The Adhesion Between Castor Oil-Based Bio-Asphalt And Aggregates Under The Action Of Thermal-Oxidative Aging

Bio-asphalt,as a new environmentally friendly material,has advantages over petroleum asphalt in terms of low-temperature crack resistance,adhesion,and water stability.It has a wide range of application prospects.However,the high content of oxygen elements and light components in bio-oil make bio-asphalt generally susceptible to aging.Nowadays,there are few studies on the change of bio-asphalt-aggregate adhesion properties under external environmental aging,which to some extent limits its large-scale application in road engineering.To investigate the adhesion of castor oil-based bio-asphalt(COBA)under the effect of thermal oxygen,we use macroscopic tests to evaluate the adhesion performance of COBA-aggregate and molecular dynamics(MD)to calculate the adhesion performance of COBA-aggregate interface.Finally,the microstructural changes of COBA-aggregate were combined with the microstructural changes of COBA-asphalt to reveal the adhesion change mechanism of COBA-aggregate under thermal oxygen.The main conclusions are as follows:Firstly,three contents of 5%,10%,and 15% of castor oil-based bio-oil and petroleum asphalt were prepared to castor oil-based bio-asphalt.The high-temperature performance,low-temperature performance,and thermal stability of the COBA were evaluated based on softening point,dynamic shear rheology,ductility,bending beam rheology,and thermal stability tests.The results found that: castor oil-based bio-oil can improve the low-temperature crack resistance and stress relaxation ability of matrix asphalt,but to some extent,it will weaken the high-temperature rutting resistance,and the thermal stability is slightly better than that of matrix asphalt.Second,macroscopic tests were used to evaluate the adhesion performances of COBA-aggregate before and after thermal oxidation.Firstly,the adhesion of COBA-steel slag and COBA-limestone before and after thermal oxygen is judged by the boiling method,and then the adhesion and peeling of them are quantitatively analyzed from the perspective of energy.Finally,the failure form and failure strength of COBA and aggregate are obtained from the perspective of mechanics.The results showed that the adhesion work and pull-off strength of COBA-aggregate were increased by 5% and 11.5% respectively due to the polymerization and esterification of bio-oil in the original stage.After short-term aging,the adhesion of COBA-aggregate is further improved with the increase of polar components.However,after long-term aging,COBA becomes hard and brittle,which is easy to produce stress concentration and brittle failure under external force,resulting in reduced failure strength and adhesion work.Although the adhesion grade of COBA-limestone is the same as that of COBA-steel slag,the adhesion work and pull-off strength of COBA-steel slag are better than that of COBA-limestone,and the cohesive failure is the main failure mode of COBA and aggregate.Third,molecular dynamics was used to construct the molecular model of COBA and the interface models of COBA-calcite and COBA-quartz before and after thermal oxygenation.The cohesion energy density(CED)and diffusion coefficient of COBA and the interface energy of the COBA-aggregate were analyzed to characterize the adhesion strength of the COBA-aggregate interface before and after thermal oxygenation from the atomic scale.The results show that the CED of COBA is consistent with the cohesive force,both of which increase with the deepening of thermal oxygenation,thus increasing the intermolecular interaction.Secondly,the interfacial energies of COBA with calcite and quartz are all negative,and the adsorption is good.Moreover,the interfacial energy of COBA-calcite is 11-20 times that of COBA-quartz,indicating that COBA adheres better to alkaline aggregates.However,the diffusion coefficient of COBA on the quartz surface is 1.53 times that of calcite,indicating that COBA has better mobility on the surface of acidic aggregates,but it is also easier to break free from the bondage of the aggregate interface and be destroyed.Finally,the aging of COBA and the adhesion mechanism of COBA-aggregate under the action of thermal oxygen were jointly revealed by the microscopic detection methods used to combined with macroscopic test evaluation and molecular dynamics simulation.The results show that the cohesion of COBA is consistent with the cohesion work and the values of both increased with the increase in bio-oil content.Also,the cohesion of COBA is highly linear with the pull-out strength of COBA-aggregate,indicating that the cohesion of COBA and the tensile strength of COBA-aggregate has a good correlation.However,the carboxylic acid in bio-oil will appear in the form of the association at the Carbonyl absorption peak,which improves the adhesion of COBA-aggregate,but the excess C =O also will reduce the viscosity of COBA due to the age and harden under the effect of thermal oxygen.At the same time,the“bee-like” structure of COBA and the reduction of Rq can also improve the mobility of COBA,which can better infiltrate and adhere to the aggregate interface.