Volume 26, Issue 4 (July 1998)
Asphalt-Rubber Mixture Behavior and Design (Wet Process)
The use of natural and synthetic rubber for improving the properties of conventional asphalt materials has been investigated since the 1960s. Due to the additional complexity in understanding, designing, and producing rubber-modified binders and mixtures, high production cost, and in some cases conflicting reports regarding their performance, the development and use of these mixtures has been limited over the years. Recently, due to the provisions of the 1991 Intermodal Surface Transportation Efficiency Act (ISTEA) on the use of tire rubber in federally funded projects, several laboratory and field studies were undertaken.
This paper describes the laboratory results of asphalt-rubber binder and mixtures characterization with materials used in New Jersay pavement projects. Specifically, the reaction curve (i.e., viscosity versus time) and the results of traditional binder tests are presented. The effects of oxidation and aging during binder and mixture preparation and laydown operations were also examined. The modified mixtures were designed according to Federal Highway Administration (FHWA) recommendations for dense-graded rubber-modified mixtures (wet process) and compared with conventional asphalt mixtures. While the overall objective of this investigation is the development of an improved mix design methodology, using the Strategic Highway Research Program (SHRP) recommendations for dynamic modulus testing, this paper presents the results from the initial evaluation of the mixtures. During this phase the possibility was considered of enhancing the traditional Marshall mix design with mixture behavior parameters since, first, a recent FHWA study concluded that the Marshall method provides saccessful designs for dense-graded rubber-modified mixtures, and, accond, many of the highway agencies are still using this method (and are expected to continue until the acquisition and full acceptance of the SHRP equipment in their routine operations). Thus, stiffness, toughness, and energy absorbed were evaluated for potential integration into the existing design methodology and are presented herein. The results from the tensile strength and resilient modulus evaluation are presented.