Viscoelastic Behavior Analysis Of Asphalt Concrete Structure Considering The Temperature And Aging Gradient

Asphalt concrete structures in highway and railway engineering exhibit significant continuous gradient viscoelastic properties within the structure under the effects of ambient temperature and aging.The effect of such gradient properties is generally ignored in conventional analysis or little considered,which are approximated by subdividing the structure into sub-layers and specifying material parameters for each layer separately.However,in assigning material properties,the elements are still regarded as homogeneous and material properties of each node remain unchanged,which obviously does not truly reflect the continuous gradient variation of material properties.This paper takes typical asphalt concrete structures in highway and railway engineering as research objects.By integrating the functional gradient theory,elastic and viscoelastic mechanics,gradient finite element theory,and dynamic performance tests of the asphalt concrete,the temperature and aging gradient viscoelasticity inside asphalt concrete structures have been characterized.Finite element analysis procedure based on the unit node level considering the continuous gradient material properties has been developed.The gradient mechanical behavior of typical structures has been investigated.Main research contents and results are as follows.(1)Analytical models of temperature fields of asphalt concrete structures were established based on the principle of heat transfer.Gradient temperature fields of typical structures were studied.Combined with dynamic tests of asphalt concrete,a quantitative characterization method of the temperature gradient viscoelasticity was established.Results show that the internal temperature distribution along the depth at the moment of the most significant temperature gradient conforms to the form of a cubic parabolic curve.The collocation method and the relaxation time-shift method can effectively characterize the distribution of the relaxation modulus and relaxation time spectrum of asphalt concrete under the influence of temperature.The change of parameters of generalized Maxwell model of asphalt concrete with temperature is essentially the result of relaxation spectrum shifting along relaxation time axis at different temperatures.When the temperature is high,the viscosity decreases,the relaxation spectrum shifts to the direction of decreasing relaxation time,the relaxation modulus of asphalt concrete decreases,and vice versa.(2)The “composite specimen method” was designed to obtain gradient aging specimens.By using dynamic property prediction theory and DSR tests of asphalt binders,the gradient distribution of dynamic properties within the specimens was obtained,and the horizontal aging shift factor was further constructed to accurately characterize the aging gradient viscoelasticity.Results show that the aging effect causes a continuous gradient in the material properties of the specimen along the depth,and the gradient aging has a significant effect on the dynamic modulus of asphalt concrete,especially at low temperatures and high frequencies.However,it has little effect on the phase angle and the temperature shift factor.The variation of the horizontal aging shift factor function with depth based on the specific gradient aging condition is generally in the form of an exponential function.The degree of aging can be quantified by its horizontal aging shift factor relative to the reference aging condition.If the shift factor is positive,the relaxation spectrum shifts to the direction of increasing relaxation time,and the relaxation modulus of asphalt concrete increases.Besides,the gradient effect caused by aging is smaller than that caused by the temperature field.(3)Based on elastic and viscoelastic mechanics and the generalized isoparametric gradient finite element theory,the elastic gradient and viscoelastic gradient finite element analysis procedures were complied.A various gradient edge value problems and the deformation characteristics of simply supported gradient beams under concentrated load in mid-span were study.Results show that the elastic gradient finite element program is suitable for the edge value problems with various gradient variations of material elastic properties,and the calculation results are in good agreement with the analytical solutions.Compared with the uniform and layered approximation models,it has higher calculation accuracy and efficiency in solving the mid-span deflection of simply supported beams.The viscoelastic gradient finite element procedure can accurately and efficiently analysis the creep characteristics of the simply supported beam,while the uniform and layered approximation models are less accurate.The calculation error decreases with the increase in the number of layers,but gradually increases with the duration of the load,which has a “cumulative effect”.(4)A study was conducted to investigate the mechanisms of temperature and aging gradient conditions on the mechanical behavior of typical asphalt concrete structures in the highway and railway engineering.The differences in the response of key structural mechanical indicators to external loads were revealed.Results show that the temperature gradient has the most significant effect on the transverse stresses,followed by the shear stresses and the weakest effect on the vertical stresses in the semi-rigid base asphalt pavement.Calculated results of tensile stresses in the uniform model are generally smaller than those in the gradient model,which may lead to unsafe structural design.While the layered approximation model falls in between and shows the “step effect” of transverse stresses at the sub-layer interface.The temperature gradient has little influence on the time history and magnitude of the structural design control index of full-thickness asphalt concrete.However,ignoring the temperature gradient will lead to inaccurate results.When considering both the temperature and aging gradient,there is no obvious changes in the amplitude and relative error of the settlement.The internal mechanism is that the superimposed effect of temperature and aging gradient does not definitely increase or decrease the overall deformation resistance of the structure.Under the positive gradient condition,the superimposition weakens the gradient effect,making the asphalt surface more homogeneous.While for the negative gradient,the inhomogeneity will be amplified.