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Minnesota Department of Transportation
An unbonded concrete overlay (UBCO) system is a Portland cement concrete (PCC) overlay that is separated from an existing PCC pavement by an asphalt concrete (AC) interlayer. Current UBCO design procedures are based on empirical equations or highly simplified mechanistic models. To overcome the limitations, fracture mechanics concepts, specifically the finite element method-based cohesive zone model (CZM), are introduced in this research as a new paradigm for analyzing UBCOs with the ultimate goal of establishing a more rational design procedure. To illustrate the advantages of a fracture mechanics-based approach to design, specific attention is paid to but one type of failure associated with pavement structures: reflection cracking. The design against reflection cracking approach relies on a load-carrying capacity equivalency between the designed UBCO and a reference newly designed single layer PCC pavement. An illustrative fracture mechanics-based design procedure for UBCOs is developed and proposed by a large number of crack propagation simulations of both the UBCO composite and the reference single layer pavement. Preliminary comparisons of the results with field observations suggest that the fracture mechanics paradigm offers promise for improved design of UBCOs against reflection cracking and other potential loading conditions that could be analyzed using nonlinear fracture mechanics models. It is recommended that an experimental program be established to assess the accuracy of the model predictions, and additional experiments and three-dimensional fracture mechanics simulations be considered to provide additional insights as to whether UBCOs can be "thinned-up."
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