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

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

Appendix C. FWD SN 129, November 3, 1995–April 14, 1996

This previously reported sensor position error was also identified using the automated version of SLIC (see appendix B). This procedure incorporated 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. The first graph shown in this appendix (figure 12) is a plot of all the SN #129 d7 sensor position predictions during 1995 and 1996 for lane 1, drop height 4 FWD tests.

In figure 12, 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 due to the relatively large distance between d6 and d7. However, during the period of time in question (November 3, 1995–April 14, 1996), the average predicted position of d7 is clearly around 121.9 cm (48 inches) (average SLIC prediction for all flagged test dates = 123.2 cm (48.5 inches)). In fact, a sensor holder was positioned at 121.9 cm (48 inches).

The second figure in this appendix (figure 13), clearly shows that the SN #129 predicted positions for sensor 7 in the fall of 1995 are extreme outliers relative to the predicted positions for sensor 7 when other (correctly configured) FWDs are used. For the six test sections shown (from two different FWDs), the average predicted position of d7 was 157 cm (61.8 inches), while the average prediction for SN #129 during the period of time in question was 124.7 cm (49.1 inches).
In the two following figures (see figures 14 and 15), the same results are shown graphically, with the lines and data points labeled 11/9/1995 showing the SLIC plot for d7 in both its actual (121.9 cm (48 inches)) and protocol but incorrect (152.4 cm (60 inches)) offset position. The portions of the 11/9/1995 lines that are parallel to the rest of the protocol position data are the correct plots, with d7 set at 121.9 cm (48 inches).

Because of this information, and the other sensor position information supplied to FHWA, 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 November 3, 1995, and April 14, 1996.

Figure 12. Graph. Predicted position of D7, unit number 129, 1995 to 1996. The figure is a scatter plot of all serial number 129 D7 sensor position predictions during 1995 and 1996 for lane one, drop height four, falling weight deflectometer tests. The X-axis is the date of testing and ranges from September 23, 1994, to March 11, 1997. 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 the November 1995 to April 1996 period, the average prediction for the 2-year period is between 52 and 65 inches parenthesis 132.1 and 165.1 centimeters end parenthesis. The average prediction for the November 1995 to April 1996 period is 48 inches parenthesis 121.9 centimeters end parenthesis.

Figure 12. Graph. Predicted position of d7, unit #129, 1995–96.

Figure 13. Graph. Same section data for D7 position, two different falling weight deflectometers. The figure is a scatter plot illustrating predictions of D7 for two FWDs, including serial number 129. The X-axis is the date of testing and ranges from November 3, 1988, to June 4, 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 shortly after September 8, 1995, the average prediction for the 2-year period was approximately 61 inches parenthesis 154.9 centimeters end parenthesis. The average prediction for the testing by FWD serial number 129 shortly after September 8, 1995, was approximately 48 inches parenthesis 121.9 centimeters end parenthesis.

Figure 13. Graph. Same section data for d7 position, two different FWDs.

Figure 14. Graph. SLIC plots for section 34–0503 including unit number 129, November 1995. The figure is a line graph showing the SLIC plots for falling weight deflectometer testing on section 34–0503 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 1.0. With the exception of the testing by FWD serial number 129 on November 9, 1995, 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 November 9, 1995, is initially flatter, then is parallel to the other plots for a short range, then again becomes flatter. In other words, it is not parallel to the other plots for its entire length.

Figure 14. Graph. SLIC plots for section 34–0503 including unit #129, November 1995.

Figure 15. Graph. SLIC plots for section 34–0507 including unit number 129, November 1995.

Figure 15. Graph. SLIC plots for section 34–0507 including unit #129, November 1995.

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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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