Hot-Recycling of Tar-Containing Asphalt Pavements

ABSTRACT Hot recycling in pavement construction leads to the emission of hazardous compounds when tar-containing recycled asphalt pavement (RAP) is used. This is due to the relatively high content of substances such as phenols and polycyclic aromatic hydrocarbons (PAH) in tar. The latter are of special interest, because some PAHs have carcinogenic and/or mutagenic properties. High tar content in combination with high pavement temperatures during construction increases the potential health risk for workmen. Switzerland does not currently consider a complete ban of RAP in hot recycling for economical and ecological reasons. Since RAP might always contain some tar, there is a need to fix a scientifically sound limit for the maximum tar content. However, scientific data on the emissions of PAH, as well as the resulting occupational health risk are rare, which motivated the present research. In the laboratory, several parameters, such as temperature, PAH content and viscosity, were varied in an experimental setup to quantify their effect on the resulting emissions. It was shown that temperature is the major factor determining the emissions in the 140 to 240°C range. A PAH content of up to 20000 ppm EPA-PAH in RAP had no significant effect on total particulate matter (TPM) emissions. However, emissions of PAH in the fumes increased proportionally with the PAH concentration in the RAP. Three field tests were carried out with RAP containing up to 5600 ppm EPA-PAH. Occupational health data for the workmen were collected with personal samplers. TPM concentrations were between 2–4 mg/m3, close to the occupational health limit value for bitumen fumes (10 mg/m3). Concentrations of Benzo(a)pyrene (BaP), which is often used as a lead compound, were in all cases below the occupational health limit value of 2 μg/m3 by at least a factor of three. This study indicates that the current Swiss recommended maximum value of 5000 ppm for EPA-PAH in the binder of RAP may be considered as adequate. However, conditions in the field trials did not include a worst-case scenario and the margin to occupational health limit values is small.