Effect of Confinement on Dynamic Modulus of Porous Asphalt Mixtures

Abstract: The Dynamic Complex Modulus (DCM), represented by |E^* |, is the measure of asphalt mixture stiffness and serves as the primary input for flexible pavement design. In conventional dense-graded asphalt mixtures, the effect of confinement on |E^* | is generally less pronounced at 7 percent air voids. Therefore, the DCM test is run under an unconfined condition. On the other hand, the higher air void content (>16%) and open gradation of Porous Asphalt (PA) mixtures significantly influence the |E^* | values at different confinement levels. This study aims to evaluate the effect of varying confinement levels on the |E^* | values of Modified Open Graded Friction Course (MOGFC) and Modified Asphalt Stabilized Drainage Course (MASDC) mixtures. Both MOGFC and MASDC mixtures were plant-produced, lab-compacted, and composed of a polymer-modified PG 76-22 binder with loose cellulose fiber additives. Testing was conducted under confined conditions (10, 20, 30, and 40 psi) and unconfined conditions (0 psi) on full-size (100 mm diameter and 150 mm height)samples at five temperature levels (-10, 4.4, 21.1, 37.8, and 54.4°C) and six loading frequencies (0.1, 0.5, 1, 5, 10, and 25 Hz). The results showed that the |E^* | values increased significantly with the confinement depending on the testing temperature and frequency of loading. The |E^* |values exhibited a marked increase at lower loading frequencies than higher frequencies, particularly at higher temperatures. Across all testing conditions, the confined |E^* | value increase from the unconfined |E^* | ranged from 30% to 900%. At an intermediate temperature level (21.1˚C) and loading frequency (10 Hz), the confined |E^* | value was 50% higher than the unconfined |E^* | value. For ease of understanding, the variations of |E^* | values in the entire temperature regime were explained with the help of a master curve. The findings of this study are particularly important in the mechanistic-empirical approach of flexible pavement design using PA mixtures. The confined and unconfined |E^* | values are used as Level-1 inputs in the AASHTOWare ME design. Comparing these two values showed the effect of confinement on the final thickness of PA pavements. The outcome of this study will provide guidelines for determining the most appropriate |E^* | values representing the field confinement condition that will aid in optimizing the thickness design of porous asphalt pavements.