Theoretical Investigation Of Asphalt Aging Mechanisms And Prevention Technologies Based On Molecular Dynamics And Quantum Chemistry

The reduction in the cracking resistance of mixtures caused by thermal and ultraviolet(UV)aging of asphalt is an important factor that triggers cracking failure of asphalt pavements,and proactively inhibiting asphalt aging can slow down the cracking of asphalt pavements.Various asphalt anti-aging technologies have been proposed,but they are difficult to be applied on a large scale due to high cost or poor compatibility with asphalt.Therefore,the use of low-cost,highly compatible organic anti-aging agents has become a promising direction for the development of asphalt anti-aging technologies.The key point is to screen or develop organic materials that can efficiently block the asphalt aging pathway based on the accurate investigation of the asphalt aging mechanisms and its targeted blocking mechanisms.Another measure to address asphalt aging is to recycle reclaimed asphalt pavement(RAP)materials and reuse them.However,due to the imperfection of the regenerative activation theory of aged asphalt,the current selection of rejuvenators is mostly based on extensive testing for forward screening rather than reverse development based on a rigorous theoretical basis.Therefore,the quality of the rejuvenators varies greatly,and in engineering practice,the recycled asphalt mixtures still face challenges such as limited RAP dosing and ease of cracking.Unraveling the mechanisms of asphalt aging behavior and further analyzing the aging blocking mechanisms of virgin asphalt and the regenerative activation mechanisms of aged asphalt will provide a solid theoretical basis for the development of asphalt anti-aging technologies and RAP rejuvenation technologies that are scientific,efficient,and easy to promote.Therefore,this dissertation adopts a multidisciplinary intersection approach combining molecular dynamics and quantum chemistry,down to the temporal and spatial scales of femtosecond(10-15s)and angstrom(10-10m),to conduct a comprehensive and in-depth theoretical study of key issues including asphalt aging reaction pathways,aging trait evolution,aging behavior blocking,and regenerative activation of aged asphalt,focusing on the two prevention ideas of “active inhibition of virgin asphalt aging” and “efficient promotion of aged asphalt rejuvenation”.Firstly,reactive force field molecular dynamics(Reax FF MD)method that can predict chemical reactions were used to simulate the aging behavior of asphalt under different temperature and oxygen concentration conditions.While analyzing the kinetic properties of asphalt aging reactions,information on aging processes such as oxidation and degradation of asphalt was obtained by following the evolution of the molecular structure.The results showed that the oxygen content and the degree of dissociation of aged asphalt molecules both increase with temperature and oxygen concentration,and a large amount of carbonyl groups as well as a relatively small amount of sulfoxide and hydroxyl groups are generated during aging.Due to the limitation of the theoretical basis,Reax FF MD simulations failed to accurately reveal the essential mechanisms of asphalt aging,but based on this study an overall understanding was developed for the aging characteristics of asphalt.Secondly,based on ab initio molecular dynamics(AIMD)and density functional theory(DFT)under the framework of quantum chemistry,the chemical reaction pathways,thermodynamic driving mechanisms and environmental influence mechanisms(especially UV aging mechanisms)of asphalt aging behavior were comprehensively analyzed.The results showed that asphalt aging involves subreactions with different energy barriers,such as hydrogen abstraction by oxygen,cycloalkane aromatization,formation of oxygen-containing groups and side chain homolysis.The behavioral tendency of the asphalt molecule to reduce its energy through oxygen absorption and isomerization after the loss of hydrogen atoms is the fundamental driving force behind its continued aging.High temperatures,high oxygen concentrations and humid environments promote asphalt aging by increasing molecular collision energy,increasing collision frequency and decreasing reaction energy barriers,respectively,while UV radiation causes asphalt molecules to jump to excited states or directly induces chemical bond breakage,thereby significantly accelerating asphalt aging.Thirdly,to guide the screening and development of asphalt anti-aging agents,the critical step of asphalt aging reaction and its potential blocking targets were analyzed,and organic hydrogen atom donor materials such as plant phenols were proposed as asphalt anti-aging agents.In addition,two plant phenol modified asphalts(i.e.,quercetin modified and lignin modified)were prepared to test their anti-aging properties,while quantum chemistry calculations were performed to reveal the factors affecting the anti-aging properties of plant phenols.The results showed that phenolic substances rely on the lower bonding strength of hydrogen atoms and the stronger electrostatic attraction to oxygen/radicals to exert anti-aging effects,and quercetin performs better in both aspects due to its more phenolic hydroxyl structure,thus its anti-aging activity is much higher than that of lignin.Based on the findings and guided by the structure of quercetin,a theoretical orientation was proposed to stimulate the anti-aging activity of lignin,a low-priced plant phenolic substance.Fourthly,to understand the regenerative activation mechanisms of aged asphalt and to guide the development and utilization of rejuvenators,the agglomeration and deagglomeration behaviors of asphalt molecules during aging/rejuvenation was deeply analyzed by calculating the parameters of molecular polarity,electrostatic potential distribution and non-covalent interactions based on a combined approach of quantum chemistry,molecular dynamics and like-dissolves-like theory.The results showed that the agglomeration of aged asphalt molecules increases in organic solvents and there is a conversion of resins to asphaltenes.while the agglomeration and component conversion are partially reversed after the addition of rejuvenators.The reason for this is that the polar oxygen-containing groups produced by aging cause strong interactions such as hydrogen bonds to form between asphalt molecules,thus increasing the intermolecular bonding energy,but rejuvenators capture and occupy the polar groups on the aged asphalt molecules through strong polar carboxyl groups,which weakens the intermolecular attraction of the aged asphalt.Finally,considering that aging/rejuvenation has the most significant effect on the cracking resistance of asphalt materials,the tensile failure behaviors of the asphalt-aggregate interfacial system under different loading conditions and aging/rejuvenation states were investigated based on molecular dynamics simulations in order to understand the cracking failure mechanisms of recycled asphalt mixtures.The results showed that the aged interfacial system has better resistance to fracture failure under single loading and worse fatigue resistance compared to the virgin interfacial system,which is attributed to the fact that the aging increases the intermolecular bonding of asphalt and makes the interfacial system less flexible.However,the rejuvenators combine strong polar carboxyl groups and non-polar chain alkyl groups,which become an intermediate medium to promote the compatibility of strong polar groups and nonpolar structures within the aged asphalt,thus enhancing the mobility of the aged asphalt and partially restoring the flexibility and fatigue resistance of the aged interfacial system.