Effect of pavement thickness on Superpave mix permeability and density

Abstract

This research study was conducted to determine the influence of maximum aggregate size, lift thickness, and aggregate source on the density and permeability of asphalt mixtures designed according to the Superpave criteria. The project included results of 16 mixes used on 9 field projects in which critical variables affecting the density and permeability of hot mix asphalt (HMA) were varied. The in-place density and field permeability were measured using the nuclear gauge and the NCAT field permeability device, respectively. Field cores were taken from completed field pavements for measuring permeability in the laboratory by using the ASTM D5084 method. In addition laboratory compaction was used to prepare and test samples in the laboratory from loose mixtures recovered from the field. The results from field study indicate that density and permeability of Superpave mixes are affected by project-specific variables including gradation, base type, source, and Ndesign level. For fine-graded mixes the t/NMAS ratio showed a lower density particularly below a ratio of 2 for gravel-source mixes and a ratio of 3 for limestone-source mixes. No clear relationship was found, however, between t/NMAS ratios and permeability for fine-graded mixes. For coarse-graded mixes no specific trends for effect on density were found but there were effects on permeability. Low t/NMAS ratios for limestone-source were found to increase permeability, but no trend could be found for the gravel-source mix. It was also found that there is a correlation between aggregate gradation and permeability. As the ratio of (%P1/2-%P3/8)/(%PNo.4-%PNo.8) increases, the permeability decreases, and as the gaps between the coarse aggregates (%P1/2" and %P3/8") and/or the fine aggregates (%P4 and %P8) increase, the permeability increases. This trend could be used in the mix design process to limit permeability by either reducing the difference between the coarse sieves, fine sieves, or both. In the laboratory study, two compaction procedures, called Method A and Method B, were used to produce specimens using the Superpave Gyratory Compactor (SGC). The result indicates that Method B, which is based on using Ndesign gyrations for different sample sizes, can be used to produce samples that give permeability values similar to values measured for field cores. The results indicate a good relationship between field permeability (using the NCAT device) and lab permeability measured on field cores of fine-graded mixes with amount of passing No. 8 sieve (P8) higher than 45%. However, the relationship between field permeability and lab permeability measured on field cores of coarse-graded mix (P8 lower than 40%) is very poor. It is therefore concluded that the NCAT permeability device could possibly be used in the field for fine-graded mix (with P8 higher than 45%) to measure a permeability index that is related to the true permeability of field cores as measured by the ASTM D5084. However, to measure the field permeability of coarse-graded mix (P8 lower than 40%), an approach to prevent water leakage along the sealant due to rough pavement surface should be established. For coarse graded mixtures, there appears to be no alternative better than taking field cores and testing them in the lab. For estimating permeability during mixture design, a method for preparing and testing permeability of SGC specimens and interpolating based on expected field density is introduced. The results represent a good estimate of the expected in-place field permeability. The recommendations from this study included no changes in the selection of minimum pavement thickness and t/NMAS ratios in the specifications. However, this recommendation does not ensure achieving density nor limit permeability. It is also recommended that for the permeability and density criteria for Superpave mix designs, the target permeability and density values should be developed from in-service pavements with recorded performance histories. For further study, the warranty projects with proven record of performance can be used to define target density and permeability criteria for HMA pavement in Wisconsin.