Investigation Of Mechanisms Of Top-Down Crack Initiation And Propagation In Asphalt Pavements

Surface-initiated longitudinal wheel path cracking which is named top-down fatigue cracking,has recently attracted numerous interests and reported as a widespread mode of failure in asphalt concrete pavements.Top-down crack(TDC)is initiated at the pavement surface and propagate downward through the asphalt layer.This new distress model,has been incorporated in the newly developed Mechanistic-Empirical Pavement Design Guide(MEPDG).Therefore,accurate determination of responses plays a critical role in a mechanistic-empirical pavement design.Comprehensive study on literature review has demonstrated that several researchers have presented hypotheses that attempt to explain the crack initiation and/or propagation phenomenon,but a complete identification for the TDC initiation and propagation mechanisms that considers factors found in the field does not exist.In this dissertation,numerous elastic and viscoelastic analyses have been conducted to investigate the mechanisms of TDC initiation and propagation.The asphalt concrete(AC)was considered as viscoelastic materials in this study and complex modulus tests of AC were performed on three different AC mixtures.Using Modified Havriliak-Negami(MHN)model,dynamic modulus mater curves are constructed.Consequently,viscoelastic material parameters were calculated from complex modulus to incorporate into the mechanistic analyses of TDC.By identifying significant factors,the mechanism of TDC initiation has been explored based on the critical horizontal tensile strain at the pavement surface.Due to the poor convergence of analytical layered elastic solutions in the vicinity of the pavement surface which may result in large errors in response results,an analysis procedure was used in this study to effectively solve this problem.According to the proposed approach,effect of temperature,interface bonding conditions,loading frequency,asphalt layer thickness and aging on TDC initiation were explored.Viscoelastic analyses of TDC initiation in asphalt pavements have been conducted by developing the method explained for elastic analysis.The results from viscoelastic solution were successfully verified.The responses at the top and bottom of AC layers were computed under the stationary and moving loads.The procedure developed in this study provides an effective tool for accurate determination of viscoelastic response.Various factors such as temperature gradient,vehicle speeds and AC thicknesses were identified as predominant factors contributing TDC initiation.The results of viscoelastic and elastic analyses showed that traffic loading may induce significant horizontal tensile strain in the transverse direction at the surface,which is a major cause of the top-down cracking.The critical location of tensile strain was just outside tyre edges.It has been pointed out that although the newly developed MEPDG uses horizontal strain at the surface for the prediction of top-down cracking,the analysis locations adopted in the MEPDG may not be able to identify the maximum tensile strain.In addition,in the pavements with cement-treated base(CTB),the dominant type of fatigue cracking was TDC,while the bottom-up cracking is unlikely due to the strong support from the base.But,in the pavements with granular base(GB),the bottom-up cracking was more likely to occur at medium or low temperatures,while the cracking was more prone to initiate from the surface at high temperatures.Analyses of TDC propagation have been conducted by using FE method which is well equipped with fracture and continuum mechanics.Using 2D elastic and 3D viscoelastic analyses,mechanism of TDC propagation has been investigated.Crack growth rate in pavement structures were evaluated by calculating stress intensity factors and J-integral through the elastic and viscoelastic analyses,respectively.According to the elastic analysis results,stress intensity factors in Mode I and II were not distributed uniformly within the AC depth.Also,TDC growth rate significantly depends on AC thickness and base layer type.Proportion of shear and tensile Mode of fracture in TDC propagation was significantly influenced by crack location relative to tire edge.In addition,the number of load repetitions for TDC propagation rate at different transverse locations is predicted based on Paris’ law equation.Based on viscoelastic analyses results and non-uniform tire pressure,three J-integral Modes have been computed at various transverse locations.The results calculated at high temperature demonstrated that J-integral can accurately determine the crack growth rate.Crack depths and AC layer thickness have tremendous effect on crack propagation in the pavements with CTB and GB.Moreover,critical location of TDC propagation was similar to the crack initiation location.Using realistic tire-pavement contact stresses causes TDCs at very close distance to the tire edge to propagate rapidly due to the strong mixed-Mode Ⅰ+Ⅱ fracture energy.By identifying the mechanism of TDC initiation and propagation,this study provided a beneficial insight to analysis and design of asphalt pavements and overlays and also revealed a roadmap for developing a mechanistic-empirical model for incorporation into the MEPDG procedures.