LTPP Guide to Asphalt Temperature Prediction and Correction

Background

The deflection under the center of the load plate (referred to in this document as "defl0") is critical for calculating the effective structural number (SN_eff) of a pavement, according to the AASHTO 1993 design method.

As shown in Figures 28 and Figures 29, defl0 (as measured on flexible pavements) is temperature dependent.

Figure 28. Sample Deflection Basins Measured at the Same Point

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Figure 29. defl0 from Deflection Basins in Figure x11

The sensitivity of defl0 to temperature, however, varies according to the thickness and stiffness of the HMA layer and, to lesser extent, the underlaying base and subgrade layers. Therefore, while the relationship of defl0 to temperature is reasonably good, as shown in Figure 30, a simple relationship between the two factors could not be found for all pavements.

Figure 30. defl0 Factor vs. Temperature for a Single Test Location

Temperature Correction

To remove the influence of the base and subgrade layers, the Delta36 was selected. The methodology is to calculate Delta36 at the reference temperature (T_r) and the measured temperature (T_m), and add the results back to the measured defl₃₆ to provide an estimate of defl₀ at both T_m and T_r. The ratio of these two estimates is then used as a temperature adjustment factor for the measured delf₀, as shown in the following equation:

TAFDefl₀ = (defl₃₆ + Delta36_r) / (defl₃₆ + Delta36_m

Where:

defl₃₆ = Measured deflection at an offset of 915 mm (36 in.)

Delta36_r = Delta36 factor calculated for the reference temperature (T_r)

Delta36_m = Delta36 factor calculated for the measured temperature (T_m)

The above equation can be used to develop a temperature adjustment chart for a specific pavement section, as shown in Figure 31.

Figure 31. Example of a Temperature Adjustment Chart

Source code for implementing TAFDefl₀ as a function in MS Excel VBA is available here.

Sample data for checking code is available here.

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