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27 100 80 1 < 1.0 % Crack Length 60 2 = 1.0 3 > 1.0 40 -1 e = 36.8% 20 0 1 2 3 - 2,000 4,000 6,000 8,000 10,000 No. of Days Figure 21. Parameters in reflection cracking severity model. H and H for high severity level, of which is shown in Figure 24 (28). These sections included H and H for medium and high severity levels, and HMA overlays over cracked asphalt surface layers in Amarillo LMH and LMH for low, medium, and high severity levels. and HMA overlays over jointed concrete pavements in Mar- lin. Geosynthetic materials were used to reinforce all of the The reflection cracking models were calibrated using field overlays in both of these sites. Reflection cracking data for the data for overlay test sections which have sufficient crack LTPP sites, New York test sections, and Texas test sections are measurements. Figure 22 shows the crack amounts expressed presented in Appendix J. as percentages of the original crack length observed in LTPP section 270506, at three severity conditions: high; high or Calibration of Field Reflection medium; and high, medium, or low severity. Cracking Model The same type of data was obtained for the test sections in New York City (27); example is shown in Figure 23. These One objective of the study was to calibrate the reflection sections included different types of geosynthetic materials cracking amount and severity model with reflection crack reinforcing the overlays. data observed in the field. The calibration refers to the Another set of reflection cracking data was found in the mathematical process through which the total error or dif- geosynthetically reinforced test sections in Texas, an example ference between observed and predicted values of distress 100 90 80 70 % Carck Length 60 Severity Level 50 L+M+H 40 M+H 30 H 20 10 0 0 1000 2000 3000 4000 5000 No. of Days Figure 22. Reflection cracking amount versus number of days for LTPP section 270506.