Evaluation Of Deflection Analysis Methods For Asphalt Pavements And Deflection Analysis Of Cracked Asphalt Pavements

Falling weight deflectometer(FWD)testing is a major technique to assess the structural performance of asphalt pavements,and it was introduced into the specifications for design of asphalt pavements of China in 2017.It is particularly important to scientifically interpret and analyze FWD deflection data.Layer elastic theory(LET)is the method basis for pavement design and analysis in China.The basic assumption of LET is that each pavement layer is linearly elastic,weightless,and continuous.Thus,the traditional deflection analysis methods based on LET cannot consider the influences of dynamic effect,viscoelasticity,and other complex mechanical properties of asphalt pavement under FWD loading,as well as the discontinuities caused by cracks,on the surface deflections.Therefore,in order to scientifically assess the structural performance of asphalt pavements using FWD deflection data,it is necessary to evaluate the accuracy of existing deflection analysis methods and to investigate the deflection characteristics of cracked pavements.To this end,the following researches were carried out in this dissertation:(1)The dynamic and static deflections were computed using the spectral element method and LET,respectively,and the dynamic effects caused by FWD loading on deflections were investigated.Then,the dynamic deflections were analyzed using the traditional static backcalculation procedures,and the statically backcalculated results were compared with the actual ones to investigate the errors caused by the static backcalculation procedures for ignoring the dynamic effects.Finally,the statically backcalculated moduli were used to calculate the critical pavement responses and the effects of errors in statically backcalculated moduli on responses were investigated.The results show that the dynamic effects on deflections are significant for asphalt pavements under FWD loading,and thus traditional static analysis procedures can cause significant errors in backcalculated results and calculated responses for ignoring the dynamic effects.(2)Based on the actual engineering sections,the dynamic effects caused by FWD loading on the acceptance deflections were investigated,and the reasonableness of the static acceptance deflections in current design specifications was evaluated.The results show that the static acceptance deflections extend the acceptance criteria to a certain extent.A simple procedure to predict the dynamic deflections using the static deflections was proposed and applied in the actual engineering projects.The proposed procedure provides a more effective approach for the acceptance of FWD dynamic deflections.(3)The dynamic viscoelastic deflections were calculated using the spectral element method for asphalt pavements under field measured FWD loading.The calculated deflections were used to backcalculate the properties of AC layers using dynamic backcalculation procedure.Then,according to the theory of viscoelasticity,a procedure to determine the effective frequency range excited by FWD loading was proposed.The effective frequency ranges of various FWD loading histories were determined.In general,the effective frequency range excited by a FWD loading history is 5 ～ 65 Hz for asphalt pavements.The results reasonably interpret the effective information contained in FWD deflection data and then the reliability of backcalculated viscoelastic properties of AC in the frequency domain was evaluated.(4)Three-dimensional finite element models were developed to simulate the asphalt pavements with various cracking conditions,and their dynamic viscoelastic deflections under FWD loading were computed and compared to investigate the effects of transverse cracks and longitudinal cracks on deflections.The results show that transverse cracks and longitudinal cracks have great influences on deflections,and the deflection characteristics of pavements with different cracking types are significantly different.Accordingly,the nondestructive procedures were proposed to distinguish the transverse-cracked layer and longitudinal-cracked layer using FWD deflection data,respectively.(5)The deflections of cracked asphalt pavements were analyzed using the dynamic viscoelastic backcalculation procedure,and the effects of transverse cracks and longitudinal cracks on backcalculated results were investigated.The results show that cracks in surface layers can result in the underestimated moduli of both surface and base layers or the overestimated moduli of surface layers and the underestimated moduli of base layers,indicating that the backcalculated results cannot be used to evaluate the cracking conditions of asphalt pavements.For pavements with cracks in both surface and base layers,the backcalculation procedure cannot find a set of reasonable layer properties to match the backcalculated deflection curves with the measured ones.These observations indicate that the backcalculation methods,which assume that the pavement is intact and continuous,cannot accurately evaluate the layer conditions and structural performances of cracked asphalt pavements,and backanalysis of cracked pavements is not recommended.