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12 CHAPTER THREE Falling Weight Deflectometer Calibration This chapter discusses FWD calibration practices and calibrated individually, but a new reference calibration proce- recommendations. If FWDs are not calibrated, the conse- dure allows multiple sensors to be calibrated simultaneously. quences can be financially significant. According to a study SHAs typically perform reference calibrations once per year by the Indiana DOT (INDOT) (Yigong and Nantung 2006), (Appendix B, Question 14). An annual reference calibration overestimating a deflection by 0.0254 mm (1 mil) resulted in is also recommended by LTPP (Schmalzer 2006). 26% more undersealing area. This error resulted in $20,000 in unnecessary drilling and $29,000 in additional asphaltic materials. By simulating a 0.0508 mm (2 mil) deflection Calibration Procedures overestimate, $37,000 of additional drilling and $54,000 Relative of additional asphaltic materials were deemed necessary, although they were actually unwarranted. Similar trends were observed on an asphalt concrete (AC) overlay project; Differences between FWD models, sensor manufacturers, additional deflections of 0.0254 mm (1 mil) led to additional and available technology have led to several relative calibra- $11,187.50 per lane-km ($17,900 per lane-mi) for asphal- tion methods. tic materials, and 0.0508 mm (2 mil) errors led to $23,625 per lane-km ($37,800 per lane-mi) of additional materials. According to a survey conducted for this synthesis, 55% Conversely, underestimated deflections led to significantly of SHAs use a relative calibration procedure developed by reduced pavement design life. Underestimating deflections Strategic Highway Research Program (SHRP)/LTPP. This by 0.0254 mm (1 mil) translated to an AC layer 25.4 mm procedure is detailed in the Long-Term Pavement Perfor- (1 in.) thinner than needed, resulting in a decrease of 2.8 mil- mance Program FWD Reference and Relative Calibration lion equivalent single axle load of pavement life. Manual (Schmalzer 2006). Conversely, 35% of respondents said they follow their Calibration Types FWD vendor's relative calibration procedure. If the ven- dor is Dynatest or JILS, then the LTPP procedure is being Relative followed. According to information provided by KUAB, they recommend that their clients in the United States Relative calibrations ascertain sensor functionality and rela- follow the LTPP procedure and that their Swedish clients fol- tive accuracy. All sensors should produce the same output low the Swedish Road Administration (SRA) method. The when in the same position at the same site location ("Stan- SRA method places all of the FWD's sensors into a holder dard Test Method for Deflections . . ." 2005). To achieve this, and subjects them to five consecutive drops. Calibration is SHAs typically perform relative calibrations once per month successful if the largest and smallest measured deflections (Appendix B, Question 15). A monthly relative calibration differ by no more than 2 m (0.0787 mils) plus 1% of the is also recommended by LTPP (Schmalzer 2006). Relative measured value. calibrations can be performed at any location, in situations in which pavement layers are adequately strong. For example, Section 7.3.1 of ASTM D4694-96 describes a relative 44% of survey respondents stated that they perform relative calibration procedure, which uses a vertical sensor holding calibrations on a "calibration pad," a specially designed PCC tower. In a manner similar to the SRA method, five deflec- floor, and 33% stated that relative calibrations are done on an tions must be measured per sensor, and if they differ by no "in-service pavement" (Appendix B, Question 16). more than 0.3% from the average deflection then no correc- tion is required. Section 7.3.2 recommends repeating the Reference procedure some distance away from the load plate so that "if any differences in average deflection greater than 2 m (0.08 These calibrations are done at specially designed calibration mils) are found, the device should be repaired and recali- centers. Reference calibrations aim to ensure sensor accuracy brated according to the manufacturer's recommendations" according to defined benchmarks. Occasionally, sensors are ("Standard Test Method for Deflections . . ." 2005).