Volume 33, Issue 2 (March 2005)
Quantification of Potentially Odorous Volatile Organic Compounds from Asphalt Binders Using Head-Space Gas Chromatography
A gas chromatograph (GC) methodology was developed that considers film thickness, aggregate surface area, and mix temperatures, since these variables are expected to influence the production of potentially odorous volatile organic compounds (VOCs). Fifteen grams of 3 mm diameter glass beads (used to simulate a single fine size aggregate) is placed in a standard GC vial, and then coated with 0.5 g of asphalt to produce a film thickness of about 11 µm. This is representative of a typical dense graded hot mix asphalt (HMA) film thickness. The sealed vials are stored in the headspace sampler oven at the desired test temperature for 1 h. During this time, the maximum agitation option on the automated headspace sampler is used to simulate the exposure of the aggregate surface to air when the mix is either mixed in the rotary drum dryer, loaded into the haul trucks, or transferred from the trucks into the paver.
This methodology was used to identify and quantify more than 80 VOC compounds found in the vial headspace at 170°C (338°F); this temperature is at the high end of typical temperatures for the majority of HMA production operations. Individual compounds were grouped based on common characteristics: alkanes, heterocyclics, phthalates, aromatics, polyaromatic hydrocarbons (PAH), sulfur-containing, chlorinated, and organic acids. Of these groups, the aromatic, sulfur-containing, heterocyclics, and organic acid compounds have the highest potential for generating odors sometimes associated with HMA production operations. Five pairs of asphalt binders (one with and one without odor complaints) were evaluated using this methodology. In all cases, the asphalt with the reported complaints had either more compounds that exceeded odor thresholds or higher concentrations, or both.