Skid resistance, the road surface’s contribution to friction, is a crucial property of a road surface course required to maintain a safe and serviceable road network. Measurement of skid resistance is restricted by the need to measure the forces acting on a rubber wheel or slider while it is dragged across the surface. If the skid resistance of the road could be determined without the need for contact then measurement could be cheaper and more thorough. One route to achieving this goal is by measurement of the texture of the road that generates the friction experienced by a sliding tyre. However, the form and scale of the texture required is not well defined. The work presented in this thesis attempts to establish a robust methodology for measurement of texture on the surfaces of aggregate particles (the main constituent of the road surface course) so that it can be compared with friction. The stages of development are described in detail and the methodology is employed to examine the changing texture on two types of aggregate. The mechanisms by which these aggregates polish, methods for characterising their surface texture, and the consequences for the friction they are able to generate are explored.

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.

Road pavement design in Kurdistan is based on ASSHTO 1993. However, it seems not to be entirely satisfactory since it is unable to take full account of properties of local soils or those which have been stabilised. To address this, a design procedure applicable to different material and environmental conditions was developed. The associated research consisted of a suite of laboratory experiment allied to the development of a finite element model. The laboratory work was undertaken on three types of subgrade soils found in Kurdistan to determine their permanent deformation behaviour, UCS and resilient modulus for a range of moisture contents. The experimental investigation considered soils stabilised with 2%, 4% cement content and a combination of cement and lime with 2% cement plus 1.5% lime and 4% cement and 1.5% lime. The results were used to develop empirical equations to: (i) predict resilient modulus values of deteriorated modified soils as a function of different stabiliser contents and types; (ii) correlate resilient modulus values of soils with their UCS and stress state; (iii) determine the accumulation of permanent deformation in modified subgrade soils subject to weathering. These relationships, together with the developed finite element model were used to establish the design procedure.

Flexible pavements are environmentally sensitive elements of infrastructure and their performance can be influenced by climate. Climate change poses a challenge to design and management of flexible pavements in the future. Climate change can occur worldwide and thus all flexible pavements can be exposed to the impact. However, an assessment framework is not available to evaluate the impact of climate change on flexible pavements in terms of performance, maintenance decision-making and the subsequent life-cycle costs (LCC). This research has attempted to develop such a framework. Case studies on six flexible pavement sections from the United States were performed to demonstrate the application of the framework. The framework started with the investigation of climate change using IPCC’s (Inter-governmental Panel on Climate Change) climate change projections. Combinations of climate change projections and local historical climate were adopted as climatic inputs for the prediction of pavement performance. The Mechanistic-Empirical Pavement Design Guide (MEPDG) was used for prediction of pavement performance because it can provide reliable performance predictions with consideration of climatic factors. Pavement performance predictions were applied to schedule maintenance interventions. Maintenance effects of treatments were considered in maintenance decision-making. Maintenance effect models of International Roughness Index (IRI) and rutting were validated using pavement condition survey data from Virginia. With selected climate related LCC components, three maintenance interventions were optimised using a genetic algorithm to achieve the minimum LCC. Eventually the outputs of the system including pavement performance, intervention strategies, and LCC can be compared under various climate change and baseline scenarios. Hence, the differences in performance, decision-making, and LCC due to climate change can be derived. The conclusions were drawn based on the scheme of maintenance decision-making. If flexible pavements are not maintained (Alternative 0), an increase in LCC will be incurred by climate change due to an increase in road roughness (IRI). For pavements maintained with strict thresholds (Alternative 1), climate change may lead to a significant reduction in the service life when the maintenance is triggered by climate sensitive distress. However, benefit can be gained from decreasing LCC as the earlier triggered maintenance may result in less average IRI. As a consequence, user costs, which can be associated with IRI, can be reduced. Hence, LCC can be reduced as user costs usually dominate LCC. However, the net present value (NPV) of agency costs can be increased due to the early intervention. For pavements with optimised maintenance (Alternative 2), the LCC is almost unaffected by climate change. However, the type or application time of interventions may need to be changed in order to achieve this. Furthermore, the balance between agency and user costs did not seem to be influenced by climate change for Alternative 2. Agencies should be aware that maintenance optimisation can significantly reduce the LCC and make the best use of treatments to mitigate the effects of climate change on flexible pavements. Pavement maintained with strict triggers may require earlier interventions as a result of climate change but can gain benefit in LCC. However, this indicates that a responsive maintenance regime may not take full advantage of interventions and that maintenance could be planned to be performed earlier in order to achieve minimised LCC. Due to climate change, road users may spend more on fuels, lubricants and tyre wear on flexible pavement sections that do not receive any maintenance treatments.

