Asphalt pavements are usually constructed in several layers and most of pavement design and evaluation techniques assume that adjacent asphalt layers are fully bonded together and no displacement is developed between them. However, full bonding is not always achieved and a number of pavement failures have been linked to poor bond condition Theoretical research showed that the distribution of stresses, strains and deflections within the pavement structure is highly influenced by the bond condition between the adjacent layers. Slippage at the interface between the binder course and the base could significantly reduce the life of the overall pavement structure. If slippage occurs within the interface between the surfacing and the binder course, the maximum horizontal tensile strain at the bottom of the surfacing becomes excessive and causing the rapid surfacing failure. This condition becomes worse when a significant horizontal load exists. This thesis is concerned with the assessment of bond between asphalt layers. The main objective of this thesis is to provide guidance for assessing bond between asphalt layers, in order to facilitate the construction of roads with more assurance of achieving the design requirements. Further modification to the modified Leutner test has been performed. An investigation regarding the torque bond test and the effect of trafficking on bond have also been undertaken. A bond database on the modified Leutner test has been developed. An analysis has been performed to estimate the achievable values of bond strengths on typical UK road constructions obtained from the bond database. The values were then compared to the results from an analytical analysis to predict the required bond strength at the interface and other standards in Germany and Switzerland to recommend specification limits of bond strength for UK roads.

This study assesses the contribution of some selected stress absorbing membrane interlayers (SAMIs) on overlaid pavement performance in delaying the offset of reflective cracking using laboratory and full scale testing. Materials characterization were carried to have knowledge of the properties of the SAMIs and overlay and some of the properties were required as input for the finite element modelling. The characterization tests include the particle size distribution, penetration and softening point tests, dynamic mechanical analysis, indirect tensile stiffness modulus test (ITSM), indirect tensile fatigue test (ITFT) and repeated load axial test (RLAT). The interface bond was investigated using the Leutner shear test and pull off test. The assessment of the contribution of selected SAMIs on overlaid pavement performance in delaying offset of reflective cracking was carried out using a wheel tracking test supported by finite element modelling, a large scale pavement test facility test and a thermal cycling test. The Leutner shear test and pull-off test were used to examine the strength and stiffness of the overlay-SAMI interface. The interface strength/stiffness was determined because it is one of the factors that influence the crack resistance of SAMIs. The wheel tracking test was carried out to evaluate the effects of the thickness and stiffness of SAMI, thickness of overlay, SAMI composition, interface stiffness, load level and temperature on the performance of SAMIs under traffic loading. To study the performance of SAMIs under conditions close to the field, a large pavement test facility test was carried out. The finite element analysis of the wheel tracking test was carried out to evaluate the deflection, stress and strain distribution in a cracked pavement with and without SAMIs. The performance of SAMIs under thermal loading (temperature variation) was investigated using the thermal cycling test. The study shows that SAMI composition, SAMI thickness and stiffness, overlay thickness, interface stiffness, temperature and load levels influence the performance of SAMIs under traffic loading. It also demonstrates that the main factor that influences the performance of SAMIs under thermal loading is the interface stiffness. Design guidelines for the successful use of SAMIs against reflective cracking were prepared and the OLCRACK software was used to demonstrate the benefits of SAMIs in an overlay over a cracked pavement.

