The maintenance of road pavements in England has become a costly necessity, due largely to the large volume of commercial vehicles using the roads which cause pavements to deteriorate quickly, and makes their repair more difficult to carry out. These roadworks incur not only direct works costs, but also indirect costs from factors such as congestion, motor accidents and pollution. There is obviously a need for cost-effective maintenance that minimises the occurrence and duration of these disruptions. To strengthen pavements bituminous overlays are often used, but may crack prematurely when placed over a layer with discontinuities such as cracks or joints, or deform excessively under wheel loading. The problem of 'reflective cracking' is widespread and reduces the life of maintenance treatments considerably. To increase the time before cracking appears on the surface of a pavement, a (more expensive) thicker overlay may be used, but this can lead to problems with property thresholds and bridge clearance. One possible option of reducing the thickness of overlays by making them more resistant against cracking and deformation, is to place a layer-of reinforcement within or at the bottom of the overlay. Although this approach has been used occasionally to reinforce overlays, over 40 years or so, it is not favoured with many road authorities, as the results of these treatments are difficult to anticipate, and may not be cost effective. This thesis describes an investigation into the effect of reinforcing thin bituminous overlays to identify key factors that significantly influence their performance. By identifying these factors, optimum use of reinforced asphalt should be possible, and thus maintenance of the road network made more cost effective. The investigation was principally carried out in the laboratory using beam tests, shear box tests, tensile tests on reinforcement and large-scale wheel tracking tests. 2-D Finite Element Analysis was used in the analysis of test results. Results show that properly constructed reinforced overlays can be between two or three times more resistant to cracking, and have less than half the permanent deformation of unreinforced materials.

At the present time (1978) new roads in the United Kingdom are designed empirically. However, over approximately the last twenty years pavement technologists throughout the world have been advocating fundamentally based procedures. Such procedures, when applied to flexible pavements, require a detailed knowledge of the two main modes of failure, fatigue of the bituminous bound layers and excessive permanent deformation of the pavement as a whole. The research described herein was aimed at improving knowledge of the permanent deformation behaviour of flexible pavements, and at developing a technique for predicting this deformation which could be incorporated in a fundamental design procedure. Three laboratory experimental pavements were trafficked with a rolling wheel facility under conditions of constant temperature, load and speed, and the resulting deformations were monitored. These deformations were compared with predictions using models developed from the results of repeated load characterisation tests on the pavement materials and utilising a digital computer. A review of previous work is presented ill two parts, the first considering the resilient and permanent deformation response of pavement materials, the second considering proposals for the prediction of permanent deformation in pavements. The development of the wheel loading facility, and the construction, instrumentation and measurements from the pavements are described briefly. More attention is given to the theoretical approach, materials characterisation testing and the prediction of resilient and permanent deformation response of the pavements. The prediction techniques and their accuracy are assessed, the problems involved in the development of such procedures are discussed, and the conclusions which can be drawn from the work are presented. A number of suggestions regarding further work in this field are made. Finally, an Appendix considers an alternative to the method of modelling permanent deformation presented in the main part of the text.

Bitumen emulsion mixtures, or cold mix, offers certain advantages over hot bituminous road mixtures in terms of potential cost savings, environmental factors, energy savings and easing of logistical difficulties inherent with hot mix. For these reasons, they are currently used in a number of regions including the United States of America, Continental Europe, Scandinavia, Southern Africa and Australia. However, cold mix has not been seriously evaluated or utilized in the UK until comparatively recently. This is due to the slow rate at which it builds strength or cures and its susceptibility to rainfall, particularly during this curing period. The overall aim of this research was, therefore, to study the behaviour of cold mix in terms of mechanical properties and the mechanisms involved in emulsion breaking and mixture curing to gain an insight into how performance may be improved. Mechanical properties were shown to be affected by a number of parameters, including binder grade, void content, curing time and additives such as cement. Fatigue tests showed that without cement, the performance in-situ of cold mix would be very poor. Pavement design calculations revealed that, with cement, emulsion mixtures could be expected to perform similarly to equivalent hot mix. Field trials have indicated that cold mix can be manufactured using conventional hot mix plant and laid using similar techniques. However, problems are still to be overcome in terms of the control of mixture composition and laying in adverse weather conditions. Fundamental tests have shown that emulsion droplet coalescence (which is an essential part of the curing process) was affected by pressure, bitumen type, emulsifier level, cement and temperature and that cement can cause emulsion charges to become more positive (or less negative) but other parameters had no effect on charge. It was also proved that cement can cause bitumen emulsion to set completely and that the residual binder has an increased viscosity compared with unmodified binder. This was shown to be due to the formation of a composite bitumen cement binder.

