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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 4. Setup


The physical setup described in this manual is mandatory for all LTPP-owned FWDs. Other FWDs collecting data on behalf of LTPP may not be capable of the setup described. In those cases, the setup should be followed as closely as possible.

4.1.1 Geophone Locations

For all LTPP testing, maintain the same placement of the geophones on the FWD. Measure geophone offsets from the center of the load plate to the center of the geophone holder. Measure the location of each geophone directly from the center of the load plate to avoid accumulated error. Offsets in front of the load plate (i.e., in the direction of the hitch) are considered positive. Offsets behind the load plate (i.e., in the direction of the rear bumper) are considered negative. The required offsets are shown in Table 1.

Table 1. Deflection sensor offsets for nine sensor FWDs.
Deflection SensorOffset
D10 mm
D2203 mm
D3305 mm
D4457 mm
D5610 mm
D6914 mm
D71,219 mm
D81,524 mm
D9–305 mm
1mm = 0.039 inch

If testing is to be performed on behalf of LTPP by an FWD that mounts only seven deflection sensors, use the sensor spacings shown in Table 2.

Table 2. Deflection sensor offsets for seven sensor FWDs.
Deflection SensorOffset
(Flexible Pavements)
(Rigid Pavements)
D10 mm0 mm
D2203 mm–305 mm
D3305 mm305 mm
D4457 mm457 mm
D5610 mm610 mm
D6914 mm914 mm
D71,524 mm1,524 mm
1mm = 0.039 inch

Non LTPP-operated FWDs and LTPP-operated FWDs that have undergone overhaul or replacement of the sensor bar or deflection sensor holders must have the deflection sensor offset measured and set accurately using the following procedure.

  1. Raise the FWD load plate and engage the transport locks and hitch pin in front sensor bar guide. Make sure the "Man Key" on the control panel is switched to "ON."
  2. Check the springs and foam rubber guides on all deflector holders to ensure they are in good condition. Make sure that spring tensions are properly adjusted so that a force on the end feeler can move the holder and feeler upward until the feeler is at least 5 mm (0.2 inch) inside the bottom of the holder body, and that it returns easily when released. (If this does not occur, apply a few drops of silicone oil to the top of the guide rod).
  3. Use a steel tape measure with 1 mm (0.039 inch) graduations and 3 meters (m) (9.84 ft) or greater in length, and zero it on the center geophone feeler that projects through the bottom of the loading plate. Apply constant positive pressure on tape to eliminate any sag in tape throughout its length. Measure the location of every other defection sensor from this zero point to the center of the sensor’s feeler. It is not adequate to measure only from center to center between individual deflector sensors; this measurement alone will result in an accumulating error. By measuring from the rear of the contact screw, a more repeatable and accurate measurement can be taken. To do this it is necessary to compensate in the measurement for the distance from the outer edge of the contact screw to the center. Check position measurements at least twice.
  4. For LTPP-operated FWDs, when the location of the deflection sensor holders is properly set, lock them in place. To do this, drill through the top of the deflection sensor holder where it passes over the sensor bar and through the sensor bar itself with a 6.7-mm (17/64-inch) bit. Tap the sensor bar itself to accommodate a 6M x 1.0 mm (thread pitch) screw. The deflection sensor holder can now be secured with a 6M x 1.0 mm to 20mm stainless steel hex-head screw. Retain the screw with a flat metal washer and medium thread-locking compound.

4.1.2 Weight Package

Configure the weight package the same for all LTPP testing. For Dynatest model 8002 FWDs, use three standard weights per side. Use two buffers per side. When new buffers are installed on the FWD, fill out form F04 and submit it to the FHWA LTPP FWD task leader.

If testing is to be performed on behalf of LTPP by an FWD other than a Dynatest model 8002 FWD, select a combination of buffers and weights that achieves the load requirements described in manual section 4.2 and comes as close as possible to a 13-millisecond (ms) pulse rise time.


Four load levels are defined here for LTPP testing. The acceptable load range for each drop height is between 90 percent and 110 percent of the target value. Experience has shown that drop loads for a given drop height tend to decrease slightly over the course of a day of testing. Setting the drop load at 103 percent of the target load at the beginning of the day will minimize the deviation over the course of the day for most cases.

Each drop height must be within the acceptable range shown in Table 3 for all testing. Table measurements are in kilonewtons (kN) and kips (1 kip = 1 × 103 lb). Drop heights may not be adjusted during a test pass.

Table 3. Target loads and acceptable ranges.
Drop HeightTarget Load, kN (kips)Acceptable Range, kN (kips)
126.7 (6.0)24.0 to 29.4 (5.4 to 6.6)
2 40.0 (9.0)36.0 to 44.0 (8.1 to 9.9)
3 53.4 (12.0)48.1 to 58.7 (10.8 to 13.2)
4 71.2 (16.0)64.1 to 78.3 (14.4 to 17.6)
Note: Drop height 1 is not used for testing on rigid pavements.


This section includes specific software settings required for LTPP testing. Instructions on how to enter these settings into the data collection software are given in the LTPP FWD Data Collection Software Manual.

4.3.1 Units

All FWD data collected for LTPP should be in International System (SI) units, with the exception of station units, which should be in feet. Specifically, temperature should be recorded and displayed in Celsius (C), load in kilonewtons (kN), deflection in micrometers (µm) (microns), and deflection sensor offsets in millimeters (mm).

4.3.2 File Format

Data collected with LTPP FWDs should be in the FWDWin MDB format. Data collected with non-LTPP seven -sensor FWDs should be in the R80-20 format, where possible. For FWDs not supporting either of these formats, contact the FHWA LTPP FWD task leader for instructions with a copy to the Technical Support Services Contractor (TSSC) before testing begins.

4.3.3 Filters

Collect data with all filters and smoothing turned off.

4.3.4 Data Checks

Enable the following checks:

Further information on what these checks are and what to do if they fail appears in manual section 6.

4.3.5 Drop Sequences

Two different drop sequences are used for LTPP testing:

Where C is a seating drop (no data saved) from drop height 3,
 1 is a drop from drop height 1,
2 is a drop from drop height 2,
3 is a drop from drop height 3,
4 is a drop from drop height 4,
H indicates that the full-time history for that drop is to be saved.

In addition, one of the following drop sequences is used to warm up the buffers before testing:

Furthermore, the reference calibration and the relative calibration drop sequences from the "SHRP-LTPP FWD Calibration Protocol" should be preprogrammed into the LTPP-owned FWDs.

4.3.6 File Naming

The file name for an FWD data file should be eight characters in the following format: XXYYYYZN

Where XX is the state code for section location,
 YYYY is the LTPP section ID for the section,
Z represents the site visit (A for the first visit, B for the second visit, and so on), and
N represents the pass number.

For example, for the first pass of the first visit to section 3807 in North Carolina, the file name should be 373807A1. Test passes are described in detail in manual section 5.2.

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