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This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-06-132
Date: December 2006

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

Chapter 6. Other Measurements Associated With Fwd Testing


Up to five holes should be drilled in the bound layers of the pavement for measuring sub-surface temperatures. Only holes that terminate in an unbound layer may be eliminated. Table 7 shows the depths to which the holes should be drilled.

Table 7. Temperature hole depths.
Hole NumberHole Depth (mm)
1 mm = 0.039 inch

All holes should be drilled as close as possible to the depths listed in table 7. The tolerance for hole number 1 is ±5 mm (±0.2 inch). The tolerance for holes 2 through 5 is ±10 mm (±0.39 inch). The holes should be drilled in decreasing order of depth to allow extra time for the heat generated during drilling to dissipate.

If a hole is within ±25 mm (±0.98 inch) of the reported bottom of the bound layers, its depth should be decreased to 25 mm above the reported bottom of the bound layers.

The holes should be drilled in the center of the OWP, but they should be offset at least 0.5 m (1.64 ft) from each other in the longitudinal direction. For GPS sections, the holes should be located on both ends of the test section, just outside the monitoring area. Typically the holes should be located near station 0-001 (0-03) and 0+153.4 (5+03). For SPS sections, holes should be located at only one end of the section, just outside the monitoring area. The operator should choose the end of the section that is most representative of the monitoring area.

Holes should be drilled with a portable hammer drill using a 13-mm (0.51-inch) diameter bit. After the hole is drilled to the required depth, it should be cleared of debris and dust by blowing through a short piece of plastic tubing or other suitable device. The actual depth of the hole should be measured after it is cleared of debris. Then it should be filled with 15 to 25 mm (0.59 to 0.98 inch) of mineral oil to provide thermal conductivity between the pavement and the temperature probe. The hole should be covered with tape (such as duct tape) and the tape slit to allow the probe to be inserted.

In addition to subsurface temperature measurements, an infrared (IR) surface temperature measurement should be obtained at the same time that subsurface temperature readings are obtained. The location of the measurement should also be in the OWP and at least 0.5 m (1.64 ft) from the nearest hole in the longitudinal direction. The operator should take care that the area where the IR surface temperature is taken is free of oil, dirt, and other foreign debris. During temperature measurements the hand-held IR device should be held at a height consistent with the height of the FWD-mounted IR measurement device. IR temperate measurements should be taken three times in quick succession. The results should be averaged and entered on form F01.


Joint/crack width measurements should be taken only on PCC surfaced pavements. An exception is testing done in accordance with test plan 11. On flexible surfaced pavements, cracks should be noted according to the instructions in manual section 6.3.

On PCC surfaced pavements, joint/crack width measurements should be taken for at least 25 percent of the load transfer tests; however, operators are encouraged to perform such measurements for all load transfer tests if time permits.

Joint/crack width measurements should be performed using calipers with tapered jaws for measuring inside dimensions. The resolution of the calipers should be at least 0.3 mm (0.01 inch).

On transverse cracks, the goal is to measure the minimum width of the opening that extends through the pavement. If the cracks are spalled, the width of the opening may need to be estimated.

On sawed joints, the goal is the measure the sawn width (as opposed to the actual opening). It may be necessary to depress the joint sealant to measure the opening, especially if the joints are spalled.

Joint/crack width openings should be measured at several points in the OWP, and the measurements averaged and rounded to the nearest 1 mm (0.039 inch). This average should be entered into the FWD data collection software in the appropriate field.

Measurements less than 1 mm are hard to make with calipers because the jaws are too wide to enter the opening. In these situations, the measurement should be recorded as 1 mm (0.039 inch). Measurements in excess of 25 mm (0.98 inch) should be recorded as 25 mm (0.98 inch).


The type and severity of pavement distress may influence the deflection response of a pavement; therefore, FWD operators should record any distress within the lane tested from about 0.3 m (11.81 inches) in front of the forward most deflection sensor to 0.9 m (35.4 inches) behind the load plate and from 0.3 m (11.81 inches) on the left of the load plate to 0.3 m (11.81 inches) on the right of the load plate. The operator should enter this information at the "Comments" prompt immediately following the test sequence. Where possible, operators should use standard abbreviations listed in tables 8 and 9. Operators should avoid other abbreviations unless they are necessary because of space limitations.

Table 8. Standard abbreviation list—distress.
CategoryFull Word or PhraseStandard Abbreviation
Block crackingBCR
Corner breakCBRK
Durability crackingDCR
Edge crackECR
Fatigue crackFCR
Joint seal damageJTSD
Lane shoulder dropoffLSDROP
Longitudinal crackLCR
Map crackingMAP
Polished aggregatePOLAG
Rough surfaceRSURF
Transverse crackTCR
Transverse construction joint damageTCJTD
Low severityLSEV
Moderate severityMSEV
High severityHSEV
Table 9. Standard abbreviation list—equipment, lane geometry, miscellaneous.
CategoryFull Word or PhraseStandard Abbreviation
EquipmentLoad plateLP
Deflection sensorDS (or DS1, DS2 …)
Excess variationEXVAR
Nondecreasing deflectionsNDD
Sensor barSBAR
Air temperatureATMP
Surface temperatureSTMP
Lane GeometryInner wheel pathIWP
Outer wheel pathOWP
Both wheel pathsBWP
Outer lane edgeOLE
Inner lane edgeILE
Lane stripeLSTR
Shoulder stripeSHSTR
Longitudinal jointLJT
Transverse jointTJT
Core holeCHOLE
Due to time limitationsDTTL
Due to weather conditionsDTWC
Moved forwardMVFRD
Test pitTPIT
Truck trafficTRKTRF


In addition to distress comments, other unusual conditions or events deserve comments. Operators should comment on data with nondecreasing deflections, excess variation, or other software-generated errors that could not be cleared by following the instructions in manual section 6. Operators should also comment on other events such as delays in testing because of breakdowns or weather, pavement changes within the section, moisture seeping out of cracks, or other conditions that could affect deflection measurements.

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