Evaluating factors that affect interface shear strength of an asphalt concrete

Asphalt concrete (AC) interfaces represent potential weak zones in pavements when interlayer bonding is inadequate. Adequate bonding ensures effective load transfer. This study systematically evaluates ten factors influencing interface shear strength (ISS) in AC using direct shear tests. Tests examined temperatures (20 °C and 55 °C), curing times, moisture damage, normal stresses, loading rates, tack coat rates/ types (SS-1 h, CSS-1 h, trackless), surface roughness (mean texture depth), surface texture orientation relative to shear direction, and surface aging. Key quantitative results show that ISS peaks at an optimum residual application rate of 0.04 gal/yd² for SS-1 h emulsion. Strength increases exponentially with curing time and becomes asymptotic after 120 min. ISS declines by up to 70% at 55 °C, and by 16% and 22% under AASHTO T283 and Moisture Induced Stress Tester (MIST) conditioning, respectively. Surface roughness positively correlates with ISS, while surface texture orientation relative to shearing has a significant effect (perpendicular orientation yields the highest median ISS of 974 kPa). Internal friction angles rise from 25° to 50° as loading rates increase from 0.1 to 5 mm/min. Aged surfaces exhibit lower ISS than unaged surfaces. Analyses included one-way ANOVA, Tukey HSD post-hoc tests, two-way ANOVA, and three-way ANCOVA (with mean texture depth as covariate). All ten factors are statistically significant (p < 0.05). Ranking by statistical significance and large effect sizes (η²) identifies loading rate, temperature, moisture damage, tack coat type, and normal stress as the top five factors.