Adhesion is defined as the molecular force of attraction in the area of contact between unlike bodies of adhesive materials and substrates that acts to hold the bodies together. In the context of asphalt mixtures, adhesion is used to refer to the amount of energy required to break the adhesive bond between bitumen (bitumen-filler mastic) and aggregates. Thus, adhesive failure can be considered as displacement of bitumen (bitumen-filler) mastic from aggregates surface, which might indicates low magnitude of adhesive bond strength. Adhesion is considered as one of the main fundamental properties of asphalt mixtures, which can be correlated with quality, performance and serviceability. However, despite its significance, research on adhesion of asphalt mixtures is limited and yet there is no established testing technique and procedure that can be used to quantify the adhesive bond strength between bitumen (bitumen-filler mastic) and aggregates. Only in the past few years, some efforts have been conducted in developing testing techniques and procedures for measuring the adhesive bond strength of bitumen and aggregates. However, the developed testing techniques and procedures have not enjoyed universal success and acceptance, and not yet established. Hence, emphasis of this study is focused on the development of laboratory adhesion test method that can be used to directly measure the adhesive bond strength between bitumen (bitumen-filler mastic) and aggregates. Also, adhesive bond strength and failure characteristics of various combinations of asphalt mixture materials over wide ranges of testing conditions were evaluated in order to validate the reliability and efficiency of the developed laboratory adhesion test method. This study was divided into three parts. In Part 1, a detailed review of literature on various testing techniques and procedures used to measure the adhesive bond strength in numerous areas of scientific literature and international standards was performed, in order to assess and thus to propose the most suitable and realistic approach for development of laboratory adhesion test method for asphalt mixtures. In Part 2, the proposed adhesion test method was subjected to evaluation, mainly based on trial and error experimental approach, in order to adapt and thus to develop the criteria and procedures for test setup and apparatus, specimen preparation, testing and data analysis. The established criteria and procedures were then used for detailed evaluation in Part 3, in order to quantify the test results of various combinations of asphalt mixture materials (i.e. bitumen (bitumen-filler mastic) and aggregates) over wide ranges of thicknesses of adhesive layer of bitumen, aspect ratio of specimens, testing conditions (i.e. deformation rates and test temperatures) and conditioning procedures (dry and wet conditionings). Results of the study were subjected to comparative analysis in order to determine the effect of various variables and parameters on the test results, to propose suitable testing conditions and to validate the reliability and efficiency of the laboratory adhesion test method. Upon completion of the study, a draft protocol was developed as guiding principles in conducting the laboratory adhesion test method.

The formation of ice and snow on pavement surfaces is a recurring problem, creating hazardous driving conditions, restricting public mobility as well as having adverse economic effects. Current winter operations primarily consist of the correctly timed application of de-icing chemicals to the pavement surface to prevent hazardous conditions occurring. It would be desirable to develop new and improved ways of modifying the pavement surface, to prevent or at least delay the buildup of ice and to weaken the pavement-ice bond; making the ice which forms easier to remove. This development could lead to economic, environmental and safety benefits for winter service providers and road users. Recent research has identified “promising” chemical additives, which appear to have the potential to provide suitable anti-icing performance, as well as meeting requirements relating to the pavement surface life, economic and environmental factors. However, research relating to performance and durability of chemically modified asphalt for anti-icing purposes and the mechanisms by which the chemicals are transferred to the pavement surface is severely limited. The research described in this thesis attempts to contribute to the field by assessing the impact that the “promising” chemical modifications of sodium formate and sodium silicate have on the anti-icing performance and durability of asphalt. The research provides extensive data on how the chemically modified asphalt behaves in terms of the compactability, stiffness, fatigue, permanent deformation and skid resistance, relative to standard asphalt surface courses, under standard testing conditions and after high moisture absorption. The research provides a better understanding of how the chemical additive can be transferred from bitumen mastics and bituminous materials to the pavement surface. The study also assesses the potential reductions in freezing point and ice adhesion that can be produced by the specific chemical concentrations. The study evaluates laboratory test results in conjunction with three full scale site trials in the UK designed to better replicate the variability in winter weather conditions and trafficking. The study concludes that the addition of de-icing chemical formulations consisting of sodium formate and sodium silicate do not significantly reduce the asphalt performance when subjected to a number of standard asphalt test methods, including compactability, stiffness, permanent deformation and skid resistance. The inclusion of de-icing chemical formulations can however, lead to an increased susceptibility to deterioration in the presence of water, combined with an ability to absorb moisture from the atmosphere. Site trials have demonstrated that this can lead to an increased risk of early life failure and reduce service life of the pavement surface course. The thesis recommends that revised screening tests and bituminous mix design procedure is developed for assessing potential de-icing chemical formulations, which places particular emphasis on the performance of asphalt in the presence of moisture/water. The study concludes that de-icing chemical formulations can be transferred from within the bitumen mastic to the pavement surface. The chemical transfer to the pavement surface is heavily dependent on the relative humidity and the number and arrangement of surface voids. The transfer of de-icing chemical formulations to the pavement surface can reduce the freezing point of the pavement surface and/or reduce the ice adhesion.

