1、Standard Practice for Calibrating the Load Cell and Deflection Sensors for a Falling Weight Deflectometer AASHTO Designation: R 32-11 (2015) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-5a R 32-1 AASHTO Standard
2、 Practice for Calibrating the Load Cell and Deflection Sensors for a Falling Weight Deflectometer AASHTO Designation: R 32-11 (2015) 1. SCOPE 1.1. This standard practice covers the annual calibration of the load cell and the deflection sensors and monthly relative calibration of the deflection senso
3、rs of a falling weight deflectometer (FWD) or a heavy weight deflectometer (HWD). It is used to establish calibration factors for correcting FWD and HWD measurements. For this practice, the term “FWD” also refers to an HWD unless otherwise noted. 1.2. This procedure is not applicable to the calibrat
4、ion of lightweight deflectometers or of cyclic loading and other types of pavement deflection testing equipment. 1.3. Annual calibration is performed at least once per year or as soon as possible after a deflection sensor or a load cell has been replaced on the FWD. It shall be performed by a certif
5、ied technician. 1.4. Monthly relative calibration is performed on the deflection sensors at least once per month and immediately after a deflection sensor is replaced. A certified technician is not required. 1.5. The procedure results in calibration factors that are entered into the FWD software as
6、multipliers. When the FWD measurements are multiplied by the calibration factors, the result is a set of measurements that have been corrected to agree with the calibration instrumentation. 1.6. Calibration procedures may vary slightly among FWD types. This procedure can be used for all types of FWD
7、s with minor modifications within the limits of the reference calibration equipment. 1.7. This procedure has been automated in the software package WinFWDCal. It is required that the computer program be used to carry out the procedure. 1.8. To use this procedure, the FWD computer shall be capable of
8、 electronic data transfer. 1.9. Data files for all types of FWDs can be read by the WinFWDCal software. The PDDX file format (Section 3.1.11) is required for data input. FWD data files that are in native format can be converted to PDDX format using PDDXconvert, which is an integral component in WinF
9、WDCal. 1.10. The values stated in SI units are to be regarded as standard. U.S. Customary units given in parentheses are for information purposes only. 1.11. This standard practice may involve hazardous materials, operations, and equipment. It does not purport to address all of the safety concerns a
10、ssociated with its use. It is the responsibility of the user of this standard practice to consult and establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. 2015 by the American Association of State Highway and Transportation Officia
11、ls.All rights reserved. Duplication is a violation of applicable law.TS-5a R 32-2 AASHTO 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: R 33, Calibrating the Reference Load Cell Used for Reference Calibrations for a Falling Weight Deflectometer T 256, Pavement Deflection Measurements 2.2. Other Docu
12、ment: FHWA-HRT-07-040, FWD Calibration Center and Operational Improvements: Redevelopment of the Calibration Protocol and Equipment 3. TERMINOLOGY 3.1. Definitions of Terms Specific to this Standard: 3.1.1. certified technicianan individual who has demonstrated proficiency at performing FWD calibrat
13、ions during an annual quality assurance review and been issued a certificate of compliance. The certification procedure is described in detail in Report No. FHWA-HRT-07-040. 3.1.2. data acquisition systemthe signal conditioner, data acquisition board, data acquisition software, computer, and cabling
14、. It is connected to either the reference load cell or the accelerometer. 3.1.3. drop sequencethe sequence of replicate drops at one or more load levels used during reference or relative calibration. 3.1.4. final gain factorthe calibration factor for a load or deflection sensor at the end of the cal
15、ibration procedure. 3.1.5. FWDfalling weight deflectometer; pulse loading device for measuring pavement structural response. The peak load is adjustable over a range from 13 kN (3000 lb) to 125 kN (28,000 lb). 3.1.6. FWD deflection sensordevice used to measure the pavement deflection response for a
16、given load (e.g., geophones, seismometers, accelerometers, or other devices). 3.1.7. FWD load celldevice located under the loading mechanism in an FWD that measures the load response of the FWD system. 3.1.8. HWDheavy weight deflectometer; pulse loading device for measuring pavement structural respo
17、nse. The peak load is adjustable over a range from 27 kN (6000 lb) to 270 kN (60,000 lb). 3.1.9. initial gain factorthe calibration factor for a load or deflection sensor that was present in the FWD operating program before the start of calibration. 3.1.10. interim gain factorthe calibration factor
