Falling Weight Deflectometer FWD Reference Calibration An Overview Demonstration

1. Falling Weight Deflectometer (FWD) Reference Calibration An Overview Demonstration

3. Falling Weight Deflectometer (FWD) Calibration Procedures A Product of the Long-Term Pavement Performance Program

4. What is a FWD? Comprised of a tow vehicle and/or trailer Designed to impart a dynamic load to a pavement structure Measures deflection of the pavement surface Simulates a moving wheel load.

5. What is it Used For? Structural capacity/remaining life estimates Load transfer efficiency between PCC pavement joints Void detection in PCC pavements Network level pavement management Project level pavement investigations Many research applications.

6. Why Calibration? FWD is a complex system Standards needed to ensure consistency between FWD units High cost of obtaining bad data (economic and political).

7. FWD Calibration Components LTPP FWD Calibration Centers LTPP FWD Calibration Equipment Center Equipment Calibrations Implementation and Training Calibration Center Certification Program Reference Calibration Procedure FWDREFCL Software Relative Calibration Procedure FWDCAL Software.

8. FWD Calibration

9. FWD Calibration Purpose - Calibration of FWD unit to known, or reference, standards Reference standards are NIST traceable where applicable Procedure completed in one day (assuming no problems) Reference Calibration should be performed at certified FWD calibration center for best results.

10. Product Development Background 1987 - Strategic Highway Research Program (SHRP) selects FWDs, need for calibration established 1988 – Original FWDs procured 1989 – Calibration procedures developed 1989 – FWD testing begins 1992 - FHWA assumes leadership of LTPP.

11. Calibration Centers Four LTPP sponsored – State owned Minneapolis, MN - North Central Region Reno, NV - Western Region Harrisburg, PA - North Atlantic Region College Station, TX - Southern Region (All LTPP Certified 1999) Non-LTPP Kansas State University (Kansas DOT) Purdue University (Illinois DOT) Dynatest (FWD manufacturer)

12. Calibration Centers Equipment In-place - Status Unknown South Africa Australia Cornell University [original developer] Potential Areas of Interest Brazil Saudi Arabia South Korea Australia New Zealand China.

13. Calibration Process Reference Calibration of FWD unit to known, or reference, standards Relative Comparison of FWD deflection sensors to one another Reference and relative calibration performed at the same time Relative can be performed on its own.

14. Frequency of Calibration Reference Once per year After equipment (load cell, deflection sensor, computer components, etc.) have been repaired or replaced Whenever data is suspect.

15. Frequency of Calibration Relative Immediately after reference calibration AND………….. Once per month After deflection sensor(s) has been repaired or replaced After a period of equipment inactivity Prior to a major project Whenever data is suspect.

16. Reference Calibration Equipment General Equipment List Building to house calibration center Isolation (test) pad Reference load cell LVDT (deformation measurement device) Aluminum I-beam Micrometer calibration stand Data acquisition/signal conditioners Computer/printing capabilities Calibration software.

17. Reference Calibration Equipment

18. Reference Calibration Equipment

19. Reference Calibration Equipment

20. Reference Calibration Building Building Requirements Indoor space Easy access for FWD and tow vehicle Floor large enough for FWD and tow vehicle to be level and completely indoors Constant temperature (50-100 F) Constant humidity (40-90 %) Adequate security to protect investment.

21. Reference Calibration Test Pad

22. Reference Calibration Process What to Expect Output is a set of multipliers used to adjust “raw” sensor readings Calibration center staff run reference calibration equipment and software FWD operator runs the FWD and responsible for updating FWD calibration parameters FWD operator must be able to competently run the FWD.

23. Reference Calibration Process What NOT to Expect Perfect calibration results - calibration has an allowable level of error Calibration center to provide spare parts Calibration center to serve as a repair shop.

24. Reference Calibration Process Preparations for FWD FWD in good operational condition Maintenance up to-date Proper FWD software Proper programmed drop sequence Adequate supply of spare parts Relative calibration stand.

25. Reference Calibration Process

26. Reference Calibration Pre-Calibration Preparation Perform buffer warm-up procedures Position FWD on test pad Allow FWD temperature to stabilize Move reference computer as close as possible to FWD computer Load FWDREFCL into reference system computer Turn off FWD filters Disengage “peak smoothing” option.

27. Reference Calibration -Load Cell FWD Load Cell Calibration Procedure performed at least twice and results averaged Set FWD mass and drop heights to produce loads within 10 percent of:

28. Reference Calibration -Load Cell FWD Load Cell Calibration Position FWD load plate near center of test pad Position reference load cell under FWD load plate Complete reference system zero process.

29. Reference Calibration -Load Cell FWD Load Cell Calibration Perform following drop sequence 3 seating drops @ height 1, pause 5 drops @ height 1, pause after each drop 5 drops @ height 2, pause after each drop 5 drops @ height 3, pause after each drop 5 drops @ height 4, pause after each drop Note: 6 drops may be used to provide a “spare” for data analysis Load plate should not be raised at any time Record data from FWD load cell and reference system Repeat.

