure is examined by means of fatigue bending tests with the imposed strain.

Failure strength and strain data under direct tension are also given, and this test is compared with the Brazilian test. The tensile and compression test allows the determination of variations in cohesion and internal friction with the loading rate and with temperature.

Resistance to rutting in asphaltic materials is also examined by means of creep tests under compression, or by a test based upon the wheel tracking test.
03019 Applied Rheology of Asphalt Mixes – Practical Application
G. Chomton, P. J. Valayer

The rheological properties of asphalt mixes are usefully studied when they emphasize the durability of a road. This durability is related to:
– Stress distribution in the road, for which rheological properties must emphasize "stiffness" of the material.
– Failure criteria related to cracks, for which rheological properties must emphasize "fatigue".
– Failure criteria related to large deformations for which the rheological properties must emphasize creep under repeated shears.

A brief description of the equipment used in Mont Saint Aignan is given.

The stiffness is expressed in terms of complex moduli and is related not only to frequency of stress application and temperature but also to mix characteristics such as filler and bitumen content and also grain size distribution. General laws are checked for fatigue and the expression of fatigue versus stiffness is very attractive ; also, new laws are given, involving the energy consumption during a fatigue test.

Finally, a vibrating creep test is described and criteria are issued from this test to select the materials which will not develop rutting in the roads. The laboratory results are then processed in terms of road performance by the use of strain distribution calculated from a computer. The modulus is an essential parameter: therefore all the results are presented versus the modulus of the layer examined.

Both fatigue and vibrating creep tests are processed this way; the fatigue test by means of the conventional tensile strain at the lower part of the layer considered and the vibrating creep test by means of the maximum shear strains calculated in the layer.

Finally, an important discussion is given on the limitations of this method : examples (rotating stress tensors under the moving load) illustrate the criticism. In conclusion, fundamental mechanical testing can be used for the selection of materials on roads : examples of practical applications are given.
03020 Strength of Bases and Subbases
Charles R. Foster

The results of in-place CBR tests made at various heights above the subgrade are reviewed to determine if the CBR at a given level in an untreated material is a function of the CBR of the underlying material and the height of the test above the underlying layer.

The data presented show conclusively that the above postulation is correct; however, material characteristics affect the CBR and a general relationship could not be developed.

The data do show that the thin layers of base do not develop significant strength. The author believes the water these thin layers collect and hold weakens the subgrade with the result that a pavement made with a treated base course may be structurally weaker if a thin cushion course is used than if the treated base course had been placed directly on the subgrade.

The data also show that the modulus of an untreated base course will increase with thickness. Thus, thickness equivalencies of treated materials which have fairly constant moduli with thickness, will decrease with an increase in thickness of base. This pattern has been reported for the data developed at the AASHO Road Test.
03021 The Behaviour of Bituminous Mixtures in Laboratory Tests and Under Road Conditions
R. Guericke, F. Weinert

The fatigue behaviour, the expansibility and the stiffness of bituminous mixtures are important parameters for the dimensioning of road structures. After work of about five years informative values for the various bituminous mixtures can be given. The determination of flexibility in the fatigue test has remained problematic. Not sufficiently known are the effects of the frequency of load cycles and the influence of the intervals between the load cycles. As long as these influences are not exactly studied it is recommended to test the fatigue of bituminous mixtures with frequencies of at most 10 c.p.s. (Hz). Moreover, it was found that the results of fatigue tests with constant deformation amplitude are in better agreement with the field conditions than the results of fatigue tests with constant power amplitude.

The determination of flexibility in the fatigue test generally requires much time and work. It was for this reason that in parallel with the fatigue tests the stress and strain were determined in axial tensile tests. As can be concluded from the results obtained so far there exists a close connection between the flexibility in the fatigue test (i.e. number of load cycles until rupture at a given deformation) and of the expansibility in the axial tensile test. The axial expansibility at low temperatures is, moreover, an important parameter with a view to the danger of the formation of cracks due to cooling in bituminous road surfacings.

In parallel with the laboratory tests the behaviour of road pavements under standing and rolling loads has been investigated. In this connection the determination of the radius of curvature of the deformation trough has proved particularly effective and informative. The curvature meters developed by Franz Muller gives exact and excellently reproducible measured values, it is smaller and easier to handle than the Benkelman beam. One measurement takes only one or few minutes.
03022 Relations between Mix Design and Fatigue Properties of Asphaltic Concrete
J. M. Kirk

A great deal is known about the fatigue properties of bituminous materials, but this knowledge is based on tests carried out under various conditions. Consequently, it is difficult to obtain a general picture of how the fatigue properties are influenced by the most important parameters such as binder content, void contents, grading, temperature etc. Therefore, in this paper an attempt is made to generalize as far as possible and to isolate the influence of the most important parameters. The first step is to show that the influence of the three parameters, the penetration grade of the binder, the temperature and the loading time can be combined by use of one single parameter, the stiffness of the binder. Then is shown how the fatigue properties of a mix is influenced by this parameter. Next is shown that the fatigue properties improve with increasing the maximum size of the aggregate and how to isolate this factor. The next step is to show the effect of the binder content and how it is influenced by the content of voids, and a correction curve for this effect is shown. Furthermore, it is shown that a mix must contain a certain minimum of filler in order to obtain good fatigue properties. This leaves only one parameter, the shape of the gradation curve, which is of importance in so far as it determines the number of large voids in the mix, and a large number may lead to poor fatigue properties,

While the fatigue properties are important in thickness design, when a bituminous base is used, they are only part of the problem in mix design, where other important points as the stiffness of the mix, which determines the load spreading capacity, the stability, the workability and the economy must be considered. This will often fix a limit to how good fatigue properties it is possible to obtain, but it is hoped that this paper will enable the mix designer to get close to that limit, which will allow the thickness designer to calculate with a high permissible strain in the material.
03023 Relationship Between Pavement Structural Integrity and Hardness of the Asphalt Cement
Norman W. McLeod

In colder climates, it is of questionable value to concentrate a great expenditure of effort on the design of a conventional, deep strength, or full depth asphalt pavement structure, if the integrity of the structure is to be impaired or destroyed by low temperature transverse pavement cracking. Several years of research on the problem of low temperature transverse pavement cracking in Canada, where this is currently the most serious pavement performance problem, have indicated that it can be most easily and inexpensively remedied by using softer grades of asphalt cement. Field and theoretical evidence are presented to support this conclusion. Finally, to bridge the gap between research and practice, a chart is provided to enable an engineer to select a grade of asphalt cement that will preserve the integrity of an asphalt pavement structure by avoiding low temperature transverse pavement cracking throughout the pavement’s service life, provided the asphalt pavement has been properly designed and constructed.
03024 Tensile Behavior of Asphalt-Treated Materials Under Repetitive Loading
Raymond K. Moore, Thomas W. Kennedy

This paper summarizes the findings of an experimental program designed to evaluate the tensile and behavioral characteristics of high quality asphalt-treated materials subjected to repeated tensile stresses by means of the indirect tensile test. The objectives of the study were:
(1) to determine whether the indirect tensile test can be used for the study of the behavior of asphalt-treated materials subjected to repeated tensile stresses,
(2) to define the general nature of the relationship between applied tensile stress and fatigue life and to evaluate the effect on fatigue life of certain mixture and compaction variables, and
(3) to investigate the possibility of estimating the fatigue life of asphalt-treated materials subjected to repeated applications of a tensile stress either by developing a predictive equation or establishing a correlation with other material characteristics.

Results of the study indicated that the indirect tensile test can be used satisfactorily to evaluate the fatigue characteristics of high quality asphalt-treated materials under repeated tensile loadings. In addition, the general nature of the relationship between applied tensile stress and the fatigue life was determined along with the inherent variation associated with fatigue life. The tensile stress-log fatigue life relationships were essentially linear with failures occurring at tensile stresses ranging from 8 to 40 psi, which were approximately 6 to 30 percent of the static indirect tensile strength. Significant variation in fatigue life occurred, and it was found that the standard deviation varied linearly with the mean of fatigue life, with the coefficient of variation ranging from 30 percent to in excess of 75 percent.

The tensile fatigue characteristics were found to be affected by asphalt content, type of asphalt cement, compaction temperature, and mixing temperature. Within the range tested, it was found that fatigue life was increased by using a more viscous asphalt cement, higher compaction temperature, and higher mixing temperature. It was also concluded that there is an optimum asphalt content for maximum fatigue life. In addition, a simple predictive equation was developed which adequately described the fatigue life of the specimens tested.

Fatigue life was found to correlate with initial stiffness, initial tensile strain, and tensile stress-strength ratio, but these correlations were associated with a large amount of variation. No correlation was found to exist between fatigue life and percent air voids.
03025 Applicability of a Linear Viscoelastic Characterization for Asphalt Concrete
Keshavan Nair, Wayne S. Smith, Chin-Yung Chang

The objective of this investigation was to determine the validity of using a linear viscoelastic constitutive equation to characterize asphalt concrete in the design of pavement systems. The investigation was conducted in two phases. In the first phase, creep tests in compression, tension and torsion, and repeated loading tests were performed on cylindrical specimens of asphalt concrete for various axial loads at different confining pressures and different temperatures to determine the response functions, and establish the degree of linearity and evaluate the time-temperature equivalence of the response. On the basis of these tests creep functions and complex modulus values for the asphalt concrete were determined. The second phase of the investigation was conducted to check if the characterization of the asphalt concrete obtained in the first phase of the program could be used to predict the behaviour of asphalt concrete under stress states which are similar to those that might exist in actual pavements by testing beams and slabs and a Winkler foundation. In this paper only the results of the first phase of the investigation are presented and discussed.

It was found that the type of test (e.g. uniaxial, triaxial, torsion) influences the magnitude and nature of the viscoelastic functions. For one type of test, the results obtained are consistent. Based on the results of tests under hydrostatic stress states it was observed that the samples exhibited a substantial degree of anisotropy. It is hypothesized that this is due to the method of compaction utilized in preparation of the samples. It was also observed that the volumetric response of asphalt concrete was tine dependent and that the usual assumption of incompressibility was questionable. Thermorheological simplicity was found to be a satisfactory assumption for uniaxial compression tests.

In order to reduce the influence of stress level, stress state, anisotropy and other effects to a level that will make linear isotropic viscoelasticity an acceptable characterization of asphalt concrete, it has been suggested that a limit be placed on the strain level that can occur in the asphalt concrete. This was done using the experimental data on the basis of a subjective evaluation. It is strongly recommended that sensitivity studies on the basis of pavement performance be conducted to determine the acceptable variations for ideal material characteristics and hence establish the degree of refinement required in the characterization of naterials for the design of pavement systems.
03026 Dynamic Structural Properties of Asphalt Pavement Mixtures
Charles A. Pagen

An approach to the structural design of pavements is advanced, based on the utilization of the in-service mechanical properties of road materials in the computerized viscoelastic and elastic multi-layered theories. Design and performance of roadways are discussed in light of the fundamental structural properties of the component materials. The primary goal of this program was to evaluate the significant changes in the dynamic mechanical properties of asphaltic concrete mixes used in highway pavement systems.

This study has quantitatively evaluated the effect of laboratory and roadway compaction on: the rheological creep and dynamic moduli, the elastic properties and the ultimate unconfined compressive strength of asphaltic concrete mixtures. The influence of compaction on the loading time and temperature-dependent strength and deformation properties of asphaltic mixtures was investigated in laboratory experiments on the phenomenological level. Such laboratory and roadway correlations are essential in order to evaluate the influence of construction techniques on pavement performance, asphalt mixture design procedures, thickness equivalencies and the structural design of road and airport pavements. The data indicate that highway test studies are required to supplement laboratory and theoretical research work.

The experimental phases of the study involved the testing of three dense asphaltic concrete mixes prepared in the laboratory by gyratory compaction and comparable field core test specimens obtained from a test section in U.S. Route 42 near Delaware, Ohio. Laboratory experiments were performed to validate the application of material science concepts to roadway asphalt mixtures and to determine the limitations of such approaches to highway design and strength evaluation programs. Experiments have quantitatively evaluated the influence of method of compaction, loading time or frequency, age of test specimen and temperature on the failure, and rheological complex and elastic moduli of the materials investigated. The field and laboratory compacted test specimen data have indicated that the application of the linear viscoelastic theory and mechanistic models to asphaltic concrete is valid, as well as that the time-temperature superposition concept is applicable for the research data obtained. Long-term creep experiments and dynamic tests have provided additional and independent checks of the concepts employed. Significant differences in the mechanical properties of the essentially identical laboratory and field compacted test sample were noted for the complex moduli and other strength parameters investigated.

Discussions on the utilization of the research procedures for evaluation of pavement thickness equivalencies, and suggestions for potential highway engineering utilization of the results in the structural design of flexible pavement systems, future studies of asphaltic concrete mixture properties, and acceptance of construction work are presented.
03027 Deformations in Asphalt Concrete Wearing Courses Caused by Traffic
W. D. O. Paterson

The transient and permanent deformations caused in dense-graded asphalt concrete wearing courses by traffic loads have been measured and are discussed in this paper. The study is concerned mainly with wearing courses ranging in thickness from 2 cm to 10 cm and tested on a concrete-based testing track with controlled conditions of load and temperature. A comparative study made on a local motorway provides some correlation to real conditions.

The deformations were measured using pairs of induction coils 25mm diameter with a high speed chart recorder for readout. Deformations were measured vertically, longitudinally and transversely to give comprehensive results. A technique of coil placement is described.

The asphalt concrete materials used were of high stability and had a fixed dense grading of crushed aggregate and two maximum aggregate sizes of 16mm and 9.5mm. The results of trafficking showed the quantitative effects of load, temperature and pavement thickness on the densification of the asphalt concrete. As the density increased the apparent dynamic modulus and the stability of the mix increased. The apparent dynamic modulus also increased markedly as the pavement thickness decreased towards twice the maximum aggregate size, leading to a quantitative assessment of the role of pavement thickness. Vertical and horizontal strains decreased as the density increased and at lower temperatures vertical strains can be less than middepth horizontal strains. Vertical strains are greatly increased by a rise in temperature while horizontal strains increase only slightly.
03028 The Characteristics of Materials for the Design of Flexible Pavement Structures
P. S. Pell, S. F. Brown

An important part of the development of a structural design approach to the design of flexible pavements is adequate characterisation of the constituent materials in the context of the part they play in the pavement structure.

This characterisation should take the form of the determination of elastic constants and failure criteria so that linear elastic theory may be used to compute critical stresses and strains and the acceptability of these may be assessed in terms of the anticipated life of the pavement.

The characteristics of bitumen bound, cement bound, unbound materials and also cohesive soils are discussed against the background of current knowledge. Some results from recent research projects on bituminous materials and soils are briefly presented.

The importance of reproducing in situ stress conditions in laboratory tests is emphasised and present test methods are critically reviewed.

Some of the more important areas for future research are outlined as a result of this review of the current state of the art. The most important of these is considered to be correlation of laboratory determined results and performance in the road, so that the results of laboratory materials testing can be used with confidence in design.
03029 Mechanical Response of Bituminous Mixture Under Various Loading Conditions
Teruo Sugawara

This paper describes the test results concerning the mechanical response of bituminous mixtures which are required in rational designing and performance studies of pavement structure.

A stress-strain relationship of bituminous mixture was investigated under various loading conditions at various temperatures. A relaxation modulus under the constant rate of strain was computed from the stress-strain curve as a function of loading time.

In the flexure test, mode of fracture, relation between flexural strength vs. temperature, strain at rupture vs. temperature were obtained. As a result of beam flexure test, the movement of the transition point at which the mode of fracture shows a change in mode from brittle to ductile, was measured.

A convergence of the strain at rupture in the ductile fracture zone (2 to 5 x 0.01), in the brittle fracture zone (1 to 2 x 0.001) and at the transition point (4 to 6 x 0.001) was obtained respectively, throughout this investigation. The effect of the rate of strain, binder content, type of mixture and type of binder on the rheological properties of the mixture was also investigated.

At higher road temperatures, the resistance to deformation is one of the primary factors in the pavement performance. In this study, simulated wheel tracking test was carried out. The systematic relationships between the resistance to deformation and moving speed. tire contact pressure, temperature and binder property were obtained: And also the relationship between: the Marshall Stability and the resistance to deformation by the Wheel-tracking test was found. This result may shows a difference between-static and dynamic test on the flow properties of the mixture. A kneading action caused from the wheel passage was briefly discussed.

An electro-hydraulic apparatus was developed to investigate the dynamic response and the fatigue properties of the bituminous mixture, and the dynamic response under the programmed strain wave was discussed.
03030 The Fatigue of Bitumen and Bituminous Mixes
W. van Dijk, H. Moreaud, A. Quedeville, P. Uge

We have carried out laboratory investigations into the phenomenon of fatigue both in bitumen (‘asphalt cement’) itself and in mixes of mineral aggregate bound with bitumen as used in asphalt pavements. The objectives are to determine the role of the binder and to attempt to establish a method of predicting road performance from laboratory fatigue tests.

Our research into the fatigue of bitumen has been carried out on thin film specimens that have been subjected to sinusoidal shearing over a range of temperatures and frequencies. In tests with a constant stress amplitude, the initial strain for a given fatigue life has been found to depend strongly on the complex modulus of the bitumen, and the slope of the fatigue curve (log life against log initial strain) to be related to the phase angle between the stress and the strain. When plotting the slope of the fatigue curve against the phase angle there is a maximum which is more pronounced for bitumens of low temperature susceptibility. The fatigue life of bitumen can be interpreted in terms of energy: the energy dissipated per cycle of stress being related to the fatigue damage per cycle, and the fatigue resistance may be regarded as the capacity to dissipate this energy into heat.

Films of filler/bitumen mixtures have also been tested in the same apparatus and it appears that a filler bitumen behaves like an unfilled one of harder grade.

