Study On Properties And Crack Propagation Behavior Of Recycled Aggregate Asphalt Mixture

The increasing scarcity of natural sand and the increasing accumulation of construction waste have become one of the main contradictions in the field of construction in China.The recycled aggregate produced by construction waste after processing can be used as the raw material of asphalt mixture,which is conducive to the efficient utilization of construction waste.At present,the research on recycled aggregate asphalt mixture mostly focuses on road performance,and the research on mechanical properties and crack propagation characteristics is not comprehensive.Therefore,this paper intends to explore the influence of recycled aggregate replacement rate on the mechanical properties of asphalt mixture,and study the influence of recycled aggregate on the crack propagation behavior of asphalt mixture.In this paper,the physical and mechanical properties and particle shape index of recycled aggregate produced by E-type crushing and counter-impact crushing were tested and analyzed.The results show that the recycled aggregate with counter-impact crushing has small water absorption,less needle-like content,large apparent relative density,small capillary volume relative density and small particle shape index,while the recycled aggregate with E-type crushing has less mud content and smaller crushing value.After comprehensive comparison,the recycled aggregate produced by counter-impact crushing is selected as the raw material for subsequent experiments.Secondly,the mix proportion of asphalt mixture with recycled aggregate replacement rates of 0%,17%,37%and 56.7%is designed.Based on the large difference between the volume relative density of recycled aggregate and natural coarse aggregate,the volume correction method is used to adjust the gradation of recycled aggregate asphalt mixture to meet the requirements of the target mix ratio.The Marshall test is used to determine the optimal asphalt-aggregate ratios of four kinds of recycled aggregate asphalt mixtures with replacement rates are 4.4,5.0,5.2 and 5.8,respectively.The addition of recycled aggregate has a great influence on the optimum asphalt-aggregate ratio,and the asphalt content is obviously improved.However,within a reasonable range of volume parameters,the increase of replacement rate leads to the increase of effective asphalt content and the increase of effective thickness of asphalt film.Then,the road performance of recycled aggregate asphalt mixture was tested by high temperature rutting test,low temperature beam test,immersion Marshall test and freeze-thaw splitting test.The results show that recycled aggregate can improve the high-temperature stability and water stability.When the substitution rate is 37%,the dynamic stability reaches the highest,but its low-temperature crack resistance decreases.At the same time,the mechanical properties of recycled aggregate asphalt mixture were tested by uniaxial penetration test,indirect tensile cracking test,semi-circular bending test,dynamic modulus and dynamic creep.The results show that compared with natural aggregate asphalt mixture,the shear resistance,cracking resistance and deformation resistance of recycled aggregate asphalt mixture are improved.Finally,digital image speckle（DIC）technique was used to evaluate the crack propagation characteristics of recycled aggregate asphalt mixture,such as average crack propagation velocity and fracture toughness.The results show that the higher the replacement rate of recycled aggregate is,the smaller the loading period is,and the early cracking of aggregate is increased.Compared with natural aggregate asphalt mixture,the average crack propagation speed of recycled aggregate asphalt mixture is increased by more than 7.3%,and the fracture toughness is reduced by more than 3.9%.It shows that the bonding between recycled aggregate and surface mortar is weak,the internal micro cracks and the strength of pure mortar block are low,which leads to the direct penetration of cracks and reduces the anti-cracking performance of asphalt mixture.In this paper,the influence of two crushing methods on the physical properties,mechanical properties and particle shape of recycled aggregate,and the influence of recycled aggregate replacement rate on the volume parameters,road performance and mechanical properties of asphalt mixture are tested.At the same time,the crack propagation law of recycled aggregate asphalt mixture is studied,which provides experimental basis for the road application of recycled aggregate asphalt mixture and is of great significance to improve the utilization rate of recycled aggregate.