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Publication Number: FHWA-HRT-06-132
Date: December 2006

Long-Term Pavement Performance Program Manual for Falling Weight Deflectometer Measurements

Chapter 7. Error Conditions

As described in manual section 4.3.4, several data checks should be enabled in the FWD data collection software. They are described in the following paragraphs.

7.1 ROLL-OFF

"Roll-off" is an error condition that results when the deflection of the pavement surface as recorded by a deflection sensor does not return to near zero within 60 ms of the trigger activation. Unless the pavement structure is weak enough to be permanently deformed by the load pulse, this is not a believable measurement. All LTPP test sections have sufficient strength that roll-off can only be to the result of measurement error.

Roll-off can be caused by poor contact between the deflection sensor and the pavement surface. This error can be incorrectly triggered when magnitude of the deflection approaches the resolution of the geophone. Thus, roll-off when the peak deflection is less than 25 µm (0.83 inch) is not necessarily indicative of error.

If the roll-off error is triggered on a deflection that is greater than 25 µm (0.83 inch), then the operator should follow the error resolution instructions in manual section 7.6. Otherwise the measurement should be accepted and testing should continue.

7.2 NONDECREASING DEFLECTIONS

"Nondecreasing deflections" is an error condition that results when the deflections measured by the deflection sensors do not decrease with increasing distance from the load plate. Deflections should always decrease with increasing distance from the load plate.

This condition can sometimes occur legitimately if there is a transverse crack or other discontinuity between the two sensors that exhibit the nondecreasing deflections. If the operator observes such a crack or discontinuity between the two flagged sensors, then the test should be accepted and the observation should be recorded at the "Comments" prompt.

Nondecreasing deflections can sometimes be triggered incorrectly when the magnitude of the deflections approaches the resolution of the deflection sensor. If the larger of the two deflections is less than 10 µm (0.39 inch), then the test should be accepted, and testing should continue.

Nondecreasing deflection may sometimes occur between deflection sensors 1 and 2 on extremely weak pavements because of permanent deformation of the pavement by the FWD. Nondecreasing deflections between deflection sensors 1 and 2 have also been observed on very stiff PCC pavements where the deflection basin is fairly uniform near the load plate and the difference in deflections in that area is less than the random error inherent to the deflection sensors.

If the deflections are not small and there are no cracks or discontinuities, or the nondecreasing deflections are not between sensors 1 and 2 and the pavement is not very weak or very stiff, then this error usually indicates poor seating between one or both of the flagged sensors and the pavement surface. The operator should follow the error resolution instructions in manual section 7.6.

7.3 OVERFLOW

"Overflow" is an error condition that results when a measured deflection exceeds the range of the deflection sensor. For the FWDs operated by LTPP, this range is 2,000 microns (80 mils, or 0.08 inch). Deflections of that magnitude are only expected on extremely weak pavements or when testing using extremely high load levels. For testing at an LTPP test section using the load levels specified in this document, it is not reasonable to expect that the deflections will exceed 2,000 microns (80 mils, or 0.08 inch).

If this error is encountered during testing, it is likely that the flagged sensor does not have good contact with the pavement surface. The operator should follow the error resolution instructions in manual section 7.6.

7.4 LOAD VARIATION

"Load variation" is an error condition that results when the peak load for repeat drops at the same drop height varies by more than the amount specified in manual section 4.3.4. This condition can occur legitimately on weak pavements where the structure is damaged by FWD testing, or on pavements with a saturated base or subgrade layer (such as during the spring thaw in wet-freeze zones). It may also occur if the load plate is not seated properly on the pavement surface, either because of loose debris or irregularities in the pavement surface.

If this error occurs during testing, the operator should follow the error resolution instructions in manual section 7.7.

7.5 DEFLECTION VARIATION

"Deflection variation" is an error condition that results when the load-normalized peak deflections for repeat drops vary by more than the amount specified in manual section 4.3.4. This condition can occur legitimately only if the stiffness of the pavement is changed by the FWD testing itself. This generally occurs only if the pavement is extremely weak or the unbound layers are saturated.

Deflection variations can occur as a result of uneven pavement surface conditions causing poor seating of the load plate or deflection sensors or vibrations generated by heavy equipment operating nearby, especially trucks traveling in adjacent lanes.

If this error occurs during testing, the operator should follow the error resolution instructions in manual section 7.6.

