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. It is an increasingly popular method for highway pavement rehabilitation. However, 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. Pavements can fail as a result of a wide variety of loadings, including thermal and mechanical. Furthermore, the failures can be produced by different forms of cracking, be it of the fast or the fatigue types. 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. Consequently, the research begins with the two-dimensional analysis of the single layer pavement. A dimensionless equation relating the pavement's load capacity to its material properties and geometric dimensions is obtained from the results of a large number of failure simulations. The single-layer exercise is followed by a three-layer model that includes the dimensions and material properties of the overlay and the interlayer, wherein a preexisting crack exists in the existing PCC pavement.
A 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.
University of Minnesota Ph.D. dissertation. October 2011. Major: Civil Engineering. Advisor: Roberto Ballarini. 1 computer file (PDF); ix, 105 pages, appendices A-C.
Towards fracture mechanics-based design of unbonded concrete overlay pavements..
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