Moisture Diffusion Behavior And Damage Characteristics Of Asphalt Mixture Within Multi-scale Domain

Moisture diffusion widely occurs in real asphalt pavements during the whole service-life.Moisture-induced damage inevitably causes the performance deterioration and durability degradation of asphalt mixture.The diffused moisture persistently interacts with multiscale compositions of asphalt mixture and leads to distinct characterisitics of moisture-induced damage within multi-scale domains.At nano-scale domain,water molecule strongly interacts with the bitumen and mineral molecules.The intermolecular non-bonded potentials between bitumen and aggregates are changed by water molecules,resulting in the dissociation of bitumen-aggregate interfacial structure.At micro-scale domain,the diffused moisture causes the degradation of rheological properties of bitumen-aggregate interfacial and bulk mastics,which leads to local adhesive failure and micro defects at mastic-aggregate interface.At meso-scale domain,the micro defects accumulate at the mastic-aggregate interface and the properties of mastic degrade progressively.The meso mechanical behavior and crack propagation of asphalt mixture are changed.At macro-scale domain,the damage induced by the diffused moisture results in the performance deterioration and loading capacity degradation of asphalt mixture.Multiscale compositions of asphalt mixture lead to different characteristics of damage induced by moisture diffusion at different scales.The evolutions of moistureinduced damage in asphalt mixture and their relationships among different scales could hardly be comprehensively understood from the investigation of moisture damage at an individual scale.Consequently,the formation and growth mechanism of moistureinduced damage in asphalt mixture is unclear.In view of the lack of multiscale methodology to investigate damage induced by diffused moisture in asphalt mixture from a multiscale level,this study investigated nano molecular interaction,micro interfacial defects,meso mechanical behavior and macro performance degradation to reveal the formation and growth mechanisms of damage induced by diffused moisture in asphalt mixture at multi scales.The study aims at proposing a multiscale methodology to investigate multiscale characteristics of moisture damage and their relationships at different scales.Long-term interaction of water with bitumen-aggregate system induces the change of nano-colloidal structure.To reveal the formation of nano-moisture damage,molecular dynamics was utilized to inverstigte molecular behavior and structural reorganization in bitumen-aggregate system with the presence of water.Results show that water molecules form self-aggregates and destroy the original bitumen colloidal structure.Interacting with water molecules,the polar bitumen molecules migrate and form a bitumen-wateraggregate triangular or lamellar structure.Water molecules in the new structure increase the distance between bitumen and aggregates molecules.The water molecules weaken the non-bonded interaction between bitumen and aggregates molecules,and result in the nano-interfacial adhesive damage.Fourier Transform Infrared Spectrometer(FTIR)was empoloyed to measure the chemical composition and Atomic force microscopy(AFM)was used to detect the morphology of bitumen-aggregate interfacial zone before and after water treatment.Results suggest that water molecules cause the migration of polar bitumen molecules,accounting for a shrinkage of boundary and micro pits and bumps on bitumen surface.The results demonstrate the migration of poar bitumen molecules and reorganization of colloidal structure.The new nano-strucutre with the presence of water molecules induces micro interfacial defects.To quantify the microscopic moisture-induced degradation of rheological properties at mastic-aggregate interface and in bulk mastic,a parameter characterizing the micro interfacial properties is needed.Based on the mastic rheological behavior on aggregate substrates,the effects of mastic-aggregate interface on mastic’s mechanical properties were investigated.By introducing an exponential fuction of mastic modulus spatial distribution,a mastic rheological model considering interfacial effects was established.The rheoligical model is able to isolate the interfacial and bulk mastic modulus.The degradations of interfacial and bulk mastic modulus caused by moisture were investigated.Results shows that a wet aggregate surface does not significantly decrease interfacial mastic modulus.However,long-term water action increases bulk mastic modulus but decreases phase angle.Based on the mastic micromechanical theory,the change of bulk mastic’s rheological properties is abscribed to the hardeness of bitumen and the increase of effect mineral filler content.Micro interfacial water film hinders the bitumen-aggregate interaction and causes micro interfacial effects,which significantly decreasing micro interfacial adhesion.Interaction between water molecule and micro pore wall influences micro moisture diffusion in fine aggregate matrix(FAM).To precisely measure the diffusion coefficient of moisture in FAM,gas flow,surface flow and capillary condensation flow in micro pore need to be considered.Combining with moisture adsorption chracteristics in FAM,micro moisture diffusion models containing surface adsorption flow and capillary condensation flow in FAM were proposed.A static penetrating method was applied to investigate the moisture diffusion in FAM under different concentration gradients and sample thicknesses.Results demonstrate that mosture diffusion under various concentration gradients consist with the moisture diffusion model containing surface adsorption flow.The moisture diffusion in the FAM samples with various thicknesses can be characterized by the moisture diffusion model containing capillary condensation flow.Provided the diffusion coefficient in FAM,meso moisture diffusion model in asphalt mixture was built to simulate meso moisture diffusion behavior depending on moisture concentration.Results suggest that the evolution of normalized moisture concentration and diffusion flux depends on the moisture concentration boundary.The evolution of moisture diffusion coefficients decreases exponentially with the time,which explains the non-Fickian moisture diffusion in asphalt mixture reported in previous researches.Introducing the degraded properties of micro interfacial and bulk mastic induced by long-term water treatment,a meso mechanical model of asphalt mixture containing micro moisture damage was established to investigate the change of meso mechanical behavior.Results show that moisture damage increases the local stress and aggravates the stress concentration.Digital image correlation(DIC)method detected strain distribution of asphalt mixture in semi-circular bending test.Results indicate that the moisture damage increases the area of high strain level.Moisture damage produces numerous micro cracks and deteriorates the mascro tensile strength and J-integral fracture toughness of asphalt mixture.The mechanism of multi-scale moisture damage in asphalt mixture is summarized as: At nano-scale domain,bitumen-water-aggregate molecular interaction destroys the original dry nano colloidal structure and reorganizes new molecular interaction pattern and new bitumen colloidal structure.The new structures degradate the mechanical properties and interfacial nano-adhesion.At micro-scale domain,interfacial water film hinders the bitumen-aggregate interaction and form interfacial loacal adhesive failure.Meanwhile,the new nano-structure destroys the micro structure of mastic and degrades the mechanical properties of mastic.At meso-scale domain,moisture damage aggravates the stress concentration at the bitumen-aggregate interfacial transiton zone of asphalt mixture,and decreases the micro interfacial adhesive strength.At macro-scale domain,moisture damage results in the decrease of macro tensile strength.Based on the multiscale moisture damage mechanism,some improvement measures,such as adjusting nanocomposition,optimizing colloidal structure,increasing interfacial adhesion and improving meso-structure of mixture,could be conducted to increase the moisture stability and durability of mixture.