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48 CHAPTER 3 Interpretations, Appraisal, and Applications Introduction severity. The process by which these sets of model calibration coefficients were developed is described in Chapter 2 (more This project developed a HMA overlay design program details are provided in Appendices L, M, and N). which is based on calibrating mechanistically predicted reflec- Twenty two sets of calibration coefficients were developed, tion cracking lives to observed overlay reflection cracking two sets for each combination of overlay and climatic zone distress. The program is written to be a subprogram to the (for which data were available). MEPDG software for new pavements. The program uses the same input and type of input that the MEPDG software requires and additional data that is appropriate to the design of an over- The Model Development Process lay. This includes the condition of the pavement at the time The application of mechanics to the prediction of reflection of the overlay construction and the properties of the layers cracking through HMA concrete overlays involves a number of of the old pavement, which may be assumed or measured by steps including the use of finite element analysis of crack growth nondestructive testing means. The design subprogram per- and the modeling of those results with an ANN algorithm to mits the user to make exploratory runs with trial mix designs, speed up the computational time. The computational task of thicknesses, and reinforcing or strain-absorbing interlayers. determining the material properties of the overlay under a vari- As with the MEPDG, the overlay design subprogram requires ety of loading conditions and temperatures, including traffic the traffic to be input as an axle load spectrum. Over 150 sep- and thermal stresses, must be done rapidly to make the design arate weather databases were assembled in the course of this model a practical tool for design. A third part is to generate project and are incorporated into the files available to the user accurate weather characteristics that can be used to provide of this subprogram in addition to others that may be generated realistic material properties and stresses throughout each day using the Enhanced Integrated Climatic Model (EICM). All and over the observed service life of an overlay. A fourth part of data may be input at one of three levels of detail selected by the the assembly of this model is to develop a consistent means of user (Levels 1, 2, or 3). describing the distress observed in the field. The fifth part was The project has also developed a model calibration program to devise a means to relate the predicted distress to the observed which may be used to create a set of calibration coefficients, distress in a simple way, and produce predicted distress that for use in the design model, to design overlays calibrated to matched well with what was actually observed in the field. A specific local or regional conditions and practices. discussion of these five steps follows. The design program is arranged as a single mechanistic crack growth model with different sets of calibration coefficients for Mechanistic Prediction of Crack Growth overlays in different climatic zones or with a different pavement structure, such as overlays over cracked asphalt surfaces or over The model was selected for the Design Program based on jointed concrete pavements. A total of 11 separate types of pave- several factors. One of the most important factors was the ments were identified with a sufficient amount of quality data speed with which daily crack growth could be computed to to develop sets of calibration coefficients. The final set of cali- facilitate consideration of several material, thickness, and re- bration coefficients for each type of overlay was selected to inforcing options in the overlay. This led to the decision to use reproduce, as closely as possible, the actual field observations of the ANN algorithms to compute both the changing modulus the growth of reflection cracking distress in both its extent and of the overlay mix and the growth of the cracks up from the