Estimation of hot mix asphalt concrete rutting potential from asphalt binder tests
In recent decades, the Strategic Highway Research Program (SHRP) suggested new performance based specifications for asphalt binders known as Performance Grade (PG) Specifications which suggest performing tests at the service temperature rather than a set temperature. To meet these new PG-specifications, manufacturers either altered manufacturing practices like air blown asphalt, or added modifiers like polymers. In general, the addition of modifiers improved the performance of hot mix asphalt concrete (HMAC), while the air blown or acid modified asphalt decreased the durability of the mixes. In general, the new performance tests are unable to differentiate between polymer modified asphalts and acid modified asphalt. To overcome this problem, the elastic recovery test has been specified by the highway agencies because it can differentiate between asphalt binder consisting of modifier and acid modified asphalt binders. However, the discussions with the asphalt producers identified that the asphalt modified with Styrene Butadiene Rubber (SBR) does not pass elastic recovery test but performs well in the field. In addition, the test does not provide any fundamental property of asphalt binder. Recently, a new test called repeated creep test has been proposed by the Federal Highway Administration (FHWA) to measure the fundamental properties of asphalt binders and can be used to identify the presence of modifiers. Therefore, the objective of this study was to evaluate the capability of the repeated creep test in identifying the presence of modifiers. Typically, rutting is one of the major causes of premature failure of HMAC, especially within the state of Texas. To identify rutting potential of hot mix asphalt concrete, the Hamburg Wheel Tracking Device (HWTD) test has been specified by the Texas Department of Transportation (TxDOT). If HWTD test results identify a mix to be rut susceptible, the test results do not necessarily indicate whether the mix design and/or asphalt binder is the major factor responsible for rutting potential. It may be possible that the repeated creep test results can identify the rutting potential; thus, identifying whether the asphalt binder is the contributing factor or not. Therefore, another objective of this study was to identify rutting potential of a mix consisting of rut prone asphalt binder using repeated creep tests. If a correlation can be established between the properties of the asphalt binders used in the HMAC and the rutting potential displayed in laboratory testing, the premature failure can be minimized. To perform this study two mix designs used by the Texas Department of Transportation (TxDOT), Type-D and Coarse Matrix High Binder (CMHB-C), were selected. The Type-D mix was obtained from the Austin District and the CMHB-C from the Bryan District. Both of the chosen mixes have shown success in the field and have recently been placed using modifiers. The modifiers assessed in this study include, Styrene-Butadiene-Styrene (SBS), Styrene-Butadiene-Rubber 9SBR), Tire Rubber (TR), and Elvaloy. Since the mix types are rut resistant and mix design are the same, the major factor contributing towards rutting of HMAC can be traced to the asphalt binders which is the only component that is altered between the mixes. To validate test results, two additional mixes (CMHB-C and Superpave from El Paso District) consisting of one binder type were evaluated. The test results suggested that the presence of modifiers can be identified using repeated creep tests and can be used to identify the rutting potential of HMAC. In addition, a relationship between mix types and accumulated strains (from repeated creep tests) has been proposed. The developed relationships were further evaluated using different mix designs not evaluated in the original study. The test results suggest that repeated creep test results have the potential of being used as the test to identify rutting potential of HMAC.
Hrdlicka, Gina M, "Estimation of hot mix asphalt concrete rutting potential from asphalt binder tests" (2006). ETD Collection for University of Texas, El Paso. AAI1439467.