United States Department of Transportation
i
Understanding The Performance of Modified Asphalt Binders in Mixtures: Low-Temperature Properties
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2002-10-01
[PDF-1.01 MB]
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Edition:Final Report October 2000 - December 2001
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Abstract:The objective of this study was to determine if the Superpave low-temperature rheological properties of polymer-modified asphalt binders correlate to asphalt mixture low-temperature resistance as measured by the Thermal Stress Restrained Specimen Test (TSRST). An emphasis was placed on evaluating polymer-modified asphalt binders with identical (as close as possible) low-temperature grades. This would indicate what types of modification provide properties that are, or are not, correctly captured by the current Superpave asphalt binder specification. Eleven asphalt binders were obtained for this study: two unmodified asphalt binders, an air-blown asphalt binder, and eight polymer-modified asphalt binders. All asphalt binders were tested with a diabase aggregate. Four asphalt binders were also tested using a limestone aggregate, a granite aggregate, and the granite aggregate treated with hydrated lime. Four asphalt binders were used in a study to determine the effect of the mixture short-term oven aging (STOA) period on low-temperature cracking resistance. The correlations between the TSRST fracture temperatures and asphalt binder cracking resistance based on the critical cracking temperature (Tcr), bending beam rheometer (BBR) creep stiffness, BBR m-value, and the BBR limiting temperature, were poor to weak. However, the correlation using Tcr was good after eliminating the data for ESI. The r-squared increased from 0.54 to 0.85. Aggregate type generally had no significant effect on the average TSRST fracture temperature. The effect was only significant in three cases involving hydrated lime. Elvaloy with granite had a significantly higher (poorer) fracture temperature compared to Elvaloy with diabase, limestone, and the granite aggregate treated with hydrated lime. This means that adding hydrated lime to the granite aggregate was beneficial. Based on the average fracture temperatures, the inclusion of lime provided no benefit for the mixtures with the three other asphalt binders used in this part of the study. In fact, it increased the average fracture temperatures of two mixtures. The variability of the TSRST fracture temperatures from replicate to replicate specimen was generally higher for the granite aggregate compared to diabase and limestone, but the addition of hydrated lime to the granite aggregate tended to reduce this variability. Initially, mixtures with ESI, Elvaloy, and SBS Radial Grafted had lower TSRST fracture temperatures than the mixture with the unmodified PG 70-22 asphalt binder. However, increasing the STOA period from 2 hours to 24 hours aged the polymer-modified asphalt binders, but not the PG 70-22 asphalt binder. After 24 hours, all four mixtures had fracture temperatures that were not significantly different. The use of softer asphalt binders when formulating the polymer-modified asphalt binders may have led to hardening from a loss of volatiles during STOA, while volatilization for the PG 70-22 asphalt binder was low.
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