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Publication Number: FHWARD98085 
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
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 (T_{r}) and the measured temperature (T_{m}), and add the results back to the measured defl_{36} to provide an estimate of defl_{0} at both T_{m} and T_{r}. The ratio of these two estimates is then used as a temperature adjustment factor for the measured delf_{0}, as shown in the following equation:
TAFDefl_{0} = (defl_{36} + Delta36_{r}) / (defl_{36} + Delta36_{m}
Where:
defl_{36} = 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 TAFDefl_{0} as a function in MS Excel VBA is available here.
Sample data for checking code is available here.
Topics: research, infrastructure, pavements and materials Keywords: research, infrastructure, pavements and materials TRT Terms: Pavements, AsphaltUnited StatesTemperature, Pavements, AsphaltPerformanceUnited States, LongTerm Pavement Performance Program (U.S.), Adjustment factors, Asphalt pavements, Backcalculation Updated: 04/23/2012