Flexible road pavements often fail due to excessive rutting. as a result of cumulative vertical permanent deformation under repeated traffic loads. The currently used analytical approach to flexible pavement design evaluates the pavement life in terms of critical elastic strain at the top of the subgrade. Hence, the plastic pavement behaviour is not properly considered. Shakedown analysis can take into account the material plasticity and guarantee structure stability under repeated loads. It provides a more rational design criterion for flexible road pavements. Finite element analyses using the Tresca and Mohr-Coulomb yield criteria are performed to examine the responses of soil half-space when subjected to different loading levels. Both shakedown and surface ratchetting phenomena are observed and the residual stresses are found to be fully-developed after a limited number of load passes. The finite element results are then used to validate the solutions from shakedown analysis. The main focus of current research is concerned with new solutions for static (i.e. lower-bound) shakedown load limits of road pavements under both two-dimensional and three-dimensional moving surface loads. Solutions are derived by limiting the total stresses at any point (i.e. residual stresses plus loading induced elastic stresses) to satisfy the Mohr-Coulomb yield criterion. Previous analytical shakedown solution has been derived based on a residual stress field that may not satisfy equilibrium for certain cases. In this study, a rigorous lower-bound shakedown solution has been derived by imposing the equilibrium condition of residual stresses. The newly developed shakedown solutions have been applied to one-layered and multi-layered pavements. It was found that the rigorous lower-bound solution based on the self-equilibrated residual stress field is lower than the analytical shakedown solution for cases when the critical point lies on the surface or at the base of the first pavement layer. The results showed that the theoretical predictions of pavement shakedown load limit generally agree with the finite element and experimental observations for pavement behaviours. The shakedown solution has been further extended to study the influence of the shape of contact load area for pavements under three-dimensional Hertz loads. It was found that the shakedown load limit can be increased by changing the load contact shape from a circle area to an elliptical one. A new pavement design approach against excessive rutting has been proposed. The pavement design is suggested by plotting thickness design charts using the direct shakedown solutions and choosing the thickness combination based on the design traffic load.

Asphalt mixture is mainly used for the construction of roads throughout the world. Large amounts of capital are spent for construction and maintenance of roads. Water is one of the major contributors towards the damage of the road structure. It is considered as the worst enemy of a pavement structure by directly causing a distress or indirectly magnifying a distress and hence damaging the road structure. Asphalt mixture loses its strength in the presence of water either through loss of cohesion within the bitumen or loss of adhesive bond between bitumen and aggregate. All the conventional techniques that are used for the determination of the moisture susceptibility of an asphalt mixture assess the material as a whole by using some mechanical testing technique without taking into account the individual physicochemical characteristics of both the bitumen and the aggregates. The surface energy properties of the materials, which are used to quantify their interfacial adhesion, play an important role in the final adhesive bond strength between these materials. The aim of this research is to produce detailed experimental techniques to measure the surface energy properties of bitumen and aggregate, and then combine them with a mechanical moisture sensitivity test procedure. This can greatly contribute towards the development of a powerful material screening protocol/tool for selection of bitumen-aggregate combinations that are less susceptible to moisture damage. This thesis describes the work that was carried out towards the development of a physico-chemical laboratory at the Nottingham Transportation Engineering Centre (NTEC). Four types of equipment were used, namely goniometer and dynamic contact angle analyser for determining the surface energy properties of the bitumen samples, and the dynamic vapour sorption and microcalorimeter systems for the surface energy properties of the aggregates. Large amount of material testing was carried out with these equipment and testing protocols were developed and improved over the course of experimental work. It was found that the dynamic contact angle technique and dynamic vapour sorption technique provides consistent results for bitumen and aggregates respectively as compared to the other two test equipment. The surface energy properties of the bitumen and the aggregates were then combined thermodynamically to determine the adhesive bond strength between the two materials, and the reduction in the adhesive properties if water is introduced into the system. The results showed that these thermodynamic properties generally correlate well with the moisture damage performance of these combinations from the laboratory testing. SATS mechanical test technique was used to determine the moisture susceptibility of different bitumen-aggregate combinations. The virgin material and the recovered material from the SATS tested cores were tested for the surface energy properties. It was found that the surface energy properties combined with SATS results can be used, with some exceptions, to identify compatible bitumen-aggregate combinations and hence improved moisture damage performance of the resulting asphalt mixture.

