This project developed comprehensive specifications, practices, and test methods for the design and construction of Stone Matrix Asphalt (SMA).

SMA is defined as a gap-graded hot mix asphalt that maximizes both the asphalt binder content and the coarse aggregate fraction. It provides a stable stone-on-stone skeleton that is held together by a rich mixture of asphalt cement, filler, and stabilizing agent.

SMA was developed in Europe and is widely used in the United States in durable, rut-resistant, long-life pavement surface layers. The original SMA design approach in the United States was to duplicate SMA mixtures used in Europe, using domestic materials, equipment, and asphalt industry expertise. The FHWA, state transportation agencies, and the asphalt industry identified a need for better definition of the specified components and properties of SMA mixtures and for new mix design, testing, and construction procedures. In particular, better information was needed regarding the type and amount of stabilizer(s), the required compactive effort, and the mixture volumetrics for design and placement. New tests to measure properties such as durability, rut resistance, and moisture susceptibility were also needed.

The following specifications, practices, and test methods were developed in this research and submitted to the AASHTO Highway Subcommittee on Materials for consideration as provisional standards: a practice for designing SMA, a specification for designing SMA, a practice for constructing SMA, a method for determining draindown characteristics of uncompacted asphalt mixtures, a practice for testing hot mix asphalt (HMA) mortars, and a sample SMA mix design. NCHRP Report 425 contains a summary of the research results, a mix design method, construction guidelines, and QC/QA procedures for SMA. The entire five-volume final report is available on CD-ROM as CRP-CD-1: Bituminous Materials Research Series.