University of California Pavement Research Center

This technical memorandum summarizes a literature review update, elements of the construction of a test track to assess various aspects of gap-graded rubberized asphalt concrete (RHMA-G) mixes with and without the addition of reclaimed asphalt pavement (RAP) as aggregate replacement, and a first-level analysis of the results from the first three Heavy Vehicle Simulator (HVS) tests.

Four different RHMA-G mixes were placed on seven sections on the test track at the UCPRC. Mixes differed by nominal maximum aggregate size (NMAS, 1/2 and 3/4 in.) and the addition of 10% RAP by weight of the aggregate as an aggregate replacement. Single and double lifts of each mix were placed. Apart from the addition of RAP, the mix designs all met current Caltrans specifications. Although Caltrans currently does not permit more than one lift of RHMA-G on projects, the placement of each lift of each mix on the test track met current Caltrans specifications for RHMA-G layers.

The first three HVS tests discussed in this technical memorandum covered the control section (0.2 ft. [60 mm], 1/2 in. NMAS with no RAP), a section with a single lift of 1/2 in. mix with RAP, and a section with two lifts of a 3/4 in. mix with RAP. Results from these first three HVS tests, which focused on rutting performance, indicated the following:

• Performance of all three mixes was satisfactory in terms of the level of trafficking required to reach a terminal average maximum rut of 0.5 in. (12.5 mm).

• The addition of RAP as a coarse aggregate replacement did not appear to have a significant influence on the test results.

• The back calculated stiffnesses of the RHMA-G layer(s) on each section before and after HVS testing indicate that the trafficking did not cause any significant damage (i.e., loss in stiffness) in any of the three test sections. Stiffnesses increased after trafficking on two of the three sections, which was attributed to a combination of aging and densification of the layers under traffic. Some blending of reclaimed asphalt binder with the asphalt rubber binder over time on these two sections, both containing RAP, may have contributed to this stiffness increase.

• No cracks were observed on any of the sections after trafficking.

Given that only three sections have been tested to date, no recommendations on RHMA-G layer thicknesses or permitting the use of coarse RAP in RHMA-G mixes can be made at this time. These recommendations will be made after all the sections have been tested and the forensic investigations and associated laboratory testing have been completed.