| Fatigue factor is one of the main reasons for the damage of asphalt pavement.With the increase of fatigue degree,diseases such as structural strength decrease,cracks,etc.,will occur to the asphalt pavement,which seriously affects the safety and service life of the road.Based on asphalt mixture micro test and macro-meso-micro multi-scale correlation model of asphalt pavement,the paper studies the changing trend of various characteristics of asphalt pavement under the effect of fatigue factor from the microscopic level.The main research contents are as follows:Firstly,four-point bending fatigue tests with 33,000,66,000,and 98,000 loadings were carried out,and four asphalt mixture specimens with different fatigue levels were obtained by simulating actual working conditions.Through the micro-scratch test and nano-indentation test,the change trend of fracture toughness and elastic modulus of each specimen was obtained,and the influence of fatigue on the micro-mechanical properties of asphalt mixture was analyzed accordingly.Secondly,the macroscopic model of the asphalt pavement is established,and the dangerous area of the asphalt pavement is determined by analyzing the stress response.Based on the multi-scale correlation theory and cohesive force constitutive model,a local mesoscopic model of the dangerous area and a microscopic fracture model of the ITZ area considering the material gradient change were established respectively,and the crack initiation position of the asphalt surface layer was explored from the mesoscopic and microscopic levels.Finally,the meso-micro experimental data are combined with the macro-meso-micro multi-scale correlation model,and three evaluation indicators of crack growth evolution,including crack initiation time,crack growth degree and crack growth rate,are defined.By comparing the calculation results of different models,the influence of fatigue factors on the evolution of micr-meso cracks of asphalt pavement is evaluated from the perspectives of time,degree and speed.The study shows that the parameters in the ITZ area show gradient variation,the thickness is about 10～30μm.Fracture toughness and elastic modulus change gradient from high to low with the transition from aggregate to asphalt mortar.No fatigue factor was found to have a significant effect on the thickness of ITZ area during the test.From0 to 98,000 times of fatigue,the average fracture toughness of ITZ area decreased from0.18MPa?m1/2 to 0.08MPa?m1/2 with a decrease of 57.79%;asphalt mortar decreased from 0.03MPa?m1/2 to 0.02MPa?m1/2 with a decrease of 17.60%;the average modulus of elasticity of ITZ area decreased from 9.37GPa to 7.73Gpa with a decrease of 17.48%;asphalt mortar decreased from 0.33GPa to 0.22GPa with a decrease of 33.45%;The aggregates did not change significantly.It shows that the influence of fatigue factor on asphalt mixture is mainly concentrated in ITZ area;the elastic modulus and fracture toughness have a nonlinear negative correlation with the fatigue degree;the early fatigue times have a greater effect on elastic modulus and fracture toughness.Slight cracks usually originate from ITZ area-the tip of the aggregate,and then continue to propagate either along ITZ or toward the asphalt mortar,and finally multiple cracks expand and connect to form through-cracks.As fatigue times increasing from 0 to 33000,66000 and 98000 respectively,the crack growth rates are 1.89,3.89,7.14 and 8.06,with an expansion rate of approximately 2.05 times,1.83times and 1.13 times;and the peak crack growth rate was 1.68,2.88,4.00 and 4.08,with an increase of 71.4%,38.9%and 2%respectively.It shows that the influence of fatigue factors on the evolution of crack growth is obvious,and the influence of early fatigue is greater.From the directions of experiment and finite element model,the paper analyses the influence of“fatigue”factor on micro-meso characteristics of asphalt pavement by comparing the change trend of material parameters and crack propagation evolution,which provides ideas and references for subsequent research on the influence of fatigue factors. |
