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1 CHAPTER 1 Introduction and Research Approach Introduction Scope Reflection cracking is one of the primary forms of distress Because overlays are expected to perform differently in dif- in hot-mix asphalt (HMA) overlays of flexible and rigid pave- ferent climatic zones, the Long-Term Pavement Performance ments. In addition to affecting ride quality, the penetration of (LTPP) database for the four climatic zones (Wet-Freeze, Wet- water and debris into these cracks accelerates the deteriora- No Freeze, Dry-Freeze, and Dry-No Freeze) will be reviewed tion of the overlay and the underlying pavement, thus reduc- carefully to determine if there are a sufficient number of over- ing service life. This project developed mechanistic-based lay test sections of a given structural type to warrant develop- models for use in both analysis and design of HMA overlays. ing a set of model coefficients for that type of overlay. As a rule The product of this research will help to account for the of thumb, a minimum of 20 test sections, all of which have effects of reflection cracking on performance, improving the begun to experience reflection cracking, are needed to develop analysis and design of HMA overlays of flexible and rigid a reliable set of calibration coefficients. pavements. The relationship between this reflection cracking model and the MEPDG software is that of a subprogram which has many Objective of the same inputs and the same format for input screens and output. Figure 1 illustrates how this model is compatible with The objective of this study was to identify and develop the MEPDG. (a) mechanistic-based models for predicting reflection crack- ing in HMA overlays of flexible and rigid pavements and (b) associated computational software for use in mechanistic- Organization of the Report empirical procedures for overlay design and analysis. This This report is organized into four chapters. The first chap- chapter summarizes the approach that has been used to ter presents the introduction and research approach used in accomplish these objectives. The research plan was developed the research project. The second chapter presents the major with the expectation that the reflection cracking models and findings. These include the definition of reflection cracking; associated software will ultimately be used for routine asphalt a brief review of the available reflection cracking models; overlay thickness design and for reflection cracking perfor- criteria for selection of the best presently available model; mance analysis of asphalt overlay structures. The plan was also and a description of the various categories of data and algo- guided by the realization that reflection cracking models and rithms that were used in assembling the model developed associated input must be compatible with the Mechanistic- in this project. The data categories include pavement struc- Empirical Pavement Design Guide (MEPDG) (1), since reflec- ture, traffic, weather, observed distress, and materials prop- tion cracking is only part of the strategic objective of designing erties. The algorithm categories include methods to predict reliably performing asphalt overlay pavements. The body of temperatures; material properties as they vary with temper- this report describes the approach that was used, the major ature and loading rate; thermal stress; traffic stresses and findings, the interpretation, appraisal and applications of the the growth of cracks up from the cracks or joints in the old results of this study, and, finally, the conclusions and suggested overlaid pavement. Finally, the second chapter describes further research that would be beneficial to the objectives of how the model predictions are calibrated to the observed this research. field data.