Tests have also been carried out on various asphalt mixes, including a dense wearing course type and an economical base course type with a low binder content. The object has been to correlate the fatigue behaviour of the mixes in bending with that of the bitumen films on the one hand, and with the development of cracking in a slab of mix loaded by a rolling wheel on the other. This last test simulates the tri-axial loading that occurs in practice in an asphalt pavement. Two types of loading have been used in the bending tests: sinusoidal to provide a link with the usual mode of laboratory testing, and intermittent to give strains whose wave-shapes are similar to those that have been recorded under moving traffic in asphalt layers of pavements. It has been found that the rest periods between successive groups of waves, corresponding to the spacing between vehicles, give fatigue lives that are longer than those with continuous sinusoidal loading. In this way, realistic data are being developed for incorporation into asphalt pavement design procedures.
03031 A Fundamental Structural Design Procedure for Flexible Pavements
S. F. Brown, P. S. Pell

The main aim of the conference is to work towards improved methods of structural design for flexible pavements wherein the empirical content of particular methods will be reduced and replaced by analytical methods backed by a knowledge of relevant material properties, details of traffic loading and pavement performance. Such approaches have come to be known as ‘rational design methods’, though an empirical procedure may in fact be no less ‘rational’. ‘Improved design methods’ or the ‘structural design approach’ are better descriptions of the aim of this work.

At the 1967 conference, the question was asked: how much more research must we do before such a method of design emerges?

This paper is an attempt to bring together various research efforts and incorporate them in a structural design approach to flexible pavement design. The approach is based on treating the layered pavement system as a structure in the traditional civil engineering manner and designing it accordingly. A framework for the design procedure is presented and is outlined in a simple flow diagram.

The design process involves the consideration of traffic loading, material characteristics under repeated dynamic loads, the computation of load induced stresses and strains and their comparison with the maximum allowable values for the various materials in the structure. An iterative procedure which modifies an initial estimate of layer thicknesses and materials is used in order to produce a pavement which is satisfactory both structurally and economically.

The difficulty of applying this method lies in the complexity of the structure, materials, loading, environment and criteria of performance. However, certain simplifications can be made and the procedure presented is based on a simple fundamental approach to the problem. The basis of the method is the availability of computer solutions for the linear elastic stress analysis of the structure. More sophisticated programmes are being developed to deal more realistically with the complexities both of the structure and the materials but as yet sufficient information is not available on the material characteristics and performance criteria to warrant their use. It is the Authors’ opinion that linear elastic theory may be used in the manner presented with some confidence and a design example of the procedure is given.

One of the aims of the paper is to highlight those areas where more research effort is needed, such as the important aspect of pavement performance and its correlation with allowable stresses, strains and deformations in the constituent materials of the pavement.
03032 Calculation of the Deformations Caused by Vehicles to Flexible Pavements
Paolo Ferrari

The repeated flexural deformations that vehicles produce in flexible pavements are the most frequent cause of failure of these pavements. This type of rupture is due to a fatigue phenomenon that no asphalt pavement can escape, because of the nature of the materials of which it is constructed, so that even correctly executed pavements crack after a more or less long period.

It is therefore important to design flexible pavements in such a way that they can withstand the flexural deformations caused by vehicles for a certain time without cracking.

Recent experimental research has made it possible to know the relations existing between the flexural deformations caused to an asphaltic concrete and the number of repetitions of this deformation necessary to bring it to rupture. Starting from this relation, it is possible to forecast the life of a pavement subjected to a certain type of traffic, if the deformations caused by the vehicles are known.

The calculation of the deformations is the subject of this paper. For this purpose, the pavement-subgrade system is schematised as a semi-infinite visco-elastic body, whose mechanical parameters can be determined by a special plate bearing test, described in the paper.

The results of the calculations made in this paper clearly indicate the effects on the deformations in the surface courses of a bituminous pavement produced by weight, number and arrangement of vehicle axles and the speed of such vehicles. These results therefore make it possible, among other things, to more rationally assess the influence of the type of traffic and properties of materials in designing flexible pavements. They also make It possible to obtain the knowledge of maximum strains in the surface course, which is necessary for an essentially rational design of flexible pavements, considering the fatigue processes as a determinant cause of failures.

However in the paper it is pointed that, before this design approach can be fully developed, much more theoretical and experimental work needs to be done, essentially in order to investigate the fatigue behavior of the bituminous concrete under the actual road conditions, where at any point of the surface course a series of strains take place, all different from each other and at irregular time intervals.
03033 Applications of Theory in the Design of Asphalt Pavements
F. N. Finn, Keshavan Nair, C. L. Monismith

Pavement design, like other aspects of engineering design, requires that engineers have the ability to analyze pavement structures in terms of significant system parameters. Moreover, it is necessary that such analyses incorporate essential features of observed pavement performance and appropriately measured values of the parameters to make the necessary quantitative evaluations required for design. It is generally recognized, however, that the parameters involved together with their interrelationships are complex.

In recent years a number of attempts have been made to formulate, in a systematic manner, pavement design systems which bring these factors together as a part of the development of improved methods of pavement design, methods which will have the capabilities to:
(1) accommodate the continually changing loading requirements;
(2) better utilize available materials;
(3) accommodate new materials which might be developed;
(4) better define the role of construction; and
(5) improve the reliability for performance prediction (or of the design estimate).

While it is difficult to develop one pavement design system which, at this stage in time, will incorporate all desirable factors, it is possible to develop a series of subsystems the goal of each of which is to minimize a particular form of distress.
To minimize the effects of various distress mechanisms, design frameworks (subsystems) have been established and specific formats are developed. These subsystems parallel the design approach widely used in Civil Engineering practice in which a structure is selected (designed), its behavior under anticipated service conditions analyzed, and its adequacy with respect to a specific distress criterion determined.

A discussion of the most recent methods available to examine each of these subsystems is included. Considering this information, the authors recommend a specific technique to solve for each distress mode recognizing present limitations in materials characterization techniques and availability of solutions for boundary value problems representative of pavement structures as well as present limitations in knowledge of traffic, environmental, and construction effects.

The concluding section is concerned with a discussion of factors not now included in the subsystems described in the report. Consideration is given to optimization and the applicability of dynamic programming techniques to optimizing the solution of the pavement design problem is discussed. Included in the discussion is the ability of dynamic programming to consider the following factors:
(a) Adaptive System – the incorporation of new information gained through observations of performance to predict future performance,
(b) Sensitivity – the influence of individual parameters on the total system, and
(c) Stochastic Processes – the ability to evaluate the degree of uncertainty in the information that forms the basis for design.
03034 Prediction of the Resilient Response of Pavements Containing Granular Layers Using Non-Linear Elastic Theory
R. G Hicks, C. L. Monismith

A laboratory-field study to better define those properties of granular base materials which significantly contribute to the resilient response of pavement structures under repetitive loadings has been conducted.

Results of the laboratory study, previously reported by the authors, indicated that the stress deformation properties of granular materials when measured in repeated load triaxial compression are non-linear with their modulus values and Poisson’s ratios dependent on stress state to a considerable degree.

Field studies included measurements of the response of a prototype pavement, 20 ft. by 20 ft. in plan, to repeated-load plate tests and of an in-service pavement in San Diego County, California to an actual truck load. Both pavements consisted of an asphalt concrete surface, a granular base course, and a clay subgrade. Instruments were placed at various positions in the pavement structures to measure deflections, stresses and strains.

For the prototype pavement, the plate load tests were conducted at the pavement surface with the base course in both a partially saturated and saturated state. Loads were applied through rigid plates at durations representative of moving traffic and in sufficient numbers to assure that a reasonable measure of the resilient behavior could be obtained. These responses were then compared with computed values which were obtained from three different non-linear elastic analyses and laboratory determined stress-strain properties. The results indicated that the predicted stresses, strains and displacements compared reasonably well with field measurements; the responses yielded by the different numerical methods were similar; and the resilient responses of the system were only slightly affected by degree of saturation of the base course.

The approach developed to predict pavement responses for the prototype pavement was extended to a full-scale test road in San Diego County, California, to verify its application to conditions representative of moving traffic on highways. Measurements of strains and displacements were obtained under a slow moving truck and compared with predicted values. As with the prototype pavement, results of the analyses indicated that the predicted and measured responses were in good agreement.
03035 The Modulus of Asphalt Layers at High Temperatures: Comparison of Laboratory Measurements Under Simulated Traffic Conditions with Theory
A. Hofstra, C. P. Valkering

In order to investigate whether elastic theory can be applied to the behaviour of flexible pavements under moving wheels, particularly at high temperatures, we have measured in our Laboratory Test Track the stresses and strains imposed by a rolling wheel. The test pavements consisted of an asphalt layer on top of a sand subgrade; the temperature of the asphalt layer has been varied between 20 and 60°C. The results have been compared with the theory. Apart from a certain degree of asymmetry of mainly the strain signals, the shapes of the experimental signals correspond well with those predicted by theory. The asymmetry has been found to be little dependent on temperature.

The maximum values of the stresses and strains are also compared with those predicted by theory. In this comparison, for the asphalt layer a Young modulus is used which is dependent on the temperature of the asphalt layer and on the width of the signal (or the loading time). In this way the influence of wheel speed and penetration grade of the binder on the maximum strains as well as the difference in magnitude between the longitudinal and transverse strains in the wheeltrack could be explained.

The comparison of the experimental results with theory indicates a condition of the interface between the asphalt layer and sand subgrade which is intermediate between slip end complete friction.

It is concluded that elastic theory can be used to describe the behaviour of flexible pavements subjected to moving wheels even at the highest temperatures that are likely to be experienced in pavements.
03036 A Comparison of Plate Load Testing with the Wave Propagation Technique
D. A. Kasianchuk, G. H. Argue

The repetitive static plate load test method has been used by the Canadian Department of Transport since its development in 1947 for the design and evaluation of the airport pavements within its jurisdiction. The vibratory loading wave propagation technique, of more recent development, offers some advantage over the plate load test in several aspects of the testing procedures. In order to determine the degree of correlation that could be expected between these two methods in the evaluation of the load carrying capacity of pavements at Canadian airports, a pilot test program was conducted during the summer of 1970.

Thirty-five vibratory tests were performed using the Shell Canada Limited equipment and technique at six Canadian airports. The test sites were selected from among those included in the annual plate load test program to represent a wide variety of the asphalt pavement types found in Canadian airports. The wave propagation measurements were analysed to provide values of elastic modulus to be used in the prediction of pavement response by layered system elastic theory. These predicted values were compared to those measured at each site in the actual plate load test.

Although the comparison obtained indicates some relationship between these approaches, further work is required to more accurately assess the predictability of plate load tests from vibratory test results.
03037 Design of Flexible Pavements for Major Highways
A. M. Krivissky

On the basis of comprehensive theoretic and experimental research carried out in the Soviet Union a method has been developed for the design of flexible pavements with asphalt concrete or similar surfacing. Structural design of this type of pavements should provide for their performance to be only in the phase of recovery (elastic strain).

Special investigations have shown that evaluation of stress-strain condition of such pavements can be obtained by applying solutions of elasticity theory for lmred semi-infinite space. The following physically real conditions are used in design as limiting criteria:
– Shear limit equilibrium in subgrade soil or in moderately cohesive materials of structural layers;
– Maximum allowable bending-tensile stress in monolithic layers (asphalt concrete, cement-bound materials) for given loading conditions.

The design procedure involves both strain (E,mu ) and strength characteristics of materials and soils the latter being shear resistance of soils and moderately cohesive materials (Y, C) and also bending-tensile strength of monolithic materials (R). Design characteristics are normal and can be obtained by means of testing samples, their state simulating the condition of materials or soils in pavement structure.

Although the design procedure includes application of relationships which are rather complex for calculation it has been found possible to draw nomograms that bring complicated calculations to simple graphical solutions. The suggested method makes it possible to design not only a total pavement thickness, but also a required thickness of each structural layer, with detail consideration of properties of the materials used.

The new structural method is well founded by the evidence of good performance of pavements under various climatic and service conditions.
03038 The In Situ Determination of the Elastic Moduli of Layered Pavements Using SH-Wave Propagation
M. Kurzeme

A method is presented of determining the in situ elastic properties of layered pavement materials through the excitation and observation of horizontally polarized shear waves (SH-waves). The method consists of generating continuous SH-waves at the frequency and the location desired, detecting and observing the wave motion over some distance from the wave generator, measuring the surface velocity of phase propagation at each imposed frequency, and interpreting the observed variation in surface phase velocity with change in imposed frequency of vibration to deduce the in situ elastic properties of the pavement materials.

Theoretical dispersion relationships have been developed that describe the variation in surface phase velocity with change in imposed frequency of SH-wave motion in idealized layered pavement structures.

The existence of modes of propagation predicted by the theoretical dispersion relationships were investigated on laboratory models of layered pavement structures. These tests confirmed the presence of the predicted modes.

The propagation of SH-waves was then investigated on real pavement structures. A torsional vibrator, capable of generating predominantly SH-waves, was constructed and a phase velocity measuring system and a field procedure were developed. The relationships between the surface phase velocity of SH-waves and the imposed frequency as observed on a number of two- and three-layer pavements are presented.

To deduce the shear wave velocities within the individual layer materials, the observed relationships have been interpreted by matching with the theoretical dispersion relationships of comparable idealized structures. A knowledge of the material densities allows the calculation of dynamic shear modulus of the materials.

Using the theoretical relationships presented, it is concluded that in favourable cases observations on real pavements can be interpreted, to deduce the in situ dynamic shear moduli of the layer materials. The relevance of the material properties so obtained is dependent on how closely the theoretical model used approximates to the real structure.
03039 A Study of Stress and Strain in the Asphalt Pavement of Tomei Highway
Yuji Miura

The Tomei Highway of 346 km. was constructed in 1969, between Tokyo and Nagoya, serving for the heavy traffic of more than 25,000 vehicles per day, Having been designed by the CBR method adopting the Structural Number and Layer Equivalency that were the results of AASHO Road Test, the whole distance was paved with asphalt concrete of 10 to 15 cm. thickness, that included surface and binder courses, over asphalt stabilized base course of 15 to 22 cm. thickness laid on the various type and thickness of subbase.

On the highway, 17 observation spots were settled, and a lot of electric resistance strain gauge, pressure cells, electrical deformation devices and thermocouples were laid into the pavement. The stress and strain of the pavement, including those in certain depth of the subgrade, were observed in the field under the controlled wheel loads. The purpose of this investigation is to know the behavior of the pavement and to obtain the data for rational design of asphalt pavement structure,

In this paper, three out of 17 observation spots are taken for the study, and not only the details of the paving and measuring techniques but also the properties of the materials, that have been determined both in the field and in the laboratory, are elucidated.

On the other hand, the measured values of stress and strain under the wheel loads are analyzed minutely, the measured data being compared with computed values according to the multi-layered elastic theory. The comparisons are made by the following two methods:
1) Assuming that each pavement is composed of three elastic layers and loaded with single circular load, the elastic modulus of each layers is arranged and determined so as to be equal approximately to the computed and measured values of stress and strain of the pavement, and then compared with the measured modulus taken in the field or in the laboratory.
2) Assuming that each pavement is composed of four or five elastic layers and loaded with dual circular load, the stress and strain of the pavement are calculated by using the elastic modulus obtained from the field measurement or the past data, and compared with the measured values, especially with the lateral strain of the asphalt treated layers. In addition, the results of the investigations and theoretical analysis are discussed concerning the center, longitudinal and traverse position of the loads.

From the comparisons, made by the said two methods, some conclusions are drawn as for the propriety of analysis of the pavement structure by using the multi-layered elastic theory, and that the theory is able to predict not only the distribution of vertical stress and displacement of the pavement but also the distribution of horizontal strain in the asphalt treated layers under the traffic loads.
03040 A Stochastic Approach to Analysis and Design of Highway Pavements
F. Moavenzadeh, J. F. Elliott

A stochastic approach is proposed for the development of a rational method of analysis and design for flexible pavement structures. The approach utilizes a three-layer viscoelastic model, a cumulative damage theory, and systems simulation techniques. In this approach the environmental variables, properties of layered materials, geometry of the pavement structure, and loading variables are described in a stochastic manner utilizing the Monte Carlo simulation techniques.

Through the use of these three interrelated models, the pavement structure and its structural integrity is simulated continuously throughout its design life. The results of the study is used to demonstrate (for a given pavement structure), the nature of damage, the manner in which damage accumulated, and the probable time that damage accumulation exceeds the allowable limits and the pavement requires reconstruction. For the design, several possible alternatives can be simulated, and the one which has the most desirable characteristics can be chosen.
03041 Applications of Computer Codes to the Analysis of Flexible Pavements
Raman Pichumani

The capabilities and limitations of three computer codes suitable for static analysis of flexible pavement structures are presented in this paper. One of these codes, WIL67, is used for the analysis of single-wheel loading; the other two, BISTRO and AFPAV, can analyze the pavement response due to the multiple wheels of very heavy landing gears of modern jumbo jets such as the U.S. Air Force transport aircraft C-5A and the Boeing 747 commercial aircraft. The WIL67 and AFPAV codes are based on finite element structural analysis technique; the BISTRO code makes use of Burmister’s layered-system concept.

The BISTRO code, developed by the Shell Oil Company, has made it easy to apply Burmister’s general theory for calculating stresses, strains, and displacements at any point in a linear elastic multilayered pavement system due to normal surface loads. This computer program predicts the pavement response due to one load at a time at a designated point in any layer and then uses the principle of superposition to determine the multiple load response. The WIL67 code, developed at the University of California at Berkeley, is a two-dimensional finite element program which solves the layered-pavement problem by treating the pavement as an axisymmetric solid subjected to an axisymmetric load. Although there is a version of this code which can analyze the effects of multiple loads using the superposition principle, the program studied in this research effort is restricted to the analysis of a single wheel load. The AFPAV code is an extended two-dimensional finite element program capable of analyzing prismatic solids. This program idealizes the layered pavement system as a layered prismatic structure and expresses the applied loading as well as the resulting displacements in terms of Fourier series. Therefore, the accuracy of the solution and the computer time required by this code is governed by the number of Fourier terms used. However, unlike the BISTRO code, the linear elastic response of the pavement system due to multiple wheels can be determined by this code in one step without recourse to the superposition principle. Therefore, the AFPAV code is far more efficient and economical than the BISTRO code for analyzing layered pavements loaded by multiple wheels, particularly with increasing number of layers, and increasing number of points where stresses and displacements are required.

Theoretical pavement response predicted by the AFPAV code is compared with field data from a full-scale flexible pavement test section which was constructed by the U.S. Army Engineers Waterways Experiment Station (WES) at Vicksburg, Mississippi and statically loaded by a 12-wheel assembly of a C-5A landing gear. The dependence of the pavement response on the elastic constants of various layers is demonstrated by a parametric study, thus emphasizing the importance of accurately determining these parameters. In conclusion, it is shown that it is necessary to continually update the capabilities of the AFFAV code to consider the more realistic characterizations of the pavement layers as they become available.
03042 Experimental and Theoretical Studies of Pavement Behaviour Under Vehicular Loading in Relation to Elastic Theory
E. N. Thrower, N. W. Lister, J. F. Potter

The paper describes a series of experiments to determine the degree to which multi-layer elastic theory can be used to predict stresses, strains and deflections of pavements under moving vehicles.