18、for a deflection sensor resulting from averaging all trials of reference calibration. 3.1.11. Pavement Deflection Data Exchange (PDDX)the file format required by WinFWDCal. This format is described in Report No. FHWA-HRT-07-040. 3.1.12. reference calibrationthe calibration of either the FWD load cel
19、l or deflection sensors against a separate reference measuring system. For load cell calibration, the reference system is a custom-made reference load cell, and for deflection sensors, it is a precision accelerometer. 2015 by the American Association of State Highway and Transportation Officials.All
20、 rights reserved. Duplication is a violation of applicable law.TS-5a R 32-3 AASHTO 3.1.13. reference gain factorthe calibration factor for a load or deflection sensor determined by one trial during reference calibration. 3.1.14. relative calibrationa calibration procedure in which the deflection sen
21、sors are calibrated relative to one another. No outside reference system is used. 3.1.15. relative gain factorthe calibration factor for a deflection sensor determined by one trial during relative calibration. 3.1.16. WinFWDCalthe software required for data acquisition and data analysis for FWD cali
22、bration. 4. SUMMARY OF METHOD 4.1. In annual calibration, the deflection sensors and load cell from the FWD are first calibrated against independently calibrated reference devices. This calibration process, which is called “reference calibration,” can be performed at a regional calibration center or
23、 any other properly equipped location. Information on the location of these centers is available from the Long-Term Pavement Performance (LTPP) Program.14.2. The FWD load cell is calibrated against a custom-made reference load cell, which enables the FWD load cell to be calibrated without being remo
24、ved from the testing equipment. 4.3. The FWD deflection sensors are removed from their holders on the FWD and mounted in a rigid stand where they are calibrated against a precision accelerometer (with the accelerometer signal being double integrated by WinFWDCal). The sensors are stacked vertically
25、in the stand, one above another in one or two columns, so that all sensors, including the accelerometer, are subjected to the same pavement deflection. 4.3.1. Next, the calibration of the FWD deflection sensors is further refined by comparing them to each other in a process referred to as “relative
26、calibration.” In annual calibration, the same sensor stand is used for both reference and relative calibration. 4.4. Monthly relative calibration uses a relative calibration stand supplied by the FWD manufacturer. It is a quick means to periodically verify that the sensors are functioning properly a
27、nd consistently. Monthly relative calibration assumes that the overall mean deflection, as determined from simultaneous measurements by the full set of deflection sensors, yields an accurate estimate of the true deflection. This assumption requires that the deflection sensors must have first been su
28、bjected to the annual calibration procedure. 4.4.1. The relative calibration procedure used for monthly relative calibration is different from that used for annual calibration. 4.5. Acceptance criteria for annual calibration and monthly relative calibration, based upon the repeatability of the calib
29、ration factors, are identified in the calibration procedure. If accepted, the final gain factors shall be entered into the FWD operating program. 5. SIGNIFICANCE AND USE 5.1. Calibration of both the load cell and the deflection sensors provides a means of comparison of results from different FWD typ
30、es and manufacturers. 5.2. Calibration ensures consistency in the data collection among various agency FWDs. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-5a R 32-4 AASHTO 5.3. Calibration improves
31、the accuracy of pavement layer moduli derived from FWD data by means of backcalculation and other interpretive methods. 6. APPARATUS 6.1. FacilitiesIndoor space with easy access for FWD and towing vehicle, a level floor large enough so that the FWD can sit level during the test, a reasonably constan
32、t temperature (between 10 and 40C (50 and 100F), heated, but not necessarily air-conditioned, and good security for the calibration equipment. 6.2. Test AreaIsolated concrete test slab having a smooth, crack-free surface with a clear zone around the perimeter for maneuvering FWDs and the reference d
33、ata acquisition system. The test area may be isolated (by impregnated felt bond breaker, sawed and caulked joint, or similar means) from the surrounding floor area. While an isolated concrete test slab is recommended, it is not required, provided that all other facilities requirements, especially th
