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8 Figure 6. Flow chart of the process of constructing a calibrated reflection cracking model. computed for the loading times for thermal stresses and the Overlay Sections with Sufficient passage of traffic loads representing single, tandem, tridem, Data for Model Development and quadrem axles. · Develop a computational technique for calculating the total After a thorough review of the data available in the LTPP crack growth caused by single, tandem, tridem, and quadrem database and for test sections in New York and Texas, the axles passing over a growing crack, and include the healing researchers concluded that there is sufficient high quality shift factor that increases with the length of time between data to develop sets of calibration coefficients for the reflec- traffic loads. tion cracking model for each major climatic zone in the · Develop a program to calculate the viscoelastic thermal United States. Data were collected for a total of 11 pavement- stresses in the overlay. structure-overlay-climatic zone sets representing 411 over- · Develop a computational technique to calculate the growth lay sections, as shown in Table 1. of a crack caused by daily thermal stresses. The LTPP sections provided the bulk of the data that were · Write a supervisory program that will combine the compu- used for modeling reflection cracking in the different types of tations of crack growth by each of the separate mechanisms pavement structure. The distribution of these sections within (thermal, bending, and shearing). the different climatic zones is given in Table 2. The sections · Write an interface program to permit the user to input data were well distributed throughout the United States and in the same format used in the MEPDG. Canada, in 37 states, the District of Columbia, and six Cana- · Calculate the number of days required for each of the three dian provinces as listed in Table 3. fracture mechanisms to cause a reflection crack to grow through the thickness of the overlay. Collection of Pavement · Develop sets of computation-to-field calibration coefficients Structure Data for each type of pavement-overlay structure and climatic zone. The pavement data used were the layer thickness of each pavement layer and the Falling Weight Deflectometer (FWD) The work performed in each of these steps to develop the data for each test section prior to the placement of the over- hot-mix overlay reflection cracking model and its calibration lay including the temperature at which the FWD data were coefficients is summarized in the following sections. obtained. The layer moduli of the old pavement were back-

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9 Table 1. Overlay sections for model development. Data Set Pavement Structure* No. of Test Sections Climatic Zone+ 1 AC/mill/AC OL 62 WF 2 AC/mill/AC OL 47 WNF 3 JCP/AC OL 69 WF 4 AC/AC OL 59 WF 5 AC/AC OL 33 WNF 6 AC/SAMI/AC OL 26 WF 7 CRC/AC OL 21 WF 8 AC/AC OL 16 DF 9 AC/mill/AC OL 16 DNF 10 AC/SAMI/AC OL 12 WNF 11 AC/Grid/AC OL 50 NY, Texas Totals 411 *The abbreviations are listed in order from the old pavement surface layer upward to the overlay. AC = existing hot mix asphalt surface layer, JCP = jointed concrete pavement, CRC = continuously reinforced concrete surface layer, Mill = old surface layer was milled before the overlay was placed. SAMI = (Strain Absorbing Membrane Interlayer) indicates that a compliant interlayer was placed between the old surface layer and the hot mix overlay. A reinforcing interlayer was placed between a leveling course and the hot mix overlay. AC OL = hot mix asphalt overlay. + WF, DF, WNF, and DNF designate Wet-Freeze; Dry Freeze; Wet-No Freeze; and Dry-No Free, respectively calculated using the program MODULUS (13). The LTPP overlay was available, including the volumetric composition of data included the deflections measured at many equally the asphalt mixture, the gradation of the aggregate, and some spaced locations within each test section. The mean of the indication of the grade of the asphalt binder. The grade was backcalculated moduli for each layer was used as the modu- used to determine the six characteristics of the master curve of lus of that layer for the entire test section. the binder according to the CAM model (the glassy shear mod- In addition to the layer thickness and the backcalculated ulus, Gg, the crossover frequency, R, the rheological index, R, moduli of the old pavement, the mixture design data of the the defining temperature, Td, and the two time-temperature Table 2. LTPP test sections used for calibration. No. of Test Sections Pavement-Overlay Total Test at Each Climatic Zone Description Structure Sections WF DF WNF DNF AC/AC OL AC, then AC overlay 108 59 16 33 - AC/Mill/AC OL AC, then Mill+AC overlay 125 62 - 47 16 CRC/AC OL CRC, then AC overlay 21 21 - - - JRC/AC + JPC/AC OL JRC or JPC, then AC overlay 69 69 - - - AC, then seal coat or friction course AC/SC or FC/AC OL 38 26 - 12 - +AC overlay Total 361 237 16 92 16