Measurements of stress and deflection in flexible pavements of realistic design have been made under pilot-scale conditions over a wide range of wheel loads, speeds and temperatures, and similar tests have also been carried out on a simple concrete slab. Results are presented showing the effects of temperature and wheel load on the measured behaviour and this is compared with that predicted from multi-layer elastic theory using data derived from both laboratory and in-situ testing. The results show that elastic theory can be used to predict the dynamic behaviour of relatively stiff pavements, but that deviations occur under high temperature conditions, where it is difficult to establish an effective modulus for bituminous materials because of the rapid change in dynamic modulus with the timescale of the loading, and for pavements which derive a large part of their-structural stiffness from granular materials.
03043 Some Considerations on the Theoretical Estimation for Deflection of Pavement Structures
K. Ueshita, T. Arakawa, Y. Watanabe

Empirical relations between surface deflection and thickness factor of pavement structures show that the effect of thickness of pavement to reduce deflection is more than the one estimated from the theory of elasticity. Extreme case is shown by the soil-cement/subgrade two-layer system which can never been explained by the theory of elasticity. The authors tried to explain these phenomena by using the finite element analyses assuming non-linear elastic properties for subgrade materials. By these computations, experimental relations of the soil-cement pavement could be almost explained. Besides, a useful equation was introduced to formulate the relationship of deflection versus thickness of non-linear elastic pavements.
03044 Effects of Multiple Wheel Systems and Horizontal Surface Loads on Pavement Structures
C. P. Valkering

The theory of elasticity has been applied to layered systems representing flexible pavements. The distributions and the maximum values of the tensile strain in the asphalt layer as the compressive strain in the subgrade under multiple-wheel systems have been calculated. The concerted action of the wheels depends, not only on the wheel spacing, but also to a significant degree on the structure which renders weighting traffic for design purposes difficult.

A thickness design method based on these strains as criteria is equivalent to one that is based on shear or deformation energy criteria, provided that the permissible values are obtained at the same stress condition as prevails in the pavement.

Under normal end tangential surface forces the influence of asphalt layer thickness on the normal and shear stresses at the asphalt/base interface has been investigated: the stresses decrease with increasing thickness. At high asphalt temperatures the shear stresses might be critical for the adhesion between, particularly, a thin asphalt layer and a stiff base.
03045 Design of Full-Depth Asphalt Airfield Pavements
M. W. Witczak

A theoretical design procedure for Full-Depth asphalt concrete airfield pavements is presented. The design is based upon the use of multilayered elastic theory and utilizes the concept of limiting strains to prevent repetitive permanent deformation and/or shear failure within the subgrade layer and repetitive load cracking within the asphalt bound layer. Development of the allowable strain levels for both failure modes are presented.

The method utilized to obtain limiting strains associated with the subgrade was to theoretically analyze flexible (granular base) pavement thickness requirements as defined by the newly revised U.S.A.C.E. thickness design method. These revisions are related to changes in thickness requirements as well as load repetition effect upon thickness from the previous method. Results indicate that a limiting vartical strain of 1460 microinches per inch, evaluated at a limiting asphalt concrete modulus of 100,000 psi is capable of withstanding 1,000,000 strain repetitions.

Limiting strains associated with the asphalt bound layer have been established from results developed by Kingham from Full-Depth asphalt concrete pavements of the AASHO Road Test study. An allowable tensile strain of 76 microinches per inch will allow 1,000,000 repetitions when evaluated at a critical asphalt concrete modulus of 1,450,000 psi.

Because of the extreme dependency of the stress and strain distributive characteristics of thick Full-Depth asphalt pavements to temperature, monthly cumulative damage techniques were used to develop thickness adjustment factors (TsubF) to adjust thickness requirements for both failure modes due to differing environments. The limiting or critical modulus utilized in the vertical subgrade strain analysis is related to an average annual air temperature of 75°F. For cooler environments, a thickness reduction may be used. The maximum suggested reduction proposed is 10% (TsubP = 0.90) for environments having a 50°F average annual air temperature. Maximum percentage thickness reductions of 13% are suggested for the tensile asphalt concrete strain analysis. This is equivalent to a maximum TsubF value of 1.00 at 40°F (average annual air temperature) and a minimum TsubF value of 0.87 for 60°F environments.
03046 Sensitivity Analysis of Various Cost Elements in Flexible Pavement Design
Eldon J. Yoder, Farideh Ramjerdi, William L. Grecco

This paper presents a method for predicting the optimum initial service life and optimum periods of resurfacing for flexible pavements. The method is based on consideration of total pavement costs including the cost of initial construction, routine maintenance and major maintenance and increased road user costs resulting from the maintenance operations.

Standard economic analyses techniques were used for determining the average annual cost of alternate designs. A modification of the Radzikowski model was used for estimating routine maintenance cost of flexible highway pavements. The pavement design method developed by the Corps of Engineers was utilized in estimating initial design as well as required major maintenance (resurfacing). A method was developed which presented an estimation of road user costs due to maintenance and resurfacing operations.

Variables evaluated in this paper included:
(1) subgrade type;
(2) initial traffic volume;
(3) rate of traffic growth; and
(4) rate of interest on the investment.
Solutions were made for both 2-lane and divided 4-lane highways.

The results of the study are presented in the form of graphs which indicate the initial design period which results in least cost for combinations of the variables given.
03047 Low-Temperature Pavement Cracking Studies in Canada
Roads & Transportation Association of Canada, Soils & Materials Committee, and Pavement Design & Evaluation Committee

Cracking of pavements at low temperatures has become a serious and extensive problem in Canada. During the past decade, various highway departments, producers and others have devoted considerable effort towards finding a solution. This paper summarizes their progress in finding the causes of the problem and their development of some practical, engineering solutions.

Field inventories were initially conducted to determine the nature and extent of the cracking. From these, and other observations, the bituminous component seemed in most cases to be the major variable. Subsequent field sampling, plus full-scale experiments such as the Ste. Anne Test Road in Manitoba, the Alberta Test Road, the Saskatchewan Test Road and the Arkona Test Road in Ontario, and laboratory investigations confirmed that certain asphalt cements were primarily involved.

Several design approaches were formulated from these findings, the earliest being that of modified specifications. Later, asphalt and mix stiffness or strain limits, fracture temperature calculation procedures, and most recently a cracking frequency estimation technique, were developed as design guides.

The paper demonstrates that while this progress has been significant, some key aspects of the problem remain. One of these concerns the treatment of the many miles of existing, cracked pavements.
03048 The Fatigue of Flexible Pavements
Bonner S. Coffman. George J. Ilves, William F. Edwards

Five asphaltic concrete pavements, each of which measured 20×30 ft. and contained two test areas, were constructed on a 48" compacted clay subgrade. Certain of these pavement areas were fatigue loaded through concentric rings composed of truck tire rubber by superimposing one dynamic 10 Hz haversine pulse on a small static load every second to simulate a continuous line of wheel loads traveling in identical wheel paths 50′ apart at 40 MPH. Surface tangential strain, surface deflection and temperature sensors were placed at a number of radii from the load plate centerline and recorded periodically throughout each test. The fatigue of four test areas, as evidenced by visible cracking, was observed closely and noted in a log book. Asphalt and compacted subgrade samples were returned to the laboratory for the determination of structural strength and physical properties. The results of these tests were entered into the Chevron n-layer program along with a number of hypothetical moduli for the natural subgrade underlying these layers. Theoretical strain and deflection profiles obtained from these calculations were compared to measured profiles to determine the best average apparent modulus for this semi-infinite layer. Trapezoidal specimens were sawed from asphalt pavement blocks and fatigue loaded with one 10 Hz haversine pulse per second over a wide range of temperatures and strain levels. An equation relating these quantities to fatigue life was developed and coupled with a theoretical pavement fatigue model to predict the time of initial surface cracking on the four test areas where visible cracks were observed.
03049 Failure Criteria for Flexible Pavements
D. Croney

The term ‘design life’ when applied to a road pavement implies a terminal or ‘failure’ condition beyond which the performance of the pavement will be regarded as unacceptable. For design procedures based on past experience a relatively loose definition of failure has been acceptable, but with the growing interest in structural design procedures, failure criteria expressed in more exact physical terms are essential.

This paper discusses the definition of the ‘failure’ condition which has been accepted for flexible pavements in Britain. The ‘critical’ condition at which overlaying to extend the life of the pavement should be carried out, is also considered.

Both these performance criteria have, for flexible pavements, been expressed in terms of permanent deformation either expressed as a rut-depth or as total deformation from the original pavement level. Observations made on normal in-service roads and on closely observed experimental roads have shown that the criteria ore not markedly different for pavements using lean concrete, bituminous macadam or unbound stone bases.

The Present Serviceability Index concept is not regarded in Britain as very applicable to the structural design problem because of its heavy dependence on riding quality factors, not necessarily associated with traffic stresses. However an approximate correlation between the British approach and P.S.I. values is given.
03050 Permanent Deformation of Flexible Pavements Under Simulated Road Traffic Conditions
A. Hofstra, A. J. G. Klomp

In a circular laboratory test track the rutting of flexible pavements has been studied at various temperatures under well-controlled conditions.

It has been found that temperature has a great influence on the depth of rutting, the increase in rutting over a temperature range of 20°C-60°C being much larger than the increase in calculated elastic deformation. The permanent deformation per wheel passage correlates with the stiffness of the asphalt binder used and decreases with increasing number of wheel passages.

An increase in thickness of the asphalt layer leads to a distinct reduction in subgrade deformation. The change in thickness through rutting is not larger for a 20 cm than for a 10 cm asphalt layer. Proper mix design proves to be an important factor in relation with permanent deformation.
03051 Strain and Curvature as Factors for Predicting Pavement Fatigue
Y. H. Huang

The fatigue cracking of asphalt pavements is caused by the repeated applications of excessive tensile strains in the asphalt-bound layer. To predict fatigue, it is necessary to determine the maximum tensile strain at the bottom of the asphalt-bound layers. Two methods are suggested for determining the maximum tensile strain, based on the two-layer elastic theory. The use of two-layer theory, instead of the conventional three- or multiple-layer theory, is based on the fact that the tensile strains at the bottom of the asphalt-bound layer depend on the property of the asphalt-bound layer relative to that of the underlying layers. Any multi-layer systems can thus be reduced to a two-layer system, if an average modulus of elasticity is used to represent the combined effect of all the underlying layers, Fortunately, the moduli of untreated granular materials and soils generally fall within narrow ranges, and typical values can usually be assumed.

To facilitate the application of the two methods, simple charts are presented for determining the maximum tensile strain under a set of dual tires. The first method, which can be used for pavement design, requires a knowledge of the elastic moduli of both layers and the thickness of the asphalt-bound layer. By entering these variables into the chart, the maximum tensile strain can be easily determined. The second method, which can be used for pavement evaluation, requires the measurement of curvature on the pavement surface. Knowing the curvature, the modulus ratio, and the thickness of the asphalt-bound layer, the maximum tensile strain can be determined from the charts. It was found that for asphalt- bound layers of 4 in. thick or more the curvature-tensile strain ratio is practically independent of the modulus ratio. By simply measuring the curvature on the surface, the maximum tensile strain can be estimated, and the adequacy of the pavement to withstand fatigue evaluated.

Examples are given to illustrate the use of these charts for pavement design and evaluation.
03052 A Design System for Minimizing Fatigue, Permanent Deformation and Shrinkage Fracture Distress of Asphalt Pavements
D. A. Kasianchuk, R. L. Terrel, R. C. G. Haas

The design of asphalt pavements includes three major structural subsystems: load-associated fracture, load-associated permanent deformation, and shrinkage fracture. In order to accelerate progress towards a rational design system it is desirable to accelerate improvements in the technology of these three subsystem areas.

This paper suggests a series of needed research and development tasks for each design subsystem. In addition, some discussion is devoted to briefly justifying the recommendations and to summarizing the current state of design knowledge for the three areas.

The paper includes in flow chart form the interrelationships between subsystems and attempts to place the subsystems within an overall pavement design and management framework.
03053 Failure Criteria Developed from AASHO Road Test Data
R. Ian Kingham

Theoretical models of pavement deformation behavior such as elastic-layered theory can only be used for design purposes when failure criteria are specified. Although such models can be used to predict stress and strain states, they in no way indicate whether the material in the pavement can withstand the predicted deformations. For elastic-layered theory, limiting values of strain or stress need to be defined before the theory can be used to assist practicing engineers in the design of asphalt pavements.

There is general agreement in the literature that horizontal tensile stress or strain at the bottom of a thick asphalt layer is the controlling criterion for design to prevent repetitive load cracking. Although such strains were not measured at the bottom of the asphalt layer at the AASHO Road Test, they can be inferred from a knowledge of the material characteristics and the measured deflections. Repetitive load cracking was observed to be the predominant mechanism of initial failure at the Road Test. Since the bituminous base sections provided a complete range of performance, from failures to survivors of over 1 million load repetitions it was possible to describe the strain history of these test sections in terms of performance.

The bituminous base sections fell into three performance classifications, depending upon whether they failed the first spring of testing, survived the testing period with a low serviceability rating or survived the testing without any change in serviceability. The horizontal tensile strain, horizontal tensile stress and vertical strain on top of the subgrade data were computed for each test section in each performance classification. Asphalt moduli for a wide spectrum of deflection measurements were input into the stress and strain computations. Moduli values were determined from dynamic loading in compression. Subgrade moduli were inferred from the deflection measurements.

The results of the elastic-layered computations showed that there were indeed large differences in horizontal tensile strain, horizontal tensile stress and vertical strain in the subgrade, depending upon the performance classification. Secondly, the level of strain or stress for each performance classification was a function of the asphalt base stiffness at the asphalt layer bottom. From the horizontal strain results it was apparent that asphalt pavements can tolerate higher strains at lower stiffnesses.

The horizontal tensile strain and stress relationships with asphalt stiffness were converted into "load repetition to failure" relationships by relating two performance classifications to the number of load repetitions to failure. A log-log relationship was assumed. The resulting family of "fatigue-like" curves for a range of asphalt stiffnesses has been used by Witczak and is the subject of another paper to this conference.
03054 Deflection Criteria for Asphalt Pavements
Sadao Nagumo, Minoru Tsukinari, Seiichi Tanimoto

The structural design of asphalt pavements in Japan is based on the CBR method, but recently attempts have been made to utilize surface deflection in the design of pavements for lightly trafficked roads and those on weak subgrades as well as in the determination of overlay thicknesses.

The Ministry of Construction carried out performance surveys of pavements on about 750 sites of national highways and about 3,600 sites of lightly trafficked roads during the period 1963 – 1965, and 1964 – 1967 respectively.

Full-scale experimental pavements have been constructed successively since 1967. The results of observations of those experiments have indicated that the stabilization or treatment of base, or subase materials with binders is quite effective in reducing the surface deflection and that the surface deflection is much influencing on pavement failures.

From the results of performance surveys on existing roads and experimental pavements, the authors obtained the following formula which expresses the relationship between the critical deflection for asphalt pavements and the accumulated traffic volume:

log 10N = 0.179 d2 – 1.117 d + C

where N: Accumulated traffic volume of heavy commercial vehicles, when the pavement becomes to require heavy repairs . . . (vehicle/in one direction)
d: Critical deflection expressed by Benkelman Beam rebound deflection using a wheel load of 4.1 tons . . . (mm)
C: Constant, 6.772 for general national highways and 6.385 for lightly trafficked roads, might be a little varied depending upon the layer thickness of asphalt mixtures and the types of bases and subbases.

When this relationship is applied to the surface deflection of the newly constructed pavements of national highways, it is predicted that no large-scaled repair works will be necessary for at least five years after construction for medium trafficked roads (traffic volume of heavy commercial vehicles: 1,000 vehicles /day, in one direction).

In the design of pavements of lightly trafficked roads which consists of a comparatively thin bituminous surfacing and a base on the top of the existing gravel roads, the authors obtained a design curve which makes it possible to determine the additional layers thickness to give a target deflection on the surfaces.
03055 The Analysis and Design of the Flexibility of Pavements
D. V. Ramsamooj, K. Majidzadeh, E. M. Kauffmann

This paper deals with the application of fracture mechanics to the problem of fatigue cracking and failure of flexible pavements. It describes experiments conducted on four-foot diameter bituminous slabs supported on an elastic foundation in order to verify Paris’ crack propagation law. The crack pattern and the actual crack lengths were obtained by X-ray photography and the techniques for analyzing the rate of crack propagation for multiple cracks are presented.

Excellent agreement has been found with Paris’ law dc/dN = AK4, where dc/dN is the rate of crack propagation, K is the stress-intensity-factor, which measures the general load transmission to the vicinity of the crack tips, in accordance with the load, geometrical and boundary conditions, and A is shown to be a materials constant. On the other hand comparisons made on the basis of the bending stress at the interface of the slab or beam and foundation showed discrepancies. This is not surprising since the stress-intensity-factor is a much more powerful parameter than the bending stress, being able to take into account much more fully the boundary and geo- metrical conditions in accordance with the load and the amount of cracking in the pavement.

The criteria for fatigue failure of pavements is defined as the time for the stress-intensity-factor at the tip of the largest crack to reach its critical value, since rapid crack propagation would then occur, or the time for the total area of cracking to exceed ten per cent of the area of the pavement surface, whichever occurs first.

