Impacts of Aggregate Gradation on Volumetric and Performance of Airfield (P-401) Asphalt Mixtures

Abstract: The P-401 specification for asphalt mixtures used in airfield pavements requires full gradation control at all sieve sizes, which differs from the Superpave (AASHTO M323) specification with control points on only four sieves. Additionally, the P-401 specification specifies fine-graded gradations, with the design gradation mostly above the maximum density line, and it requires a minimum void in mineral aggregate (VMA) that is 1% higher than the Superpave specification. The fine-graded graduation requirement reduces the permeability of the compacted asphalt layer, and the higher VMA requirement yields a higher binder content, improving the mixture’s durability and resistance to cracking. However, the restrictive gradation requirements make it challenging to design P-401 mixtures with higher VMA, especially with local aggregates. Therefore, this study was initiated to evaluate the effects of gradation inside and outside of the restrictive gradation bands on the volumetric and performance properties of airfield asphalt mixtures.
The study selected P-401 mix designs that were approved but had gradation near the upper or lower gradation limits and had difficulty meeting the volumetric requirements. For each approved mix design, the design gradation (i.e., falling between the required gradation bands) was labeled as In-Spec 1. This gradation was then adjusted using the Bailey method to have similar volumetric properties (i.e., VMA within ± 0.5% of In-Spec 1) but to fall outside the required gradation bands and was labeled as Out-Spec 1. Additionally, two other gradations were designed, one centered within the gradation bands labeled as In-Spec 2, and the other further outside the gradation bands with a 2% higher VMA than In-Spec 1. Consequently, each mix design was used to develop four gradation designs: two inside the gradation bands (In-Spec 1 and In-Spec 2) and two outside the bands (Out-Spec 1 and Out-Spec 2). Furthermore, all mix designs had a similar binder type and content to eliminate the effect of binders on performance.
Each gradation underwent a comprehensive testing program to compare the performance of In-Spec and Out-Spec mixtures. This evaluation included testing for rutting resistance, cracking, moisture susceptibility, durability, and permeability using various tests such as the Asphalt Pavement Analyzer, IDEAL-CT, Illinois Flexibility Index, Disk-Shaped Compact Tension, Tensile Strength Ratio, Cantabro, and Florida permeability tests.
The results have been obtained for gradations close to both the lower and upper limits. The findings indicated that adjusting the P-401 gradation to be coarser (below the lower limits) or finer (above the upper limits) increases the VMA. In addition, gradations that were both within and outside the specified limits and had similar volumetric properties demonstrated statistically similar mixture performance characteristics. Moreover, the coarser gradation significantly enhanced resistance to intermediate-temperature cracking while maintaining low-temperature cracking resistance, rutting resistance, moisture resistance, durability, and permeability of the mixture.