Dense graded hot mix asphalt (HMA) mixtures are designed to have low permeability to resist excessive penetration of water and avoid durability problems. With the introduction of Superpave mixes, ...there is a general concern that the coarse graded mixes are more permeable, at similar void levels, compared to fine graded mixes. However, at present there is a lack of a simple tool for measuring the in-place permeability of asphalt mixes. A simple permeability test was developed to determine the effect of voids and gradation on permeability. The field permeameter was used for testing projects with 9.5 mm, fine and coarse, 12.5 mm coarse, 19 mm coarse, and 25 mm coarse graded mixes. Testing of cores taken from location of field testing were also conducted in the laboratory. The results from the in-place permeability tests were found to be consistent with experience with fine and coarse graded mixes. The results indicated that mixes with different gradations and nominal maximum aggregate size have significant increase in permeability at different voids in total mix content. Field testing showed that 25 mm coarse, 19 mm coarse, 12.5 mm coarse and 9.5 mm coarse mixes show significant increase in permeability at 5, 6, 7 and 8 percent voids in total mix respectively, whereas a 9.5 mm fine mix showed a significant increase in permeability above eight percent voids in total mix. Field permeability of 19 mm and 25 mm coarse graded mixes are significantly higher than laboratory permeability, at similar voids in total mix content, most likely due to presence of horizontal flowpaths and high horizontal permeability. It is recommended that field permeability tests be conducted for all mixes, in order to get the best indication of permeability of these mixes.
The aggregate specifications for Superpave designed hot mix asphalt (HMA) mixtures includes a restricted zone which lies along the maximum density line, through which gradations were recommended not ...to pass. This research project was undertaken to determine under what conditions, if any, compliance with the restricted zone requirement is necessary when HMA meets all other Superpave requirements such as fine aggregate angularity (FAA) and volumetric criteria. The following factors were evaluated: two coarse aggregates, ten fine aggregates, two nominal maximum aggregate sizes (9.5 and 19.0 mm), five aggregate gradations, and three compactive efforts. Of the five gradations utilized, three passed through the restricted zone and two passed outside the restricted zone (control). Three mechanical tests were used to evaluate the effect of the five gradations on permanent deformation of the HMA mixes. Based upon the analysis of test data, mixes with gradations passing through the restricted zone were not necessarily more susceptible to rutting than mixes with gradations outside the restricted zone. Based on the test results and analyses, the deletion of the restricted zone as a guideline or requirement in the Superpave mix design system is recommended.