Finally, assuming knowledge of the load-time history of random loading, and the variation of the material constants (such as the Young’s modulus of each layer, the crack propagation constant, A, and the critical-stress-intensity factor, KlC, of the bituminous material) with temperature, rate of loading, age and moisture content, etc., a method of analysis and design is proposed to guard against flexibility failures in pavements. Further simplifications are then introduced to illustrate the effect of using thicker bituminous layers to replace granular material that contribute the same strength according to the AASHO method of design, and to obtain the load equivalency factors for various magnitudes and configurations of wheel loads.
03056 Rut Depth Prediction in Asphalt Pavements
J. E. Romain

A computational procedure and a computer program are presented that enable the highway designer to predict the rut depth, after a given number of vehicle passages, of a road structure considered for design. The ingredients to be fed in are: the stress distribution throughout the layered road structure under a wheel load; the characterization of the vehicle axles; the statistical distributions of the magnitudes of the wheel loads and of their locations across the road surface; the number of vehicle passages to be withstood; the deformation law of an element of each material under repeated stressing.
03057 Evenness and Serviceability of Roads
W. Schwaderer

Evenness and serviceability are terms about which, in colloquial language, everybody, can make himself an idea, but which can hardly be defined scientifically. Highway engineers always knew that there is a relation between evenness and the degree of serviceability. At the beginning of the thirties a geometrically -abstractly formulated evenness criterion was introduced in Germany for acceptation purposes, which is still valid today. In default of profound knowledge, people tacitly thought that this criterion guarantees a maximum serviceability. Investigations by Carey and Irick brought a serviceability standard and showed that there can only be a relation between evenness and serviceability if evenness is appropriately defined. While evenness can be defined in an entirely mathematical-geometric way and thus stays independent of epochal influences, serviceability is submitted to temporal ideas. Extensive investigations in Germany confirm the American results. Other investigations pointed out the existence of serviceability index limits which for traffic reasons should be strictly kept at the moment of pavement construction (acceptance) as well as at the end of life of a road.
03058 Control of Design of Pavements through Elastic Layers Method Using Real Dynamic Modulus Values
Jorge Tosticarelli, Luis M. Zalazar

The paper describes the first experiences made in Argentina to measure elastic dynamic properties of road materials and the use of actual modulus values so determined to the control of flexible pavements through Elastic Layers Method.

The first part of the paper describes the technique and instrumentation developed to make in situ measurements of elastic properties of the different layers by the Surface Wave Method and laboratory test on cores by non-destructive Resonance and Pulse methods.

The second part of the paper refers to the control of several flexible pavement designs by Elastic Layers Method using real dynamic modulus values obtained by the Surface Wave Method.

The flat terrain of Santa Fe, the unfavourable soil profile and the lack of coarse granular materials in the zone, makes a difficult situation for an economical pavement design.

The used procedure has permitted to clarify the properties of very peculiar structures of pavements as "Sand-Soil Asphalt Emulsion Stabilization" which was quite a pioneer process to make stable subbase and base layers with fine local materials.
03059 Moduli and Critical Strains in Repeated Bending of Bituminous Mixes. Application to Pavement Design
J. Verstraeten

The purpose of this paper is to present results concerning moduli and fatigue strength of bituminous mixes tested in sinusoidal bending (controlled stress tests) with a view to their use in pavement design.

The domains of variation for temperature (T), frequency (f) and number of cycles (N) are respectively: -2O°C <equal to/ less than> T <equal to/ less than> +30°C; 3 Hz <equal to/ less than> f <equal to/ less than> 100 Hz; 10super3 < N <equal to/ less than> 10super6.

The 27 mixes differ by their composition and by the type of bitumen.

The data indicate that the apparent activation energy relation (Arrhenius equation) may be used to evaluate the frequency and temperature dependent response of bituminous mixes. It is also important to note that the apparent activation energy is approximately the same for all the mixes investigated (46 kcal/mole) It is also established that, within the limits of the mixes investigated, there exists a general relation between the ratio |E*| /V sub A (ratio between the modulus of the mix and the percentage volume of aggregate in the mix, voids included) and the product |E*|sub b x gamma (product of the corresponding modulus of the bitumen and a factor which depends on the penetration of the bitumen). The interest of such a relation is to permit, for practical purposes, to estimate the values of the modulus |E*| of a mix knowing its composition, the corresponding modulus |E*| sub b and the penetration of the bitumen.

For the fatigue strength in bending it is established that the critical strain, epsilon sub r (N), for a given number N of cycles depends principally on the percentage volume of bitumen in the mix (voids included) and also on the asphaltene content of the bitumen and the kind of stones. As found previously epsilon sub r (N) is practically independent of temperature and frequency.

On the basis of these results and therefore within the limits of the mixes investigated a general formulation of the fatigue law of bituminous mixes is given and the statistical aspect of the phenomena is taken into consideration for the application of this law.

It is also concluded that in practice the choice of a composition must be made in the general frame of the road conception and not only on the basis of a single property.
03060 Pavement Serviceability Measurements for Pavement Evaluations
Roger S. Walker, W. Ronald Hudson, Freddy L. Roberts

The primary operating characteristic of a pavement at any particular time is the level of service that it provides to the users. One of the important factors influencing levels of serviceability is roadway roughness and one of the generally accepted methods for characterizing roadway roughness is the serviceability index concept. This paper is concerned with methods of achieving this measure for Texas highways through the use of the serviceability-performance concept. This concept requires a correlation between objective, physical measurements of pavement characteristics, and subjective measures of pavement quality by highway users.

To obtain these correlations, physical measurements were performed with the SD Profilometer while subjective measurements were provided by a 15-member rating panel. Regression models, relating these measurements, are presented and field evaluations and subsequent modifications of these models are included. Correlations of these models with an inexpensive roughness measuring device (the Mays Road Meter) are also discussed. This device provides an economical means for rapidly obtaining routine performance information. The SD Profilometer is used as a measurement standard, thus maintaining compatibility in performance measurements.

The use of spectral analysis as another means of providing a more comprehensive method of identifying pavement characteristics and characterizing pavement performance is presented. An example of pavement identification in construction control and a brief discussion of current research using spectral analysis for providing a more comprehensive serviceability model are discussed.
03061 Spanish Full-Scale Road Test on the Madrid-Barcelona Road
E. Balaguer Camphuis, J. A. Fernandez del Campo

On the Madrid-Barcelona road and in the period 1962-1964, a full scale test road with flexible and concrete pavements was constructed. The length of this test road is about seven kilometers. The flexible pavements were intended for the determination of the behaviour of different types of pavements in real conditions on a very heavy traffic road. 48 different sections of flexible pavements were constructed with three types of bases (macadam base, continuous graded granular base and asphalt base), two types of binders, and eight types of surface courses. In this road, traffic is especially heavy and carefully controlled in that concerning composition and number of vehicles. An electronic dynamic scale was installed.

Hitherto, the evolution of pavements has been followed pointing out their damage on account of the accumulation of traffic effects, and studying the evolution of enough series of parameters to define, in a moment, the conditions of road pavements.

For this purpose studies have been established on:
– Evolution and seasonal variations of elastic deflections measured with the Benkelman beam on the different pavement structures. Also were established correlations between the deflections and pavements temperatures, and also between the deflections obtained by different systems of measure.

– Variations of bearing capacity of the pavements under static and dynamic loads and its changes with factors such as, temperature, moisture content, etc. For this purpose, special instruments for measuring stresses were installed in the pavements.

– Variations of geometric surface by the periodic leveling of studs included in the surface of pavements and by the use of a longitudinal profilometer – (viagrafo) and a transversal profilometer.

– Settlements at different depths in the pavement structures.

– Influence lines of strains under the circulation of heavy axles at different speeds. Consequently, some idea of "response" of the pavements to dynamic loads.

– Evolution of surface texture of the wearing course.

The results obtained have served to establish some laws of behaviour of tested pavements in terms of traffic, some judgments on the road conditions and some laws on the influence of ambience factors in the test results.

The behaviour of the pavement structures with the two types of granular bases is relatively as different under slow loads as quick loads. For slow loads the structures with continuous graded granular bases give moduli of rigidity greater than the structures with macadam base. For quick loads the moduli are very much more similar, but those of the macadam base are slightly greater. The behaviour of the structures with an asphaltic base are very dependent on the season of the year for its correlation with the pavement temperature. For each speed of load linear logarithmic laws are established between the bearing capacity and the temperature of the pavement.

For all structures also logarithmic laws are established between elastic Benkelman deflexions and pavement temperatures. The parameters for the different structures are included. Conclusions about critical Benkelman deflexions of different structures show the differences in the behaviour of the structures with granular bases and those of asphaltic bases.

Laws are established for the evolution of the transversal profiles (surface deformation) of the different structures and the number of vehicles N passing the road.
03062 Main Findings from the Experimental Research in Mexico
Santiago Corro-Caballero

Since 1962 the Secretaria de Obras Publicas sponsors a research program on flexible pavement design, which is conducted through the Institute of Engineering of the Universidad Nacional Autonoma de Mexico (UNAM). Main purpose of the program is to obtain better methods of design for roads with low traffic volume, taking into account regional conditions of the country in such aspects as materials, traffic characteristics, climate, environment, specifications, construction procedures, safety factors and investment policy.

The experimental studies include three closely interrelated aspects:

1) Research on the performance of three test roads located on two Federal Highways. Project 1 is in a tropical zone; projects 2 and 3 are in a cold steppe region and they were constructed on the same road with only a short transition in between. There are 80 structural sections in the three projects, with pavements ranging from 10 to 50 cm in total thickness. After seven years of testing under actual traffic 72 of the original sections are still in good condition, with present serviceability ratings of 2.5 or more. Each section is one-lane wide and it is 30 m long.

2) Study of the actual performance of existing pavements located in three climatic zones representative of the conditions of the country. Total length under study is of the order of 6000 km, from which 96 field sections will be studied in detail in a factorial experiment. Main variables of the study are climate, traffic, subgrade strength, composite strength and years in service. Length of each section will be 500 m.

3) Research on full-scale pavements tested in a 13 m diameter circular track, under the action of 10 Ton single axle loads applied through conventional 10.00-20 dual tires. The facility was especially designed as part of the program and it has been in operation since December 1970. Each test ring has three structural sections and it is possible to study four rings per year.

After nine years of research, the following conclusions can be stated:

a) There is an urgent need for developing fundamental parameters which can be used for pavement design. In the meantime, it would be convenient to standardize the methods of test to interchange information on an international basis; for the same reason a uniform climatic classification is suggested, in combination with other environmental studies such as water table level and moisture equilibrium.

b) AASHO system of present serviceability rating has been found useful. Acceptance and rejection levels (2.5 and 2.0) have been found adequate for the conditions of the country.

c) Deflection basins determined by use of Benkelman beam are close to Boussinesq’s prediction, and maximum rebound deflections did not correlate clearly with pavement design but with the composite strength; this fact should be considered to predict performance or to design overlays.

A tentative design chart is presented based on the experimentation conducted to date. AASHO load equivalence factors were applied; the track will be used in further stages to establish coefficients for other soil types
03063 A Fresh Look at the Interpretation of Pavement Serviceability and an Experiment to Measure the Riding Quality of Roads in South Africa
P. C. Curtayne, R. N. Walker

The need has arisen in South Africa for evaluating the condition of pavements in a systematic manner in order that maintenance can be more effectively managed and financed, In order to make effective decisions on maintenance it is necessary not only to evaluate those aspects of pavement condition which are functional and relate to the comfort and safety requirements of road users, but also those mechanistic aspects which relate to the structural condition of the pavement.

This paper approaches the evaluation of pavement condition from a functional point of view and describes the features and contributing factors that influence the present pavement serviceability; it also suggests methods for their assessment. The term ‘present serviceability’ as used in the AASHO Road Test related only to the aspects of the general definition concerned with riding quality. The paper concentrates on the assessment of riding quality – a function which is important to the comfort of road users and also to their safety if the road is in a very poor condition. The formulation of the equation for the measurement of the present serviceability index (PSI) is examined by means of a step-wise regression analysis of the original data and it is shown that the inclusion of the term for cracking and patching is not significant. Where the rut depth is less than 10 mm (0,4 in) it need not be considered when evaluating the PSI. It is concluded that the effect of rutting on safety by causing pending of water in the wheel paths should be considered separately from the effect of rutting on the PSI. Limits for rut depth from safety considerations should be set objectively for different climatic conditions.

The paper also describes the results of an experiment for evaluating methods of determining the riding quality of a pavement. A large number of sections of roads were rated subjectively by a large panel of assessors in order to determine the riding quality of each section as accurately as possible. Although the mean ratings for the roads were considered sufficiently reliable, the results of the individual raters were biased to reduce the errors that usually occur in subjective judgements; it was found, however, that this made very little difference to the mean ratings or to their reliability. The results of correlations of the mean ratings with roughness measurements, obtained by means of the BPR Roughometer and the PCA Road Meter, and with the slope variance of the simulated CHLOE profilometer, indicated that each of these instruments can give satisfactory results. The PCA Road Meter results, however, were found to depend on the type of vehicle in which it was used, This implies that the PCA Road Meter should be calibrated against a subjective rating when it is used in different vehicles, Although this is impractical, it is shown that the PCA Road Meter can be satisfactorily calibrated against slope variance provided enough measurements are used.
03064 Comparison between Measured and Calculated Stresses and Strains in Flexible Road Structures
R. Dempwolff, P. Sommer

At two full-scale flexible road structures of a test track extensive measuring programmes were carried out to determine stresses and strains in structures under controlled dynamic load conditions. A rail guided load vehicle, driven automatically, enabled the application of defined rolling loads of a single lorry wheel in the range between 0.5 and 2.0 tons. For testing the influences of loading time (speed) and environmental conditions, the load device was run on each test structure in the speed range between 5 and 50 km/h, and during all seasons of the year at asphalt temperatures between 0°C and 30°C.

Radial strains (longitudinal and transverse) and vertical strains in the asphalt layer, and vertical stresses in the unbound base and the soil were measured at different levels of depth in the structures. The applied transmitters (strain gauges, pressure cells) had been tested successfully in situ for years.

The two investigated rood structures with a total thickness of the asphalt of 22 cm differed only in the stability properties of the asphalt base materials. Therefore direct comparisons could be made between the two structures under the same conditions. As a general result, the differences between radial strains in the asphalt were bigger than the differences between the vertical stresses in the u&ound base and in the soil.

For the comparisons between measured and calculated values a so-called effective dynamic stiffness modulus of the asphalt materials was introduced which accounts for the material properties, the temperature and the "effective" loading time. The numerical data were calculated by the computer programme BISTRO, which is based on the theory of elasticity for multilayer systems. The dynamic moduli of the materials were determined in the laboratory by a known vibration method, and in situ by the wave propagation method.

The best correlations between measured and calculated values were found for strains in the lower part of the asphalt layer and for the stresses in the unbound base and the soil, even when the stiffness conditions for the asphalt layer were low at asphalt temperatures of 30°C. When asphalt temperatures exceeded 25°C, maximum tensile strains were observed in the middle part of the asphalt pavements at the interface of two layed asphalt courses.
03065 Load-Deformation Characteristics of a Pavement with Cement-Stabilized Base and Asphalt Concrete Surfacing
Per E. Fossberg, J. K Mitchell, C. L. Monismith

This paper describes studies concerned with the evaluation of the usefulness of elastic theory for predicting the behavior of soil-cement pavements under loading, and with determination of the influence of various thicknesses of asphalt concrete on the response of such pavements to load, For these studies, two square soil-cement test pavements, each 20 ft. by 20 ft. in plan and 8.5 in. thick, were constructed on a soft clay subgrade. One of the two pavements was loaded to a level resulting in cracking of the soil-cement layer. Both were tested under repeated loading with 0, 1, 3 and 5 in. of asphalt concrete surfacing. Stresses, strains, and deflections measured in the test pavements were compared with values predicted by both elastic layer and finite element solutions using material parameters determined in the laboratory.

For all the repeated load tests, the deflections of the base increased nearly linearly with the plate load, were primarily dependent on the total load applied, and tended to decrease with increasing plate size. A similar dependency existed between applied load and vertical stresses recorded in the subgrade. Vertical stresses in the surfacing and in the base directly under the loaded area were, however, primarily governed by the plate pressure, regardless of plate size. Generally, horizontal strains in the base and surfacing increased with increasing plate size for any one plate pressure, and increased with decreasing plate size for any one plate load.

For both the cracked and the untracked pavement, providing 5 in. of asphalt concrete surfacing reduced the deflections of the base by approximately 20 percent; the same reduction was obtained in subgrade stresses. This relatively small reduction resulted because the asphalt concrete course had a stiffness of only about 1/8 that of the soil-cement for the conditions of test. The asphalt concrete had a large effect, however, in reducing the vertical stresses in the base, and also in reducing the horizontal strains, particularly in the upper part of the base.

The study showed that both elastic layer theory and finite element analyses can be used to predict pavement performance under loading. In this study, both methods tended to underestimate vertical deflections, suggesting a potential shortcoming of deflection as a criterion for pavement performance. While both methods were satisfactory for predicting vertical stresses, the finite element solution proved superior in predicting horizontal strains in the surfacing and base, since it permitted the reinforcing effect of the loading plate to be taken into account. Using the finite element analyses with an incremental loading procedure it has also been shown that for a pavement structure with a relatively stiff base or surfacing, assumption of linear stress-strain characteristics of the subgrade is perfectly adequate for analytical purposes.
03066 The Structural Behaviour of Bituminous Surfacings in an Experimental Asphalt Pavement
Charles R. Freeme, Claude P. Marais

In many of the advanced theoretical methods of pavement design – a subject which is currently being studied throughout the world – attempts are being made to include the effects of the fatigue of the bituminous layers of a pavement structure. The consensus of opinion is that this will result in improved design procedures which will reduce the occurrence of this type of distress. In order to engender confidence in these procedures, it is first necessary to make a comparison of theoretical prediction techniques with in situ field measurements taken under actual loading and environmental conditions.

A detailed study of one section of a heavily trafficked experimental asphalt pavement with a granular crushed-rock base and a 100 mm thick qap-graded surfacing was undertaken in which both linear-elastic and non-linear elastic finite-element computer techniques were applied to predict elastic deflection at various depths within the pavement structure, and maximum tensile strain at the surfacing/base interface. These results were examined mainly on the basis of a comparison between the measured and predicted behaviour of the pavement layers. In addition, a less extensive investigation was made on other sections where both gap-graded and asphaltic concrete surfacings of various thicknesses had been constructed on the same experimental pavement structure. In situ pavement tests, such as CBR and wave propagation measurements, were combined with information acquired from a laboratory investigation to establish the most realistic modulus for each pavement component.

The modulus of the granular crushed-rock base was shown to be stress dependent and had a major effect on the variation of elastic deflection with depth below the surface of the pavement structure. This characteristic of the base material must also be taken into account when strains at the surfacing/base interface are computed using theory, for loadings (wheel load and tyre pressure) over a wide spectrum.