34、e deflection characteristics of the test area, are achieved. Note 1Slab dimensions of 4 by 5 m (12 by 15 ft) with a clear zone around the perimeter of 2.5 m (8 ft) wide are suggested. Note 2A modest amount of hairline cracking in the slab is permissible. If the test area develops cracks that are vis
35、ibly open (1.5 mm (0.06 in.) or more), it should be replaced. 6.2.1. The deflection characteristics of the test area shall be deemed satisfactory, provided that the maximum acceleration and maximum number of replicate drops required by the WinFWDCal software are satisfied. Note 3For preliminary scre
36、ening of a potential test area, the concrete slab should have a deflection of 300 m (12 mils) or more due to a 70-kN (16,000-lb) load at the position of the deflection sensor calibration stand when the FWD is in the specified position for calibration. In general, a concrete pavement on a relatively
37、weak subgrade will yield the required deflection amplitude. Note 4Calculations indicate acceptable fatigue life can be achieved with a 125-mm (5-in.) thick portland cement concrete slab resting on a 200-mm (8-in.) open-graded crushed aggregate base. A layer of filter fabric should be placed below th
38、e base to protect it from intrusion of subgrade fines. To achieve adequate deflections, the subgrade modulus should be less than 80 MPa (12,000 psi) when bedrock is deeper than 8 m (25 ft). Where bedrock exists at depths of 3 to 8 m (10 to 25 ft), the subgrade modulus should be 50 MPa (7500 psi) or
39、less. Test areas located where bedrock is less than 3 m (10 ft) deep are likely to be very sensitive to minor variations in subgrade moisture and hence are not advisable. 6.2.2. The calibration stand holder (ball-joint anchor) should be located not closer than 300 mm (12 in.) from the edge of the te
40、st area, but it is not required, nor is it possible, that the test area shall deflect uniformly across the entire area of the pad. Note 5Placing the stand holder too close to the edge of the slab may result in excessive accelerations due to the nature of the surface waves near the edge of the slab.
41、6.3. EquipmentThe following equipment is needed in the calibration facility: Note 6Drawings of each of the special items of equipment, cabling diagrams, and the data acquisition software, WinFWDCal and PDDXconvert, are available from the LTPP Program.1 The signal conditioner and data acquisition boa
42、rd described in the following sections are required for compatibility with the WinFWDCal software. 6.3.1. Signal ConditionerMeasurements Group Inc. Vishay Model 2310 signal conditioner2with power cable and stabilizer bar. 2015 by the American Association of State Highway and Transportation Officials
43、.All rights reserved. Duplication is a violation of applicable law.TS-5a R 32-5 AASHTO 6.3.2. Data Acquisition Board (DAQ)Keithley Model KUSB-3108 data acquisition board.3Note 7The DAQ board should be placed in a box housing to protect the wire connections. 6.3.3. Connecting CablesAccelerometer to s
44、ignal conditioner load cell to signal conditioner, signal conditioner to DAQ, and DAQ to computer USB port. A push-button cable powered by the DAQ is also required for communication with the WinFWDCal software. 6.3.4. ComputerA laptop or desktop computer capable of running Windows XP or higher softw
45、are, having at least two and preferably four USB ports. 6.3.5. Reference Load CellCustom-built reference load cell (300-mm diameter) calibrated annually to a 106 kN (24,000 lb) capacity. 6.3.6. Reference AccelerometerSilicon Designs Model 2220-005 5 g accelerometer mounted in a custom-built aluminum
46、 box housing. 6.3.7. Accelerometer Calibration PlatformCustom-built aluminum platform capable of being leveled. Used for daily calibration of the accelerometer and for storage of the accelerometer when it is not in use. 6.3.8. Data Acquisition SoftwareWinFWDCal and PDDXconvert.16.3.9. Deflection Sen
47、sor Calibration StandsCustom-built welded aluminum stands with shelves and associated hardware for ten deflection sensors and one reference accelerometer. One design is used with geophones and another with seismometers. Used for annual calibration. 6.3.10. Ball-Joint AnchorCustom-built hardware, bol
48、ted to the floor or test area, used for coupling the deflection sensor calibration stands to the pavement. 6.3.11. Relative Calibration StandProvided by the FWD manufacturer with as many positions as the number of active deflection sensors. Used for monthly relative calibration. 7. FALLING WEIGHT DE
49、FLECTOMETER SETUP 7.1. The FWD shall be in good operating condition prior to performing reference calibration. Particular attention shall be paid to cleaning the deflection sensor bases, where appropriate, to ensure that they seat properly in the sensor stand. Also, verify that the FWD load plate is firmly attached to the load cell and the swivel (if so equipped) is well lubricated. All cables and electrical connectors shall be inspected and, if necessary, cleaned and firmly seated. Verify that the cables are connected to the correct cha
