Study On Reflection Cracking Characteristics Of Warm Mixed Recycled Asphalt Mixture Under Aging And Freeze-thaw Cycles

With the rapid development of transportation in our country,the development of high-grade highways have entered a prosperous and maintenance stage,a large number of high-grade pavement need maintenance,during the process of maintenance will produce a large number of recycled asphalt pavement（RAP）.At present,the renewable utilization rate of RAP is less than 30%^[1],such a low utilization rate will waste a lot of non renewable resources.The combination of recycling and warm mixing technology can not only increase RAP content and reduce construction temperature,but also improve the road performance of recycled pavement,which is of great significance to improve the development of the transportation industry and reduce the waste of resources.The majority of our pavement semi-rigid base pavement structure,and the recycled asphalt mixture is mostly used in the middle and lower surface layer of pavement structure,its reflection cracking problem is more serious.At the same time,the pavement surface will be affected by complex climate and environmental factors during service,among which long-term aging and freeze-thaw cycles have a great impact on the pavement surface.Therefore,the reflection cracking characteristics of warm mixed recycled asphalt mixture under long-term aging and freeze-thaw cycles were studied in this paper.Firstly,the Overlay test（OT）was carried out on the warm-mix recycled asphalt mixture with different RAP content（30%,50%,70%）and hot-mix asphalt mixture（0%）under the aging and freeze-thaw cycles.Based on the macro scale,the reflection cracking characteristics were evaluated by the load loss rate and the change rate of load loss rate.Based on the theory of damage mechanics,the damage factor D was defined by the relative change rate of load loss rate,and the evolution law of reflection cracking damage of recycled asphalt mixture was analyzed before and after different conditions,and the reflection cracking damage model was established.Based on the fractal theory,the fractal characteristics of OT specimen reflection cracking were explored,and the influence degree of different influencing factors on the anti-reflection cracking performance of recycled asphalt mixture was calculated by gray correlation analysis method.The results show that the anti-reflection cracking properties of recycled asphalt mixtures with different RAP content are decreased after different conditions,but they all meet the use requirements,and RAP content has the greatest influence on them.The model has a high fitting accuracy and can accurately describe the evolution process of reflection cracking damage of recycled asphalt mixture.Through the analysis of the fractal characteristics of the reflected cracks,it is shown that the reflected cracks of the recycled asphalt mixture have fractal characteristics in statistical significance.Secondly,digital image correlation technology（DIC）was used to collect the deformation process during OT test loading to obtain the deformation field and its characteristic cloud image.The effects of different conditions on the resistance to reflection cracking and the evolution process of reflection cracking damage of recycled asphalt mixture were analyzed by means of vertical strain density（D_E）and damage factors defined based on vertical strain.The average crack propagation velocity and crack tortuous degree were calculated from the reflected crack propagation process images collected by DIC technology,so as to study the expansion of reflected crack as.The results show that after the aging and freeze-thaw cycles,the anti-reflection cracking property of the recycled asphalt mixture decreases,the damage rate increases,and the average crack propagation rate increases.After the freeze-thaw cycle,the crack tortuous degree of the recycled asphalt mixture is the largest,while after long-term aging,the crack tortuous degree of the recycled asphalt mixture is the smallest,and the overall crack tortuous degree of the recycled asphalt mixture decreases with the increase of RAP content.Finally,the correlation analysis of macro and meso fracture resistance indexes and fracture propagation behavior indexes is carried out.The results show that there is a good correlation between macro and meso crack resistance indexes,and the correlation coefficient between D_Evalue and load loss rate of recycled asphalt mixture is larger than that of fractal dimension on the whole.It is recommended to use load loss rate to evaluate its anti-reflection cracking performance.In addition,the correlation coefficients between macro and meso crack resistance indexes and crack propagation behavior indexes are about±0.95,indicating that they are highly correlated.