Research to improve the performance of asphalt mixtures through the addition of crumb rubber using the dry process has continued worldwide because of its potential as a recycling option for used tires. For decades, dry mixed rubberised asphalt mixtures have performed inconsistently in field trials and laboratory evaluations. However, current research has revealed that the performance of asphalt mixtures is highly dependent on the characteristics of its internal structure or phase constituents. A comprehensive methodology has been developed in this study to characterise the microstructural properties of dry mixed rubberised asphalt mixtures and correlate them with the mixtures' macroscopic response to compression and fatigue. The proposed methodology combines a non-destructive imaging technique; X-ray Computed Tomography (CT) and image processing and image analysis procedures to quantify the properties of air voids and cracks as well as the rubber distributions within the rubberised asphalt specimen. A gap graded mixture of Hot Rolled Asphalt (HRA60/20) containing different percentages and gradings of crumb rubber particles was compared to a conventional mixture (unmodified or control mix). The results showed that the addition of crumb rubber affects the formation and distribution of air voids in an asphalt mixture. Correlations between the internal structural damage parameters and the mechanical behaviour of the asphalt mixtures were unanimous in concurring that adding rubber improves a mixture resistance to fatigue failure. The modulus of the asphalt mixtures at peak stress under compression reduced when the rubber was introduced into the mixture although in contrast, the image analysis showed less fracture within the tested specimen in comparison to the control mix. The imaging procedures developed in this text are recommended as a guide to characterise the internal structure of rubberised asphalt mixtures.

Flexural fatigue is one of the main failure modes in asphalt mixtures. After reviewing and critiquing the current literature about fatigue in pavement engineering, this research project focused on dissipated energy approach because it takes into account the evolution of the material during a test. A comparison between the traditional method and several dissipated energy methods was made by using statistical analysis. Further research investigation involved the understanding of fatigue testing machines. Different categories of tests give different fatigue life values; so which one best represents the real world? This project focuses on pure fatigue tests and diametrical fatigue tests. The main innovative contribution of this thesis is the development of a new fatigue test: ITFT in strain controlled mode. It is a simple fatigue test widespread in UK often used by civil engineering firms to characterise stiffness and fatigue properties of asphalt materials mostly for construction and maintenance sites. Currently, the ITFT characterises the behaviour of asphalt material under repeated constant load; so no ITFT data obtained in strain control mode exist. To overcome this lack, ITFT in strain control mode was developed; this allows comparing results between simple flexural tests and diametrical tests. Results show that fatigue lives obtained by means of the ITFT are smaller than fatigue lives obtained by pure fatigue tests, this is due to the accumulation of permanent deformation during the ITFT; however ITFT results are reliable and statistically not different from 4PB results. It is true that pure fatigue does not really exist in real life; failure is a more complicated phenomenon. Thus, developing ITFT in strain control mode could reduce the gap between research in laboratory (where pure fatigue tests often are used) and in the fields (where experience showed that quick and simple tests are preferred by engineering consultancies).

This research presents a methodology that accounts for variability of key pavement design input variables and variations due to lack-of-fit of the design models and assesses effects on pavement performance (fatigue and deformation life). Variability is described by statistical terms such as mean and standard deviation and by its probability density distribution. The subject of reliability in pavement design has pushed many highway organisations around the world to review their design methodologies to evaluate the effect of variations in materials on pavement performance. This research has reinforced this need for considering the variability of design parameters in the design procedure and to conceive a pavement system in a probabilistic way, similar to structural designs. This study has only considered flexible pavements. The sites considered for the analysis, all in the UK (including Northern Ireland), were mainly motorways or major trunk roads. Pavement survey data analysed were for Lane 1, the most heavily trafficked lane. Sections 1km long were considered wherever possible. Statistical characterisation of the variation of layer thickness, asphalt stiffness and subgrade stiffness input parameters is addressed. A model is then proposed which represents an improvement on the Method of Equivalent Thickness for the calculation of strains and life for flexible pavements. The output is a statistical assessment of the estimated pavement performance. The proposed model to calculate the fatigue and deformation life is very fast and simple, and is well suited to use in a pavement management system where stresses and strains must be calculated millions of times. The research shows that the parameters with the greatest influence on the variability of predicted fatigue performance are the asphalt stiffness modulus and thickness. The parameters with the greatest influence on the variability of predicted deformation performance are the granular subbase thickness, the asphalt thickness and the subgrade stiffness.

