Study On Mesoscopic Characteristic Of Asphalt Mixture Based On Particulate Matter Mechanics

At present, the research of asphalt mixture has focused on the experience methods basedon phenomenology and laboratory tests, and the commonly used continuum model isapplicable to analyze the dense granular systems under quasi-static conditions and researchthe mechanical properties within these systems. But the size effect of component medium inthe systems is not considered by this model. Therefore, the relationship betweenmeso-structure and macroscopic performances cannot be revealed in analyzing asphaltmixture consisting of composite material based on continuum methods and models. However,the mesomechanics method studies an inhomogeneous media with multiscale mechanicaltheory, and asphalt mixture belongs to particulate matter material in material composition, andit is found that particulate matter systems have the loading sensitivity, nonlinear responses andself-organization behaviors from a large number of tests and numerical simulations, there areessential differences between the mesomechanics and continuum theory, macroscopicappearance results. In view of the above results, it provides the evidence in constructing themesomechanical models and studying meso-responses of asphalt mixture by mesomechanicsand particulate matter mechanics.Mesoscopic characteristics of asphalt mixture were taken as a mainline in thisdissertation. The distribution characteristics of force chains were analyzed for aggregate andasphalt mixture structures. Digital specimens of asphalt mixture were constructed to simulatethe indirect tensile test numerically and the influence factors of mesomechanics characteristicsof asphalt mixture and distributions of force chains within specimens were analyzed. Internalstructure of asphalt mixture specimen was excavated and dynamic indicators of asphaltmixture and sand mastic were measured to provide the mesoscopic viscoelastic parameters formesomechanical model of asphalt mixture. Stress and strain changes with time weremonitored, and the distribution changes of contact force chains, displacement field andvelocity field with time were traced inside the component particles during loading process,and the interaction at the interfaces between aggregate and sand mastic, and the relationshipbetween internal component and external loading, deformation were interpreted.The main research contents are as follows: (1) Photoelastic stress test and indentation test for particulate matter were used to detectthe force chains among aggregate particles and obtain the distribution law of force chains;(2) Mesoscopoic models of aggregates and asphalt mixture were constructed based ondiscrete element method, and the force chain field and displacement field were obtained bynumerical simulation, and digital specimen of asphalt mixture was constructed to analyze theinfluence factors of mesomechanical properties of asphalt mixture and force chain evolutionwithin specimen under servo-loading conditions;(3) Asphalt mixture specimens were prepared by SGC (Superpave Gyratory Compactor)and sand mastic specimens were prepared by static compaction, the dynamic moduli andphase angles of asphalt mixtures and sand mastic were determinated by Simple PerformanceTest (SPT) at different temperatures, frequencies;(4) Asphalt mixture specimens prepared by SGC were cut, the cross-sectional imageswere processed by digital picture processing technique, and the two-dimensional image wasimported discrete element software to generate the digital specimen of asphalt mixture.Simple Performance Test was simulated with semi-sinusoidal loading numerically,comparative analysis was taken to achieve the difference of dynamic modulus and phaseangle between simulated results and tested results. Finally, the force chain evolution withinspecimens and the changes of displacement field and velocity field with time were traced.The conclusions of this dissertation are follows:(1) In photoelastic stress test, the brighter the contacts among particles were, the greaterthe contact force was. Regular array structures under vertical loading only formed the verticaltransmission path of contact force, irregular array structures existed four transmission pointsof contact force among internal particles while the two structures had no transmission ofcontact force in horizontal direction basically;(2) In indentation test, the probability distribution of contact force chains increasedfirstly and then decreased, and there existed a significant probability peak value, and thedistribution of force chains decreased in negative exponential over the peak value;(3) For the aggregates meso-structures with lateral restraint, the force chain networkshowed a symmetrical distribution and only normal contact force formed among particleswhich was consistent with the results of photoelastic stress test, and the displacements of upper particles along Y direction were greater and decreased with the increase of depth;(4) For the aggregates meso-structures without lateral restraint, the regular arraystructures only formed compression force chains, the contact forces in irregular arraystructures were mainly compression forces and tension force area was scattered which mainlylocated in the middle and lower parts and was asymmetrical distribution, and thedisplacements of upper particles were greater and decreased with the increase of depth;(5) In the influence of asphalt nature on the meso-responses of asphalt mixture, stresspeak values and splitting strengths increased with the increase of bond stiffness ratio, andcoordination numbers had small changes. Stress peak values and splitting strengths decreasedwith the increase of bond strength ratio, and coordination numbers had small changes, too.The influences of bond strength ratio on splitting strengths, coordination numbers andmicro-cracks were greater than those of bond stiffness ratio;(6) In the influence of aggregates on the meso-responses of asphalt mixture, the greaterthe friction coefficients were, the greater the peak stress values and splitting strengths werewhile the smaller the coordination numbers were, and the number of micro-cracks increased;(7) In the influence of loading conditions on the meso-responses of asphalt mixture,stress peak values and splitting strengths increased with the increase of loading rate, and thecoordination numbers decreased. Contact forces among particles become larger and the strongforce chains in contact force chains networks transfered greater stress, their extensiondirections were parallel to the maximum principal stress direction, and the displacements weremainly along loading directions and moved to both sides;(8) Dynamic moduli of asphalt mixture and sand mastic reduced, phase angles increasedwith the increase of asphalt-aggregate ratio. Dynamic modului reduced while phase anglesincreased with the increase of temperature;(9) There was less difference between the measured values and simulated values ofdynamic moduli and phase angles;(10) Force chains within aggregates were all compression force chains, force chains ofsand mastic and aggregate/sand mastic interface included compression and tension forcechains, compression force chains dominated and strong force chains were all compressionforce chains which were along the vertical direction, but the weak force chains were smaller compression and tension force chains with larger dispersibility;(11) The maximum displacements of aggregates and sand mastic gradually increasedwith the increase of loading time, but the difference was smaller, and the displacements ofsand mastic were slightly larger than those of aggregates. Their displacements decreased fromtop to bottom gradually. The velocities of aggregates and sand mastic were both greater at theinitial stage. In addition, the velocities increased with the increase of time gradually, but thevelocity changes of sand mastic were less than those of aggregates.