Mechanism And Performance Investigation Of Crumb Rubber/SBS Composite Modified Asphalt Binder

Crumb rubber/SBS composite modified asphalt (CR/SBSCMA) technology is proposedto combine the advantages of crumb rubber (CR) and SBS. Then, the production cost of themodified asphalt can be reduced, and the waste rubber resources can be made full utilization,moreover, the performance of asphalt can get improvement. At present, researches aboutCR/SBSCMA mainly focus on the asphalt mixture prepared in-situ, road engineering andperformance evaluation. However, industrial preparation of composite modified asphalt withstable performance is the first problem for the large-scale popularization and application ofCR/SBSCMA, which requires more researches about the key subjects, such as reactionmechanism between modifiers and matrix asphalt, storage stability, etc. while related studiesare not well documented. Based on this situation, the rheological behavior, thermal oxidativeaging properties and storage stability of CR/SBSCMA have been investigated in this thesis.Moreover, the swelling mechanism and kinetics of crumb rubber in asphalt have beendiscussed. All research works and the main results are outlined as follows.To change the dosage of modifiers and compatilizer, crumb rubber modified asphalt(CRMA), SBS modified asphalt (SBSMA) and CR/SBSCMA were prepared with theoptimized process. In the case of the low content of CR and SBS, the conventionalperformance of CR/SBSCMA was obviously superior to CRMA and SBSMA withcorresponding content of modifiers, and it has good modified effects. The rheologicalbehavior of CR/SBSCMA has been analyzed from the angle of material rheology, which ismore suitable for prediction of the pavement performance of asphalt and simulates the realmechanical response of asphalt pavement. In the process of preparation and construction,modified asphalt is presented as liquid state, and the high temperature viscosity is selected asits rheological parameter. Modified asphalt is presented solid state in the usage of pavement,and the complex shear modulus G*, phase angle, storage modulus G’ and anti-ruttingfactor G*/sin as rheological parameters.The viscosity measurement of modified asphalt showed that shearing rate and rotor typehave notable influence on the apparent viscosity. When the test temperature was110℃, and when the temperature exceeded135℃, SBSMA transformed to newton fluid, while CRMAand CR/SBSCMA still presented non-newton fluid properties. Moreover, the rotor type has noimpact on the viscosity of SBSMA at the testing temperature in this study because of themacroscopic homogeneous system of SBSMA. However, because of the influence of CRparticles, the viscosity tested by S27rotor with smaller volume was larger than that with S21rotor for CRMA and CR/SBSCMA. The influence of testing condition on viscosity based onthe properties differences between matrix asphalt and modifiers has been discussed in thisthesis, and the visicosity equation was revised for modified asphalt with CR modifier. Theviscosity of heterogeneous modified asphalt with CR modifier could not be accuratelymeasured under the current test condition, and S21rotor with larger volume and high shearingrate could reduce the influence of CR particals. The viscosity with setting testing condition ofCRMA and CR/SBSCMA should be decreased further according to the viscosity-temperaturecurve.The experimental results of dynamic shear rheology showed that the G*, G’ and G*/sin of CRMA, SBSMA and CR/SBSCMA decreased with the increasing temperature. And thethree rheological parameters presented a linear relation with temperature in the semilogcoordinate. In the case of similar penetration, the G′, and G*/sin of CR/SBSCMA werebetter than CRMA and SBSMA. In addition, compared with softening point TR&B, theequivalent softening point T800and high grading critical temperature THSof modified asphalthad the more similar changing trend, therefore, T800can be used to reflect high temperatureanti-rutting performance.Rotating thin-film oven test has been used to simulate the thermal oxidative agingprocess of CRMA, SBSMA and CR/SBSCMA for research the influence of matrix asphalt,CR and SBS on the aging performance of modified asphalt. According to the convetinalproperties and dynamic rheological characteristics change degree of asphalt binder in agingprocess, the anti-aging properties order was as follows: CRMA＞CR/SBSCMA＞SBSMA＞matrix asphalt. Thus, the added modifier improved the aging resistance of asphalt. Combiningthe change of performance indexes with composition and structure of matrix asphalt and threemodified asphalts, the thermal aging mechanism of CR/SBSCMA have been analyzed andsummarized. With different from the effect of modifier in CRMA and SBSMA, in the agingprocess CR/SBSCMA, due to the co-existence of CR and SBS, the condensation reactionwould be intensified between the degradation products of two modifiers with secondarycomponents of matrix asphalt, which resulted in the decrease of aromatics and resin content, and the increase of asphaltene and toluene insoluble content. Degradation products, secondarycomponents and condensation products changed the group components and colloidal structureof CR/SBSCMA, resulting in the changes of macro properties.The segregation behavior of modifier in CRMA, SBSMA and CR/SBSCMA has beenanalyzed, and it found that SBS could be a steady state in matrix asphalt, while the sinking ofCR particles was the main reason of the segregation of CRMA and CR/SBSCMA. Theinvolvement of SBS could delay the segregation of CR particles to some extent, but this effectwas not significant. The CR/SBS blends (CRM) was prepared by twin screw extruder toincrease the compatibility of CR and SBS. The experimental results indicated that themodifier blends can significantly improve the performance of composite modified asphalt.With the function of stabilizer, the SBS part of CRM and SBS main body could form theCRM—SBS—matrix asphalt connecting structure in CRM/SBSCMA, then the storagestability of composite modified asphalt can be improved. When the CRM amount as low as1.5%, the softening point difference after segregation test of CRM/SBSCMA were below2.5℃, but the sinkage of CRM particles still appeared. The research has shown that a certaininteraction between CRM and SBS may be formed in matrix asphalt, making the performanceof CRM/SBSCMA better than CR/SBSCMA, but this interaction was not powerful enough tostop the sinkage of CRM particles, resulting in the sinkage of CRM from top to bottom in thelong time storage. Therefore, it still needs to agitate to maintain CRM particles uniform andstable distribution in the storage and transportation process of CRM/SBSCMA.The swelling effect of crumb rubber in matrix asphalt has a critical impact on theperformance of CRMA and CR/SBSCMA. The swelling process of squarely thin sheet of tirerubber in asphalt components has been quantitatively researched in this thesis. And it foundthat there is a fast-growing process for the swelling of rubber. In a relatively short period oftime, the swelling degree quickly become large, and then gradually achieved swellingequilibrium. The swelling ratio could be enhanced by raising the swelling temperature, thenature rubber content of swelling rubber or the light oil fractions of swelling mixtures.Moreover, the compatibility between crumb rubber and asphalt could be improved based onthe sufficiently swelling effect of crumb rubber.The swelling kinetics of crumb rubber swelling process has been analyzed based onpolymer swelling theory and Crank single diffusion. It found that the swelling rate constant ofrubber and the diffusion coefficient of swelling mixture components increased with the raisedtemperature. Meanwhile, the swelling rate constant of rubber was larger in swelling mixturewith high content of light oil fractions, so was the diffusion coefficient of swelling mixture components. Therefore, raising swelling temperature and light oil content of swellingmixtures can accelerate the swelling of rubber. According to the range analysis, the influenceprecedence ordering of three factors in this study on diffusion coefficient was listed as follows:group component of swelling mixtures>swelling temperature>rubber type.