Research On Fatigue And Failure Mechanism Of Cold-recycled Asphalt Mixture Based On Discrete Element Method

Cold recycling technology of asphalt pavement has been paid more and more attention and popularized at home and abroad because of economic benefits and environmental protection.However,compared with hot-mix and hot-recycled asphalt mixture,cold-recycled asphalt mixture is less in the basic research,material composition design and engineering application,especially when applied to the lower surface layer or the base,it is easier to cause fatigue failure.At the same time,the composition of cold-recycled asphalt mixture is relatively complex;its strength structure,fatigue and failure mechanism are different.In order to achieve sustainable and efficient recycling,it is imperative to study the internal interface structure,fatigue and failure mechanism of cold-recycled asphalt mixture.In this article,through multiscale approaches,such as macro mechanical test,scanning electron microscopy(SEM)microscopic test and discrete element simulation,the fracture properties,fatigue and failure mechanism of the cold-recycled emulsified asphalt mixture have been explored and analyzed.Then,the micro-scale study on different interface properties has been researched.The discrete element model of two-dimensional semi-circle bending(SCB)specimen has been established for the fracture and fatigue simulation process;at the same time,the discrete element simulation analysis on different structures of recycled asphalt particles(RAP)has been carried out.The fatigue and failure mechanism have been comprehensively analyzed by fracture interfaces combined with micro-scale weak interface,fracture and fatigue performance indicators.The results of microscopic,mesoscopic and macroscopic levels have been combined with the actual engineering practice;besides,the material performance optimization direction based on discrete element method has been put forward.The main research contents are as follows:(1)The mix design of cold-recycled emulsified asphalt mixture was completed,and the optimal mixing order of cold recycled mixture was determined by comparing the macroscopic test indexes such as indirect tensile strength,moisture stability and dynamic modulus of cold recycled mixture formed under different mixing order conditions.The subsequent semicircular bending specimens were formed in the optimal mixing order,and the macroscopic fracture test and fatigue test were carried out to study the fracture behavior and fatigue performance of cold-recycled asphalt emulsion mixture,respectively.(2)Different specimen preparation methods were used for different types of slurry,two-phase interface and three-phase interface,and scanning electron microscope microscopic tests were carried out to study the micro-morphology of different components inside the cold recycled mixture.By taking micro-scale pictures and combining with image processing technology,the distribution features of voids in the interfacial transition zone were extracted and analyzed.(3)Color emulsified asphalt was used as the binder to distinguish the RAP material and aggregate-cold recycled mortar interface,and random algorithm was applied to generate the RAP materials with different structures in the two-dimensional plane of the mixture.The macro-parameters of asphalt mortar were obtained through in-situ uniaxial test,which were transformed into the meso-parameters by the mechanical characterization formula of the microscopic contact model.According to the actual semicircular bending fracture and fatigue tests,the discrete element simulation loading conditions were realized,and the discrete element model was verified.(4)The discrete element model of two-dimensional SCB specimen,using color emulsified asphalt and the image processing technology,has been established for the fracture simulation process.First,the cracking behaviors and the influencing factors have been analyzed.Similarly,the aggregate random generation method was used to construct mixtures simulating different structures of the RAP material,and the simulation results of cold-mix mixtures were used as the control group.The virtual fracture behaviors were analyzed and compared.(5)The same two-dimensional SCB specimen was used to simulate the fatigue test.The controlled stress mode was adopted,and the half-sine curve was loaded at 5Hz loading frequence in a cyclic manner until the specimen was in a completely failure state.The fatigue simulation results were analyzed.Furthermore,the fatigue and failure mechanism were analyzed in detail from the influence factors.Similarly,the simulations of fatigue tests with different RAP structures were carried out,and the simulation results of cold mix mixture were taken as the control group as well.The virtual fatigue cracking behaviors were analyzed and compared.The above micro and macro test results indicate that the interface containing cement and its hydration products,especially when the cement and its hydration products are not uniformly dispersed,the interface bonding degree between coldrecycled asphalt mortar and aggregate is weak.The micro-morphology analysis results are consistent with the results of the performance optimization design in the macro performance test.The virtual simulation results above show that the weakest interface in cold-recycled asphalt mixture is the interface between cold-recycled asphalt mortar and aggregate.Besides,the bonding strength of cold-recycled mortar is one of the important factors affecting the strength,fatigue and failure mechanism of the coldrecycled mixture.In the fatigue simulation tests of cold-recycled mixture with different RAP structures,the relatively weak interface is that between the aged asphalt mortar outside the RAP and the cold-recycled asphalt mortar.