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Publication Number: FHWA-RD-03-093
Date: August 2006
Study of Long-Term Pavement Performance (LTPP): Pavement Deflections
APPENDIX J. FWD SN 061, JULY 17, 1995–OCTOBER 31, 1995
This previously reported sensor position error was located in the database using an automated screening version of SLIC. This screening version is tailored for sensor 7 (among others), with a close to zero overall bias and the best possible precision (see appendix B). The first three graphs shown in this appendix (see figures 38, 39, and 40) are plots of all the SN #132 d7 sensor position predictions during 1995 to 1997 for lane 1, drop height 4 FWD tests. In these graphs, it can be seen that the average prediction for this 3-year period, excepting the data for the dates in question, was around 152.4 cm (60 inches), as expected. However, during the period of time in question (July 29, 1996 to October 25, 1996), the average predicted position of d7 is clearly around 121.9 cm (48 inches) (average SLIC prediction for all flagged test dates = 122.2 cm (48.1 inches)). In fact, an empty sensor holder is generally positioned at 121.9 cm (48 inches).
In the fourth figure in this appendix, figure 41, it clearly can be seen that the SN #132 predicted positions for sensor 7 in mid-1996 are outliers relative to the predicted positions for sensor 7 when other (correctly configured) FWDs are used. For the seven test sections shown (from three different FWDs), on average the predicted position of d7 was 154.9 cm (61 inches) while the same prediction for SN #132 during the period of time in question was 121.4 cm (47.8 inches).
In the two figures that follow, figures 42 and 43, the same results are shown graphically, with the gray lines and data points showing the SLIC plots for d7 in both its actual (121.9 cm (48 inches)) and protocol but incorrect (152.4 cm (60 inches)) offset positions. The gray lines that are parallel to the rest of the lines are the correct plots, with d7 set to 121.9 cm (48 inches).
Because of this information, and the previous information supplied to FHWA, it can be concluded with certainty that d7 was positioned at 121.9 cm (48 inches) (or very close to 121.9 cm (48 inches)) on FWD SN #132 between July 29, 1996 and October 25, 1996. 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 slightly, if other tests were conducted along different lanes or at different drop heights. In any case, FWD tests conducted on or before June 6, 1996, and on or after November 5, 1996, clearly show d7 positioned at 152.4 cm (60 inches) (per protocol).
Figure 38. Graph. Predicted position of d7, unit #132, 1995.
Figure 39. Graph. Predicted position of d7, unit #132, 1996.
Figure 40. Graph. Predicted position of d7, unit #132, 1997.
Figure 41. Graph. Same section data for d7 position, three different FWDs.
Figure 42. Graph. SLIC plots for section 48–k310 including unit #132, July 1996.
Figure 43. Graph. SLIC plots for section 48–k350 including unit #132, August 1996.