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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-98-085
The deflection under the center of the load plate (referred to in this document as "defl0") is critical for calculating the effective structural number (SNeff) 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
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
To remove the influence of the base and subgrade layers, the Delta36 was selected. The methodology is to calculate Delta36 at the reference temperature (Tr) and the measured temperature (Tm), and add the results back to the measured defl36 to provide an estimate of defl0 at both Tm and Tr. 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 + Delta36r) / (defl36 + Delta36m
defl36 = Measured deflection at an offset of 915 mm (36 in.)
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.