Permeability Testing on Dense-Graded Hot Mix Asphalt (HMA) and Gap-Graded Rubberized Hot Mix Asphalt (RHMA-G) Surfaces

Falling head permeability tests were conducted on seven projects, four with dense-graded hot mix asphalt surfaces and three with gap-graded rubberized hot mix asphalt surfaces. Tests were conducted between the two wheelpaths and in the right wheelpath, and in both directions of traffic. Averaging all the test results shows that the permeability of RHMA-G is much greater than that of HMA: 2.9E-3 cm/sec versus 6.1E-4 cm/sec, respectively. However, one of the three projects with a gap-graded rubberized hot mix asphalt surface had permeabilities not consistent with the other two. When data from that gap-graded rubberized hot mix asphalt surface was removed, the average value for the surface permeability dropped over 80 percent, to 4.2E-4 cm/sec. This average is lower than the average permeability for dense-graded hot mix asphalt, and the data remaining from the six projects—four dense-graded and two gap-graded—showed no statistical difference in the average surface permeability of HMA and RHMA-G when tested at the 95% confidence level. On four of the seven projects, including all three of the RHMA-G sections, testing in both directions and in both wheelpaths locations showed no statistical difference in surface permeability. But, one HMA project showed dissimilarity in both directions and in the wheelpaths. Five pairs of cores from this project were then tested for permeability and specific gravity in the laboratory. Unfortunately, no conclusions could be drawn due to a lack of spatial information; however one of the five locations had very little variability between core pairs in either specific gravity or permeability, while another location had significant differences between the specific gravity and permeability measurements. When data from this dense-graded hot mix asphalt surface were removed along with the data from the one gap-graded rubberized hot mix asphalt surface with very high permeability, the remaining five projects—three dense-graded and two gap-graded—again showed no statistical difference in the average surface permeability of HMA (3.3E-4 cm/sec) and RHMA-G (4.2E-4 cm/sec) when tested at the 95% confidence level. It is recommended that testing of more RHMA-G mixes may be warranted to see if the high permeability on one mix in this study is unique. Construction QC/QA should also be examined to see if that mix met compaction specifications.