A Review of Changes in Composition of Hot Mix Asphalt in the United States

Abstract

This review researched the materials, methods, and practices in the hot mix asphalt industry that might impact future exposure assessments and epidemiologic research on road paving workers. Since World War II, the U.S. interstate highway system, increased traffic volume, transportation speeds, and vehicle axle loads have necessitated an increase in demand for hot mix asphalt for road construction and maintenance, while requiring a consistent road paving product that meets state-specific physical performance specifications. We reviewed typical practices in hot mix asphalt paving in the United States to understand the extent to which materials are and have been added to hot mix asphalt to meet specifications and how changes in practices and technology could affect evaluation of worker exposures for future research. Historical documents were reviewed, and industry experts from 16 states were interviewed to obtain relevant information on industry practices. Participants from all states reported additive use, with most being less than 2% by weight. Crumb rubber and recycled asphalt pavement were added in concentrations approximately 10% per unit weight of the mix. The most frequently added materials included polymers and anti-stripping agents. Crumb rubber, sulfur, asbestos, roofing shingles, slag, or fly ash have been used in limited amounts for short periods of time or in limited geographic areas. No state reported using coal tar as an additive to hot mix asphalt or as a binder alternative in hot mix pavements for high-volume road construction. Coal tar may be present in recycled asphalt pavement from historical use, which would need to be considered in future exposure assessments of pavers. Changes in hot mix asphalt production and laydown emission control equipment have been universally implemented over time as the technology has become available to reduce potential worker exposures. This work is a companion review to a study undertaken in the petroleum refining sector that investigated current and historical use of additives in producing petroleum-derived asphalt cements.

Keywords:

• asphalt
• bitumen
• historical
• hot mix
• occupational exposures
• paving

ACKNOWLEDGMENTS

The authors acknowledge support for this work provided by the Asphalt Paving Environmental Council (APEC), which includes the Asphalt Institute, the National Asphalt Pavement Association, and the State Asphalt Paving Association Executives. In addition, we would like to acknowledge participating paving contractors, SAPA representatives, state DOT representatives, and the faculty of the National Center for Asphalt Technology (NCAT) for their assistance and cooperation in completing our interviews.

Notes

^A The Rubber Pavements Association representative provided information for rubber asphalt companies in Arizona, California, and Texas.

^B In Kansas, one individual was interviewed: current SAPA representative, retired DOT.

^C One retired.

^D Two retired.

^A May include employees of non-HMA related work.

^B Allocation of work was not specified by two contractors (Texas and California); one contractor (Kentucky) did not distinguish between federal, state, and city work—only private values included here; contractors in Ohio did not distinguish between federal and state work—only private and city values included here; for two contractors (Virginia and Ohio), total tonnage was not available, and allocation was provided only as %.

^A Includes current or historical use; includes experimental or regular use.

^B “Infrequent” when reported as, e.g., “not routine,” “rare,” “a couple of jobs,” “a few small projects,” “test sections.”

^C Does not include experimental or “infrequent” use.

^D Not specifically asked about but more than one state reported use.

^E Not applicable or not specified.

^F Imidazoline and acids were included in the survey as possible anti-strip, but no state reported use of either; “Other” reported anti-strips used included latex, silicone, and foundry sand.

^G Nickel or copper slag and Boiler ash were asked about, but no state reported ever using these products. Silica fume was asked about, but only one state reported ever using this product—experimentally only.

^H Two states reporting use of “Silica” referred to it as “Natural sand”; one state reported “Steel slag” and “Blast furnace slag” as the same product; one state reported “Polyproylene” and “Polyester” as the same product; two states reported “Mineral fibers” and “Rock wool” as the same product.

^I Does not include use as an anti-strip agent.