The Blending Mechanism Of Virgin And Aged Asphalt Binders In Hot Recycled Asphalt Based On Molecular Dynamics Simulations

The recycled asphalt mixture has better rutting resistance at high temperatures,but its cracking resistance at low temperatures is not as good as that of the virgin asphalt mixture,leading to the premature degradation of recycled asphalt pavement.The properties of recycled asphalt binders,the blending degree of virgin and aged asphalt binders,and the contact characteristics of asphalt and aggregate have significant impacts on the cracking resistance of recycled asphalt mixtures.Among them,it is difficult to directly observe the blending state of virgin and aged asphalt in the recycled asphalt binder through macro or microscopic tests.Therefore,this paper adopted the molecular dynamics simulation to investigate the blending mechanism of virgin and aged asphalt on the molecular scale and try to establish the relationship between the blending state of virgin and aged asphalt and the performance of recycled asphalt binders.Firstly,the asphalt binders in different aging degrees were obtained by Rolling Thin Film Oven(RTFO)and Pressure Aging Vessel(PAV)tests.The relative content of characteristic functional groups in the aged asphalt was determined by Fourier infrared spectroscopy(FTIR)tests and then the molecular structures of the virgin and aged asphalt binders were determined.The Saturate,Aromatic,Resin and Asphaltene(SARA)test and element analysis test of asphalt binders were carried out.According to the proportion of four components and H/C atomic ratio of each asphalt,the number of different molecular structures was determined.Based on them,the molecular models of virgin and aged asphalt binders were constructed and verified.Secondly,the diffusion models of virgin and aged asphalt were assembled to conduct the diffusion simulations.The self-diffusion coefficients of different asphalt binders were calculated based on the mean square displacement of the particles,while the inter-diffusion coefficients were calculated based on the mass density profiles of virgin and aged asphalt.Results show that the increase of aging degree will pose negative impacts on the self diffusion of asphalt.For the inter-diffusion of virgin and aged asphalt binders,the diffusion coefficients of virgin asphalt are higher than that of aged asphalt,however,the diffusion depths of aged asphalt are larger than virgin asphalt.Then,the effects of the aging degree of asphalt and temperature on the self-diffusion and inter-diffusion of virgin and aged asphalt were analyzed.It is found that the asphaltene index and oxidative aging index will get larger with the increase of aging degree,which has a negative effect on the self-diffusion of asphalt,where the correlation between asphaltene index and self-diffusion is higher(R~2=0.997).However,the increase of aging degree is beneficial to the inter-diffusion of virgin and aged asphalt.The mass density differences of virgin and aged asphalt are proportional to its inter-diffusion coefficients.With the increase of temperature,the self-diffusion and inter-diffusion of molecules will be more active,but the sensitivity of each component in the asphalt to temperature is different.The saturate is the most susceptible to temperature,while asphaltene is the least susceptible to temperature.When the temperature increased from 140℃to 180℃,the inter-diffuse coefficient of virgin asphalt increased by 29.7%～93.6%,while the inter-diffuse coefficient of aged asphalt increased by 32.3%～57.3%.By comparing the energy changes of virgin and aged asphalt in the process of inter-diffusion,it is found that the aging influences the inter-diffusion by affecting the intermolecular Coulomb interactions,while the temperature influences the inter-diffusion by affecting the intramolecular potential energy and intermolecular van der Waals interactions.Finally,a method for calculating the microscopic degree of blending for recycled asphalt binders was proposed.Also,the bulk modulus and fracture energy of partially blended asphalt and fully blended asphalt were calculated.The results show that the degree of blending of virgin and aged asphalt will be higher with the increase of diffusion time.However,this degree of blending of recycled asphalt has an upper limit.For the bulk modulus of recycled asphalt,the degree of blending of virgin and aged asphalt was positively correlated with it.When the microscopic degree of blending between virgin and aged asphalt reaches 72.7%,the bulk modulus of partially blended asphalt can reach the level of fully blended asphalt.The increase of asphaltene index and oxidation aging index will lead to the increase of bulk modulus of recycled asphalt,where the correlation between oxidative aging and bulk modulus is higher(R~2=0.983).For the cracking resistance of recycled asphalt,the tensile failure mode of partially blended asphalt is different from that of fully blended asphalt.The former is prone to crack at the interface of virgin and aged asphalt,while the latter is prone to crack at the tensile end.The higher the degree of blending would have greater fracture energy so that the cracking resistance of recycled asphalt would be better(R~2=0.98).The higher degree of aging of asphalt would lead to the lower fracture energy of recycled asphalt so that the cracking resistance would be weakened.