Rutting occurs due to the accumulation of permanent deformation in a few layers in a concrete structure. Therefore, the study of the permanent deformation properties of asphalt mixtures has become the focus of research, aimed at reducing or reducing the rut of flexible pavement. The accumulation of permanent deformation in asphalt pavement layers is now considered to be a major component of rot in flexible pavements. This results in increased tire pressure and axle load, which increase the stress on the asphalt pavement layer closest to the tire-road contact area. The research work reported in this paper aims to contribute to the understanding of the physical properties in asphalt mixtures and the factors affecting permanent deformation, the mechanism of permanent deformation, and its prediction methods. Specific objectives of this research effort include reviewing and evaluating available models for permanent deformation of asphalt concrete mixes, investigating the effects of volume composition, loading and temperature conditions on permanent deformation of asphalt concrete, and establishing simple measures for permanent resistance. Involves identifying and defining potential distortion. For the purpose of the study, we performed laboratory investigations on several asphalt concrete samples with different volume compositions. 
Two test procedures were employed: repeated load triaxial and triaxial creep and recovery tests. The tests were carried out at two temperature levels, 25 and 50oC, under different stress conditions. The literature on factors influencing permanent deformity and available models for predicting permanent deformity are also reviewed. A review of the literature shows that most research work performed to date has focused on evaluating the effect of component material properties, such as total gradation, total angularity, and binder type (or grade), on the permanent deformation response. Most studies on the permanent deformation properties of asphalt mixtures have also been found to be based on different test procedures and evaluation methods, making them difficult to compare and draw firm conclusions. The literature also shows that there is currently no comprehensive model of asphalt concrete deformation. The results of the tests performed in this study were analyzed to investigate the effect of the volume composition, specifically the binder material and void material, and the loading conditions on the permanent deformation response of the mixture. Both the binder content and the void material were found to significantly affect the permanent set properties. The effects of loading conditions, that is, limiting and divergent stresses, were also found to be significant. 
Throughout the study, emphasis was placed on the methods and parameters used to evaluate the permanent deformation resistance of the mixture. Traditionally used parameters such as slope and strength assess its sensitivity to changes in the intercept volume composition of the model. This assessment is based on the premise that any measure of resistance to permanent deformation must be sensitive to changes in volume composition for it to be sufficient. Most of these parameters were found to be insensitive to changes in volume composition and therefore not suitable for comparing mixtures composed of the same material but with different component ratios. Permanent deformation in asphalt concrete is caused by both densification and shear deformation. The mode of deformation in asphalt concrete pavements, for greater part of their service life, is considered to be the shear deformation. Therefore, it is necessary to evaluate mixtures for their susceptibility to shear deformation. The shear deformation manifests itself in the form of large lateral deformation relative to axial deformation. 
It is found that one dimensional analysis, which does not take the lateral deformation into account may lead to misleading results regarding the resistance to permanent deformation of mixtures. Therefore, parameters which include volumetric and lateral strain are proposed for use in evaluation of mixtures. Substantial effort is put into modelling the accumulation of permanent deformation under repeated loading. For this purpose, two approaches were selected: the cyclic hardening model based on bounding surface plasticity concept and an elasto-viscoplastic model based on strain decomposition approach. The bounding surface plasticity approach is found to be a convenient method to model the accumulation of permanent deformation. It is demonstrated that deformations calculated using cyclic hardening model based on bounding surface plasticity fits the measured deformation quite well. The elasto-viscoplastic model, which is based on strain decomposition approach, provides a suitable method for analysis of creep and recovery test results. Deformations calculated using this model also fit the measured deformation quite well. Finally, a new composite measure of resistance to permanent deformation is developed. The resistance index is based on strain decomposition approach and is simple to calculate. 
The index incorporates a parameter related to shear susceptibility of mixtures and is sensitive to changes in volumetric composition. It is believed that this index can be used to compare and select mixtures at mixture design stage. If its applicability to other materials is proved by further research, it can also be linked to performance related specifications, as a simple measure of performance with regard to rutting. It utilizes the Discrete Element Method (DEM) technique to simulate the microstructure of three ACs captured using X-ray Computed Tomography (CT). These three mix designs (coarse-graded, gap graded, and fine-graded) prepared with hard limestone aggregate and a PG 76-22 modified binder were included in the study. Advanced digital image processing techniques were utilized to process the X-ray CT images and to suitably input their microstructure into the DEM model. The viscoelastic rheological properties of the asphalt mastics were defined by fitting Burger model parameters on frequency sweep test data conducted at 60 °C. The DEM simulation, in two dimensions, involved modeling the unconfined FN tests under a repeated stress of 690 kPa. The simulation loading was applied for 0.1 second followed by a 0.9 second rest period until 10,000 load cycles or 5 % accumulated strain was reached. The 2D DEM simulation results appear to capture the significant differences in FN properties between these three AC mixtures and hence can be used to compare their rutting susceptibility.
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