30. Reference Calibration -Load Cell FWD Load Cell Calibration If the two calibration factors agree within 0.0030, average the results If not, perform third calibration If SD of three results < 0.0030, average the three results If SD > 0.0030, repeat entire procedure.

31. Validity of Test Results -Load Cell

32. Reference Calibration - Sensors FWD Deflection Sensor Calibration Initialize data acquisition system Clean reference LVDT Ensure smooth operation of LVDT Calibrate reference LVDT Enter calibration results into computer Secure LVDT in holder on aluminum beam.

33. Reference Calibration - Sensors FWD Deflection Sensor Calibration Perform drop sequence as per load cell calibration procedure 3 seating drops @ height 1, pause 5 drops @ height 1, pause after each drop 5 drops @ height 2, pause after each drop 5 drops @ height 3, pause after each drop 5 drops @ height 4, pause after each drop Note: 6 drops may be used to provide a “spare” for data analysis.

34. Reference Calibration - Sensors FWD Deflection Sensor Calibration Valid test results No movement in calibration beam (view time history plots) No excessive noise messages for drop heights 2, 3 or 4 Standard deviations for five readings at any drop height that differ by more than a factor of three Standard error of adjustment factor < 0.0020 Average peak deflection at highest drop height < 16 mils (0.016) If any violated, repeat test.

35. Validity of Test Results - Sensors

36. Reference Calibration Data Analysis Calculations performed automatically by FWDREFCL software Perform least squares regression for all data for each measurement device Repeat any tests that did not meet the acceptance criteria outlined previously.

37. Reference Calibration Post-calibration Procedures Enter new calibration factors into FWD software FWD load cell calibration is a “final” calibration factor FWD deflection sensor are “interim” factors Final factors to be determined during relative calibration.

38. Relative Calibration

39. Relative Calibration Procedure Remove deflection sensors from FWD holders Label sensors 1 to n n = number of sensors Sensor in load plate = 1, furthest sensor from the load plate = n Label top-most sensor stand position “A”. Label lower positions in increasing alphabetical order (e.g. A, B, C, D, E, F …).

40. Relative Calibration Position sensors in stand A = sensor 1 B = sensor 2, etc. Support sensor stand in vertical position close to load plate Mark position of stand on floor Tip 1: Glue washer on the floor Tip 2: Chisel out divot.

41. Relative Calibration Select FWD drop height Goal: deflections = 16 – 24 mils Test pad can usually be used Warm up FWD buffers/condition test point Repeat series of 10 drops Load and deflection nearly uniform No increasing or decreasing trend in load or deflection.

42. Relative Calibration Lower FWD load plate Do not raise plate or move FWD during calibration Perform two seating drops Perform five replicate drops Rotate sensors Repeat until all sensors have been in every position in stand.

43. Relative Calibration Data Analysis Performed by FWDCAL3 software Three-way analysis of variance Determines source of error sensor number position in calibration stand data set.

44. Relative Calibration Valid Test Results Repeat of procedure yields calibration factors within 0.003 for each sensor In this case, average results If not perform third calibration If third calibration performed Standard deviation < 0.003, average results Standard deviation > 0.003, repeat relative calibration procedure.

45. Relative Calibration

46. Relative Calibration Select FWD drop height Goal: deflections = 16 – 24 mils Test pad can usually be used Warm up FWD buffers/condition test point Repeat series of 10 drops Load and deflection nearly uniform No increasing or decreasing trend in load or deflection.

47. Relative Calibration Post-calibration Procedures Enter new calibration factors into FWD software FWD deflection sensor are “final” factors.

48. Relative Calibration Relative Calibration Performed on Monthly Basis Find pavement to produce suitable deflections Perform as per procedure Adjustment to sensor calibrations only performed when adjustment ratios indicate need When sensors changed, perform relative calibration process.

49. FWD Calibration Reports FWD unit information (serial #, id) Transducer calibration information Load cell calibration information Printouts from FWDREFCL Printouts from FWDCAL3 Final calibration computation worksheet Diskettes with cal. data kept in FWD Backups kept at the office.

50. Who Should Use the Product? Organizations performing FWD testing State DOT’s Other Government entities Consultants Academia Organizations contracting for FWD services Should be a part of contract to have FWD reference calibrated within 1 year, relative calibrated prior to job.

51. When Do You Use the Product? General Reference – once per year Relative – After reference and once per month thereafter After a period of inactivity Other recommendations When equipment is replaced When equipment is suspect Whenever a major malfunction occurs.

52. Benefits of Use Provides guidelines for calibration of testing equipment Provides a benchmark performance standard for equipment Minimizes equipment variability Promotes greater confidence in FWD testing and resulting pavement design Minimizes risk of errant pavement rehabilitation designs.

53. What’s Ahead Calibration software to be updated to Windows-NT format Calibration software upgraded to accept PDDX format Continue to promote FWD calibration procedures.