It is further shown that the AASHO load equivalency factors, normally used in pavement design to convert mixed traffic (wheel loads) to an equivalent number of standard wheel loads, do not apply to the design of bituminous surfacings less than 100 mm in thickness. In this regard recommendations are made to take into account mixed traffic in a more realistic manner.
03067 The Structural Analysis of Asphalt Pavements from Field Loading Tests
William S. Housel, James H. Ito

The analysis of structural behavior of asphalt pavement presented in this paper started some years ago with the detailed study by Ingimarsson of the results of Hybla Valley tests reported by Benkelman and Williams. That study is continued by including all of the test sections and translating the results into terms of the stress reaction or strength developed by each of the pavement components. It was also found desirable to supplement the Hybla Valley tests by another notable program of field testing conducted by the U.S. Army Engineers Waterways Experiment Station at Vicksburg, Mississippi. This investigation featured direct measurement of pressure transmitted to the supporting soil from loads applied at the surface with deflections measured at various depths and lateral offsets. The results reflect the characteristics of pressure distribution and the geometric limits within which a soil of given strength can sustain the stresses imposed by surface loads.

Thus these two outstanding investigations provide basic data on stresses in the soil mass or pressure distribution as it is generally known. The Vicksburg tests provide the basis for isolating the lateral distribution of applied load by shear transfer on the perimeter surface through the pavement structure from the load transmitted directly to the subgrade by the central column. These two stress reactions designated as perimeter shear and developed pressure are expressed by the linear equation for bearing capacity used successfully for many years in the design of soil supported structures.

The magnitude, sequence of development, and variation in these stress reactions with the size of the bearing area are determined in the analysis of the Hybla Valley field loading tests. The limiting values of these stress reactions and the settlement at which they occur are controlling factors in the structural design of flexible pavements. Results from those test sections at Hybla Valley which were loaded to capacity have been worked out by the solution of sets of linear equations and are reported in the paper. With the quantitative value of these stress limits established, the linear equation can be translated directly into terms of the pavement thickness required to reduce the contact pressure on the pavement surface to the permissible pressure transmitted to the supporting subgrade.

The primary objective of this paper was to present observed stress limits in flexible pavements from full-scale field loading tests as the background for design. However, space limitations do not permit more than a brief discussion of the obvious application to flexible pavement design. An example is given of a balanced design in which the full strength of all pavement components are combined assuming flexibility of the pavement structure sufficient to mobilize perimeter shear in unison with the full supporting capacity of the subgrade. Analysis shows that the Hybla Valley test sections were not so designed and the rigidity or high resistance to punching shear of the thick well-constructed bases prevented the most economical combination of subgrade bearing capacity with pressure distribution through the pavement structure. Comparative analysis of this approach with other methods of design and field experience is available but must be presented later.
03068 Nondestructive Vibratory Pavement Evaluation Techniques
A. H. Joseph, J. W. Hall jr.

The U.S. Army Engineer Waterways Experiment Station (WES) has performed several studies since the Second International Conference on the Structural Design of Asphalt Pavements in an effort to develop techniques and criteria for use in the nondestructive evaluation of flexible pavements and the prediction of performance of the pavements under various aircraft loads. This paper covers the results of three studies:
(a) development of criteria relating elastic deflection beneath prototype loads to performance of pavements subjected to trafficking with those loads,
(b) development of techniques of extrapolation of deflections under steady-state vibratory loads to predict elastic deflections beneath static prototype loads, and
(c) development of an automated nondestructive testing system and utilization of this system in accumulation of data from airfield pavements throughout the United States that confirm the nondestructive evaluation procedures by comparison with conventional Corps of Engineers (CE) criteria.

Relationships between the elastic deflection of a pavement beneath a static wheel load to the number of repetitions of that load to cause failure of the pavement have been developed for flexible pavements. These relationships were made from data accumulated over a period of years from full-scale airfield and highway test pavements. These relationships are not only important in the nondestructive evaluation procedures, but may also provide the link for theoretical treatments of layered systems to predicted performance of these systems.

The multiple-wheel heavy gear load (MWHGL) test sections at WES consisted of instrumented pavements that were tested with various single- and multiple-wheel loads. Pavement deflections measured beneath the steady-state vibratory loadings were extrapolated with a fair degree of accuracy to predict deflections under static loading with the test load carts. Load-deflection ratios obtained with a large mechanical vibrator were related to performance of the test sections under traffic and to conventional pavement strength parameters.

Airfield pavements throughout the United States, which provided a range of pavement types and environmental conditions, were evaluated by both nondestructive techniques and conventional test pit methods, and comparisons of these results are presented,
03069 Laboratory Fatigue and Its Relationship to Pavement Performance
R. Ian Kingham, B. F. Kallas

The traffic testing of asphalt pavements in a Washington State University test track provided an opportunity to explore the fatigue mode of failure in asphalt pavements. Observations of 8-foot wide pavements surviving 247,000 applications of a dual-wheel load weighing 10,600 lbs. showed that alligator crack patterns were confined to the bottom of the asphalt bases. The experiment was designed to compare the performance of an asphalt concrete base, a sand asphalt base and a crushed stone base.

The principal objective of this complimentary study was to determine if test track failures could be predicted from the results of a laboratory, flexural beam, fatigue test. Secondly, the performance results allowed a check to be made on field failure criteria derived from the AASHO Road Test. The cumulative damage hypothesis of Miner was used to predict the time to failure for each test section using either the laboratory or the field criteria.

The analysis procedure required the computation of horizontal tensile strains associated with each application of load. Complex modulus tests on asphalt bases and surfacing, and modulus of deformation tests on undisturbed samples of subgrade soil provided input into the Chevron elastic-layered program. For the laboratory fatigue tests, controlled stress and controlled strain tests were completed at three temperature levels to bracket most of the testing temperatures at the test track. Equations were derived to express load repetitions to failure as a function of strain and temperature for each asphalt mixture. Similarly, the field failure criteria developed previously from AASHO Road Test data related load repetitions to failure strain and the stiffness of the asphalt mixture.

The extensive program of laboratory fatigue testing has added considerable knowledge to the behavior of asphalt mixtures in the flexural beam test. For both constant stress and constant strain tests, the effect of temperature was particularly significant. The laboratory fabricated test specimens provided the same results as those specimens cored from untraveled portions of the test pavements. This finding suggested that lab fabrication can produce a specimen representative of the field.

Both stress controlled and strain controlled flexural beam test results tended to over-predict the actual number of repetitions to failure. Predictions for the stress controlled test were closer than those for controlled strain and very close to those observed for three of the six Full-Depth asphalt test sections. The field failure criteria gave slightly closer predictions than laboratory results. This finding offers some promise for the application of the field criteria to asphalt mixtures that have different dynamic modulus – temperature relationships than that for the AASHO Road Test bituminous bases. Neither the field failure criteria nor the laboratory fatigue criteria predicted satisfactorily the lives of the crushed stone base test sections. For this base type, the use of elastic layered theory, combined with Miner’s hypothesis is suspect.
03070 Practical Design Applications Based on Washington State University Test Track Results
Milan Krukar, John C. Cook

The Washington State University Test Track is a practical apparatus for the evaluation of the strengths of different pavements and materials. Tests run on different base materials, untreated and treated, during a six-year span reveal that many ideas about the value of the use of some materials need revision.

The description and results from the four test rings are briefly presented. Design equivalencies for the different materials are developed. The results from the test track seem to cast some doubt on the benefits of the use of untreated rock bases. Results indicate that increasing the thickness of untreated base does not necessarily increase the pavement life. Under certain economic conditions and surface condition criteria, cracked pavements with untreated bases may be preferable to pavements with treated bases.

Studies on asphalt bases show that treated bases are no substitute for a good subgrade. It does very little good to construct thick asphalt pavements on weak non-uniform subgrades. The practicability of laying thin asphalt concrete surfacing on a good subgrade without a base has been demonstrated and can be applied in the construction of low-volume, low-cost roads.

Comparison of results from field data with elastic n-layer theory reveals that results are, in general, comparable. Differences may not be due to faulty instruments, but to the nature of the theory itself, which requires assumptions based on laboratory results which may not duplicate actual conditions.

The value of the use of inferior aggregates treated with asphalt cement has been demonstrated. The superiority of deep strength asphalt pavements has been shown.

Environmental conditions during construction and testing have played a more important role in the life of a pavement than either the type or thickness of base and/or pavement.
03071 Traffic Tests of Airfield Pavements for the Jumbo Jets
R. H. Ledbetter, H. H. Ulery jr., R. G. Ahlvin

A flexible pavement test section was constructed and tested to failure under full prototype aircraft loadings. These tests were designed to gain information pertaining to the behavior of flexible pavement structures and subgrades under multiple-wheel heavy gear loads (MWHGL), such as the C-5A and the Boeing 747. The test section was constructed to carefully controlled strengths to a full 12-ft depth, and five items of various thicknesses were incorporated above the 4-CBR subgrade.

Gradations, plasticity restrictions, minimum thicknesses, and similar specification controls were consistent with design requirements for a U.S. Air Force "medium-load" airfield. This is an airfield capable of supporting KB-50, KC-135, and similar aircraft. The test pavements would thus be generally considered to be of very good quality. The C-5A was designed for operation on medium-load airfields.

The loadings and multiple-wheel configurations indicated the need for determining wheel interaction effects both in the pavement structure and to the same depth within the subgrade soil. Therefore, two of the items were instrumented with stress, strain, deflection, pore pressure gages, and temperature probes. The stress and deflection instrumentation was placed at various depths throughout the full 12-ft depths of the items; stress and deflection were the primary measurements made.

The instrumentation was loaded both statically and dynamically (slowly moving vehicle) with one main 12-wheel landing gear of the C-5A, a 6-wheel component of the C-5A gear, a twin-tandem component of the Boeing 747 landing gear, and a single wheel, which was one wheel of the C-5A gear. Various test loadings were conducted on each of these gears with the test loads per wheel ranging from 6000 to 60,000 lb.

An analysis of the soil behavior patterns investigated to date has resulted in the following:
a. Both stress and deflection distributions showed differences in pattern between the two instrumented items.
b. Assuming elastic behavior, theoretical predictions of deflection versus depth or of offset versus deflection are not good except for a single-wheel load.
c. Stress and deflection distribution patterns under a single-wheel load are different from those under a multiple-wheel assembly.

Behavior of the test items was determined in terms of the traffic applications to produce failure under each of the loads applied. In all, over 20 combinations of load and test item structure were studied, permitting comparisons of behavior related to thickness, type of structure, and single versus 4- and 12-wheel loadings.

In general, behavior of the test pavements under multiple-wheel heavy gear loads was better than simple extensions of prior pavement design criteria would indicate for higher repetition levels. The 12-wheel C-5A aircraft loading seemed to yield elements of pavement behavior somewhat different from behavior recognized previously under one-, two-, and four-wheel loads.
03072 Evaluation of Flexible Pavements by Nondestructive Tests
H. A. Balakrishna Rao

A vertical oscillator operating on the surface of a layered medium generates dispersive waves. The elastic properties of the various layers are determined from the characteristics of the surface waves (dispersion curves).

This paper deals with two asphaltic concrete test sections on which vibration tests were performed. The elastic properties of the various layers are used to theoretically predict the deflection basin under a loaded plate. A comparison between the computed and measured displacement fields shows that the computed deflection basin is considerably shallower than the measured deflection basin. This discrepancy is attributed to the low strain level input generated during the determination of layer properties by the vibration method.

A technique to modify the elastic properties of the layers based on the actual strain level inputs generated by field loading conditions is introduced. This technique was recently developed at the University of Kentucky and, at the present time (1971), is primarily used to modify the elastic properties of the subgrade only. However, the principle of this modified procedure greatly strengthens the concept of nondestructive testing as a practical tool for evaluating the performance of airfield pavements.

The average modified elastic property of the subgrade and the elastic properties of surface and base courses as determined by the vibration method are used to compute the new displacement field which is compared with the measured displacement field.

Further modifications of the proposed method to account for the actual distribution of strain within the pavement system would upgrade the nondestructive testing procedure to provide a meaningful and practical evaluation of the load-carrying capacity of a pavement.
03073 Pavement Performance Using Residuals in the Deflection Test
A. J. Scala

The paper describes investigations into permanent deformations arising in a pavement during the Benkelman Beam Test. The study is particularly concerned with pavements giving low deflections – subgrade deformation is only a minor part – and hence any permanent movement is probably due to a lack of strength in one or more of the pavement layers.

Although the elastic deflection or curvature values may be satisfactory on a road for the particular traffic, in the absence of an indication of permanent movement it may not be presumed that the structure or any layer is behaving elastically. When the front legs and probe of the beam are within the deflection bowl at the start of a test using the original W.A.S.H.O. procedure and no other factor is influencing the residual deflection, a positive residual reading should be observed. On the other hand, if a negative reading (equivalent to a permanent rise in the pavement) is recorded it can be symptomatic of adverse behaviour and should be significant in an evaluation of a pavement by elastic deflection.

The study shows with a number of examples that the residual deflection can provide an indication of permanent movement. A performance study undertaken on a number of sections indicates that a measure of permanent deformation is equally as important as rebound deflection in the evaluation.
03074 The Alconbury Hill Experiment and Its Relation to Flexible Pavement Design
P. D. Thompson. D. Croney, F. W. H. Currer

In a paper given to the First Conference on the Structural Design of Asphalt Pavements results obtained from the major full-scale pavement design experiment at Alconbury Hill during the first four years were given. The present paper summaries the conclusions after 13 years.

Although the major trends – such as the superior performance of dense bituminous base material and of rolled asphalt surfacings – have continued to be apparent, certain of the earlier tentative conclusions, and particularly those relating to the role of the subbase, have needed revision.

The traffic using the experimental road has been weighed by an electronic weighbridge and the number of cumulative 8200 kg (18,000 lb) axles has been calculated using the A.A.S.H.O. equivalence factors. This will allow a direct comparison to be made with the results of the A.A.S.H.O. Road Test for the forms of construction common to both experiments.

A simple structural analysis based on elastic theory has been made for certain of the sections using lean concrete, wet-mix slag and rolled asphalt road bases. Although the broad conclusions are in agreement with the experimental findings the analysis illustrates some of the practical problems entailed in applying structural analysis techniques to the design of road pavements.
03075 Pavement Performance in the S12 Road Experiment, An AASHO Satellite Test Road in South Africa
D. J. van Vuuren

Experimental sections of asphalt pavement totalling 2 km in length were incorporated during the construction of a heavily trafficked route between Johannesburg and Witbank with the object of assessing the suitability of various pavement materials and structures for carrying heavy traffic in the local environment. The experiment was planned as an AASHO Road Test satellite project to be used in an attempt to adapt the AASHO Road Test findings to South African conditions.

After two years of traffic, certain performance trends have become apparent. In particular, the early failure of a section with a cement-treated base, following the development of severe shrinkage cracking and pumping, has been closely followed. It is likely that at least 10 years will elapse before firm conclusions can be drawn on the performance of the remaining sections.

In order to obtain earlier results from road experiments of this type, a completely automatic heavy-vehicle simulator has been designed and constructed which is capable of applying 300 000 equivalent 80 kN axle load passes per day. The maximum wheel load is 80 kN, and the machine simulates the weaving action of normal traffic across the road. Initial results obtained with the simulator on the experimental sections are reported.
03076 Flexible Pavement Performance in New York
John M. Vyce

New York State recently completed an extensive investigation of flexible pavement performance. This study involved serviceability measurements on 175 pavements located throughout the state and subsequent comparisons with a number of design, operating, and environmental variables. The results are summarized here with emphasis on two areas:

1. The data are compared with AASHO Road Test results, and differences are discussed along with apparent causes.

2. Performance data are related to design criteria. The most important factor in determining proper asphalt thickness is rate of traffic loading, with climate a secondary influence. Soil support values had little influence on pavement performance and were excluded as a design factor. Graphs relating traffic to performance are presented, along with a design procedure developed using data from this study.
03077 On Structural Characteristics and Performance of the Calcareous-Soil-Sand-Asphalt Base Courses Used in Argentina
Laboratorio de Investigaciones Viales Facultad de Ingenieria-Universidad de Buenos Aries – Departamento de Tecnologia-Direccion Nacional de Vialidad-Buenos Aries, Argentina

The jobs of enlargement and strengthening of the highway network in the littoral region of Argentina, have always come across the problem of the high price and scarcity of conventional aggregates. This is due to the scanty production centres and to the need of carrying the material over distances sometimes over 250 miles. The above mentioned circumstances have brought about the need to use local aggregates which do not meet the quality requirements corresponding to the aggregates for conventional asphalt mixes particularly to asphalt concrete. On the basis of these non-conventional aggregates it has been necessary to develop asphalt mixes for the base courses for new pavements, especially for strengthening of roads which must be adapted to stand the extraordinary increase in load and frequency of vehicles registered in last decade. moreover, it has been necessary to estimate, based on experience, a criterion of the equivalences between these non-conventional mixes and the conventional ones, allowing the structural design of the new roads and of the strengthenings.

Among materials available, we have natural siliceous sands of fine and uniform gradation, which do not permit the obtention of asphalt mixes of adequate stability. We also have boring-pits of soils A-4 (AASHO) of the 3rd horizon. Their peculiarity is having undergone a not altogether developed process of calcification which determines the presence of varying percentages of soft and porous calcareous noduli. The mixture of calcareous soil, coarsely pulverized, with approximately equal amounts of sand, processed in the plants, shows 20% retain on sieve #4, dry way, whilst in wet sievage, the lumps of soil disintegrate and a percentage of passing #200 of about 30% is reached. The incorporation in hot of 8-9 per cent asphalt of penetration 70-100 originates the mixes called "calcareous-soil-sand-asphalt" widely used in Argentina as the base course in new pavements and strengthenings. Conventional aggregates are only used for the wearing course. The amount of the former mixture used in the last 15 years is estimated in 2.5 million tons.

We have studied the behaviour of the calcareous-soil-sand-asphalt mixes during the compaction process, both under laboratory (Marshall method) and field conditions. The job compaction initiated with pneumatic tyred equipment and completed with steel wheeled roller, determines densities corresponding to a laboratory compaction with approximately 20 Marshall blows on each face, yet, no consolidation under service is observed later. Simultaneously we have studied the mechanical characteristics in relation to compaction, especially the stiffness modulus calculated on the basis of stability and Marshall flow by the Nijboer formula.

Bearing in mind the composition of the mixture, we have studied its resistance to water, based on the swell test, as a function of the asphalt content and of the degree of compaction. Laboratory results confirm the performance and show that the fraction of calcareous soil has been adequately stabilized by the joint action of heat during the process of partial drying and that of the asphalt absorbed in the porosity inherent to the particles calcareous soil.

