Impact of Air Voids on Dynamic Modulus and Fatigue Performance Prediction in Asphalt Pavements

Abstract: The primary goal of the Additive Group (AG) experiment at the National Center for Asphalt Technology (NCAT) Test Track is the development of a framework for the rapid evaluation of dense-graded asphalt mixtures containing additive technologies. To that end, the stiffness characteristics and fatigue performance of six asphalt mixtures, each with an identical mixture design, but differing in their respective additive technologies, were evaluated in the laboratory. Wet and dry-process rubber, dry-process fiber, and wet and dry-process plastic were among the additive technologies evaluated. Full-scale test sections were paved at the NCAT Test Track and instrumented with asphalt strain gauges (ASGs). The surface performance and structural condition of the AG test sections were monitored during the two-year accelerated trafficking period (where approximately 10 million equivalent single axle loads (ESALs) were applied). Trafficking was paused with the conclusion of the 2021-2024 NCAT research cycle. An additional 10 million ESALs are planned for the next research cycle.

Following completion of trafficking in 2024, the first steps toward developing the AG evaluation “framework” were taken, namely linking the laboratory characterization and performance testing to the fatigue cracking measured in the field. WESLEA and FlexPAVETM were the primary tools utilized for this analysis. Structural responses, including peak tensile strains, were computed via WESLEA using the surveyed layer thicknesses and the backcalculated layer coefficients of the AG test sections. FlexPAVETM was used to simulate the damage evolution via bottom-up fatigue cracking, using many of the same structural inputs, in addition to the dynamic modulus and fatigue inputs directly from FlexMATTM. Because the PMLC test specimens were prepared in a controlled environment, at a specified air void percentage, their |E*| and fatigue performance characterization may not accurately reflect the in-place properties of the mixtures. The AG test sections not only vary in their average densities, but have spatially variable densities throughout the sections, and some may have significant density gradients through the depth of their AC layer. Each of these factors complicate fatigue cracking performance simulation when utilizing the PMLC test specimens for analysis.

This paper details the laboratory characterization and fatigue performance simulation of the AG control mixture (N7 (Ctrl)). This mixture was found to have 2% higher average in-place density relative to the other AG mixtures. Initial testing of the N7 (Ctrl) mixture at 7% air voids found its |E*| master curve, fitted Sapp parameter, and fatigue life transfer functions to be in the middle amongst the suite of AG mixtures. New PMLC test specimens were prepared for N7 (Ctrl) at the original 7% air void percentage, as well as the lower 4% air void percentage (matching in-place density). |E*| and direct tension cyclic fatigue testing were performed. The |E*| master curves, fatigue performance characterization (including fitted Sapp), and resulting FlexPAVETM bottom-up fatigue cracking simulations were compared at the two air void levels. These comparisons provide valuable insight to how in-situ density affects lab-measured properties and performance predictions and provides further motivation for matching field conditions in the lab as closely as possible.