Assessing Moisture Damage Resistance of Dense-Graded Mixtures Based on Lift Thickness, Permeability, Total Air Voids, and Indirect Tensile Strength

The present research highlights the impact of lift thickness on the permeability and susceptibility to moisture damage of a 19 mm nominal maximum aggregate size (NMAS) dense-graded asphalt mixture. Specimens of two different thicknesses, 50 and 75 mm, having thickness to nominal maximum aggregate size ratio (t/NMAS) of 2.5–4, were compacted at four different air voids, namely, 4%, 6%, 8%, and 10%. A total of 40 samples were compacted (2 thickness × 4 air voids × 5 replicates) in the laboratory using Superpave Gyratory Compactor in height mode. The permeability of all compacted samples was measured using falling head test apparatus as per standard FM 5-565. Further, after measurement of permeability, the samples were subjected to moisture-induced stress tester (MIST) conditioning to assess moisture damage using indirect tensile strength (ITS). The findings revealed that permeability was exponentially increased with increase in total air voids for both the selected thickness. A threshold value of 6% of total air voids was ascertained as critical total air voids having permeability value less than 125 × 10−5 cm/s, which is the maximum accepted permeability value. Therefore, a minimum compaction of 94% Gmm is recommended for 19 mm NMAS mixtures. The permeability versus ITS value of MIST conditioned samples were analyzed, and based on the permeability threshold value, a minimum requirement of ITS was found to be 1.4 MPa for moisture-resistant mix, which is found to be higher than the internationally accepted value of 0.7 MPa. The maximum allowable connected air voids value (CAVMax) was estimated, and it was found that it should have been below 1% to avoid the impact of moisture on asphalt mix. Surprisingly, the study did not find a significant effect of lift thickness on critical total air voids, minimum connected air voids, and minimum ITS value, possibly due to the time utilized for compaction (TUC) and compaction temperature was the same for both lift-thickness, leading to equivalent workability and compaction.