University of California Pavement Research Center

This research report summarizes a literature review update, 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, a first-level analysis of the results from five Heavy Vehicle Simulator (HVS) tests, and a first-level analysis of laboratory test results on the four mixes.

Four different RHMA-G mixes were placed on seven cells 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 five HVS tests discussed in this report covered the control section (0.2 ft. [60 mm], 1/2 in. NMAS with no RAP), a section with two lifts (0.4 ft. [120 mm]) of the 1/2 in. mix with no RAP, a section with one lift of 1/2 in. mix with RAP, a section with one lift of 3/4 in. mix with no RAP, and a section with two lifts (0.5 ft. [150 mm]) of 3/4 in. mix with RAP. The untested sections included a section with two lifts of 1/2 in. mix with RAP, and a section with two lifts of 3/4 in. mix with no RAP. Findings include the following:

• Performance of all four 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).• Differences in nominal maximum aggregate size and/or the addition of RAP as a coarse aggregate replacement did not appear to have any significant influence on the HVS and laboratory test results. The time that the mix was stored in the silo and the interval between construction and start of HVS testing (i.e., degree of aging of the RHMA-G) appeared to have a larger influence on results.• The backcalculated stiffnesses of the RHMA-G layer on each section before and after HVS testing indicate that trafficking generally caused some damage on the sections, as expected. An exception to this observation was noted on the first test, which was attributed in part to stiffening of the mix through diffusion of small amounts of RAP binder, which possibly countered the effect of damage by trafficking, and potentially in part to the method used for backcalculation.• A hydraulic oil spill on one of the sections had a notable negative effect on rutting performance.• No cracks or other distresses were observed on any of the sections after trafficking.

Recommendations, if justified, for changes to limits for nominal maximum aggregate size in relation to RHMA-G lift thickness, RHMA-G lift thickness, whether more than one RHMA-G lift can be considered in pavement designs, and the use of RAP as aggregate replacement in RHMA-G mixes will be made in a separate report that documents, second-level analysis, and mechanistic simulations.