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5 CHAPTER 2 Findings Introduction ing due to the movement in the existing pavements in the vicinity of joints or cracks. This movement may be induced This report addresses the issues associated with reflec- by bending or shearing action resulting from traffic loads or tion cracking and reports the development of mechanics- daily and seasonal temperature changes. In general, reflection based models for use in mechanistic-empirical procedures cracking is caused by the combination of these three mecha- for the analysis and design of HMA overlays. These mechanics- nisms. Every pass of a traffic load will induce two shearing based models are capable of predicting reflection crack- and one bending effect on the HMA overlay that are affected ing in HMA overlays of flexible and rigid pavements and by the daily temperature (see Figure 4). Thus the combina- incorporate associated computational software for use in tion of all three mechanisms is required to successfully model mechanistic-empirical procedures for overlay design and reflection cracking. In addition, crack initiation and propa- analysis. gation is also influenced by the existing pavement structure (see Figure 5) and conditions, reflection cracking counter- Reflection Cracking--Definition measures (e.g., reinforcing, interlayer), HMA mixture proper- and Mechanisms ties, the degree of load transfer at joints and cracks, and others. Therefore, these three mechanisms and these influencing factors Reflection cracking can be defined as the cracking of a must be taken into account in developing a reflection cracking resurfacing or overlay above underlying cracks or joints, with model. movement of some form in the underlying pavement as its probable cause. As illustrated in Figure 3, this cracking can result from traffic and environmentally induced causes. The Available Reflection existing joints or cracks can affect reflection cracking in two Cracking Models forms (7): Concern about reflection cracking of asphalt overlays over existing pavements dates back to 1932, when Gary and 1. The existing joints or cracks provide stress concentration Martin (10) studied this problem. Subsequently, many at the bottom of an asphalt overlay, which will lead to the studies have been conducted to address this problem and continued crack growth into the asphalt overlay layers. many models have been developed to analyze or predict reflec- 2. If the stress-concentrating effect of the existing joints or tion cracking. In general, these models can be categorized as cracks has been nullified by some means, a secondary follows: effect of the existing joints or cracks is that a maximum deflection of the pavement under a wheel load will occur Empirical model; at the crack. Thus, the maximum stresses in the overlay Extended multi-layer linear elastic model; will occur at this point, making it the most likely location Equilibrium equations based models; for crack growth to initiate as an indirect effect of the exist- Finite element plus traditional fatigue equation model; ing crack. Finite element plus fracture mechanics model; Crack band theory based model; Stress concentration, which is the first effect of existing Cohesive zone cracking model; and joints or cracks, plays the dominant role in reflection crack- Non-local continuum damage mechanics based model.