An asphalt mixture is combined of different sizes of aggregate, filler, and bitumen for application on the most common road construction materials. In asphalt pavement material there are different types of distress such as permanent deformation (rutting), fatigue cracking, ravelling, potholes, stripping, etc. There are many reasons for these types of distress, some of them related to the pavement structure, e.g. whether the underlying layers are weak, others related to the mixture properties. Other causes could be related to external conditions such as high temperature, high axle load, long duration of load application, etc. This research has focused on the permanent deformation (rutting) as a function of aggregate gradation. The aggregate gradations of more than twenty asphalt mixtures, manufactured with different gradations, were analysed by using the Bailey method of gradation analysis. The analysis was performed in relation to Repeated Load Axial Test (RLAT) testing results to study the performance of each mixture. The results showed that the Bailey method is not capable on its own to define the differences between the gradations of each mixture. Therefore, three more packing ratios were introduced to adequately describe the aggregate gradation. The aggregate particle packing was extensively studied through these packing ratios and it was shown how the different particle sizes interact with each other. Images were taken for two mixtures to validate the theory behind the ratios. The five packing ratios (two of Bailey and three new ratios) were used in Artificial Neural Network (ANN) and Adaptive Neuro-Fuzzy Inference System (ANFIS) techniques for all the mixtures as input data to predict the mixture performance (RLAT permanent deformation and Indirect Tensile Stiffness Modulus ITSM stiffness modulus) and they showed good prediction capability. After establishing the impact of aggregate packing on the performance, six mixtures were re-manufactured and re-tested with different variables; the selection of the mixtures was made to cover a range of different gradations (ratios). The aim of this step was to understand the effect of these variables on the asphalt mixture in the light of the packing ratios. The variables that were used were binder content, testing temperature and compaction effort. The binder content results showed an interesting effect on the permanent deformation and stiffness of the asphalt mixture. The packing of aggregate was very helpful in understanding the different mixture behaviour with different binder content. The effect of aggregate packing was not shown at relatively low testing temperature, but as the temperature rises the aggregate packing effect starts to appear. The effect of compaction which was represented by the number of gyrations in gyratory compactor was inconsistent; results show over-compaction can lead to poor performance. Finally, a linear viscous method was introduced aiming to predict the rutting in an asphalt mixture. The method was based on using a multilayer linear programme (BISAR) and using viscous parameters of the mixture as input. The non-linear properties of the material were incorporated by using the RLAT test. For this purpose, six mixtures were used and tested in a wheel tracking machine. The predicted results were compared with the wheel tracking rut depth in the laboratory and showed good agreement at different temperatures. However, at high temperature (50 °C) the material properties in the RLAT test did not behave as linear viscous, which resulted in a much poorer prediction. Trials were made to predict field rut but it was found that special requirements were needed for the approach which were not available at the time of the research. However, for the available field data, the method was found to be a good predictor.

Hot Mix Asphalt (HMA) is the most commonly used material in the construction of asphalt pavements. Approximately 650 million tonnes of asphalts for road pavements were produced, worldwide, in 2014. However, the HMA industry is responsible for a substantial consumption of energy, the creation of health and safety issues and has a negative impact on the environment. These shortcomings initiated substantial discussion within the industry with the aim to develop more environmental friendly, sustainable and economic pavement materials. These have resulted in the development of cold bitumen emulsion mixtures (CBEMs). However, to date, low early life stiffness, a slower rate of curing, the length of time necessary to achieve full strength, high air void contents and the presence of moisture in these mixtures have prevented them from being fully embraced by pavement authorities. This led to them being considered inferior to HMA because of a lack of essential mechanical properties. Currently, the use of CBEMs in pavement construction is limited to low traffic road surface course, reinstatement works and footways. Because of this, the development of CBEMs with high early strength and minimal time delay requirements before structural loading, would be considered as a breakthrough in CBEM research. This research aims to develop a novel, fast-curing and environmentally friendly, cold binder course emulsion asphalt (CBCEA) for heavily trafficked roads. The new CBCEA mixture comprises the same gradation as conventional dense bitumen macadam (DBM) mixtures which are normally used as a binder course and base in road pavements in the UK. The new CBCEA incorporates a new cementitious material, alkali activated binary blended cement filler (ABBCF), made from Paper Making Sludge Ash (PMSA) and a Fluid Catalytic Cracking Catalysts Residue (FC3R) activated by a waste NaOH solution (W-NaOH). Incorporation of the PMSA and FC3R was achieved through the replacement of conventional limestone filler (LF), while W-NaOH replaced the pre-water necessary to wet the aggregate in the CBCEA. It was found that the glass phases of the new filler particles were broken and reacted with Ca(OH)2 creating C-S-H gel through the hydration process. This results in a very high early strength and improved mechanical properties. Balanced oxide compositions, within the new filler, were identified as responsible for an enhanced hydration reaction. A laboratory programme of testing measured the stiffness modulus, conducted at 1, 3, 7, 14, 28, 90 and 180 days. Susceptibility to temperature, wheel track testing to establish rutting resistance, fatigue resistance measured by a four-point beam bending test, fracture resistance testing via semi-circular bending tests, moisture damage resistance and ageing tests were successfully performed. Advanced techniques for microstructure assessment, i.e. Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD), were used to provide scientific data to provide a deeper understanding of the microstructure and internal composition. An environmental investigation was performed using a Toxicity Characteristic Leaching Procedure (TCLP) test. The new ABBCF mixture offers a significant improvement in stiffness modulus compared to HMA and the reference cold binder course mixture containing conventional limestone filler (LF). Target stiffness, according to British and European standards, can be surpassed after less than one day of curing. The new ABBCF mixture offers a stiffness modulus which is 27 times better than the LF mixture after 3 days. This will overcome restrictions caused by the length of time required to achieve acceptable stiffness by traditional CBEMs. More remarkably, the new ABBCF mixture is 78% better than mixtures treated with Ordinary Portland Cement (OPC) in terms of ITSM after 3 days. Furthermore, the impact of a rise in temperature on stiffness modulus from 5 to 45°C, was much larger in LF and both HMA mixtures in comparison to ABBCF, revealing the potential to use these mixes in severe conditions, both hot and cold weathers. ABBCF mixtures displayed considerably reduced susceptibility to permanent deformation, demonstrating the potential advantage of using this material on heavily trafficked roads. Fatigue resistance was noticeably improved by the use of ABBCF in comparison to the reference LF and HMAs. Improved water sensitivity for progressive hydration with the new ABBCF was also established resulting in an enhanced long ageing performance meaning that these mixtures can be considered durable. SEM observation and XRD analysis confirmed the formation of hydration products at various curing times. The concentration of heavy metals in the samples incorporating ABBCF was observed to be less than the regulatory levels determined for hazardous materials. Microwave treatment has proven to be an effective technique to reduce the air void contents of the ABBCF mixture and achieve acceptable levels of porosity. Finally, achieving the aim of the current research will theoretically increase the application of such mixtures and allow them to be used as structural pavement materials. On a further positive note, the inclusion of waste and by-product materials in CBEMs results in more sustainable practice and eliminates disposal problems.