The triaxial test shows that the behaviour of the calcareous soil-sand-asphalt mix, under load, greatly depends on its high non-viscous cohesion, or Nijboer initial resistance (independent from the deformation rate, but depending on the temperature).

The work hypothesis that has uphold these studies and is confirmed by the experimental results, is that the structure of the calcareous-soil-sand-asphalt mixes is different from that of conventional mixes. In these, the finest fractions of the total aggregate are dispersed in the asphalt, constituting the binder medium covering the coarser fractions in the loose condition, and are displaced towards the voids between the particles of the granular structure formed b) the compaction process. In this way, the finest fractions do not reach a high densification condition, since the stresses determining compaction are mainly borne by the granular structure. In the calcareous soil-sand-asphalt mixes, the effective asphalt content is only sufficient to cover with thin coats the lerge surface area existing, and to partially occupy the void between the fine particles in the high degree of densification that these reach, because of directly bearing the compaction stresses, since there is no granular structure acting as a matrix. It is therefore understood that the coarser fractions (calcareous noduli and soil lumps) not hard enough, remain dispersed in the compacted asphalt mortar, in which there is a granular microstructure answering for the frictional resistance and especially for the non-viscous cohesion. The properties of this asphaltic mortar are the ones that govern the characteristics and behaviour of the calcareous soil-sand-asphalt mixes placed as base course.

The set of results snows that the quality criteria and demands adopted for the conventional asphalt mixes for base courses cannot be extrapolated to the calcareous soil-sand- asphalt mixes. The optimum job compaction corresponds to a mean density in the Marshall test with a number of blows between 15 and 20. The control of the increase in density as a function of time under traffic will be continued until the final density is reached. The variation of the stiffness modulus as a function of time under traffic shows, in every case, first a decrease of the modulus after 30 days and then an increase in the modulus at an age of 300 days under traffic.

This evolution may ue explained by the production of fines though granulometric degradation and their later incorporation to the asphalt binder. The coefficient of equivalence of the thicknesses of calcareous soil-sand-asphalt relative to a conventional material such as asphalt concrete, has been determined by the ratio of the R values of both materials proposed by Ruiz. The values found tend to a maximum. We intend to continue with periodical measurements in order to reach a long range comparative value of the behaviour of the calcareous soil-sand-asphalt, as compared with asphalt concrete, which will objectively back up the estimation of thickness as which should be used in the future.
03078 Sensitivity Analysis of Three Flexible Pavement Design Techniques
T. R. Buick, J. C. Oppenlander

To better understand the art and science of pavement design, three highway design techniques were studied to determine the relative effects of various design parameters on pavement thickness. The analysis phase of this investigation consists of formulating each technique into a comprehensive mathematical or graphical thickness model. An evaluation of the influences on thickness of the major design factors was accomplished by a sensitivity analysis with a theoretical and two practical measures of parameter importance.

The theoretical measure of parameter importance reveals considerable differences among techniques as to the process of resolving design thicknesses and to the relative theoretical influence of various parameters that estimate the same major design factors. However, the practical parameter importance measures, which account for variations in parameter values as well as the formulated parameter-thickness relationship, demonstrate a greater similarity in the importance of generic factors among the design methods. For flexible pavement design, traffic load and subgrade support are the more influential factors in the determination of pavement thickness.
03079 Output Measurements for Pavement Management Studies in Canada
Roads & Transportation Association of Canada, Pavement Design & Evaluation Committee

The Pavement Design and Evaluation of the Roads and Transportation Association of Canada has conducted extensive, nationwide studies of pavement performance for several years. The results have been widely applied to design purposes. In recent years, a pavement management system has been defined to provide a logical framework for the activities of the Committee.

A significant portion of these activities has been in the area of pavement output measurements. The paper summarizes this work. It is concluded that the present serviceability measure of Riding Comfort Index is one of the primary outputs that should be known on a mass inventory basis.

Procedures for acquiring such mass inventory data on Riding Comfort Index, using a Car Road Meter for estimating purposes, are outlined in the paper. The important considerations and limitations of the approach are discussed, and the extension of the approach through a photo inventory technique is briefly considered.
03080 The Catalogue of Structures of the Direction des Routes et de la Circulation Routiere Francaise (French Highways Authority)
M. Chantereau, Ph. Leger

Present methods of structural design, based on a combination of theoretical and empirical considerations, do not seem to be entirely satisfactory for several reasons:

– They very rarely take account of certain elements of "good practice" which are essential for the proper behaviour of pavements.

– They do not easily allow of extrapolation to pavements of a new type or to a different environment.

– They give the designer a fallacious impression of accuracy whereas in fact they are very imprecise and depend to the highest degree on the quality of the materials and their application.

Consequently we have turned in France to the idea of a catalogue of structures, giving possible structures for a certain number of categories of soil and traffic, and giving for these different solutions the advantages and drawbacks liable to be encountered either at the time of carrying out the work or during the life of the pavement. The catalogue also indicates the required composition of materials and the necessary conditions of performing the work, together with the characteristics of predictable non-destructive testing (deflexion, speed of wave propagation, etc…). Structures for overlaying of existing pavements are also included in the catalogue.

Where traffic is concerned, account has been taken only of heavy vehicles (without bringing in the notion of equivalent traffic), and 4 classes are proposed, each corresponding to an order of magnitude of the total heavy traffic which the pavement must be able to bear before being totally remade.

Subgrades have also been placed in 4 categories, on the basis of the U.S.C.S. classification and in certain cases additional geotechnical characteristics (index of plasticity, C.B.R. index) of hydraulic conditions (depth of the water table, drainage) and of depth of freezing.

In the case of overlaying, the subgrades categories are replaced by 4 other categories, defined in the light of deflexion (overall properties of the existing pavement), where necessary corrected in the case of particularly frost-susceptible soils, and in the light of the properties of the pavement courses (radius of curvature of the deflected pavement, pollution of courses, etc…).

The structures proposed are then designed taking account of:
– Existing theoretical elements (stress on the subgrade bending stress in bounded courses)
– Specific characteristics of the materials used (e.g. rigidity under relaxation or adaptation to minor settlement, the effects of temperature, etc…)
– Problems of construction (number of courses, compacting, roughness)
– Data available on existing pavements.
03081 The Increase of Asphalt Topping Stability
0. K. Dobozy, B. Bartha

We give account of a rather less regarded impairment of asphalt roads, of a decomposition of bitumen taking place at the building of casted or prefabricated asphalt toppings by the hot method which is a primary damage. We report on a process which enables to protect the toppings against this and another method against the secondary corrosion of asphalt caused by the chemical removal of snow and ice. We show the deterioration of asphalt toppings, to what extent it may be reduced and the stability of toppings increased by these processes.
03082 Properties of Materials Used for Surfacing and Stabilized Base Structures of Pavements for Heavy Trafficked Roads
N. V. Gorelyshev, V. N. Kononov

The large part of the USSR territory is notable for its continental and sharp continental climate. Even in the moderate climatic zone the year round temperature changes ere equal to 60 to 70 deg. C and in the eastern and northern regions they are as high as 80 to 90 deg. C. In some regions the transition of temperature over a zero point takes place up to 80 times. Such severe climatic conditions influence a pavement not less than traffic loads and imply the necessity of not only designing the depth of surface and base courses but also regarding their thermo-physical properties.

Till the end of 50’s a two-layered bituminous concrete surfacing on a crushed stone base was prevailing as a pavement structure. The depth of the crushed stone base varied from 20 to 40 cm due to design loads. An additional (underlying) course was made of sand or gravel with the purpose of draining end protecting the subgrade against excessive freezing.

In 60’s such structures had ceased complying with demands of ever- growing traffic due to gradual wear of crushed stone during deflections of bases and rapid loss of surface evenness. For heavy trafficked roads the pavement structures on bases only from stone materials stabilized with different binders were proposed. Whatever the type of a binder, all the above mixes are claimed to comply with one general demand: the mix grading must provide maximum density and shear resistance of the material in the base course.

The requirements of grading, strength and wear resistance of rock materials stabilized have been specified. Another general and obligatory requirement to all mixes is their frost resistance specified according to a climatic zone.

Depending on the type of a binder, the base course is of different deformability, crack resistance, elastic modulus, thermal conductivity, bearing and distributing capacities.

As far as deformability and crack resistance are concerned, the bases of bituminous mixes are the more preferable the more severe and continental the climate is. But even in the warm climate their advantages over other bases are beyond doubt.
The least deformability and crack resistance are inherent in the mixes in which the crushed stone materials are treated with small portions of cement. Even in cases of minor temperature changes the cement-bound bases show cracks.

The intermediate deformability, strength and distributing capacity are inherent in stone materials treated with granulated blast-furnace slag or a mixture of slag and fly-ash. The various bases for heavy trafficked roads are constructed of the above stabilized materials.

In this case the most typical structures of pavements are as follows:

A. – The two-layered bituminous concrete surfacing with the 9-10 cm overall thickness or single-layered bituminous concrete one 7-8 cm thick.
– The base of bituminous mix (with crushed stone of gravel) to be compacted in one lift 15-18 cm thick.
– The subbase of crushed limestone to be watered in course of rolling in 15-16 cm thick layer or ash-slag mixtures 15-16 cm thick.
– The draining layer made of sand, as a rule. The design-depth is due to the required water-thermal regime of the subgrade.

B.- The same surfacing as for the structure type A.
– The base of bitumen-coated crushed stone 8 cm thick (this structural layer is omitted in the regions of moderate and soft climates).
– The base (or the subbase) out of crushed stone or crushed gravel with the addition of 15 to 20% of granulated blast-furnace slag or of 4 to 6% of Portland cement. The depth of the layer is 20 to 25 cm.
– The draining course.

Such structures have proved to be unsettleable and crack-resistant. At the same time the procedure of compacting bituminous mixes in one layer up to 20 cm deep was developed.

The compositions and properties of bituminous concrete represent a particular problem. In the case of large loads end sharp changes of temperature and humidity bituminous concrete should meet high requirements of its strength, deformability and skid-resistance. With due regard for these requirements a new standard has been worked out.
03083 Design of Flexible Pavements in Czechoslovakia; Recent Research Works
I. Gschwendt, I. Poliacek

In 1961 a recommendation for the design of flexible pavements was presented for approval of the Central Road Administration in Slovakia. The design method is the result of the work on a research project performed by a team of research workers.

The method contains design principles used for selection of the structure and the total thickness of the pavement as well as the material and the thickness of the different layers. The total thickness is determined taking into consideration the protection of the pavement and subgrade against frost effects. In the design of the structure of the pavement the following factors are considered:
– the influence of traffic during the whole service life of the pavement
– the bearing capacity of the subgrade
– the characteristics of materials used in the pavement structure
– the mechanics of the behaviour of the pavement under a load and
– the method of construction.

The designed pavement is assessed with regard to the required performance characteristics (the service life) and the stresses to which the various layers will be subjected. The design criterion for serviceability is the deflection of the pavement. The stresses in the pavement are assessed according to the tensile (bending) stress arising from the action of a design load at the bottom of the surfacing or at the bottom of one of the base courses. In this case the criterion is the permissible tensile (bending) stress of the material.

When calculating the deflection and the stresses, the real pavement is simulated by an ideal three-layer system consisting of perfectly bonded layers of flexible materials. The elastic properties of the various materials are considered in design (table) values; the elastic modulus of the subgrade is derived from a CBR value established in the laboratory.

The correctness of the design can be checked by load bearing tests performed on the finished pavement. In view of the fact that the structural design uses design values instead of real (instantaneous) values of the deformation properties of the materials in the pavement and the subgrade, the deformations measured in the bearing test will be lower than the calculated ones and similarly, the real stresses in the pavement will differ from those established by a calculation.
03084 Current Design and Construction Procedures in Switzerland
U. Kunz, M. Blumer

Various peculiarities such as great frost depth, heterogeneous soil, unsatisfactory topographical conditions and the large variety of available materials, excludes the unlimited application of foreign results and procedures. The tendency in Switzerland is towards empirical design methods which are suited to the specific conditions of the site and which take own experience into consideration. Standards for routine designing, described in this report, were published in 1971.

Systematic observation on the performance of roads during the periods of frost and thaw have shown that:
1. Large deformations through frost-heave can be avoided if the depth of the pavement structure amounts to at least 50% of the frost depth.
2. Decrease in stability during the thaw period is only considerable if frost has clearly penetrated the roadbed.

On sufficiently designed roads, influence of frost may be neglected as long as the design is based on the most unfavourable stability strength during the thaw period. Exceptions are highly frost-sensitive soils with equally unsatisfactory environmental conditions. In such cases measures must be taken to avoid frost damage, e.g. replacement of material, stabilisation of the soil foundation layer or the inclusion of a heat insulation layer on the subgrade.

In order to lay down thickness equivalency values for currently used materials in Switzerland, the strength of various asphaltic concretes was compared with gravel on a test road. The results showed that the relative strength not only depended on the characteristics of the material but also on the strength of the layers underneath and the thickness. With the aid of thickness equivalency values which only take consideration of the characteristics of the materials but not of the other facts it is barely possible to compare the strength of various materials. Diagrams, however, are suitable for this purpose (e.g. fig. 3 of this report).

Designing is undertaken step by step according to the suggested method:

1. Determination of the soil (frost sensitiveness) and environmental conditions (depth of frost and hydrological conditions). Decision on whether construction can be carried out on the natural soil or whether constructional measures must first be undertaken to avoid frost damage (fig. 2).

2. Determination of the type and thickness of the base course in function of soil stability (CBR value) and the daily equivalent 18-kip axle loads (TF). The base is to form a permanent stable support for the bituminous surface; it may be made up of gravel or stabilised material (fig. 4).

3. Determination of the minimum thickness of asphaltic concrete with crushed aggregates as a function of the daily equivalent 18-kip axle loads (TF). It must be assured that the strength is sufficient even following a great number of axle load passings to avoid cracking through frost or fatigue (fig. 4).

At the conclusion of the report, a method for strengthening of existing roads is described. It starts by determining the Spring deflection permitted (d sub crit) in function of the number of axle load passings (N) during the design period. This is followed by measuring the existing Spring deflections (d sub 0) on the road and extracting from a diagram the required thickness of the bituminous cover (fig. 6).
03085 The Current Design Procedure for Flexible Pavements in Britain
J. V. Leigh, D. Croney

The paper first reviews briefly the development of design standards for flexible road pavements prior to 1960.

Since 1960 recommended standards of design for such pavements have been given in the Road Research Laboratory’s Road Note No. 29 (A guide to the structural design of flexible and rigid pavements for new roads). The designs, which cater for all classes of road from motorways to those associated with housing estate developments, have been widely accepted throughout the country. They are based almost entirely on the performance of a large number of experimental roads constructed in England, Scotland and Wales.

Early editions of the Road Note classified traffic in terms of the number of commercial vehicles per day likely to be using the road 20 years after construction. Six traffic categories only were considered, leading to discontinuities of design between the various categories. As more information has become available from the road experiments some refinements of design have become possible.

In the 1970 edition of the Road Note traffic is expressed in terms of the number of 18,000 lb (8200 kg) axles to be carried by the pavement during the chosen design life. Axle-load spectra studied on typical roads using permanently installed weighbridges have been converted to equivalent 18,000 lb (8200 kg) axles using published AASHO equivalence factors.

An intensive analysis of the results from the road experiment has enabled continuous curves to be developed relating the cumulative number of standard 18,000 lb axles to be carried with the thicknesses of subbase, base and surfacing. The design charts are reproduced in the paper.
03086 Structural Design of Full-Depth-Asphalt-Pavements and Field Tests in Comparison with German Standardized Asphalt Pavements
G. Leykauf, W. J. Kawohl

The "Institut fur Bau von Landverkehrswegen" of the Technical University Munich has carried out upon order of "Mobil Oil AG" investigations to determine the necessary thickness and the fatigue life of "Full-Depth" asphalt pavements.

In the scope of a theoretical investigation "Full-Depth" constructions have been designed in analogy to the German Standardization of asphalt pavements for five traffic classifications from a very slight traffic to a very heavy one, applying elastic theory to multi-layer systems. The thickness of Full-Depth constructions was determined in a way that the vertical stressing on subgrade is the same as for the corresponding standardized pavements with an asphalt base course of round aggregate. The stressing was computed by the equations of Odemark and Boussinesq, whereby asphalt pavements of all constructions have been divided into several layers with different dynamic moduli of elasticity corresponding to the average temperature in these layers. To allow an economical use of the "Full-Depth" constructions the necessary thickness was computed for three different bearing capacities of subgrade.

Also for these determined "Full-Depth" constructions the radial stressing at the bottom of the asphalt pavements were calculated according to seasonal influences by aid of the tables of Jones; in connection with the hypothesis of Miner it is possible to predict the cumulative stressing and the expected fatigue life. By a comparison with the corresponding standardized constructions it is evident that "Full-Depth" constructions are expected to have a longer fatigue life.

Field tests were carried out on two test sections, one with an asphalt pavement using normal design standards for slight traffic and a bituminous round aggregate base (Field 8 ) and the other with a Full-Depth asphalt pavement on a poor subgrade and for slight traffic as designed by the institute (Field 9 ) .

In both test fields the clear dependence of temperature could be observed by measurements of strains and deflections. The increased stiffness or load-carrying capability of the Full-Depth asphalt pavement at rising temperatures was proved. The Full-Depth asphalt pavement design can therefore be expected to have a longer fatigue life. The temperatures measured during the 1970/71 winter revealed that the Full-Depth asphalt pavement had a greater thermal insulating effect than granular base.

The field tests showed that the Full-Depth method is superior to the standardized asphalt pavement construction in load-carrying capacity. In spite of the extreme susceptibility of the subgrade to frost damage, frost heave occurred to almost the same extent in both sections, also the heave which occurred in Field 9 was much more uniform and there was less surface irregularity or distortion.
03087 Developing an Operational Pavement Design and Management System and Updating it with Elastic Theory
B. Frank McCullough, W. Ronald Hudson, Ramesh K. Kher

Pavement design is a complex soil structure interaction problem involving variables in the broad categories of load, environment, construction, maintenance, materials, economics, geometric configuration, and performance. A great deal of effort has been expended in the study of pavement behavior and performance, including full-scale testing such as the AASHO Road Test. Unfortunately, no long-term coordinated effort has been made to develop comprehensive methods of pavement design and management. Governmental agencies involved with highways are usually divided administratively into design, construction, and maintenance sections. This typically results in dissection of the physical problem with little or no attention paid to the interrelationship between design, construction, maintenance, and management.

