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

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

Appendix D. FWD SN 129, April 15, 1997–May 21, 1997

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 done 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 7 with a close to zero overall bias and the best possible precision (see appendix B). The first graph shown in this appendix (figure 16) is a plot of all the SN #129 d7 sensor position predictions during 1997 and 1998 for lane 1, drop height 4 FWD tests.
In figure 16, it can be seen that the average prediction for this 2-year period was around 152.4 cm (60 inches), as expected. The predictions are somewhat scattered, mainly due to the relatively large distance between d6 and d7. However, during the period of time in question (April 15, 1997–May 21, 1997), the position of d7 is clearly around 121.9 cm (48 inches) (average SLIC prediction for all flagged test dates = 121.9 cm (48 inches)), where in fact an empty sensor holder was usually positioned.

In figure 17, it can be seen clearly that the SN #129 predicted positions for sensor 7 in the spring of 1997 are outliers relative to the predicted positions for sensor 7 when other (correctly configured) FWDs are used. For the five test sections shown (from two different FWDs), the average predicted position of d7 was 154.2 cm (60.7 inches), while the average prediction for SN #129 during the period of time in question was 123.2 cm (48.5 inches).

In the two following graphs in this appendix, figures 18 and 19, respectively, the same results are shown graphically, with lines and data points labeled 5/13/1997 in figure 18 and 4/15/1997 in figure 19 showing the SLIC plot for d7 in both its actual position (121.9 cm (48 inches)) and the protocol but incorrect offset position (152.4 cm (60 inches)). The portions of the 5/13/1997 and 4/15/1997 lines that are parallel to the rest of the data are the correct plots, with d7 set at 121.9 cm (48 inches). Because of this information, it can be concluded with certainty that d7 was not positioned correctly at 152.4 cm (60 inches); rather, it was positioned at 121.9 cm (48 inches) (or very close to 121.9 cm (48 inches)) on FWD SN #129 between April 15, 1997 and May 21, 1997, inclusively. These dates correspond to the dates when lane 1 tests were conducted at drop height 4. This period of time may need to be extended somewhat if other tests were conducted along different lanes or at different drop heights. In any case, FWD tests conducted on or before March 25, 1997, and on or after June 3, 1997, clearly show d7 positioned at 152.4 cm (60 inches) (as per protocol).

Figure 16. Graph. Predicted position of D7, unit 129 for 1997 to 1998. The figure is a scatter plot of all serial number 129 D7 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 D7 in inches and ranges from 0 to 80 inches parenthesis 0 to 203.2 centimeters end parenthesis. With the exception of April and May 1997, the average prediction for the 2-year period is between 55 and 65 inches parenthesis 139.7 and 165.1 centimeters end parenthesis. The average prediction for April and May 1997 is 48 inches parenthesis 121.9 centimeters end parenthesis.

Figure 16. Graph. Predicted position of d7, unit #129, 1997–98.

Figure 17. Graph. Same section data for D7 position, two different falling weight deflectometers. The figure is a scatter plot illustrating predictions of D7 for two falling weight deflectometers, including serial number 129. The X-axis is the date of testing and ranges from January 17, 1989, to August 18, 1998. The Y-axis is the predicted position of sensor D7 in inches and ranges from 40 to 70 inches parenthesis 101.6 to 177.8 centimeters end parenthesis. With the exception of the testing by FWD serial number 129 around April 5,1997, the average prediction for the 2-year period was approximately 60 inches parenthesis 152.4 centimeters end parenthesis. The average prediction for the testing by FWD serial number 129 around April 5, 1997, was approximately 48 inches parenthesis 121.9 centimeters end parenthesis.

Figure 17. Graph. Same section data for d7 position, two different FWDs.

Figure 18. Graph. SLIC plots for section 24–0509 including unit number 129 in May 1997. The figure is a line graph showing the SLIC plots for falling weight deflectometer testing on section 24–0509 on four 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 0.5. With the exception of the testing by FWD serial number 129 on May 13, 1997, the plots are linear, almost parallel to each other, and rise from left to right. The plot for the testing by FWD serial number 129 on May 13, 1997 is initially parallel to the other plots, then becomes flatter. In other words, it is not parallel to the other plots for its entire length.

Figure 18. Graph. SLIC plots for section 24–0509 including unit #129, May 1997.

Figure 19. Graph. SLIC plots for section 36–4017 including unit number 129 in May 1997. The figure is a line graph showing the SLIC plots for falling weight deflectometer testing on section 36–4017 on five 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 0.5. With the exception of the testing by FWD serial number 129 on April 15, 1997, the plots are generally linear, almost parallel to each other, and rise from left to right. The plot for the testing by FWD serial number 129 on April 15, 1997, is initially parallel to the other plots, then becomes flatter. In other words, it is not parallel to the other plots for its entire length.

Figure 19. Graph. SLIC plots for section 36–4017 including unit #129, April 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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