Rheology And Morphology For The Heterogeneous System Of Polymer Modified Asphalt

Polymer modified asphalt is widely applied in the paving industry as an adhesive for mineral aggregates.Asphalt determines the performance of pavement to a great extent.However,rapid increasing traffic volumes,heavier vehicle loadings as well as unexpected extreme weather during last decades are serious challenges to the consistency of pavement.In this sense,there are many distresses in bituminous roads,including rutting,thermal-cracking and fatigue.In fact,these problems are directly related to the rheological properties and viscoelastic behavior of asphalt.Therefore,rheology is a foundmental way to improve the performance of asphalt and to resolve the distress of pavement.The structure of polymer affects the performance of modified asphalt significantly.It provides important guidance for practical application to study “structure-property” relationships of polymer-modified asphalts.In this work,dynamic shear rheometer(DSR),bending beam rheometer(BBR),fluorescence microscopy(FM)together with statistical analysis method and phase field theory(PFT)were used to study viscoelasticity,morphology,compatibility and storage stability of polymer modified asphalt.Furthermore,the relationship of polymer structure with viscoelasticity as well as the correlations of morphology with viscoelasticity was analyzed deeply.Rheological criterion used to evaluate compatibility between polymer and asphalt was developed.Also,modelling and numerical simulation was developed for polymer modified asphalt with the aim to describe and predict the phase separation behavior.The main points were shown as following:In order to reveal“structure-property” relationships of four typical polymer-modified asphalts,comprehensive viscoelastic characterizations were conducted on dynamic shear rheometer(DSR)and bending beam rheometer(BBR)for asphalt modified by polyethylene(PE),styrene-butadiene-styrene copolymers(SBS),ethylene-vinyl-acetate(EVA)copolymers and crumb rubber(CR)with different structures.The time-temperature superposition method and generalized Maxwell model were applied to rheological results in order to have the deep discussions.Thus,the influences of block ratio in SBS,density of PE,VA content in EVA and diameter of CR particle were obtained based on above evaluations.The results revealed that the maxium value in modulus and zero-shear-viscosity(ZSV)corresponds to the optimal S-B structures in SBS.As the increase of block ratio S/B,asphalt is less susceptible to shear forces and more brittle at low temperature.Medium density polyethylene(MDPE)significantly improves the high temperature performance of asphalt and endows asphalt with the most obvious shear-thinning behavior.Ostwald model describes such shear-thinning behavior fairly well.Besides,LLDPE greatly improves the low temperature performance of asphalt.For EVA modified asphalt,there is the optimal high temperature performance while low temperature performance enhances monotonously as the increase of VA content in EVA.Regarding to CR modified asphalt,the increase in particle size causes an enhancement in viscosity and modulus.Moreover,increasing particle size leads to more obvious non-Newtonian flow behavior and less viscosity-temperature susceptibility.Compatibility between asphalt and polymers was evaluated by viscoelastic parameters curves,including master curves,isochronal plots,black diagrams,Han diagrams and Cole-Cole plots.It was found that the symmetry of the Cole-Cole curve has the most direct correspondence with the compatibility of the system and symmetrical curves correspond well with good compatibility,which also has a fairly good accuracy and sensitivity.Then Cole-Cole plots were used to study the effect of polymer structure on compatibility of asphalt.The results showed that moderate value of block ratio as well as LDPE is most compatible with asphalt,while increasing VA in EVA and decreasing particle size of CR improve the compatibility.Storage stability of polymer modified asphalts was characterized by rheological method.It was found that asphalt modified by SBS with block ratio of 30/70 is most stable during hot storage period.Meanwhile,increasing VA in EVA and decreasing particle size of CR benefit for stability.As a result,there is a significant positive correlation between compatibility evaluated by Cole-Cole plots and pratical storage stability.The microstructure of different polymer modified asphalt was observed by fluorescence microscopy.The dimension and distribution of polymer-rich phase was characterized by statistical analysis.Based on those measurenments,the relationship of morphology with viscoelasticity and stability was established.Three types of morphology in SBS modified asphalt,including SBS-rich phase as dispersed phase,co-continuous phase and asphalt phase dispersed in SBS,have a strong relationship with viscoelastic curves obtained from frequency sweeps and temperature sweeps,which is sensitive to morphology.Aiming to find the key parameter controlling the phase separation process,this paper employed numerical simulation and phase field theory to simulate the phase separation process of polymer modified asphalt.A phase field model was developed for polymer modified asphalt.Model parameters were determined for various modified asphalts based on experimental results.In order to simulate the real storage conditions,phase field model coupled with Navier-Stokes equations was developed in which gravity is considered.Furthermore,phase field model coupled with Navier-Stokes equations was applied to simulate morphology evoluation of SBS,PE and EVA modified asphalts during high temperature storage periods.The simulation results were compared with experimental microscopy.It was proved that phase field model coupled with Navier-Stokes equations describes the phase separation process well during hot storage period.The results monitor the whole separation process between polymers and asphalts well.More importantly,the results obtained numerical simulations are consistent with the resuts of experimental storage stability test.Meanwhile,the simulation results are similar with morphology obtained from fuorescence microscopy.