The general format of most existing procedures for pavement design revolves around the determination of layer thicknesses. Other design details are subsequently adjusted in accordance with these thicknesses. Details such as pavement type, thicknesses and schedules of overlays, maintenance and seal coat requirements, traffic handling during maintenance operations, and selection of the optimum design strategy are usually never considered in present design methods.

This paper concerns a systems method of pavement design and management which uses a computer to analyze many phases of the pavement design problem. Two major parts essential to pavement system development are included:
(1) an initial system which actually works and which can be used for initial design and further study, and
(2) a concept and functional method for continuous study and updating of the system as required.

The program presented herein used over 50 input variables and analyzes a great number of possible solutions, generated within the boundary constraints. The output is an ordered set of pavement design strategies which can be used by the administrator in making a rational design decision.

An important part of any pavement design system involves upgrading it to include the best possible technology. The feasibility of upgrading the working systems model is demonstrated through an example whereby the structural subsystem of the design method is improved using elastic layer theory, fatigue concepts, and stochastic variations of material properties. A new mechanistic model is developed, based on the primary distress manifestation of cracking. This model will replace the empirical relationship used at present to simulate a gross transformation between the input variables and the performance of a pavement.
03088 Structural and Mixture Design of Low Volume Roads Using the Elastic Theory
R. E. Root, E. L. Skok jr., D. L. Jones

The purpose of this paper is to establish mixture and thickness designs for materials to be stabilized in place on several low volume U.S. Forest Service Roads. The results of a repeated load triaxial compression test in terms of resistance to permanent deformation are used to establish appropriate mixture designs.

Thickness designs are established by the elastic theory using the stress-strain properties of the materials determined from the repeated load triaxial compression test and an appropriate failure criteria. The stress-strain properties were defined for one temperature (75F) and one loading time (1 sec) in order to reduce the amount of testing required. The failure criteria used, consists of limiting the vertical strain at the top of the embankment and the tensile strain at the bottom of the stabilized layer. These criteria were adopted from the work of others and applied to the pavement system being studied. Several assumptions were made in order to use these criteria with the limited testing schedule used. A thickness design curve is developed using an analysis of the elastic properties of the materials by the Chevron N-Layer Elastic Systems Program and the adopted failure criteria. Designs are established for three low volume roads using the developed design curve.

It is tentatively planned to use one of the designs developed in this study for the construction of a test road. The variables to be studied from the test road include: asphalt type, asphalt content and thickness of the stabilized layer. The test road will be instrumented to provide measurements of the critical parameters defined by this study.
03089 Evaluation and Revision of the AASHO Interim Design Guide for Asphalt Pavements
B. A. Vallerga, C. J. Van Til

A major objective of the AASHO Road Test was to provide information for use in developing pavement design criteria and pavement design procedures. Following completion of the Test, AASHO developed the Interim Design Guide for Flexible Pavement Structures. Details of the development of this Guide were presented to the 1962 conference. Since that time some 32 of the 52 state highway departments have reported making direct use of it, either in whole or in part. Of the states not reporting direct use, some have made some indirect use and others plan to attempt to adapt them to their use.

Because of the extensive use which has been made of this design guide, and the fact that no revisions had been made since originally issued, National Cooperative Highway Research Program Project 1-11 was initiated in 1967 to develop information for evaluating and preparing recommendations for revising the Guide. The initial effort in the first phase of this project was submitting to the state highway agencies a comprehensive request for information form consisting of 72 questions prepared to elicit specific pertinent information relative to procedures currently being used for design of flexible, rigid, and overlay pavements. Also included were theoretical analyses of the significance of the variables in the design procedure, and a comparison of the design procedures with theoretical relationships developed through recent research efforts by agencies engaged in the study of theoretical aspects of highway materials and design. The results of the first phase of this project are presented in the form of a summary of the significant findings and conclusions.

Based on the results of the first phase, an implementation phase was instituted with the objective of preparing a revised guide. This revised guide was presented to the AASHO Design Committee for their consideration. Although the revisions made no changes in the basic design equation or in the design procedure, considerable explanatory and background material was added for clarification and for assistance in selection of the appropriate values of the design parameters. A brief description of the most significant additions made in the draft of the revised guide is presented.
03090 Theoretical and Practical Advantages of Single-Pass Construction of Thick Bituminous Road Courses
R. N. Varlan

Slab effect of the bituminous road courses over a certain thickness and the good behaviour under the traffic charges of the flexible pavement structures with great thickness of the asphalt layers, have been on the one hand proved experimentally by full-scale teats (e.g. WASHO-Road Test and AASHO-Road Test) and, on the other hand, demonstrated from some theoretical studies (e.g. Jeuffroy-Bachelez’s method) already for a long time.

However, the single-pass construction of the thick bituminous courses and, especially, of the road bases which reach today usually thicknesses of 12 – 20 cm, has been applied much later, although the advantages for the practice of the utilization of such technology are not negligible. From theoretical view-point can be also demonstrated the improvement of the stress-strain conditions in the road structure.

This paper describes briefly, in its first part, the manner by which has been build in Romania, as far back as in 1965, a bituminous road base with a great thickness (14 cm) spreaded and compacted in a single-pass with medium capacity finishers (80 to/h) made in Germany. In the paper are enumerated the practical advantages obtained in the construction site and are done comparisons with the knowledges revealed in other studies published in the highway literature referring to some similar realizations (e.g. from Germany and Great Britain).

In the second part of the paper, it tries to show by theoretical way, the improvement of the stress-strain distribution in the road structure and, consequently, the increase of the road life, owing to the utilization of the thick bituminous layers. In this scope, the road structure is considered as a three-layer medium and are utilised the Jeuffroy-Bachelez’s, Jones-Peattie’s, and Shell’s methods, transposed in some original graphics, in order to permit to set off the rates variation of the tensile stress at the base of the upper layer (sigma sub t i), of the vertical stress at the subgrade level (sigma sub z) and of the deflection at the surface course (w), for the analyzed three-layered road structures.

From those presented, result some conclusions in connection with the incontestable advantages of the single-pass construction technology and the general utilization of the thick bituminous road courses.

The elaborated graphics can be also utilized for the structural design of three-layered flexible pavements.
03091 Design and Dimensions of Standardised Bituminous Pavements in the Federal Republic of Germany – Data, Requirements, Performance, Developments
Heinrich Vogt, Peter von Becker

The paper presents a survey of current practice in using standardised bituminous road constructions in West Germany. Data, requirements and wearing qualities of bituminous mixtures and road constructions are laid down in Technical Standards and Guidelines. They permit a large number of different base constructions using hot mixed bituminous material.

Methods for classifying traffic and for determining the type and thickness of construction needed for each class are discussed, as are the standardised thickness for surface and base courses. The layer equivalencies of different base materials are discussed. The five standard construction types using bituminous mixtures are presented together with the requirements concerning subsoil, subgrade and frost protection course for standardised pavements.

The paper further discusses the bearing capacity of road base and surfacing and examines the relation between construction, live-load and road serviceability. Research work carried out on test sections of standardised pavements is reported as well as theoretical considerations. Suggestions are made for amending and improving some of the standards.
03092 Danish Experiments with the French Falling Weight Deflectometer
A. Bohn, P. Ullidtz, R. Stubstad, A. Sorenson

The falling weight deflectometer is a portable apparatus used for measuring the deflection of a road surface under the influence of a load pulse of very short time duration. This pulse simulates a fast moving wheel load, and it can be adjusted to correspond to any desired wheel load of 5 tons (5000 kp) or less. The unit is built into a trailer frame, which is easily pulled by a passenger automobile. To date, about 20 falling weight deflectometers have been produced in Denmark.

The validity of comparing the effect of the falling weight to the effect of a moving wheel load has been investigated by two means, both of which showed good correlation between the two effects. One set of measurements was undertaken in Holland, where photo-electric equipment was used to measure the deflection due to a passing wheel load. Another set of measurements was undertaken in Denmark, using an accelerometer. The acceleration signal was integrated twice with an analog integrator to obtain the deflection in question.

Static and dynamic surface deflection tests have shown poor correlation to each other, especially for roads with thick asphalt concrete layers. Investigations undertaken on a Danish experimental road, on the other hand, showed good correlation between dynamic deflection measurements and the corresponding theoretical elastic deflection values.

The asphalt concrete moduli used in these theoretical calculations were the results of dynamic three-point bending tests. The moduli of the unbound layers were found by low frequency sound wave measurements taken on-site.

Curves are shown which allow conversion of measured deflections from an arbitrary temperature between 5°C and 30°C to the corresponding deflections at 20°C (standard temperature). The 20°C curve can then be used to analyse certain limiting criteria. The limiting criteria are the strain in the bottom of the asphalt concrete layer and the verticle stress on the subgrade. These can be set to whatever values are deemed reasonable at the 20°C standard temperature. Finally, the diagrams can be used to help find the necessary thickness of a possible additional asphalt concrete reinforcement layer.
03093 Study of Some Aspects of Strengthening Thin Concrete Pavements with Flexible Overlays
Mittar P. Dhir


Reported in this paper is a study on the strengthening with flexible overlays of thin concrete highway pavements existing in India. Concrete panels of 10 ft. by 10 ft. by 4 in. were cast indoors with typical ground support conditions. Strain gauges were fixed to their under-faces by mounting the gauges on precast concrete blocks which were made integral with the slab concrete. Granular overlays of 3, 6 and 9 in. W.B.M. were constructed successively on one panel for testing, and similarly bituminous overlays of 2, 4 and 6 in. A.C. were laid on the other. Static load tests were carried out at interior, edge and corner positions using 12-in, diameter steel plates with loads applied up to 9000 lbs. Strains and surface deflections were measured.

The load test data show that flexible overlays do bring about reduction in load stress in concrete slab but that reduction is relatively limited. Broadly speaking, 9-in. W.B.M. overlay or 6-in. A.C. overlay reduces the load stress to about 60 percent for all the three load positions. Analysis of the pavement as an elastic layered system corroborates the test measurements and indicates that even 4-in, concrete slab acts as quite a rigid base. Test data show that the equivalent angle of load dispersion for W.B.M. overlays increases from 33°-34° for interior loading to 37°-38° for edge loading to 45°-47° for corner loading. The corresponding values for A.C. overlays are 45°, 49°-51° and 51°-53°. One inch of A.C. overlay is seen to be equivalent to 1.3-1.7 in. of W.B.M. overlay.

Test data were also developed through measurements on outdoor sections on the effect of flexible overlays on temperature differential in concrete slab. The data show that the differential is reduced to about 50 percent by +4 in. A.C. overlay and to about 40 percent by +6 in. W.B.M. overlay.

The study indicates that 4-in. concrete pavement cannot be saved from distress with reasonable thicknesses of flexible overlays under long term application of 9000-lb wheel load. Where thick subbases already exist, as for most of such roads in India, it would appear desirable to provide the flexible overlay from the point of flexible pavement requirements, keeping reflection cracking, etc. in view. Measures to take the operation of loads away from the inside of the concrete edges would go to enhance the life of concrete slab.

Reported in the paper are also broad indications available from 5-year performance study of about 50 different specifications of flexible overlays laid as an experiment on sections with cracked and uncracked concrete slabs. In the case of uncracked slabs, reflection cracking has developed in overlays of up to 6 in. thickness. There is as yet no reflection cracking in +4.5 in. overlays on cracked slabs. Asphaltic concrete, with a high binder content, has shown good resistance to reflection cracking.
03094 Pavement Overlaying in France – Organization – Set Up
B. Fauveau, M. Siffert

1 – The first part of the communication comprises a general description of the organization of the technical highway services in France, and the original organization set up in France, and which has been in operation since 1964, to cope with the considerable increase in the need for pavement overlaying.

2 – The second part contains a detailed description of the organization of pavement overlay teems (P.A.R.’s) and of the Coordinating Centre. An organization chart is presented, and the authors define the missions of these bodies as they were originally conceived. The results achieved during the first five years of operation of the P.A.R.’s and the Coordinating Centre are given.

3 – In the light of the missions assigned to P.A.R.’s and the necessity of an evolution, the roles of the different services involved in the implementing of pavement overlay projects are than analysed. The current evolution and the predictable evolution in the near future are presented in the light of the extent of the demands which arise.

4 – A more detailed analysis of an overlay project, from its original conception through its implementation to follow-up observations, gives a clear picture of the interrelations between the various services concerned with overlays. This analysis reviews the resources available to the Administration for planning and following up its project and drawing useful information from it.

Continuous measurement methods (deflexions, roughness and skidding resistance) provide elements for assessing the quality of the work carried out and how it stands up to conditions of use over a period of time. An overlaid pavement must possess characteristics such that it comes up to what the user expects of it; operations of the "half-way" type must be practically done away with, and curative maintenance must be replaced by preventive maintenance.
03095 The Development of Overlay Design Procedures Based on the Application of Elastic Theory
M. C. Grant, R. N. Walker

In many cases it is economical and convenient to improve the structural condition of a pavement by means of an asphalt overlay. A number of different methods can be used for the design of the overlay, one of the most popular being based on deflection measurements and limiting deflection criteria. New methods based on a structural design approach have been proposed by various investigators. One of the difficulties of this approach is the measurement of the moduli of the various pavement layers to use in the theoretical calculations.

This paper presents two new charts for designing overlays developed from the results of a theoretical analysis of model pavements using linear elastic theory. The one chart is used to determine the thickness of overlay required to reduce the deflection to the design limit. Since this chart was developed using a dual-load configuration which closely simulates that used in the Benkelman beam deflection test procedure, it is believed that it will give more accurate results than existing charts developed from considerations of a single loaded area. It is proposed that design deflection limits for overlays constructed on asphalt surfaced roads with granular bases be obtained from the deflection-load repetition relationship developed by the Asphalt Institute, and the assumption is made that this will limit distortion to an acceptable degree during the design period. The other chart is designed to eliminate cracking of the overlay and gives the relationship between radius of curvature and tensile strain in the asphalt layer for various thicknesses of asphalt. It is concluded that the use of these charts could be useful for designing overlays in cases where it is necessary to use simple evaluation techniques and simple overlay design procedures. However, it is shown that the stress-dependency of materials in the existing pavement can have a significant effect on the relationships established, and this can only be taken into account if more sophisticated testing and analysis procedures are used.

Two simple methods are described for measuring the in-situ moduli of pavement layers. The first involves measurement of both deflection and radius of curvature, and on roads with thin surfacings enables estimations to be made of the base/subgrade modular ratio and of the base modulus. It is concluded that this information would be of particular value when having to make a decision on whether it would be worthwhile improving a road by means of an asphalt overlay.

The second method uses measurements of the deflection at various depths within the pavement structure using a modified Benkelman beam procedure. This not only enables the vertical strains at any point to be calculated (from the change in deflection within a finite depth), but also enables the moduli of the various materials to be calculated using elastic theory. In this way any weak layers within the structure can be identified or the effect of an overlay or new loading condition on the vertical strains within the structure can be determined. An example of the use of the method is given in the design of an overlay to strengthen an airfield runway for the coming of Boeing 747 aircraft.
03096 Dynamic Non-Destructive Testing of Pavements in France
R. Guillemin, J. C. Gramsammer

Nine non-destructive testing equipments are now in use in the French Laboratoires des Ponts et Chaussées. They were first used in the course of the year 1966.

This paper contains two main chapters. The first accounts for the various techniques of exploitation of the experimental results leading to the determination of the elastic and geometric characteristics of pavements. The second consists in a series of concrete examples giving a general survey of the applications of non-destructive testing in the field of pavements.

Chapter II: Dynamic non-destructive testing made with the light vibrator consists mainly in determining the phase curve of a vibration of known frequency in order to obtain the dispersion curve which gives the relation between the propagation velocity and the wavelength. The analysis of the dispersion curve leads to the elastic and geometric parameters of the structure.

It is shown in this paper that very often the experimental phase curves present periodical oscillations; the latter are due to interference between two or more waves of the same frequency. Several techniques are developed either graphic or with spectral analysis, it is then possible to separate the waves and identify them.

The various techniques of interpretation of the dispersion curves are examined. Since the light vibrator is systematically used in France these techniques are conditioned by the fact that they have to be easy to use and quick to get under way. Several graphical methods for interpretation of the results concerning the structures with one or two surface layers are described.

Chapter III: The first examples deal with the study of the setting of materials treated with hydraulic binders. It will be particularly seen in the first example that it is possible with dynamic non-destructive testing to detect several inadequacies in their young age.

Several examples are described concerning a posteriori controls either on new pavements or on reinforcements. The last two examples concern particular studies in which the light vibrator was appreciated. one of them deals with the study of the behaviour of a pavement submitted to frost. It is obvious with this technique that the pavement is deeply weakened during the thawing period. The other example is a study on the influence of temperature on the viscoelastic properties of an asphaltic concrete, the main result being that on one hand the elasticity assumptions remain valid in the usual frequency and temperature fields and on the other hand a relation is given between the Rayleigh speed of asphaltic concrete in function of temperature.
03097 The Use of Gap Graded Mixes in Asphalt Overlay
K. Knight, P. Groth, F. M. L. Akeroyd

A type of gap graded mix intermediate between continuously graded asphalt concrete and a high sand content gap graded rolled asphalt has been used in the Natal asphalt overlay programme with good results. A full scale overlay experiment at Umbumbulu, trial sections at Tugela and a survey of some 150 miles of overlay already completed are described and their performance evaluated.

The mix used appears to give good performance in layers thinner than called for in terms of current design methods. Explanations are given for its apparently superior flexibility and crack resistance over the more traditional asphalt concrete.

Indirect tensile strength tests have been undertaken, which indicate a high tensile strength for this mix in the temperature ranges experienced in Natal.

Economics of stage construction are possible because of the good performance of thin (less than 4") layers of gap graded mix even over old distressed pavements of high deflection levels.
03098 The Use of Deflection Measurements for the Structural Design and Supervision of Pavements
Ph. Leger, P. Autret

The value of the deflexion is a convenient though not exhaustive means of judging the quality of a pavement and of summarily evaluating the structure (type of material and thickness) of the overlay which may be necessary.

It is accepted by some engineers that the fatigue of a subgrade, i.e., its settlement under a given number of loadings, depends to a first approximation only on the vertical elastic deformation of the soil when a load passes over it; examination of the results of calculations on two-course pavements has shown that the ratio of the deflexion at the top of the soil mass to the deformation at the interface remains practically independent of the elastic moduli, provided that the thickness of the top course does not vary.