This Thesis was undertaken at the University of Nottingham which has a world class history of research into the fatigue of asphalt materials. The work described in this thesis was part of a research project funded by EPSRC, which attempts to gain a greater understanding of fatigue crack propagation in High Modulus Base (HMB) materials. Following on from Pell, Brown and Read this research has introduced the Compact Tension (CT) Test and Fracture Mechanics principles to examine the behaviour of HMB materials which had relatively little fatigue behavioural understanding on commencement of this research. This research investigated the fatigue cracking behaviour of twelve High Modulus Base (HMB) binders and mixtures. The research was instigated in response to the introduction of these HMB binders from France, as it was thought that these materials were not particularly well understood. In hindsight, this view was proven to be correct, as numerous problems have been experienced after using such materials. The Compact Tension test has been proved to be an effective means of testing bituminous mixtures for their crack propagation resistance, allowing the study of temperature effects. Crack propagation is dramatically affected by both binder hardness and temperature. In a pavement, the current approach to design, assuming a single fatigue characteristic, underestimates the life of 35 and 25 pen mixtures. However, it probably overestimates the life of 15 pen mixtures by not accounting for the effects of low temperatures.

Excessive rutting, one of the major distress modes of bituminous pavements, is mainly caused by the accumulation of load-induced permanent deformation. However, current pavement design approaches against the excessive rutting are mainly developed using the theory of elasticity. Recently, a new pavement design approach based on the shakedown concept has attracted lots of attention because it can consider plastic properties of pavement materials. However, most of the existing shakedown solutions were developed for pavement foundations composed of granular materials and soils. Very limited work has been reported on bituminous pavements. Besides, current studies usually assume homogeneous, isotropic pavement materials obeying an associated plastic flow rule (termed as standard materials in the present study), which may not be realistic for pavement materials. In the present research, a step-by-step numerical approach was used to obtain numerical shakedown limits of pavement structures under repeated moving loads. Both two-dimensional and three-dimensional problems were considered. It was found that, under the assumption of standard materials, the obtained numerical shakedown limits and residual stress fields agreed well with the available theoretical data. A static (i.e. lower bound) shakedown approach for pavements with anisotropic, heterogeneous materials was developed based on Melan’s lower bound theorem and the critical residual stress method of Yu and Wang (2012). The influence of material plastic flow rules on pavement shakedown limits was also evaluated both numerically and theoretically. It was found that neglect of the inherent material properties (i.e. anisotropy, heterogeneity and non-associated plastic flow) could overestimate the real shakedown limits of bituminous pavements. A series of tests were conducted to validate the shakedown concept for the responses of bituminous pavements under traffic loads. Two distinct phenomena corresponding to shakedown and non-shakedown were observed. Triaxial tests and uniaxial compression tests were also undertaken to obtain the stiffness and strength parameters, from which the theoretical shakedown limits can be calculated. Comparison between the experimental results and the theoretical solutions revealed that the current 3D shakedown approach for standard materials may overestimate capacities of bituminous pavements. Finally, the lower bound shakedown approach was employed to design a typical bituminous pavement. A direct comparison was made between the shakedown-based design and the current UK design method. It demonstrated that the shakedown-based design for bituminous pavements can be conducted considering the maximum contact pressure and a relatively high air temperature.