The truth and myth of fatigue cracking potential in hot-mix asphalt: Numerical analysis and validation

The aim of this study is to quantify dynamic flexible pavement responses to various tire loadings using three dimensional finite element (FE) models and implicit-dynamic analyses. In addition to presenting conclusions that satisfy this objective, this research goes further to raise valid questions and present theories about "bottom-up" fatigue cracking and its initiation. The study considered two tire configurations, dual-tire assembly (275/80R22.5) and wide-base tire (455/55R22.5), at a tire pressure of 720 kPa and three wheel loads: 35.5 kN, 45.5 kN, and 53.5 kN. The pavement designs utilized in the study include a typical interstate highway and three alternative designs with hot-mix asphalt (HMA) thicknesses of 76,152, and 305 mm. Two vehicle speeds were simulated: 8 and 105 km/hr. The calculated pavement responses were compared to field-measured responses at the Virginia Smart Road. The study concluded that the critical pavement responses, including tensile strains at the bottom of the HMA and vertical shear strain in HMA, are predicted at intermediate (25 C) or high (40 C) HMA temperatures at low vehicle speed (8km/hr), given that the pavement surface is smooth and vehicle dynamics are not changing. The vertical shear strains near the HMA surface and tensile strains at the bottom of HMA (38 mm from the surface) at low HMA temperature (5 C) under high speed (105 km/hr) resulted in greater magnitudes of responses than at the low speed (8 km/hr). At low temperature (5 C), the high-loading frequency (high speed) of the transient moving loading may exert higher acceleration and inertia forces to the pavement system responses than those at the low speed. The research shows that vertical shear strains near the surface could be responsible for what is referred to as "bottom-up" fatigue cracking; while surface tensile strains near the tire-edge rib could be responsible for top-down cracking. As a result, the study questions the validity of "bottom-up" fatigue cracking in relatively thick pavements and suggests that these cracks may instead be initiated at 50-100 mm below the HMA surface. The most important implication of this study is that it encourages a fresh investigation of where these cracks are actually initiated and suggests "near-surface" cracking as a primary cause of pavement cracking distress.