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33 100 90 80 70 % Crack Length 60 50 40 30 Measured Data 20 Model 10 0 0 1000 2000 3000 4000 5000 No. of Days Figure 30. Computed and measured reflection crack for LTPP section 55B901. Prediction of Temperature in wave radiation to the atmosphere; convective heat transfer a HMA Overlay between pavement surface; and the air close to the surface, which is enhanced by wind. Below the surface and within the A new temperature model was developed to better predict pavement and ground beneath it, heat is transferred by con- temperature variations with depth within the overlay. The duction. Not included in this model is heat transfer enhance- model differs somewhat from the model in the EICM (32, 33, ment by precipitation. Mathematical details of this model 34) that is used in the MEPDG. A comparison of the temper- follow. atures in a pavement surface as measured and as calculated by the EICM model is shown in Figure 8. Figure 9 shows a com- parison between the measured temperatures and as calcu- Heat Transfer in Pavement lated with the new model. The figures illustrate that the new Heat transfer in the pavement is governed by the classical model matches the measured temperatures more closely than thermal diffusion equation the EICM model. The new one-dimensional model was devel- oped based on radiation and conduction energy balance fun- T 2T damentals (details of the model are presented in Appendix B). = 2 (14) t x The heat transfer process is depicted in Figure 31. Sources of heat transfer at the pavement surface are solar radiation and where reflection of the solar radiation at the surface by a fraction ~, the albedo; absorption of atmospheric down-welling long- T = the pavement temperature as a function of time and wave radiation by the pavement surface; emission by long- depth below the surface (x); Outgoing long-wave Solar radiation (Qs) radiation (Qr) Atmospheric downwelling Heat convection long-wave radiation (Qa) by wind (Qc) Heat conduction Pavement Figure 31. Schematic presentation of heat transfer model of pavement.