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56 CHAPTER 4 Summary and Suggested Research Summary The calibration coefficients were developed in this project and used in the Reflection Cracking Overlay Design Model to A Reflection Cracking Overlay Design Program was cali- produce reflection crackling predictions that match the field brated using field data for more than 400 pavement test sections observations as closely as possible of the more commonly in 28 states and the four climatic zones of the United States. The used overlays. The program runs quickly enough to allow program uses a mechanistic model that predicts the reflection speedy trials of several overlay thicknesses and asphalt mixes in cracking lives of a specified HMA overlay due to bending and order to find the design that best matches the project objectives. shearing traffic stresses and thermal stresses. The relationship The computational speed is achieved partially by the use of between the computed lives and the field conditions, in terms ANN algorithms to perform the computations that must be of the extent and severity of reflection cracking, depends upon done many times in the course of a simulated time period of the characteristics of the pavement structure and overlay and 10 to 20 years. The ANN algorithms are used to compute the of the weather at the location. This relationship is expressed mixture modulus at different temperatures and loading rates as sets of calibration coefficients that depend upon the season and to calculate the stress intensity factors which drive the where the overlay was placed. A User's Guide for the Design growth of cracks. Another factor in increasing the computa- Program is presented in Appendix O. It uses dialog boxes tional speed is re-writing all of the routines and subroutines in the same format as the MEPDG and has the same three optional levels of input detail. The program is designed to run in the same C # computer language. independently or to be incorporated as a subprogram of the Several examples of predicted reflection cracking perfor- MEPDG software. mance of various overlays in different parts of the United States A total of 11 sets of calibration coefficients were developed are presented in Chapter 3 and Appendix O. These examples from the available data. The overlay performances that can be illustrate the reproduction of the field observations of the predicted with these different sets of calibration coefficients growth of the extent and severity of reflection cracking. The represent the four climatic zones in the United States. Eleven assumed form of these distress growth curves was chosen different pavement structure and overlay combinations were because it gives realistic predictions and remains within reason- identified in the preliminary surveys. These overlays include able bounds. However, extreme designs that are well outside HMA overlays over cracked asphalt pavement surfaces, jointed the range of the types of overlays that were used for calibration concrete pavement as well as asphalt overlays that use strain- may not produce realistic results. absorbing membrane interlayers and reinforcing geosynthetics. When considering the four climatic zones, there are 36 possible Suggested Research sets of calibration coefficients. The methods of developing sets of calibration coefficients for overlays described in this Another 25 sets of pavement structure-overlay and climatic report make it possible to develop sets of calibration coefficients zone combinations remain to be developed, some of which for a specific type of overlay and to design overlays using these may not be possible because they are simply not built. However, sets of calibration coefficients. A Calibration Program has been the process of developing calibration coefficients for a par- provided to facilitate this independent calibration process, and ticular type of overlay described in this report can be applied a User's Guide is provided in Appendix P. Both the Design to any overlay in any region or even within a state or region. Program and the Calibration Program have the same internal For example, sets of calibration coefficients for overlays 150 location weather databases that can be called up by the user. incorporating geosynthetic reinforcing products need to be
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57 developed (only three were developed in the project). It most sensitive predictors of overlay performance. Making would also be desirable to develop guidelines or standards for improvements in these material properties and how they are such products to provide estimates of the reinforcing stiffness measured will extend the service lives of overlays and increase of these products. the predictive accuracy of the models that are developed. The fit between the predicted and the observed reflection Performance-based specifications which are focused on these cracking distress can be improved by reducing the error in material properties will make construction quality control both the predicted and observed performance. The observed and quality assurance more effective and will extend the service field data were fitted with an S-shaped curve with two param- lives of overlays. eters: , the scale parameter and , the shape parameter. Both The Design Program is a good candidate for incorporation of these parameters were fitted by linear regression analysis into the MEPDG software framework or being a stand alone. with mechanistically predicted numbers of days for traffic Experience using the Design Program in designing overlays and thermal stresses to cause a reflection crack to grow to the will show which sets of calibration coefficients need to be surface of an overlay. There are errors that are inherent in improved and, in addition, will reveal those features of a design both the observed and the predicted values. The error in the that make the greatest extension of an overlay's service life. observed field data can be reduced by recording mean values An evaluation of available reflection cracking models was per- of the observed distress from a sampling survey. The error in formed in this project and a number of promising approaches the mechanistically predicted numbers of days of crack prop- were in the development stage at that time. Most of these agation can be reduced by having more accurate recorded models proved to require extensive computational times that pavement structure and materials properties, traffic, and would make them impractical to use in design. However, as weather and by improving the cracking-predicting process of computer speeds increase, the mechanics-based methods will the mechanistic model. require less running time and should be considered for future A sensitivity analysis of the mechanistic model of the crack versions of this overlay design function to reduce the systematic growth process will identify material properties which are the errors and simplify the calibration effort.