This leads to the assumption that deflexion is a valid criterion of longevity in the case of pavements which deteriorate mainly as a result of inadequacy of bearing capacity of the underlying soil.

Moreover, the application of the theory of elasticity shows that in an elastic two-course pavement the deflexion is roughly proportional to the deformation of maximum tensile stress as the interface of the two courses. Consequently, in the case of a two-course pavement and assuming a failure by excessive deformation of the upper course, the deflexion suffices to reflect the criterion of the limit of admissible tensile stresses within a certain range of thickness of the course.

In the case of granular course, deflexion is not an indication of the evolution of deterioration. Certain processes of deterioration have no connection with deflexion. The measurement may vary considerably with the time of year, weather conditions, and conditions of loading (speed, the distribution of stresses under the wheels of vehicles, etc.).

Very often the value of the deflexion does not alone suffice to reflect the mechanical behavior of the pavement, and gives no indication of the condition of the surface.

If deflexion is taken as a criterion of failure, i.e., a limiting value corresponding to a given number of loads, this limit (which varies with the type of structure) remains to be specified.

The paper deals with these other issues:

1/ The concept of radius of curvature.
2/ Contribution to the determination of a limiting threshold of deflexion.
3/ Analysis of structures before overlaying.
03099 Deflection Criteria for Flexible Pavements and the Design of Overlays
N. W. Lister

The long-term performance of flexible pavements can be determined by the magnitude of the transient deflections which occur under traffic. A continuous programme of deflection measurements extending over fifteen years has been carried out on the full-scale road experiments in flexible construction built by the Road Research Laboratory. Over 300 experimental sections and in addition 30 lengths of major road whose construction is of particular interest have been tested annually with the Benkelman Deflection Beam under a wheel-load of 3175 kg (7000 lb). The measured deflections have been corrected for the influence of temperature on the stiffness of the bituminous components, and ‘standard’ deflections at 20°C have been related (a) to the age of the road expressed in terms of the cumulative total of equivalent standard 8200 kg (18,000 lb) axle loads carried, and (b) to the condition of the pavement in terms of rutting and cracking. The deflection histories thus derived enable the development of critical conditions within the pavement when strengthening of the road would be most appropriate, to be related to deflection behaviour under traffic.

Well-defined relations between standard deflection values measured early in the life of pavements and their lives have been established. For lives exceeding 10^6 standard axles they are of the form:

life = 1 / (deflection)^3

For pavements with crushed stone, rolled asphalt and coated macadam bases under rolled asphalt surfacings the curves are similar, but acceptable deflection levels are lower on pavements with cemented bases.

Charts making due allowance for the slow increase in deflection which takes place during the life of a pavement are being prepared to enable prediction of road performance from deflection values measured at any time to be made. An example is given.

Overlays have been applied to certain full-scale road experiments as they have been retired from service and deflection studies have been continued. The behaviour of overlays laid to strengthen normal roads is being similarly studied. Relations have been derived for the reduction in deflection brought about by overlays of different thicknesses applied to various types of pavement on different subgrades. The results have been compared with those predicted from the application of multi-layer elastic theory. Deflection histories of overlaid pavements have been used to prepare recommendations for overlay thicknesses to achieve any required extension of pavement life. The implementation of recommendations for pavement strengthening is considered.
03100 Test Sections
R. Sauterey, M. Siffert

The growing economic importance of road traffic in modern life places fresh responsibilities on the highway engineer with regard to the structural design and overlaying of pavements. Purely theoretical or purely empirical hypotheses are no longer satisfactory for insuring that the pavement shall have a suitable length of life without entailing excessive costs. Consequently, existing hypotheses have to be permanently checked and corrected, and working methods and material must be adapted to the new demands of road traffic. In short we have to define technically and economically optimal structures.

In the context of the program undertaken by the Laboratoire Central des Ponts et Chaussees (Central laboratory of bridges and highways) for the overlaying and structural design of pavements it was seen to be necessary to organize a program of studies relating to about thirty test sections; these sections are representative of current technology applied in accordance with directives which specify thicknesses, the quality of materials and the conditions of their application.

These sections, which are located in various regions of France are studied by regional laboratories which possess adequate resources for non-destructive testing, and whose engineers are in permanent liaison with the contractors concerned. This article describes the conditions under which this program has been laid down, the different techniques represented and the principal results obtained.

The need to have a thorough knowledge of the pavement before overlaying and especially of the conditions of production of the materials involved, together with the way in which the work itself is performed were all seen immediately to be essential. The principal measurements made after completion of the work may be listed as follows:
– Measurements of deformability: Lacroix deflectograph, Benkelman beam, detectors, measurement of radius of curvature and evaluation of the Rd product; measurements of deformation of the longitudinal profile and the transverse profile.
– Use of the light vibrator: a non-destructive method for assessing the rigidity of materials and detecting doubtful zones.
– Reconnaissance of the structure and its evolution by means of core sampling and exploratory drilling and analysis of samples of materials.
– Assessment of the importance of the effects of heavy traffic on the pavements concerned.

Apart from the assessment of the immediate quality of the overlay and its evolution over a period of time, this test program has made it possible to develop certain procedures and to establish correlations betwen certain measurements; it has also enabled us to study certain deteriorations and to seek their causes; to study the influence of environmental factors (traffic, weather conditions, drainage, shoulders, hard shoulders etc.) on the behavior of the pavement. The initial results obtained enable us to give a fresh orientation to certain hypotheses relating to the conditions of production of materials and to the execution of the work, the thicknesses of materials applied etc.

The lessons drawn from this study should be immediately applicable to the improvement and development of highway techniques.
03101 Applications of a Theoretical Procedure to Airfield Pavement Evaluation and Overlay Design
C. J. Van Til, B. A. Vallerga

The results of research on theoretical aspects of pavement design and performance have been applied successfully to the solution of practical problems of evaluation of load-carrying capacity of existing pavements and the design of strengthening overlays. Procedures used included the determination of a modulus of resilience by cyclic laboratory triaxial tests of undisturbed samples of pavement component materials and underlying layers of soils; the measurement of deflections in the field; the verification of modular values by comparing measured and computed values of deflection; the establishment of evaluation and design criteria in terms of limiting values of stress or strain which are specifically applicable to pavement, traffic, and environmental conditions at the site; and the preparation of overlay design curves based on these limiting values of stress or strain and on a fatigue curve developed from past traffic and pavement performance at the site.

The procedures used are presented in the form of brief descriptions of their applications to investigations conducted at three airports used by jet aircraft: Salt Lake City Municipal Airport, San Francisco International Airport, and Fairbanks International Airport. These examples serve to show the evolution of the procedure from the early applications at Salt Lake City in 1967 to the Fairbanks investigation completed in 1971. Application to a wide range of conditions is demonstrated by the fact that traffic volumes varied from very heavy on the multiple runways at San Francisco, moderate at Salt Lake City, and relatively light at Fairbanks; and that supporting soils and climate varied from soft, compressive "bay muds" and a mild climate at San Francisco, to coarser-grained soils and severe climate at Fairbanks. An investigation procedure is presented which is based on experience to date. Further developments in the procedures are expected with future usage.
03102 3rd International Conference on Asphalt Pavements – Volume II – Preliminary pages and Table of Contents
03103 Report of Session I – Factors Influencing the Design of Flexible Pavements
Chairman: Alfredo Pinilla

A detailed discussion of the papers presented in the session, follows:

Chairman’s Opening Address
Alfredo Pinilla
President, Permanent Commission of Asphalt Buenos Aires, Argentina

It is an honor for me to have the opportunity of participating in this First Session of the Third International Conference on the Structural Design of Asphalt Pavements.

First of all I would like to congratulate The University of Michigan, the Transport and Road Research Laboratory, The Asphalt Institute and Eurobitume for the organization and arrangements of this Conference. This represents quite an effort, and is of positive benefit to all of us in the development of new ideas and research related to structural design of asphalt pavements. On this occasion, I would also thank the organizers for having invited me to act as Chairman of this First Session.

At the present Conference, as at the two previous ones, an important number of papers have been presented for discussion by over a hundred specialists from many countries for the purpose of improving basic theoretical and practical field research. The fundamental purpose of this Conference is to bridge the gap between theory and practice in bituminous structure design.

During this week over seven hundred specialists in asphalt technology from all over the world will exchange information and research results pertaining to this scientific area. There will be special emphasis on better scientific and rational design for flexible pavement structures. This is in order to provide engineers with new accurate methods and procedures for the design and construction of new roads, also for strengthening old pavements and generally improving methods for future research.

Et cetera. . .
03104 Report of Session II – Properties of Materials
Chairman: J.E. Buchanan

A detailed discussion of the papers presented in the session, follows:

Chairman’s Opening Address
J.E. Buchanan
Past President, The Asphalt Institute
College Park, Maryland, U.S.A.

Ladies and gentlemen. I invite your attention to the two pages in the printed program which describe Session II of this Conference. I have the honor and duty to serve as Chairman. My purpose will be to facilitate the meeting for maximum benefit to everyone. All of us at this Conference well know the fundamental and universal value of roads to all people and all countries in the world whether those countries be under-developed or advanced in economic status. Some 40 countries are represented here today. We also know, without belaboring the point, that asphalt is highly useful in the first stages of road improvement, to provide initial access to new areas. Asphalt is equally useful in upgrading roads as development occurs. Then comes the problem of providing asphalt pavements for heavy-duty usage, represented by present and future motor vehicles and airplane traffic.

This International Conference is focused on the structural design of asphalt pavements having in mind principally the design of pavements for heavy traffic duty. This special focus is not to be interpreted as implying any lack of appreciation of the importance of the simpler types of asphalt-surfaced roads. Certainly not. The basic fact is that the lower and intermediate types do not have the analytical and design challenges inherent in the problem of designing heavy-duty asphalt pavements.

We harbor the idea that the design of heavy-duty asphalt pavements should be susceptible to analytical and engineering procedures with a degree of reliability achieved in the design of structures made of steel and concrete.

Et cetera . . .
03105 Report of Session III – Design Theory
Chairman: Egil Nakkel

A detailed discussion of the papers presented in the session, follows:

Chairman’s Opening Address
Egil Nakkel
General Director, Department of Road Construction Techniques
Federal Road Research Institute
Cologne, Germany

Ladies and Gentlemen. In opening the Third Session of this Conference I would like to thank the organizers for giving me the opportunity of taking the Chair at this Session. It is a privilege to be here, and I am indeed honored to have been invited to come in this capacity to one of the most important international conferences where outstanding experts on asphalt pavements have come to meet from all over the world. Not only I feel privileged by this post. At the same time it credits and honors the experts from my country and all those who are here from Continental Europe.

I believe that all the participants from Continental European countries and the Moderators, too, will agree with me that the rational design of asphalt pavements is a problem very close to our hearts – -after all, our countries belong to those where pavements are exposed to the heaviest of stresses, either on account of the volumes of traffic or due to the fact that extremely heavy axle loads are permitted.

There is still another reason for taking such pleasure in acting as Chairman of a Session at this Conference. Never before has there been such good cooperation, such uniting of efforts of the organizers of the Ann Arbor Conferences (already a tradition)with the Permanent International Association of Road Congresses and the European Bitumen Association (Eurobitume) in making this Conference possible.

Et cetera . . .
03106 Report of Session IV – Failure Criteria
Chairman: Philip J. Rigden

A detailed discussion of the papers presented in the session, follows:

Chairman’s Opening Address
Philip J. Rigden
Vice President, South African Council for Scientific and Industrial Research
Pretoria, Republic of South Africa

Ladies and Gentlemen, it is for me a pleasure, and a privilege, indeed a signal honor, to have this opportunity of chairing a Session at this Third International Conference on the Structural Design of Asphalt Pavements. I would like, if I may, to take this opportunity to thank the Conference Organizing Committee for inviting me again to this "Ann Arbor in London" Conference. I think the organizers are to be congratulated on all they have done to make this Third Meeting possible and to attract so many delegates from so many countries. I can only hope that, at the end of this week, the total effort involved and the exchange of ideas and information that take place will have proved well worth while.

When I spoke as a Session Chairman at Ann Arbor in 1967, I made a plea for the Conference organizers to sit down and try to define for us the questions which we must aim to answer in 1972. I also made a plea for a measure of international cooperative effort in research in preparation for 1972. In regard to this latter point I must confess to some disappointment, as I don’t think this has been done, or achieved. Perhaps I was expecting too much; there are indeed considerable difficulties in such an exercise. In Pretoria, I recall, we gave a lot of thought to how this might be done in the field of flexibility and fatigue testing of bituminous surfacings. This was going to require a large program of exchange of samples between different laboratories in different countries. The logistics, and indeed costs, of such an operation looked rather formidable and we didn’t follow it up.

Et cetera . . .
03107 Report of Session V – Pavement Performance
Chairman: Desmond F. Glynn

A detailed discussion of the papers presented in the session, follows:

Chairman’s Opening Address
Desmond F. Glynn
Director, Australian Road Research Board
Victoria, Australia

The main objective of this Third International Conference on the Structural Design of Asphalt Pavements is to bridge the gap between theory and practice relating to the structural design and construction of asphalt pavements.

The first four sessions have reported the latest research progress in regard to environments and traffic factors, properties of materials, design theory, and failure criteria. The next two will attempt to show how such data should be used in current design and construction procedures, and in strengthening existing pavements.

The present session considers full-scale experiments which will have been planned and designed by research workers but, in most cases, built by practicing engineers. Thus many minor construction details will very often mask, or at least greatly modify, the research expectations. Research workers aim to report their latest progress in their papers and to discuss the general standing of the topic with other research workers. This has been the traditional way in which research has advanced.

Practical engineers hope to be informed about research findings relevant to each aspect of pavement practice and, if possible, for recommendations to be made about the best ways to incorporate such findings into the practice of pavement design and construction. But both have been trained to think as scholars. This session should thus provide a common watershed at which contributions from both could be pooled; thus providing a deeper understanding of the true weight to be given to each of the studies reported.

Et cetera . . .
03108 Reception and Conference Banquet

Photos of the Reception and Conference Banquet held in The Great Room, Grosvenor House, London.
03109 Report of Session VI – Current Design and Construction Procedures
Chairman: William N. Carey, Jr.

A detailed discussion of the papers presented in the session, follows:

Chairman’s Opening Address
William N. Carey, Jr.
Executive Director,
Highway Research Board National Academy of Sciences
Washington, D.C., U.S.A.

This Third Conference is aimed at bridging the gap between theory and practice in the matter of structural design of pavements. This is an end devoutly to be desired and many of the excellent papers that have been accepted for this Conference demonstrate rather clearly that we are coming closer to bridging this gap. However, much remains to be done and there will need to be a Fourth and perhaps a Fifth Conference.

To my mind at this point in time, practice in the design of asphalt pavements is ahead of theory. I say this because we do design flexible pavements and build them. They do perform their function with excellent service. We need not be ashamed that the word "design" should be in quotes since what it is that we really do is to exercise engineering judgment based on a great deal of experience with different traffic, with different soils and with other different materials. On the other hand, theory, while far advanced in recent years, is not yet ready to provide us with a comprehensive and rational asphalt pavement design system. This is clear from the discussions at this Conference. I wish to discuss very briefly some of the reasons that this is so.

The development of such a rational design system has been hampered by the understandable desire of engineers to find a relatively simple formula that will work. There has been a reluctance to face up to the truly stupendous complexity of the problem.

Et cetera. . .
03110 Report of Session VII – Strengthening Existing Pavements
Chairman: Ray Millard

A detailed discussion of the papers presented in the session, follows:

Chairman’s Opening Address
Ray Millard
Deputy Director, Transport & Road Research Laboratory
Crowthorne, Berkshire, England

This is the last of the sessions of this Conference at which we shall be considering a particular theme. There has been a logical pattern in the themes of each succeeding session. In the first two sessions the theme was analysis – to define the factors that we have to consider in designing asphalt pavements, first the external factors, the characteristics of the traffic loads that roads are called upon to bear and the influence of the wide range of climatic conditions in different parts of the world on pavement design. The analysis continued in a review of the properties of the wide range of materials available for building and maintaining flexible road pavements.

From analysis we moved to synthesis, reviewing in the next three sessions the knowledge that is available from theory and from practical experience, and attempting to fit this knowledge into a coherent framework. Now in the previous session and in this one we are attempting to review how the accumulated knowledge can be put to good use. In the last session we were developers concerned with the design of new pavements. In this session we are good conservationists seeking how to preserve the value of our existing assets.

To the road engineer new construction is infinitely more exciting than maintenance. Superficially at least there is a great deal more satisfaction to be gained from creating something new, from pushing a project through the stages of planning design and construction, to completion when it will be there for all to see and to use, a demonstrable example of the contribution that engineers can make to the good of mankind. Besides this road maintenance is very pedestrian stuff indeed.

But the wind has begun to change . . .

Et cetera. . .
03111 Report Of Session VIII – Summary Session
Chairman: Sir William H. Glanville

A detailed discussion of the Conference follows:

Chairman’s Opening Address
Sir William H. Glanville
Engineering Consultant
Northwood, Middlesex, England

We have now reached the final session of the Third International Conference on the Structural Design of Asphalt Pavements. It is the business of those who speak today to look back over what has been written and said and to try to distil the essence from it. But first of all I should like to say that in my opinion the Conference has been a great success. As you have already heard, instead of the 400 participants first envisaged the number has been 800 or more – twice as many – and some hundreds more than in 1967. The meetings have been well attended and the discussions have been lively and informative.

The atmosphere has been a happy and friendly one, and I feel sure that engineers and others who have taken time off from their normal duties will feel amply rewarded and after they have recovered from the social side of Conference activity, mentally refreshed. Several of those who, like myself, have attended many conferences, have expressed the view to me that this has been one of the most successful conferences that they have attended: the mode of operation, the circulation of reports and papers, the Moderators’ reports and the orderly discussions have been most effective. Of the alternative ways of organizing conferences, none of which by the nature of things can be perfect, this has been shown to maintain interest and to lead to the development of related thinking and to the fusion of ideas.

Et cetera
03112 Corrections By Authors To Papers Printed In Volume I

These corrections have been appended to the appropriate Acrobat files in the ISAP database of papers.
03113 List of Registrants

An alphabetical list of 3rd Conference registrants, their position and their affiliation.
03114 Index of Contributors and Discussers