Development of a Standard Test Method for Characterization of Asphalt Modification
In the last few decades, due to heavier trucks and increased volume, use of modifiers in asphalt binders increased significantly. Current Superpave Performance Grading (PG) system (AASHTO M320) was developed more than two decades ago primarily using unmodified asphalt cements. In AASHTO M320, high temperature binder grading is based on dynamic shear rheometer (DSR) tests performed within linear visco-elastic region, whereas, modified binders and failures such as rutting are engaged in non-linear region. Therefore, how PG tests and specifications reflect performance of modified binders remain largely unknown to manufactures and users. Some states use PG Plus specifications for modified binders. PG Plus tests are mostly empirical and specifications and even the test methods are often inconsistent across the states. Recently, Multiple Stress Creep Recovery (MSCR) test (AASHTO TP70) based PG system (AASHTO MP19) has been introduced. Only the high temperature binder grading requires MSCR tests and no signification changes have been made in low temperature binder grading of MSCR based PG system. To this end, this study has been initiated to (a) characterize modified asphalt binders in relation to their aging-related degradation; and (b) develop a new standard and specification to supplement the knowledge gap in the performance grade (PG) system for modified asphalt binders. In the proposed study, an extensional rheometer fixture will be used in a DSR platform to perform a suite of tests. Elongation tests and fracture tests will be performed using the extensional rheometer varying sample geometry, temperature and strain rate. To replace ductility test (AASHTO T51), final strain in elongation test will be analyzed along with other parameters. Second force peak of force ductility test (AASHTO T300) will be compared to second force peak in elongation test. A novel and direct low temperature cracking susceptibility test will be developed by analyzing fracture strength and fracture energy parameters in fracture test using extensional rheometer. Extensional stress relaxation modulus and strain hardening will be investigated to determine the influence of polymer microstructures such as linear and radial styrene-butadiene-styrene (SBS). The proposed study will also investigate and quantify (a) the degradation of modifiers with aging and (b) how polymers/modifiers influence aging susceptibility. A standard test method and corresponding specifications will be developed for immediate implementation.
Principal Investigator: Wasiuddin, Nazimuddin -- Civil Engineering
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