Asphalt paving consist of a mix of aggregate, sand, filler, bitumen and occasionally a number of additives. Asphalt road surfaces are, thus, composed of compacted aggregate and bitumen binder. Hot Mix Asphalt (HMA) is by far the most widely used, generally over 80 percent, and produces very few emissions (EAPA, 2003). Other types of road paving include cutback asphalt and emulsified asphalt, which are both liquefied asphalts (EEA, 2005). Cutback asphalts are liquefied by blending with petroleum solvents (diluents such as heavy residual oils, kerosene or naphtha solvents) and therefore show a relatively high level of emissions of CO and NMVOC due to the evaporation of the diluent. Therefore most emissions from road paving will arise from the use of cutback asphalts. Depending on the evaporation rate, three types are distinguished: Rapid-Cure (RC), using a naphtha or gasoline-type diluent of high volatility, Medium-Cure (MC) using a diluent of medium volatility and Slow-Cure (SC) cutback asphalt which use oils of low volatility. This is in contrast to so-called emulsified asphalt that contains mostly water and little or no solvent. The amount of diluent used is usually lower in warm countries than in the cooler climates, and hence lower emission factors may be expected in warm countries.
Activity data for hot mix asphalt and production of cold mixes or 'modified asphalt' can be obtained for most European and several other industrialised countries from the European Asphalt Pavement Association (EAPA) or national paving and roofing associations such as the Asphalt Institute (EAPA, 2003; Asphalt Institute, 2004). Hot mix asphalt typically contains about 8 percent asphalt cement (bitumen) (EEA, 2005), but this may differ between countries (a figure of 5 percent has also been reported). For most industrialised countries the fraction of cutback asphalt is a few per cent, however several show shares of 5 percent to 12 percent, and exceptional shares up to 20 percent, or have none (EAPA, 2002; EAPA 2003; U.S. EPA, 2004). If the quantity of asphalt paved is not known but rather the area paved, a conversion factor of 100 kg asphalt/m2 road surface may be used to calculate the mass of asphalt produced.
Gases are emitted from the asphalt plant (hot mix, cutback or emulsified), the road surfacing operations and subsequently by the road surface. The EMEP/CORINAIR Emission Inventory Guidebook provided process-specific uncontrolled emission factors for the different asphalt plants.
The asphalt roofing industry produces saturated felt, roofing and siding shingles, roll roofing and sidings: asphalt shingles, smooth surfaced organic and asbestos felt roll roofing, mineral surfaced organic and asbestos felt roll roofing and sidings, asphalt saturated organic and asbestos felts, asphalt saturated and/or coated sheeting and asphalt compound. Most of these products are used in roofing and other building applications. Asphalt felt, roofing and shingle manufacture involves the saturation or coating of felt. Key steps in the total process include asphalt storage, asphalt blowing, felt saturation, coating and mineral surfacing, of which asphalt blowing is included here. Direct greenhouse gas emissions from asphalt roofing products are negligible compared to emissions such as NMVOC, CO and particulate matter.
Asphalt blowing is the process of polymerising and stabilising asphalt to improve its weathering characteristics. Air blown asphalts are used in the production of asphalt roofing products. Blowing may take place in an asphalt processing plant or an asphalt roofing plant (or in a refinery)3. Asphalt blowing leads to the highest emissions of NMVOC and CO, more than the other process steps. All asphalt used for non-paving applications has been blown (EEA, 2005).
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