Research On Cracking Behavior And Fracture Mechanism Of Plant Produced Reclaimed Asphalt Pavement Mixtures

Due to the increasing costs of raw materials and the environmental issue,the demand to use higher percentages of reclaimed asphalt pavement(RAP)(higher than 25%)in the construction of hot-mix asphalt(HMA)pavements continues to rise.However,the decreasing cracking resistance of RAP mixtures makes the use of higher percentages of RAP in the construction of HMA pavements been limited.The major reasons of the RAP mixtures' decreasing cracking resistance derive from three aspects: the rejuvenating RAP binder property;the degree of blending between the virgin binder and RAP binder;and the mesostructure of RAP mixture.To address these above problems,this paper researched the cracking resistance of plant produced RAP mixtures and the influences which affects the cracking resistance.Based on the relationship between the crack resistance and its influencing factors,suggestions for optimal design of RAP mixtures are proposed.Firstly,the properties of RAP binder were investigated.Then the asphalt blends with different percentages of RAP binder,different types of virgin binders and different rejuvenator dosages were prepared.After that,the mechanical performance and chemical structure of the asphalt blends were analyzed.The results show that the addition of RAP binder decreases the low-and intermediatetemperature cracking resistance of rejuvenating RAP binders.The utilization of rejuvenator can improve the cracking resistance of rejuvenating RAP binders.The rejuvenating RAP binders were in a higher aging level compared with the virgin binder and with the aging level deepens,the aging level of rejuvenating RAP binders accelerated.This paper suggested that the cracking resistance evaluation of RAP binders should consider the aging level and the service life of asphalt pavements.Secondly,the degree of blending between the virgin binder and RAP binder(DOB)was investigated based on SEM/EDS and tracer technology and the evaluation indicator was proposed.It was found that the virgin and RAP binder were partially blended in the RAP mixtures and the partial blending was gradual diffusion from the virgin binder to the RAP binder.With the increase of the RAP percentage,the degree of blending decreased.The degree of blending continues to occur in the service life of asphalt pavements.The use of rejuvenator can improve the degree of blending effectively.Thirdly,the mesostructure of the RAP mixtures was researched based on Matlab and 2D scanning images.The mesostructure evaluation parameters are proposed based on the coarse aggregate contact characteristics and the asphalt mortar distribution characteristics.The results show that the RAP material agglomeration affects the aggregate contact characteristics of RAP mixtures.And the thickness of asphalt mortar in RAP mixtures is thinner than the virgin mixture and is more concentrated on smaller thicknesses.The use of rejuvenator did not improve the coarse aggregate contact characteristics and asphalt mortar distribution characteristics of the RAP mixtures.More virgin asphalt content makes the distribution of coarse aggregates in the RAP mixtures more uniform and have thicker mortar thickness,but also makes the mortar thickness uniformity of the RAP mixtures worse.Fourthly,the cracking resistance of RAP mixtures were evaluated using semi-circular bending(SCB)tests under different temperatures and different loading modes.Based on the R resistance curve theory,the R curves of asphalt mixtures in the SCB test are constructed.The cracking resistance evaluation indicators and thresholds are proposed.The test results indicated that the RAP reduces the crack resistance of the RAP mixtures.The rejuvenator can improve the crack resistance of the RAP mixtures to a certain extent,but the improvement effect is not as good as to increase the virgin binder amount in RAP mixtures.Among the 14 reclaimed mixtures evaluated in this paper,40% RAP content and 50% RAP content mixtures without rejuvenator have the worst crack resistance.And the cracking resistance of 50% RAP content mixture with rejuvenator is improved,but other methods are needed to ensure the crack resistance of the mixture.Finally,the effects of the RAP binders' performance,the DOB and the mesostructure of the RAP mixtures on the cracking resistance of the RAP mixtures were investigated.The results show that the RAP binders' performance and the DOB have a significant effect on the cracking resistance of the RAP mixtures.The worse the RAP binders' performance,and the lower the DOB,the worse the crack resistance of RAP mixtures.In order to ensure the cracking resistance of the RAP mixtures,the influence of the combined effect of the RAP binders' performance and the DOB should be considered.The expectation of the distribution of asphalt mortar has a significant effect on the crack resistance of the RAP mixture.The greater the expected value the asphalt mortar distribution,the better the crack resistance of the RAP mixture.Based on the effects of crack resistance,suggestions for the optimal design of plant-produced RAP mixtures were proposed.