Reinforcement Of Building Recycled Aggregates And Performance Of Asphalt Concrete Based On Microscopic Features

The recycled aggregate obtained from the crushing of waste building concrete is a multiphase structural material.The prepared asphalt mixture has problems such as increased oil-to-stone ratio and attenuation of road performance,which makes it difficult to popularize and apply in asphalt pavement structure.At present,the research on recycled aggregate asphalt mixture(RAAM)mostly focuses on the influence of the content of recycled aggregate on its performance,but less attention is paid to the performance enhancement of recycled aggregate and the weak interface transition zone.Therefore,this paper focuses on the experimental research on the influence of recycled aggregates before and after strengthening on the road performance of RAAM and the transition zone of weak interface,and verifies the feasibility of applying RAAM in the middle and lower layers of asphalt pavement after strengthening.Firstly,by detecting the basic performance indicators such as density,water absorption,crushing value,abrasion value,mud content and asphalt adhesion of building recycled aggregates in each particle size range,the performance differences between building recycled aggregates and natural aggregates were analyzed;And use grinding test to wear construction recycled aggregate,cement mortar has a significant influence on its basic mechanical properties;To characterize the discrete degree of basic performance indicators,the water absorption and crushing value with larger discrete degree are selected as the main indicators for quantifying the performance of recycled aggregates after strengthening.Secondly,sodium water glass,emulsified asphalt,penetrating waterproofing agent,and water-based polyurethane were used to strengthen the recycled aggregate,and SEM and EDS were used to analyze the microscopic morphology and element distribution characteristics of the recycled aggregate.The results show: Due to the existence of acid aggregate surface and dry shrinkage cracks,the water absorption rate of aggregate is high and the adhesion to asphalt is poor after strengthening by sodium water glass;Emulsified asphalt effectively reduces the water absorption rate and crushing value of aggregates,but the demulsification asphalt filled in the pores of the mortar is poor in high temperature resistance,and aggregates stick together into blocks,the water absorption rate and crushing value will increase twice;The penetrating waterproofing agent fills the fine pores of the mortar to form a staggered base surface,and the water absorption rate of the aggregate is close to meeting the requirements of the technical specification;The compound reinforcement effect of "penetrating waterproofing agent + waterbased polyurethane" is the best.The water absorption rate and crushing value of the aggregate are reduced to 1.05% and 16.9%.Aggregate and asphalt have good adhesion,and the basic properties are close to natural aggregates.Thirdly,the gradation design and road performance test of RAAM before and after compound strengthening were carried out,X-ray CT was used to obtain the micro-breaking state of the mixture under load,and the performance of RAAM of compound strengthening was analyzed.The results show: The mass conversion method eliminates the problem of RAAM gradation design deviation.The best oil-to-stone ratio of RAAM after strengthening is 4.0%,and it is reduced by 0.4% before strengthening;After strengthening,the dynamic stability,low temperature crack resistance and water stability of RAAM are significantly improved,and its road performance is close to that of natural aggregate asphalt mixture;The crack damage in RAAM mainly occurs between the cement mortar and the cement mortar-asphalt mortar interface.After compound strengthening,RAAM has no broken cracks.The compound strengthening method effectively improves the strength and adhesion of recycled aggregates.Finally,nanoindentation,SEM and EDS were used to characterize the micromechanical parameters,morphology and elements of each phase of RAAM before and after strengthening and the transition zone of the weak interface of cement mortar-asphalt mortar.The results show:Before strengthening,the elastic modulus of the RAAM interface transition zone is 8.77 GPa and the thickness is 30μm.The cement mortar phase near the interface transition zone is loose and porous,with abundant pore cracks,and the interface transition zone has a gap structure of about 5μm;After strengthening,the elastic modulus of the RAAM interface transition zone is11.16 GPa and the thickness is 20μm.The cement mortar phase near the interface transition zone forms a dense structure,and the pore crack and crevice structure basically disappears;The interface transition zone can be identified by the content of main elements.The content of C,O,Ca,and Si main elements in the interface transition zone is between the cement mortar phase and the asphalt mortar phase.The EDS "line scan" mode analysis shows that the thickness of the interface transition zone is 30μm and 17μm,which are in good agreement with the nanoindentation test results;Cement mortar and cracks are densely filled with permeable materials,the thickness of the weak interface transition zone is reduced,and the modulus is increased,which are the essential reasons for the improvement of RAAM road performance.