Study On Calculation Method Of Thermal Stress Of Asphalt Pavement In High Altitude Area

Under the action of perennial low temperature,large diurnal temperature difference,strong solar radiation and other harsh natural conditions,asphalt pavements in high altitude areas are prone to produce common diseases such as temperature shrinkage cracks,rutting deformation,differential settlement of subgrade,which seriously affect the mechanical response of pavement structure.Among them,low temperature cracking caused by thermal stress is the main form of structural diseases.At present,the researches on thermal stress are mainly realized by laboratory test and finite element method(FEM)simulation,and it is difficult to accurately describe the mesoscopic characteristics of the material.Asphalt mixture is a multiphase composite composed of aggregates,asphalt mortar and voids.Its meso-structure has a significant effect on the mechanical properties of the material.In view of this,in this paper,based on the combination of laboratory test and discrete element method(DEM)simulation,thermal stress restrained specimen test(TSRST)virtual specimen of asphalt mixture is built by DEM software PFC2 D to study the meso-mechanism of thermal stress accumulation and crack propagation in the process of low temperature cracking.The effects of aggregate characteristics,void characteristics and bond strength of aggregateasphalt mortar interface on asphalt mixture are analyzed.These effects are reflected in heat conduction,low temperature cracking resistance and crack propagation.Firstly,based on the principle of inverse stereology and the method of probability theory,the transformation relationship between three-dimensional volume grading and two-dimensional quantity grading of asphalt mixture was established,and the quantities of particles at all levels were determined.The coarse aggregate was regarded as irregular polygon.The generation algorithm of single irregular polygon was defined,and the random generation of irregular polygon considering grading characteristics was realized.Based on this,combined with the characteristics of material composition and structure,a two-dimensional discrete element virtual specimen of asphalt mixture was established,which provided the physical model basis for the subsequent analysis.Secondly,according to the characteristics of the contact models in PFC2 D,combined with the characteristics of contact behavior between the components in asphalt mixture,different contact models were given.The contact behavior between particle elements was equivalent to the mechanical behavior of elastic beam and viscoelastic beam,and the transformation relationship between macro-parameters and meso-parameters was deduced.The macro-parameters of aggregate and asphalt mortar were obtained through laboratory test,and the influence of temperature was analyzed.Finally,the meso contact parameters between different materials were obtained.Thirdly,based on the physical model and meso-parameters,combined with the laboratory test conditions,the virtual TSRST test method was established,and the accuracy of the virtual test was verified by comparing the laboratory test results.Through TSRST virtual test,the change rule of temperature field,temperature stress curve,distribution characteristics of mesoscopic contact force and meso-mechanism of low temperature cracking were analyzed.The results show that the low temperature cracking process of asphalt mixture can be divided into three stages: cracking initiation,stable propagation and unstable propagation.In the stage of unstable propagation,when the temperature reaches the fracture temperature,the thermal stress decreases by 66%,the total number of cracks increases by 132,and meso-cracks develop into macrocracks.In the study of thermal fatigue of asphalt mixture,the parallel bond damage model of rock material was introduced.Moreover,the thermal fatigue test was simulated by repeated direct tensile test at low temperature and low frequency.The simulation results proved the feasibility of thermal fatigue test by DEM,and revealed the meso-mechanism of thermal fatigue cracking.Finally,based on the TSRST virtual test method,the influences of aggregate characteristics,void characteristics and interface bond strength of aggregate-asphalt mortar on the heat conduction,low temperature cracking resistance and crack propagation were analyzed.The results show that with the increase of aggregate content,the heat conductivity and fracture strength of the specimen increase,but the fracture temperature and the number of failed contacts increase at the same time.With the increase of void ratio and void radius,the heat conductivity of the specimen decreases.In addition,the fracture strength decreases,and the fracture temperature increases,both of which show a non-linear change rule.The failed contacts increases,which are mainly reflected in the interior of the asphalt mortar,and the cracks tend to run through the asphalt mortar.When the bond strength of interface is low,increasing the bond strength can significantly improve the cracking resistance of the material.As the bond strength increases to a certain value,the change of fracture temperature and fracture strength is small,and the cracks basically pass through the asphalt mortar.In this paper,the low-temperature cracking behavior of asphalt pavement in high altitude area is taken as the research object.The thermal stress distribution and the meso-mechanism of the crack development process of asphalt mixture are analyzed,which provides certain theoretical guidance and technical measures for the prevention and control of the low temperature cracking of asphalt pavement.