The shakedown concept has been widely applied in structural and mechanical engineering numerical models. The concept is related to the response of a structure to load repetitions in a resilient manner without further permanent deformation. More than 40 wheel tracking tests were conducted with various wheel load levels for each test to check the validity of the shakedown concept in the pavement foundation. Six different types of soils with different characteristics were used in the wheel tracking tests. These were a silt (from gravel pit washings), a silty-clay (Mercia Mudstone, referred to here by its earlier name of Keuper Marl), two sands (Portaway and Langford Fill), and two crushed rocks (Carboniferous Limestone and Granite). Three different sized wheel-tracking facilities were used; a small wheel tracker (SW), a larger Slab Testing Facility (STF) and the half-scale Nottingham Pavement Testing Facility (PTF). These allowed various wheel specifications and test specimen sizes to be investigated. The test programme embraced one, two and three layered systems. The permanent vertical deformation of each system was measured after a certain number of passes. The soil is said to be under shakedown if after a certain number of passes, there is no further permanent deformation. The experimental result was compared with the theoretical shakedown prediction. A series of static triaxial tests for each soil, with the test conditions close to the wheel tracking tests, was carried out to identify the shear strength to be used as input parameters for the theoretical shakedown prediction. The theoretical shakedown limits of the various soil combinations show a good agreement with the wheel tracking test results.

This dissertation is concerned with the fatigue cracking of bituminous paving mixtures. It considers both the life to crack initiation and the life for crack propagation, including the development of a method for calculating the number of wheel load applications to either critical or failure condition. The development and subsequent validation of the Indirect Tensile Fatigue Test (ITFT) are described. Both the repeatability and reproducibility of the method are examined as well as its correlation with two other fatigue test methods. The test is shown to be a suitable method for measuring the life to crack initiation of bituminous paving mixtures as well as being an economically viable test. Poisson's ratio for bituminous paving mixtures is examined showing that, provided at least 500 conditioning pulses are used in order to achieve steady state conditions, 0.35 is an appropriate value. Prior to these early load applications Poisson's ratio is shown to be variable and often in excess of 0.50, an explanation of which is given in detail. Crack propagation was experimentally simulated using beams of material with a crack initiated on the underside. The work shows that the rate of crack propagation can be described by a power relationship between the stiffness of the mixture and the number of cycles to failure, which is mixture and binder dependent. A general equation is developed which allows the critical and failure lives of bituminous pavements to be calculated and these are compared to two traditional pavement design methods, giving equivalent results for unmodified mixtures but, giving more realistic results for polymer modified mixtures. Image analysis of the cracks demonstrates that they propagate around coarse aggregate trying to separate it from the matrix and that they travel in the straightest line possible between the point of crack initiation and the point of applied load.

