Research On Evolution Mechanism And Influence Factors Of Long-term Performance Of Emulsified Asphalt Cold Recycled Mixture

Cold recycling technology with emulsified asphalt has attracted more and more scholars’attention due to its advantages of construction at room temperature and high utilization rate of reclaimed asphalt pavement（RAP）.However,most of researches are mainly focused on the early and final pe rformance of cold recycling mixture with emulsified asphalt（CRME）at present,and few information is pub lished about its long-term performance.As a result of the influence of environmental factors and driving loads during long-term service of pavement,the characteristics of raw materials in CRME will change,such as mutual fusion between new and old asphal t on the surface of RAP,aging of new asphalt,and so on.These changes will have an important influence on long-term performance of CRME.Besides,alt hough some studies have indicated that industrial solid waste additives（e.g.fly ash）could improve pavement performance of CRME,such as indirect tensile strength（ITS）and Marshall stability,the effect of industrial solid waste additives on long-term performance is unclear.Therefore,it is essential to carry out an in-depth research on evolution mechnasim and influence factors of long-term performance of CRME,which can provide a vital theoretical basis for broadening the application of industrial solid waste additives and prolonging the service life of the corresponding pavement.In this thesis,firstly,a materials composition designing was conducted to obtain the emulsified asphalt with excellent performance,which was used to prepared CRME in the following research.Then,a laboratory accelerated curing method was employed to simulate the long-term performance of CRME,and the influence of the aging degree of RAP,gradation,additive types（e.g.cement,fly ash,rice hush,flue ash）on the pavement performance in the process of long-term curing were investigated,respectively.These test methods,such as Fourier Transform Infrared Spectroscopy（FTIR）,Optical Microscope,Environmental Scanning Electron Microscope（ESEM）and X-ray Diffraction（XRD）,were used to characterize the chemical structure and microstructure of different constituents in CRME during long-term curing,and discuss the long-term performance evolution mechanism of different CRMEs.The main research conclusions are as follows:（1）Based on the types and dosages of emulsifier,the acid types and p H values in the soap,the emulsified asphalt for cold recycling is designed,and the formula for preparing emulsified asphalt with excellent performance is determined as follows:SB slow-cracking cationic emulsifier with 2.5%dosages by weight of emulsified asphalt,p H value 2 of soap using hydrochloric aci d（HCl）.（2）During the long-term curing of cement-CRME,due to a rapid increase in the content of cement hydration products and asphalt binder in the preliminary stage,a partially continuous network structure consisting of them is formed gradually,which in turn leads to the continuously increase in ITS,Masharll stability and freeze-thaw ITS ratio（TSR）.But after 20 days,on the one hand,both the hydration of cement and demulsification of emulsified asphalt are already completed,on the other hand,the aging degree of asphalt binder is deepened,thus the ITS and TSR of CRME begin to decrease.（3）The old asphalt on the surface of RAP can improve the ITS,water stability and low temperature anti-cracking property of CRME,while have a negative effect on high temperature stability.During the long-term curing of CRME,on the one hand,aging of the new asphalt on the surface of RAP happens,on the other hand,the mutual fusion degree between new and old asphalt on the surface of RAP gradually increases.The more severe aging of the old asphalt is,the better high temperature stability and water stability of CRME during the long-term curing process are;however,if the aging of old asphalt is too serious,its improvement effect on low temperature cracking resistance can be severely weakened.（4）An optimized grading range for medium-grain central plant cold recycling mixture with emulsified asphalt is proposed firstly,and then three gradations by varying the passing rate of 4.75mm critical sieve（37%,41%and 45%）are selected in this range to explore influence of gradation on final and long-term properties of CRME.It is determined that the basic pavement performances of CRME with various gradations under optimal water content and optimal asphalt emulsion content fully meets the requirements of the specification.Comparing three gradations,it is found that the CRME with finer gradation before and after long-term curing has higher ITS,water stability,high temperature stability and low temperature anti-cracking property.（5）Compared with cement-CRME,the industrial solid waste additives-CRMEs under same dosage show the higher compactness,ITS and low-temperature crack resistance,while have the lower high temperature stability and water stability.The dynamic stability meets the requirement of no less than 2000 times/mm in the specification,but the TSR is lower than the minimum requirement of 70%in the specification.In addition,compared with the single-mixed industrial solid waste additive,the high temperature stability and water stability of CRME are evidently enhanced after the three industrial solid waste additives mixed with cement.Among them,the mixing of fly ash and cement leads to the most obvious improvement.The XRD test results indicate that the C a（OH）₂ from cement hydration continues to undergo pozzolanic reaction with fly ash,and then more C-S-H are generated.（6）As the curing time increases,the Marshall stability and TSR of three industrial solid waste additives-CRME always show a gradual increase trend.And when the curing time reaches 20 days,their TSR meet the basic requirement of no less than 70%.Compared with cement-CRME,industrial solid waste additives-CRME throughout the long-term curing process exhibits the higher ITS,low-temperature anti-cracking property and the lower high-temperature stability and water stability,and the improvement effect of fly ash to ITS and low-temperature indirect tensile strain of CRME is the most remarkable.（7）Based on Optical Microscope,XRD and ESEM tests,it can be known that,after adding fly ash in emulsified asphalt,the shell of some fl y ash particles is gradually dissolved,as well as the dem ulsification of asphalt emulsion particles is accelerated.But there is no significant change for rice husk ash and flue ash.In addition,compared to 1 day curing,after 2 days curing the connectio n between the industrial solid waste additive particles becomes relatively tighter due to the increase in demulsification of emulsified asphalt,the industrial solid waste additive particles play a"reinforcement"role in CRME at lower temperatures.After 5 days or ev en 30days curing,a dense microstructure composed of asph alt binder and industrial solid waste additive particles or their dissolved substances is finally formed.