Preparation And Performance Of Microcapsules-based Self-healing Asphalt Pavement Materials

Asphalt has been widely used as pavement material grounded on its economy along with good riding comfort and high driving safety factor of the road surface constructed with it.However,asphalt will age under the influence of traffic load and natural environment.Then microcracks will present in the asphalt matrix and the process of damage will be accelerated.As a result,diseases such as chapping,horizontal and vertical cracks will be arisen,which seriously reduce the service life of the asphalt pavement.Therefore,how to prolong the service life of asphalt pavement has become the focus of the majority for scholars.The microcapsule encapsulation method is a kind of material filling technology,which is used to prepare modified asphalt by mixing microcapsules of coated restorative agent with matrix asphalt to construct asphalt pavement.When microcracks appear in the pavement due to aging,microcapsules are punctured and the encapsulated repair agent permeates into microcracks.Consequently,the surrounding asphalt can be filled and connected through physical softening or chemical cross-linking to repair microcracks.Thus,the self-healing ability of asphalt can be improved and the service life can be extended.Nevertheless,most of the microcapsules prepared in the present study have poor homogeneity,contamination during the remediation process,and single way to assess the remediation efficiency.Based on this,two shell microcapsules（polymethyl methacrylate and urea-formaldehyde resin）encapsulated three core materials,such as rapeseed oil,epoxy resin and amine curing agent,were prepared by solvent volatilization method and one-step in situ polymerization method in this study.The optimal preparation processes were explored to ensure the homogeneity of microcapsules.Afterwards,modified asphalts were prepared by blending microcapsules containing three different core materials into the matrix asphalt using the respective optimal process parameters.The microcapsule-modified asphalt with the best self-healing performance was selected through the comprehensive evaluation of high temperature aging and fatigue aging experimental tests.Finally,the self-healing mechanism was explored.The main research contents and conclusions of this study are as follows:（1）Preparation and characterization of microcapsules with poly（methyl methacrylate）shell layer and amine curing agent core:Microcapsules with amine curing agent as core material and polymethyl methacrylate as shell layer were prepared through solvent evaporation method.It was concluded from the optimization of the preparation process that microcapsules with regular morphology,smooth and without adhesive surface,and uniform particle size distribution could be prepared when sodium dodecyl sulfonate（SLS）-gum arabic compound emulsifier was used as emulsifier and its concentration,emulsification speed,and emulsification temperature and time was 1.0 wt.%,300 rpm,40℃,and 30 min,respectively.The average particle size,wall thickness and core coverage of resultant microcapsules were24.7μm,3.2μm and 19.1 wt.%,respectively.Infrared spectroscopy and thermal weight loss tests demonstrated that the amine curing agent core was successfully encapsulated and the thermal decomposition temperature of microencapsule core was increased significantly.It increased from 90°C to 200°C as compared with the non-microencapsulated core.Thus,the stable presence of the encapsulated curing agent during asphalt mixing（180°C）and paving（160°C）could be ensured.（2）Preparation and characterization of microcapsules with urea-formaldehyde resin shell layer and rape seed oil/epoxy resin core:Microcapsules with urea-formaldehyde resin as shell and epoxy resin/rapeseed oil as core materials were prepared based on a one-step in situ polymerization method.The effect of variable process conditions on morphology and structure of microcapsules was investigated using an orthogonal test with the following process:the mass ratio of urea to formaldehyde solution was 1:2,the amount of NH₄Cl and resorcinol was 10.0wt.%of urea mass,the p H of the system was 4,the emulsification time was 30 min,and the reaction time was 4 h.The results showed that regular spherical microcapsules with dense and smooth inner and outer surfaces of the capsule without adhesion were produced based on the following conditions:the concentration of SLS-gum arabic compound emulsifier was 2.0 wt.%,the emulsification speed was 1200 rpm,the reaction speed was 700 rpm,the dilution ratio of core material was 1:1 and the mass ratio of core material to two shell materials was 2:（1:2）.The particle size distribution,average wall thickness and core coating ratio of the prepared microcapsules were 40-50μm,2.0μm and 84.0 wt.%（rapeseed oil）and 40.4 wt.%（epoxy resin）,respectively.Results from infrared spectroscopy and thermal weight loss analysis showed that the urea-formaldehyde resin shell microcapsules containing two core materials were prepared successfully.The integrity of the microcapsules could be maintained during the high temperature mixing and paving of asphalt.（3）Self-remediation behavior of microcapsules-modified asphalt with rapeseed oil cores:The microcapsule containing rapeseed oil（RO）-modified asphalt was fabricated by mixing optimal microcapsules with RO core into the matrix asphalt.It was found that the base properties of the modified asphalt were well recovered and the self-healing ability was improved after high temperature aging compared with the base asphalt when the dosing of RO microcapsules was 3.0 wt.%.Although RO microcapsules reduced the complex shear modulus of the modified asphalt during fatigue aging,the structural strength was also restored to some extent after the repair,especially at the RO microcapsule dosing of 3.0 wt.%.The recovery rates of complex shear modulus(HI^G*),energy storage modulus(HI^G’),cyclic shear number（HI^N）and cumulative dissipation energy(HI^DE)could be increased from 77.66%,74.50%,1.29%and1.10%of the matrix asphalt to 93.53%,96.37%,38.47%and 38.42%in turn.Moreover,the fatigue life could be extended as well.（4）Study of self-healing behavior of microencapsulated epoxy resin/amine curing agent cores-modified asphalt:The EA microcapsule-modified asphalt was produced by introducing EA microcapsules into the matrix asphalt,which were obtained by blending best epoxy resin core（E）microcapsules and amine curing agent（A）core microcapsules in a mass ratio of1.00:1.22.The needle penetration,ductility and softening point of the prepared modified asphalt before and after high-temperature aging,as well as fatigue-repair-fatigue properties,were investigated and compared with those of the matrix asphalt.It was found that the base properties of EA microencapsulated modified asphalt with 3.0 wt.%doping were reduced to some extent after high-temperature aging.However,the viscoelasticity,structural strength and external load-bearing capacity after fatigue aging were restored to a great extent.The HI^G*,HI^G’,HI^Nand HI^DE of 3.0 wt.%EA microencapsulated asphalt increased from 77.66%,74.50%,1.29%and 1.10%to 91.24%,87.41%,95.74%and 89.15%,respectively,as compared to the matrix asphalt.It indicated that the introduction of 3.0 wt.%EA microcapsules into the asphalt could enhance the self-healing property of asphalt substantially.