Premature rutting in SMA?

VDOT has identified a limited number of Stone Matrix Asphalt (SMA) 12.5 sections exhibiting rutting around Virginia and has conducted forensic investigations. As a result, the VDOT-VAA Statewide Asphalt Cooperative Committee formed a subcommittee to review the field and lab data collected by VDOT and make recommendations. A kickoff meeting of the subcommittee was held on July 18. VAA members and staff, along with VDOT representatives, are on the subcommittee. VDOT presented findings related to several sections in Fredericksburg District that have exhibited rutting as well as related pavement management data for the six VDOT districts utilizing SMA. Data from several sections was presented with findings related to in-place density, asphalt binder grade, and underlying layer material condition. The committee discussed some primary factors that define what entails an SMA mix, from stone on stone contact to full gradation control. Past research on rutting of SMA-9.5 mixes showed the need for adjustments to gradation bands that ultimately led to specification changes. With the SMA-9.5, the mixes met the VCA requirements but still deformed under loading. Therefore, the 3/8” and #4 sieves were modified to 65-75% passing and 25-32% passing, respectively. These changes, along with proper binder selection and compaction in the field, virtually eliminated rutting of the mix. Fortunately, there is much production data available from both VDOT and Contractor databases to assist in further analysis of the SMA-12.5 mixes.

At the conclusion of the meeting, the subcommittee decided to recommend the Statewide Asphalt Co-Op Committee request the Virginia Transportation Research Council to establish a research project to investigate the issues surrounding the rutting issue. Look for more updates in future Asphalt News articles.

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