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.

This is the second of two reports that present fatigue cracking performance models for asphalt concrete overlays placed on existing asphalt concrete pavement. The models were developed from the pavement management system (PMS) database of the Washington State Department of Transportation (WSDOT). The database included existing pavement structure, overlay thickness and type, truck traffic, and observed percent of the wheelpath cracked from annual condition surveys. Climate data was developed by the UCPRC to augment the WSDOT data. This report presents a model for crack propagation, starting from crack initiation, which was defined as 5 percent of the wheelpath with longitudinal cracking. The combined initiation and propagation models were included in a spreadsheet calculator which was used to perform an analysis of the sensitivity of crack initiation and propagation to the input variables. The models are extremely useful for predicting pavement performance. For use in California they will need recalibration of the coefficients to reflect differences in WSDOT and California practice, primarily the use of thicker overlays in California, placement of overlays at much more advanced states of cracking in the existing pavement, and possible differences in routine maintenance activities.

In the United States, the Superpave Asphalt Binder Performance Grading (PG) system proposed by the Strategic Highway Research Program (SHRP) is the most common method used to characterize the performance-related properties of conventional and polymer-modified asphalt binders. Dynamic modulus (G*) and phase angle (δ) are the two main binder properties and they are measured using a dynamic shear rheometer (DSR) with parallel plate geometry and either a 1 mm or 2 mm gap between the plates. Since these Superpave parameters were developed for binders that do not contain additives or particulates, the California Department of Transportation (Caltrans) does not use them as asphalt rubber binder specification criteria. Instead, penetration and viscosity are used as acceptance of quality control; however, these parameters do not necessarily provide a satisfactory link between the measured binder properties and potential performance in the field over a range of operating temperatures. In California, current specifications require that crumb rubber particles used to produce asphalt rubber binder in the “wet process” must be smaller than 2.36 mm (i.e., 100 percent passing the #8 sieve), and typically these particles vary in size between 1 mm and 2 mm. As a consequence, when the parallel plate geometry is used to test this type of binder, the larger rubber particles can contact the plates; if this occurs, the rubber particle rheology can potentially dominate the results, which in turn may not be representative of the modified binder as a whole. To address this problem, a potentially more appropriate DSR testing protocol using concentric cylinder geometry was investigated in this study to explore an alternative means of determining the performance properties of asphalt rubber binders. In the first phase of the study, documented in this technical memorandum, a series of tests were undertaken to compare the two geometries and to assess which binder properties influence the results from the testing approaches. The interim results indicate that there is no significant difference between the concentric cylinder and parallel plate geometries in terms of the G*/sinδ after testing on a range of different binders and asphalt rubber binders with finer crumb rubber particle sizes (i.e.  <250 μm). However, the correlations between results from the geometries were increasingly weaker with increasing crumb rubber particle size, indicating some potential influence of larger sizes on the results of the testing using parallel plates. The concentric cylinder geometry resulted in relatively lower values of G*/sinδ compared to samples tested with the parallel plate geometry. This difference is provisionally attributed to large rubber particles touching both plates, and to edge effects issues. The proposed alternative approach to measuring the rheological properties of asphalt rubber binder is considered feasible, and that with its use the edge effect and trimming issues can be eliminated. However, the concentric cylinder method requires a longer testing time and a larger binder sample than the parallel plate test method. The testing will be continued to develop proposed revised quality control procedures for testing asphalt rubber binders used on Caltrans projects. ​