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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 Historical documents were reviewed, and industry experts from 16 states were interviewed to obtain relevant information on industry practices.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. [ABSTRACT FROM AUTHOR]

The U.S. asphalt paving industry, which includes both the refining and hot mix sectors, has evolved over time to meet various performance specifications for asphalt (known as bitumen outside the United States). Additives to asphalt produced in the refinery, as well as changes in temperatures, equipment used, and materials added in the hot mix asphalt process - which are likely to vary by state - may affect exposures to workers in the asphalt paving industry. The purpose of this investigation was to document the changes in the refining, manufacturing, and hot mix of paving asphalt in the United States since World War II. The approach taken for the investigation of both the refining and hot mix sectors was similar. Historical documents, as available, were reviewed and individual experts in each industry sector were interviewed to identify current and historic practices. In general, of the refining facilities surveyed, the number reporting use of any material within a particular class ranged from none to more than half the respondents. Sixteen states were included in the review of the hot mix sector. All states reported addition of materials to hot mix asphalt currently or historically, but concentrations were generally small (less than 10% per unit weight of asphalt). Changes to paving asphalt in the United States over the past 50 years appear to have occurred, but are generally not consistent by geography or over time. Modifications that have been made to asphalt were to improve performance, and were dictated by state specifications. [ABSTRACT FROM AUTHOR]