The primary aim of this research was to investigate the behaviour of a clay subjected to a loading regime similar to that experienced by a road subgrade under traffic loading in Great Britain. The material used was Keuper Marl. The samples were anisotropically consolidated in a triaxial apparatus from a slurry which allowed careful control over the stress history and produced uniform samples. The samples were fully instrumented and the apparatus was capable of applying repeated axial and radial stresses. The test programme was designed to investigate the resilient and permanent response of the samples to a variety of stress pulse magnitudes and time periods. The main conclusions were: i) The material exhibited a marked stress softening. ii) The mean normal effective stress remained constant under a variety of total stress paths over the range of frequencies tested. iii) The resilient response was found to depend on the magnitude of the applied stress pulse and the mean normal effective pressure, and to be independent of the preconsolidation pressure. iv) The material exhibited significant thixotropy. A smaller parallel series of tests was carried out on compacted triaxial samples of three clays (Keuper Marl, Gault clay and London clay) in a simple pneumatic repeated load triaxial rig. The test programme was designed to investigate the resilient response of the samples over a range of repeated deviator stresses. The suction moisture content relationship for each clay was established, and the resilient response of the clay was found to be controlled by the magnitude of the stress pulse and the suction. A series of California Bearing Ratio tests was carried out on compacted samples of the three clays, and on anisotropically consolidated samples of Keuper Marl, to allow a comparison to be made between the resilient modulus and CBR. A review of previous work is presented.

The demands on asphalt pavements, as a result of the growth in traffic volumes, traffic loads and tyre contact pressures, has resulted in an increased interest in the use of modified bitumens, particularly over the last ten years. Of the various types of modified and specialised binders that are available worldwide, polymer modified bitumens (PMB’s) have tended to be the most popular. Polymer modification significantly alters the rheological characteristics of the binder, thereby requiring the use of fundamental rheological testing methods rather than empirical methods, to provide an indication of the performance of the binder and subsequently the asphalt mixture. This thesis is concerned with the use of a Dynamic Shear Rheometer (DSR) to quantify the fundamental rheological characteristics of various unaged and aged PMB’s. The parallel plate testing methodology used with the Bohlin Model DSR50 Dynamic Shear Rheometer requires accurate temperature control by means of a circulating fluid bath, a combination of different plate diameters and sample geometries, and the use of small strains in order to measure the linear viscoelastic rheological characteristics of a bitumen specimen. Conventional and dynamic shear rheometry testing of various penetration grade bitumens, semi-crystalline ethylene vinyl acetate (EVA) PMB’s and thermoplastic rubber styrene butadiene styrene (SBS) PMB’s have indicated that the rheological characteristics differ considerably between the unmodified and polymer modified bitumens as well as between the plastomeric EVA and elastomeric SBS PMB’s. The DSR dynamic rheological parameters of complex modulus and phase angle indicate that the modification mechanism of EVA PMB’s consists of the crystallisation of rigid three dimensional networks within the bitumen. These rigid crystalline structures increase the stiffness and elastic component of the viscoelastic balance of the PMB up to the temperature associated with the melting of the copolymer. The modification mechanism of SBS PMB’s consists of the establishment of a highly elastic network within the bitumen that increases the elasticity and stiffness, particularly at high temperatures. The higher melting temperature of the SBS copolymer allows the rheological character of the SBS PMB to extend to temperatures greater than those found for EVA PMB’s. DSR measurements of the rheological changes associated with laboratory ageing of EVA PMB’s indicate that the ageing mechanism is linked to a chemical change of the copolymer due to fusion of the crystallites. This chemical change leads to a degradation of the polymer and, therefore, a transition of the rheological behaviour towards that of an unmodified bitumen. The rheological changes associated with the ageing of SBS PMB’s is linked to a breakdown of the molecular structure of the SBS copolymer to form a lower molecular weight polymer substructure. This results in an increased viscous behaviour after ageing compared to the increased elastic behaviour found for unmodified bitumens.

