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Publication Number: FHWA-RD-98-085

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 (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.


deflection basins illustrating temperature effects

Figure 28. Sample Deflection Basins Measured at the Same Point


def0 vs. mid-depth temperature plot

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.


def0 vs. mid-depth temperature plot with regression line

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 (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

Where:

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

Delta36r = Delta36 factor calculated for the reference temperature (Tr)

Delta36m = Delta36 factor calculated for the measured temperature (Tm)

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


sample temperature adjustment factor chart

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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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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