Keywords: finite element method, reflective cracking, rigid pavement, viscoelasticity, hot-mix asphalt overlays, strain response.

Placing a hot-mix asphalt (HMA) overlay on top of a deteriorated pavement is an efficient rehabilitation measure used to improve pavement serviceability. However, HMA overlays placed on cracked-jointed pavements are prone to early reflective cracking development, leading to premature failure of the resurfaced pavement. Reflective cracking is due to horizontal and vertical movements in the underlying pavement at the vicinity of the discontinuity, which are, in turn, induced by traffic wheel loads and environmental variations.

This study examines the reflective cracking strain state of typical HMA overlays on top of PCC slabs. Horizontal strain, characteristic of mode I (opening) cracking, and shear strain, characteristic of mode II (shearing) cracking were monitored at the bottom of the HMA overlay as a function of time. A two-dimensional plane strain finite element model was implemented using the commercial code ABAQUS (v6.9). The effect of design parameters such as HMA thickness, vehicle speed, sub-base and sub-grade moduli, and pavement temperature distribution on the strain state at the bottom of the HMA overlay was examined. A linear viscoelastic constitutive (LVE) model was used to express the time-dependent properties of the HMA overlay. The LVE model was implemented using the Prony series expansion expressed in terms of shear relaxation moduli and relaxation time. Typical pavement temperature distributions were incorporated in the FE models, and the temperature-dependent properties of HMA materials were implemented using the Williams-Landel-Ferry equation. Using a haversine function, fifty wheel load passages, each with a vertical contact pressure of 700 KPa were simulated for three vehicle speeds. A logarithmic regression analysis was performed on the strain history curves obtained. The logarithmic regression model was defined through two parameters, the slope of the logarithmic regression curve, representing the rate of strain increase, and the initial strain at the beginning of the loading period.

Results obtained show that the rate of horizontal and shear strain increase at the bottom of the HMA overlay drop with higher vehicle speed, higher subgrade modulus, and higher subbase modulus. Moreover, the rate of horizontal strain accumulation increases with higher overlay thickness. Although initial strain values were higher at positive pavement temperature distributions, the corresponding rate of strain increase were higher at negative pavement temperatures. Finally, an extrapolation of the strain history curve for various pavement design parameters was used to estimate the number of cycles for bottom-up crack initiation.

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