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6 HMA overlay Crack growth Crack growth Thermal expansion Existing layer and contraction Traffic movement Sub-base Thermally induced fatigue Traffic induced fatigue Thermal contraction Crack growth Temperature gradient giving greater Warping contraction at surface Surface initiated cracking Figure 3. Mechanisms of reflection cracking (8). Selection of a Reflection The model must be capable of accepting the additional Cracking Model information that distinguishes the reflection cracking mech- anisms, i.e., load transfer and crack or joint spacing in the A comprehensive review of available models and their capa- underlying old pavement surface layer. bilities, advantages, and disadvantages is provided in Appen- The model must be capable of accepting the additional dix R. The review revealed that the finite element plus fracture layer geometry associated with the more common overlay mechanics model provides the most desirable attributes. The configurations and be capable of being enhanced in the major requirements of the selected model for compatibility future as computational capabilities increase. with the MEPDG include the following: The model must be user friendly, accepting overlay design The model must use the same input data as the MEPDG, input data in a graphical format. The model must be able to calculate the crack growth including the traffic, weather, material properties, and pave- ment layer geometry. potential directly from the input data, and it should The model must be capable of using the same level of com- permit the consideration of commonly used overlay ap- putational capability as the present MEPDG (e.g., it should proaches including interlayers, fabrics, and reinforcing not require the use of a supercomputer). layers. The computational time required by the use of the model must be short enough to allow effective use of the time in evaluating alternative overlay designs. Tip of the crack Overlay Old surface Overlay course Bending Interlayer Stresses at the tip stress B Level-up of the crack Void Shearing A Existing pavement stress Base/Subbase C Position of wheel load Figure 4. Bending and shear Figure 5. Illustration of an asphalt mechanisms (9). overlay system.