Characterization Of Evaluation Indicator And Mechanical Model Of Permanent Deformation In Asphalt Mixtures

Rutting is one of the major distresses occurring in asphalt pavements.It results from excessive permanent deformation that is accumulated in asphalt mixtures under repeated traffic loads.Accurately evaluating the resistance to permanent deformation of asphalt mixtures is therefore of great significance to prevent the rutting damage.Currently,the resistance to permanent deformation of asphalt mixtures is mainly investigated in terms of evaluation indicator and mechanical model.However,as for the evaluation indicator,the existing flow number calculation methods are not able to accurately determine the flow number values;with regard to the mechanical model,the constructed master curves of viscoelastic parameters are not in compliance with the linear viscoelastic theory,and the viscoplastic model parameters are determined in disregard of the physical significance of model parameters.This dissertation attemps to address the aforementioned deficiencies.The major conclusions are summarized as follows:（1）Flow number is employed as the indicator of resistance to permanent deformation of asphalt mixtures.According to the convex-and-concave characteristic of permanent deformation curve,a two-step secant method is proposed to accurately determine the initiation point of secondary stage and that of tertiary stage（i.e.flow number）for asphalt mixtures tested under a certain condition.The calculation results indicate that the deformation rate in secondary stage is uniquely related to flow number regardless of mixture type,loading force and test temperature.（2）The first step of viscoplastic modeling is to construct the master curves of viscoelastic parameters in frequency domain.Two approaches are put forward to develop the master curve models of viscoelastic parameters based on the approximate Kramers-Kronig relation between dynamic modulus and phase angle and that between storage modulus and loss modulus,respectively.As for each approach,the same time-temperature shift factors are applied to all master curve models so as to construct the master curves of four viscoelastic parameters that are in compliance with linear viscoelastic theory.Comparisons between these two approaches indicate that both approaches are capable of accurately constructing master curves complying with linear viscoelastic theory for viscoelastic parameters of asphalt mixtures in frequency domain.（3）The second step of viscoplastic modeling is to construct the master curves of viscoelastic parameters in time domain.The constructed storage and loss modulus master curves that are in compliance with linear viscoelastic theory provide the basis for this step.A continuous relaxation spectrum model is firstly derived with regard to the storage modulus master curve model.The relaxation modulus master curve is then generated based on the integral relationship between relaxation modulus and continuous relaxation spectrum.Subsequently,a numerical model of continuous retardation spectrum is established according to its integral relationship with continuous relaxation spectrum.The creep compliance master curve is therefore constructed according to the integral relationship between creep compliance and continuous retardation spectrum.The constructed relaxation modulus and creep compliance master curves are demonstrated to be accurate in the complete time domain.（4）The third step of viscoplastic modeling is to determine the viscoplastic model parameters based on the separated viscoplastic strain.A testing protocol is designed on account of the physical significance of each viscoplastic model parameter.In this protocol,the Aggregate Imaging System is adopted to scan the side surface of asphalt mixtures and the Material Testing System to perform the multiple stress creep and recovery test,creep and recovery tests at small stress levels in order to determine the nonlinear parameters g₀,g₂ and five viscoplastic model parametersα,β,κ₀,κ₁ andκ₂.A creep test is subsequently conducted,based on which the induced nonlinear viscoelastic strain and viscoplastic strain are separated;the remaining two viscoplastic model parametersΓand N are then solved by fitting the viscoplastic model to the separated viscoplastic strain.