The key objective of this research was to determine the minimum hot mix asphalt (HMA) lift thicknesses and minimum in-place density necessary to achieve an impermeable, durable pavement.

The research team found that the HMA pavement density that can be obtained under normal rolling conditions is clearly related to the ratio t/NMAS of the HMA. For improved compactibility, the agency recommended that t/NMAS be at least 3 for fine-graded mixes and at least 4 for coarse-graded mixes. The data for SMA mixes indicate that the ratio should also be at least 4. Ratios less than these suggested values can be used but a greater than normal compactive effort will generally be required in these situations to obtain the desired in-place density.

The results of an experiment to evaluate the effect of mix temperature on the relationship between pavement density and t/NMAS found that the more rapid cooling of the HMA is a key reason for low density in thinner sections (lower t/NMAS). Hence, for thin HMA layers the team emphasized the importance of paving rollers staying very close to the paving machine so that rolling can be accomplished prior to excessive cooling.

The project team further identified the in-place air voids content as the most significant factor impacting permeability of HMA mixtures, followed by coarse aggregate ratio and VMA. As the coarse aggregate ratio increases, permeability increases, but it decreases as VMA increases at constant air voids content. The variability of permeability between various mixtures is very high; some mixtures are permeable in the range of 8 to 10 percent air voids while others are not. However, to ensure that permeability is not a problem an in-place air voids content between 6 and 7 percent or lower is recommended. This recommendation appears applicable to a wide range of mixtures regardless of NMAS and aggregate gradation.

The final report has been published as NCHRP Report 531. The full findings of the research are available online as NCHRP Web Document 68.