Microscopic Damage Model Of Asphalt Aggregate And Numerical Test Of Mixture Fatigue

Fatigue disease of asphalt pavement is one of the main reasons for the outstanding durability of highways in my country,and it is also an important problem that needs to be overcome in the research of longevity surface pavement in my country.In the indoor fatigue test of asphalt mixture,due to the limitations of the test environment and test instruments,various test factors cannot be fully considered,and there are problems such as high test cost,long time consumption,and inability to obtain the evolution mechanism of its fatigue failure.Therefore,it is an effective method to comprehensively study the damage mechanism of asphalt mixture in the fatigue process by means of discrete element numerical test method,combined with indoor microscopic fatigue damage test and macroscopic four-point bending fatigue test.The main work and conclusions of this paper are as follows:(1)Based on the DMA test method,a set of equipment and processes for testing the resistance to fatigue damage of aggregate-asphalt adhesion specimens was designed and developed,which can be used to test the adhesion of adhesion specimens with a size of20mm*20mm*21mm.Adhesion and resistance to cyclic loading.The study found that the adhesion of limestone was 2.5% higher than that of basalt and 7.9% higher than that of granite at25℃;the adhesion of the three specimens was close at 35℃,and the difference was less than1.5%.When the loading frequency is between 5 and 15 Hz,the fatigue life of the three adhesion specimens increases,among which the increase of limestone is 36.8%,the increase of basalt is34.6%,and the increase of granite is 45.6%.Under different stress level conditions,the fatigue life of limestone decreases by 4.04% on average,and the fatigue life of basalt decreases by 2.48%when the stress level increases by 0.01.(2)Referring to the research on existing fatigue damage models,combined with the decay law of elastic modulus of different aggregates during the test,a set of fatigue damage models for aggregate-asphalt adhesion specimens that comprehensively considers the effects of temperature,frequency and load level are proposed..In the process of parameter analysis,it is found that the fatigue damage of the asphalt adhesion specimen is difficult to occur when the temperature is lower than a certain value(point D),which proves that in the low temperature environment,the fatigue cracking of the asphalt mixture is more likely to occur in the asphalt mortar,this value reflects the fatigue resistance of different adhesion specimens under temperature changes,where the value is 12.5 for limestone,13.5 for basalt,and 10.8 for granite.(3)Using the three-dimensional discrete element interface program to use the obtained fatigue damage model for the contact model,the four-point bending fatigue numerical test model of the asphalt mixture is established,and the contact model(parallel bonding model and Burgers model)is determined according to different internal structures.,which improves the program control of loading mode(strain control)and loading environment.The study simulated different test temperatures(15～45°C),different internal porosity(4～8%),different strain levels(400～800),different loading frequencies(5～15Hz),and different aggregate types(basalt).,granite and limestone),and realized the simulation of fatigue test under multi-level constant amplitude load.(4)Through the four-point bending numerical test of asphalt mixture,the contact information,particle movement amount,movement angle change and the development law of force chain of the numerical sample were analyzed.The direction of the force chain at the initial stage of loading is mainly at 60～120°,240～300°,accounting for 34.5% and 37.5%,respectively.At the end of loading,the direction of the force chain is concentrated at 75～105°,255～285°,accounting for 38.7% respectively and 42.2%.The development of the damage zone of the asphalt mixture starts from the loading rod,and gradually expands the width and height.When the depth of the damage zone exceeds 25 mm,the stiffness modulus of the numerical sample decreases rapidly.When the depth of the damage zone develops to 38.7mm,the four-point bending test is performed.Fatigue failure of the sample occurred.The fatigue phenomenon of the asphalt mixture is affected by the damage of the adhesion between the aggregate and the asphalt,which is manifested as the expansion of the damage zone and the increase of the aggregate movement in the middle position during the fatigue process of the specimen.