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Publication Number: FHWA-RD-03-093
Date: August 2006

Study of Long-Term Pavement Performance (LTPP): Pavement Deflections

Appendix E. FWD SN 058, October 15, 1997–March 5, 1998

This sensor position error was recently identified using an automated screening version of SLIC and reported in Feedback Report RNS–2M (see appendix A). This relatively short-lived misreporting of the actual position of d7 was not detected during the previous work that resulted in Feedback Report RNS–2.

The automated version of SLIC used a model that was specifically chosen to predict the position of sensor 2 with a close to zero overall bias and the best possible precision (see appendix B). The first graph shown in this appendix (figure 20) is a plot of all the SN #058 d2 sensor position predictions during 1997 and 1998 for all lane 1, drop height 4 FWD tests.

In figure 20, it can be seen that the average prediction for this 2-year period was around 20.3 cm (8 inches), as expected, albeit with some scatter. However, during the period of time in question (October 15, 1997–March 5, 1998), the average predicted position of d2 is clearly around 22.9 cm (9 inches) (average SLIC prediction for all flagged test dates = 23.4 cm (9.2 inches)).

In the figure that follows (figure 21), it can clearly be seen that the SN #058 predicted positions for sensor 2 in the fall and winter of 1997-98 are outliers relative to the predicted positions for sensor 2 when other correctly configured FWDs are used. For the four test sections shown (from two different FWDs), on average the predicted position of d2 was 20.6 cm (8.1 inches) while the average prediction for SN #058 during the same period of time in question was 22.9 cm (9 inches).

In the graph that follows (figure 22), the same result for a portion of the test dates is shown graphically, with the lines and data points labeled 11/13/1997 and 12/18/1997 showing the SLIC plot for d2 in both its protocol but incorrect (20.3 cm (8 inches)) and actual (22.9 cm (9 inches)) offset positions. The same test section was tested twice during the period of time in question, both times with FWD SN #058. The portions of the lines that are parallel to the rest of the data are the correct plots, with d2 set to 22.9 cm (9 inches).

Because of this information, it can be concluded with certainty that d2 was not positioned correctly at 20.3 cm (8 inches); rather, it was positioned at 22.9 cm (9 inches) (or very close to 22.9 cm (9 inches)) on FWD SN #058 between October 15, 1997 and March 5, 1998, inclusively. These dates correspond to the dates where lane 1 tests were conducted at drop height 4. This period of time may need to be extended slightly, if other tests were conducted along different lanes or at different drop heights. In any case, FWD tests conducted on or before September 9, 1997, and on or after April 6, 1998, clearly show d2 positioned at 20.3 cm (8 inches) (per protocol).

Figure 20. Graph. Predicted position of D2 , unit number 058, 1997 to 1998. The figure is a scatter plot of all serial number 058 D2 sensor position predictions during 1997 and 1998 for lane one, drop height four, falling weight deflectometer tests. The X-axis is the date of testing and ranges from December 1, 1996, to November 1, 1998. The Y-axis is the predicted position of sensor D2 in inches and ranges from 0 to 12 inches parenthesis 0 to 304.8 centimeters end parenthesis. With the exception of the fall and winter of 1997–1998, the average prediction for the 2-year period is between 5 and 11 inches parenthesis 12.7 and 27.9 centimeters end parenthesis. The average prediction for the fall and winter of 1997–1998 is 9 inches parenthesis 22.9 centimeters end parenthesis.

Figure 20. Graph. Predicted position of d2, unit #058, 1997–98.

Figure 21. Graph. Same section data for D2 position, two different falling weight deflectometers.

Figure 21. Graph. Same section data for d2 position, two different FWDs.

Figure 22. Graph. SLIC plots for section 10–0102 including unit number 058, November to December 1997. The figure is a line graph showing the SLIC plots for falling weight deflectometer testing on section 10–0102 on several different test dates. The X-axis is the natural logarithm of the offset in inches and ranges from 2.0 to 4.5 inches parenthesis 5.2 to 11.4 centimeters end parenthesis. The Y-axis is the natural logarithm, positive or negative, of the normalized deflection, and ranges from minus 2.5 to 1.0. With the exception of the testing by FWD serial number 058 on November 13, 1997, and December 18, 1997, the plots are generally linear, almost parallel to each other, and rise from left to right. The plots for the testing by FWD serial number 058 on November 13, 1997, and December 18, 1997, are initially flatter than the other plots, then become parallel to them. In other words, they are not parallel to the other plots for their entire length.

Figure 22. Graph. SLIC plots for section 10–0102 including unit #058, November–December 1997.

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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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