7.6 DEALING WITH DEFLECTION ERRORS

If deflection errors occur, the operator must attempt to identify the source of those errors. If the errors caused by the FWD equipment, then those problems must be fixed before testing continues. If the errors result from localized pavement conditions, the operator should reposition the FWD and comment on the condition. If the errors are the result of pavement conditions that are representative of the test section as a whole, or due to factors beyond the operator’s control such as truck traffic, the operator should accept the error and comment on the condition. Manual sections 7.1, 7.2, 7.3, and 7.5 detail possible causes of the various types of deflection errors.

If the deflection errors appear to be to the result of truck traffic in an adjacent lane, the operator should attempt to pause the test sequence to allow the trucks to pass, and then continue the sequence during lulls in the traffic. If there are too many trucks for this method to be practical, the operator should provide a comment for drop sets that were potentially affected by the truck traffic.

Operators must be very careful to not to overlook the FWD equipment itself as the source of the error. The following list is a summary of the troubleshooting process that is recommended in all cases, and the operator should deviate from the process only when confident of the source of the error, and the error source is not well suited for resolution using this troubleshooting process.

7.6.1 For the first such error at a test location, it is recommended that the operator get out of the tow vehicle and check the flagged deflection sensor (or sensors). Check that the deflection sensor is seated securely in the sensor holder, that the screws retaining the sensor magnet and sensor holder are tight, that the deflection holder is not resting on a loose stone or crack, and that the holder springs and foam bushing are in good shape.
   
7.6.2 If several sensors are flagged, it is recommended that the operator check all analog connections: sensor to control box, control box to multisignal cable and multisignal cable to signal processor.
   
7.6.3 The data should be rejected and the test repeated without repositioning the FWD.
   
7.6.4 If the errors persist, the operator must reject the test and perform the optional activities in manual section 7.6.1 and 7.6.2. If problems with the equipment are discovered and corrected, all data collected in this test pass should be discarded, and the test pass should be restarted.
   
7.6.5 If errors persist and the test being performed is a load transfer test, or if the FWD cannot be moved forward because of a joint or transverse crack, then the results should be accepted whether or not they contain errors. The operator should enter a comment stating "Error could not be resolved."
   
7.6.6 If the errors persist and the FWD can be repositioned, the operator should reject the data and move the FWD forward .6 meters (2 ft). The operator should repeat the test a fourth time. If the errors persist the operator should accept the results and at the prompt enter a comment stating "Error could not be resolved."
   
7.6.7 If the error could not be cleared, then for all subsequent errors of the same type in the same test pass the activities listed in manual sections 7.6.1 and 7.6.2 need not be repeated.

7.7 DEALING WITH LOAD ERRORS

If load errors occur, the operator must attempt to identify the source of those errors. If the errors result from problems with the FWD equipment, then those problems must be fixed before testing continues. If the errors are caused by localized pavement conditions, the operator should reposition the FWD and comment on the condition. If the errors are caused by pavement conditions that are representative of the test section as a whole, the operator should accept the error and comment on the condition. Manual section 7.4 gives details of the possible causes of load errors.

Operators must be very careful to not to overlook the FWD equipment itself as the source of the error. The following summary of the troubleshooting process is recommended in all cases, and the operator should deviate from it only if confident of the source of the error and the error source is not well suited for resolution using the troubleshooting process.

7.7.1 The data should be rejected and the test repeated without repositioning the FWD.
   
7.7.2 If the error persists, the operator should get out of the tow vehicle and check the equipment. All analog connections should be checked: load cell to load cell cable, load cell cable to control box, control box to multisignal cable and multisignal cable to signal processor. The weight height targets should be checked to ensure that they are tight. The load plate should be raised and the swivel checked to ensure that it moves easily. Check the rubber sheet and pavement surface under the load plate for debris, and remove any .
   
7.7.3 Reject the data and repeat the test without repositioning the FWD.
   
7.7.4 If errors persist and the test being performed is a load transfer test, or if the FWD cannot be moved forward because of a joint or transverse crack, then accept the results whether or not they contain errors. Enter a comment stating "Error could not be resolved."
   
7.7.5 If the errors persist and the FWD can be repositioned, reject the data and the move the FWD moved forward .6 m (2 ft). Repeat the test a third time. If the errors persist then accept the results, and enter a comment stating "Error could not be resolved."
   
7.7.6 If the error could not be cleared, then for all subsequent errors of the same type in the same test pass, the operator need not repeat the activities in step 7.7.2.

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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).
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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