This report includes the results of testing Crumb Rubber Modified (CRM) asphalt to quantify the layer coefficient values for CRM mixes. Several test sections were selected from recently constructed CRM pavement. The Falling Weight Deflectometer (FWD) data were collected at 15 meter intervals on each test section during a one month period.

The objective of this project was to develop an asphalt mix design method incorporating crumb rubber and using the "Wet" or "Dry" method of producing Crumb Rubber Modified Asphalt (CRM). Several resurfacing projects have been constructed using both the "Wet" and "Dry" methods. Based on this study, the Kansas Department of Transportation could use CRM mixes with a binder content between 7.5% and 9.0% depending on the percent air voids, with 19% to 22% rubber content. In this study, it was observed that using 24% rubber produced mixes that were too sticky to manage. With a rubber content of less than 18% combined with AC-5 it was difficult to satisfy the minimum viscosity requirements. Fracture tests can be used as a basis to determine the optimum binder content for any asphalt-rubber mix.

The objective of this study is to evaluate the fatigue behavior of crumb-rubber modified asphalt. The research involved flexural fatigue testing of beams made with hot mix asphalt rubber using the wet process where the crumb rubber reacted with the asphalt at high temperature. Two types of asphalt rubber were studied: Asphalt Rubber Surface (ARS) and Asphalt Rubber Base (ARB). The ARS was a gap-graded aggregate mix with 0.5 in. (1.27 cm) maximum size and 7% design asphalt rubber content using CRM-III (minus #10 mesh) rubber. The ARB had a denser gradation with a 0.75 in. (3.81 cm) maximum aggregate size and 7% design asphalt rubber content also using CRM-III rubber.

Pavement layer coefficients had been identified as essential parameters to use the 1986/1993 AASHTO Guide for Design of Pavement Structures (AASHTO guide) in Rhode Island. Layer coefficients of granular subbase materials were estimated using the direct method recommended by AASHTO Guide. Resilient moduli were determined using a pneumatic testing system. Coefficients based on resilient moduli ranged from 0.08 to 0.14, and the average was 0.12 for eight subbase samples in Rhode Island. The pneumatic testing system was also used to determine resilient moduli of bound layeres. Based on results of a series of testing, layer coefficients were estimated using moduli at the loading magnitude of 20% of tensile strength and loading frequency of 0.5Hz. Recommended coefficients for wearing course (Class I-1), modified binder, and modified asphalt base were 0.40, 0.42, and 0.43, respectively. If all asphalt bound layers will be analyzed as one layer, the coefficient of 0.41 was recommended for pavement design and analysis in Rhode Island. Field study including FWD testing was performed to verify laboratory test results. Laboratory determined moduli were compared with backcalculated moduli based on FWD deflection data. Generally, FWD moduli are consistently higher than laboratory moduli with a wide range of variability. For the covering abstract of this conference, see IRRD number 863140.