Evaluation of the field permeability of HMA using the NCAT tiered permeameter

The increasing levels of traffic and vehicle wheel loads that are continuously encountered by roadways has demanded the combined effort of experts in the Hot Mix Asphalt (HMA) industry to better accommodate the techniques used in designing and evaluating HMA roadways. The combined effort of these experts produced the Superior Performing Asphalt Pavement (Superpave^™) mix design and analysis system. This paper presents the results of a continuing study initiated to evaluate the field permeability of HMA mixes placed on the roadway in Tennessee. Four Superpave^™ designed surface mixes constructed in 2000, five Marshall designed surfaces mixes constructed in 2000, three Superpave^™, and three Marshall designed surface mixes from 1999 were evaluated for this study. Along with the surface mixes, two Superpave^™ binder mixes and one Marshall designed binder mix were also added to the analysis. Loose HMA was collected along with roadway cores from each project and transported to the University of Tennessee Asphalt Laboratory where further testing was performed. The instrumentation used for this study was a tiered field permeameter, which was developed at the National Center for Asphalt Technology (NCAT). The objectives of this study were to document the permeability of HMA in Tennessee, and to evaluate the merits of using the NCAT tiered permeameter (see Figure 1) in estimating field permeability of HMA. The NCAT tiered permeameter was used to develop a correlation between permeability, and in-place density (%G_mm) on the roadway. The permeability of Superpave^™ designed mixes was compared with that of the conventional 75-blow Marshall designed mixes. In addition, 3 binder mixes (2 Superpave^™ and 1 Marshall) were tested to quantify the relative difference in permeability and its reaction to change in density. The HMA surface mix permeability data were also compared with time effects and test sub-section location to achieve a better understanding of how permeability changes over time and location. The results presented within this paper confirm that a good relationship exists between field permeability and core density (%G_mm). The field permeability data obtained using the NCAT tiered permeameter were found to vary over a large range, thus making it difficult to obtain a precise value of permeability. Further, it was found that the nuclear density (%G_mm) data obtained from selected construction projects is consistently higher than actual (%G_mm) measured for cores taken from the roadway. The findings of this study document and help explain many problems associated with in-place density for HMA roadways in Tennessee. Recommendations and conclusions from this study should aid in directing more attention to achieving adequate field density on HMA roadways during construction.