The permanent deformation of a pavement due to vehicle load is one of the important factors affecting the design life as well as the maintenance cost of a pavement. For the purpose of obtaining a cost-effective design, it is advisable to predict the traffic-loadinduced permanent pavement deformation. The permanent deformation in pavements (i.e. rutting) can be classified into three categories, including the wearing of the asphalt layers, compaction, and shear deformations. In the present study, discrete element analyses have been performed to predict the permanent deformation of a pavement when subjected to moving wheel loads. Note that the wearing of the asphalt layers has been disregarded. DEM biaxial test simulations have been carried out in terms of both unbonded and bonded granular materials. The typical stress-strain response, as well as the volumetric strain development, have been reproduced, in qualitative agreement with the experimental results. The factors affecting the mechanical behaviour of granular materials have been investigated, e.g. particle stiffness, sample compaction and parallel bond strength. In addition, the elastic properties, initial yield stress, strength parameters and so on have been analysed. These compression tests provided guidance for the selection of the particle parameters for the subsequent pavement simulation. The permanent deformation in unbonded pavements was represented under moving wheel loads, and proved to be qualitatively consistent with the laboratory tests. The initial self-weight stress had a significant effect on rutting. When the initial gravity stress was relatively high, both shakedown and surface ratchetting phenomena were observed for different loading levels. However, the accumulation of permanent deformation was continual for pavements with low gravity stress, even if the wheel pressure was small. Other factors affecting the rutting have been taken into consideration, e.g. specimen preparation, interparticle friction, etc. In the case of the single-layered pavement, permanent deformation ceased after the first wheel pass. Plastic deformation increased with the decrease in the self-weight stress. For the double layered pavement, the permanent deformation continually increased with wheel passes, probably owing to compaction of the bottom unbound layer. The pavement shakedown phenomenon was not observed prior to wheel pass 300. The permanent deformation increased augmentation of wheel pressure as well as decrease in the sample density and upper layer thickness. The residual stresses in both vertical and horizontal directions can be obtained using the measurement circle. For all the pavements in the current simulations, the vertical residual stress is nearly always zero, consistent with the equilibrium condition. In the case of the unbonded pavement, the large horizontal residual stress depends on the high initial gravity stress, instead of high wheel pressure or wheel pass number. For the single-layered pavement, the peak of the horizontal residual stress was observed near the pavement surface. The residual stress rises with the augmentation of the wheel pass number and the wheel pressure. In the double-layered pavement, the residual stresses are discontinuous at the interface between different pavement layers. The peak appears near the pavement surface and increases with the reduction in the upper layer thickness as well as the rise in wheel passes and wheel pressure. Nevertheless, residual stress is not apparent in the granular base. The probability density distribution was investigated in terms of the contact and bond forces. For the normal contact force, a peak generally appeared at small contact forces, followed by a drastic decrease and, after that, the probability density progressively approached zero. For the tangential contact force as well as the bond forces, in general, a peak of the probability distribution was observed at small contact forces, and then a sharp drop followed from the two flanks of the peak point. Finally, there was a gradual decrease until the probability density decayed to zero. The factors, e.g. pavement layer, wheel pass number and wheel pressure, mainly affect the probability distribution of the small contact or bond forces. For both single- and double-layered pavements, the absolute extrema of the bond forces in the top layer increased with the augmentation of the wheel pass number and the wheel pressure. For the unbonded pavement, the sliding contact ratio was studied and it was significantly affected by the pavement layer, initial gravity stress and sample compaction. The distribution of the pavement particle displacements were demonstrated. In the unbonded pavement, factors, such as wheel pressure and initial gravity, not only affect the distribution of the relatively large particle displacements but also increase the magnitude of the particle displacements. The directions of the large displacement vectors are diverse as the large gravity acceleration is assigned to the particles but are almost downward when the self-weight stress is small. In the single- or double layered pavement, factors, such as wheel pass number and wheel pressure, merely increase the values of the particle displacements. The distribution of the displacements is hardly affected. For the single-layered pavement, the large displacements were observed near the pavement surface and their directions are almost contrary to the movement direction of the wheel. In the double-layered pavement, relatively large particle displacements are widely distributed in the pavement. Their directions are in an almost vertical direction.