As part of a European Union funded research study (the "SCIENCE" project) performed between 1990 and 1993, granular road construction material and subgrade soil specimens were tested in the four participating laboratories of the project: Laboratório Nacional de Engenharia Civil Portugal University of Nottingham United Kingdom Laboratoire Central des Ponts et Chaussées France Delft University of Technology The Netherlands The author was based the first of these and visited the other participating laboratories, performing the majority of the work described. Inaccuracies in repeated load triaxial testing based on the use of different apparatus and instrumentation are identified. A detailed instrumentation comparison is undertaken, which results in the magnitude of potential errors being quantified. The author has derived material parameters and model coefficients for the materials tested using a number of previously published material models. In order to establish these parameters a method for removing outliers from test data based on the difference between the modelled and experimental material parameters for each stress path applied was developed. The consequences of repeatability and reproducibility, variability and inaccuracies in the output of repeated load triaxial testing, on the parameters and, hence, on computed pavement design thicknesses or life is investigated using a number of material models and the South African mechanistic pavement design method. Overall, it is concluded that: • Instrumentation differences are not as critical as variations in results obtained from different specimens tested in a single repeated load triaxial apparatus. It was found that specimen manufacture difference yielded greater variation that instrumentation differences. • Variation in results has some effect on the upper granular layers, where higher stress levels are experienced, but even quite considerable variation in the results from materials used in the lower layers has little effect on pavement life. • Analytical methods to determine the stresses and strains vary considerably as do the predicted pavement thicknesses consequent on using these methods. The inaccuracies in testing (large discrepancies are found when the same material is tested in the same laboratory) and the limitations of the available material models severely limit the usefulness of advanced testing and non-linear modelling in routine pavement design. On the basis of this study it is recommended that a more simplistic pavement design approach be taken keeping in line with future developments of testing and modelling and field validation.

Little effort has been made to apply the Critical State Soil Mechanics concept to the prediction of pavement response. The aim of this research is to apply soil mechanics principles, particularly the kinematic hardening concept, to the prediction of the response of lightly trafficked pavements to repeated loading. For this purpose, the finite element critical state program CRISP is used. A comparison is made between the predictions given by the three-surface kinematic hardening (3-SKH) model and a layered elastic analysis BISAR for the resilient deformation produced by repeated loading of a thinly surfaced pavement, and the models are found to be in good agreement. The ability of the 3-SKH model to predict soil behaviour under cyclic loading, and under one-dimensional loading, unloading and reloading is also evaluated. A comparison between model predictions and experimental data obtained by other researchers shows that the 3-SKH model over-predicts the value of K0,nc and hence shear strain during monotonic loading. This problem is magnified when the model is applied to cyclic loading behaviour where large numbers of cycles are involved. The model also predicts an accumulation of negative shear strain with increasing number of cycles under some stress conditions. This will lead to unrealistic predictions of rutting in pavements. However, the model is suitable for obtaining resilient parameters for input to a layered elastic analysis program. A new model, which is a modified version of the 3-SKH model, is therefore proposed by modifying the flow rule and the hardening moduli. This model can correctly predict the value of K0,nc and reduce the amount of shear strain predicted. The model also eliminates the problem of accumulation of negative shear strain with increasing number of cycles. The new model introduces two additional parameters, one of which can be determined by one-dimensional normal compression test, and the other by fitting a set of cyclic loading data. The new model is used to design the required thickness of granular material using the permissible resilient subgrade strain and permanent rut depth criteria during construction. It is found that the new model predicts a realistic granular layer thickness required to prevent excessive rutting, thus showing much promise for use in design of thinly surfaced pavements.

Research has been conducted into the fundamental mechanical properties of a granular material. This has involved the use of both a repeated load triaxial apparatus and a new hollow cylinder apparatus, which has required development and modification. Building on the foundation of earlier research at Nottingham, models have been developed which predict the stress-strain behaviour of a dry granular material under any combination of applied stresses. This includes repeatable elastic behaviour and the development of irrecoverable plastic strain. The accuracy of these models has been tested as far as possible using available test equipment. The effects of varying particle gradation, degree of compaction, maximum particle size and mineral type have been explored using a repeated load triaxial apparatus. The effect of moisture has also been investigated with respect to both full and partial saturation. A computer program (GRANMAT) has been written, which makes use of the stress-strain equations developed, to analyse a road pavement structure consisting of thin bituminous surfacing, granular base and subgrade. The reliability of the program has been assessed and areas of necessary improvement indicated. In-situ testing at a number of road sites is described and analysed with respect to both the effect of compaction on a granular road foundation and the use of a number of in-situ test devices. Finally, the information gained, in the laboratory and on site, is brought together in a series of design suggestions for granular road foundations, with particular reference to results from the GRANMAT computer program.