Refinement of P-401 Gradation Bands and Development of Predictive Models for Airfield Asphalt Mixture Performance and Volumetrics

Abstract

Many airport runways, taxiways, and aprons are designed using asphalt mixtures following the P-401 specification from Advisory Circular 150/5370-10H. This specification mandates a preliminary mix design based on volumetric criteria, specifically 3.5% air voids and a minimum Voids in Mineral Aggregate (VMA). Final mix approval requires passing the moisture susceptibility using the Tensile Strength Ratio (TSR) test, and rutting resistance using either the Asphalt Pavement Analyzer (APA) or the Hamburg Wheel Tracking Test (HWTT). Compared to Superpave (AASHTO M323), P-401 imposes more restrictive gradation controls, requiring a full gradation limit across all sieve sizes and a higher VMA threshold. While this enhances mix quality, it introduces substantial challenges for designers in meeting both the VMA and full gradation limits simultaneously. The study focuses on two primary objectives: 1) evaluating the effects of adjusting gradation outside the P-401 gradation limits without changing volumetric properties on the laboratory performance and proposing a more flexible gradation limit that preserves overall mixture performance. 2) evaluating the effects of gradation change on the volumetric properties and laboratory performance of airfield asphalt mixtures and developing prediction models to quantify these relationships. The experimental program began with the selection of 13 P-401 mix designs representing diverse aggregate sources across four Long Term Pavement Performance (LTPP) climate zones (dry/no-freeze, dry/freeze, wet/no-freeze, and wet/freeze). Preference was given to designs compacted at 75 blows or gyrations, as this compaction level requires more stringent design requirements. Each selected mix was redesigned using measured aggregate properties to meet P-401 specifications with a gradation closer to the gradation limits, referred to as In-Spec 1. Three additional gradation designs were then developed for each mix: Out-Spec 1 outside the gradation limits with a similar VMA with In-Spec 1; In-Spec 2 centered within the limits, and Out-Spec 2 further outside the limits, both with VMA 1–2% difference from In-Spec 1. All four gradation designs shared the same binder type and content to isolate the effect of gradation. Laboratory performance was evaluated using the APA or HWTT for rutting resistance, TSR for moisture susceptibility, Cantabro test for durability, Florida permeability test, Disk-Shaped Compact Tension (DCT) test for low-temperature cracking resistance, and the Indirect Tensile Asphalt Cracking Test (IDEAL-CT) and Illinois Flexibility Index Test (I-FIT) for intermediate-temperature cracking resistance. Comparing In-Spec 1 and Out-Spec 1 showed that adjusting the gradation outside the P-401 gradation limits while maintaining volumetric properties and binder content did not significantly affect the laboratory performance of P-401 mixtures. The lower limits of the 12.5 mm NMAS mix design were revised by a 2% reduction in the No. 16, No. 30, and No. 50 sieves. Comparing all four gradation designs showed that moving the gradation further from the maximum density line (MDL) toward either the upper or lower limits consistently increased VMA and air voids. Moisture susceptibility, permeability, durability, and low-temperature cracking resistance remained unaffected. Coarser gradations improved intermediate-temperature cracking resistance. Moreover, coarser gradations maintained or significantly improved the rutting resistance. Prediction models were developed to predict changes in VMA, rut depth, CTIndex, and Flexibility Index (FI) as functions of changes in gradation and aggregate properties. The developed models were deployed as a web-based decision-support tool to assist mix designers in evaluating the volumetric and laboratory performance of gradation adjustments before laboratory testing.