Warm-Mix Asphalt Study: Evaluation of Rubberized Hot- and Warm-Mix Asphalt with Respect to Emissions

In recent years, warm mix asphalt (WMA) technologies have been used to modify asphalt binders, with the aims of decreasing production and construction temperatures, reducing fumes and emissions, retaining the properties of the mix during long haul distances, and improving workability. Reduced production temperatures at the plant and during paving and compaction are believed to reduce emissions from the asphalt. The objective of this research study was to quantify the potential environmental benefits during paving operations with respect to the reduction of volatile and semi-volatile organic compounds and polycyclic aromatic hydrocarbons to confirm or refute this hypothesis. Asphalt plant stack emissions were not assessed as part of this study. A portable “flux” chamber was designed and fabricated to capture and directly measure emissions during paving operations. An analytical method was developed to identify and quantify alkane emissions using gas chromatography mass spectrometry (GC-MS). A separate method was developed for identification and characterization of trace level polycyclic aromatic hydrocarbon compounds in the asphalt fumes. The results demonstrated that the warm-mix asphalt technology type, plant mixing temperature, and level of compaction significantly influence emissions characteristics throughout paving operations. Depending on these variables, most warm-mix technologies appear to have potential for reducing emissions during transport and paving operations. Key findings from the study include: The majority of the reactive organic gases are volatilized in the first hour after sampling initiation. There is a significant difference between emissions concentrations from loose mix and those recorded immediately after compaction. Gaseous phase PAH compounds in asphalt fumes are mainly low molecular weight compounds and are present at trace levels. The concentrations vary depending on the temperature of the mix at the time of sampling. Particulate phase PAHs were below the detection limit of this study for all the mixes. The results confirmed that typical asphalt production temperatures are not high enough to initiate significant PAH formation. The total alkane emissions from the warm-mixes were, in most instances, significantly lower than those measured from the hot-mixes (e.g., 117 µg/m3 from one of the warm-mixes compared to 2,516 µg/m3 from the hot-mix control). In some instances, specific warm-mixes had higher alkane concentrations than the hot-mix controls. Consequently, any generalization with regard to emissions reduction through the use of warm-mix asphalt is inappropriate. PAH concentrations correlated with initial mix production temperature, with those warm-mixes produced at the lowest temperatures showing the lowest PAH concentrations.