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Publication Number: FHWA-RD-02-088
Date: May 2003

Evaluation of Joint and Crack Load Transfer Final Report

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This section presents the results of an assessment of FWD deflection data used in joint/crack LTE calculation and describes the procedure used to determine representative LTE parameters. 

Deflection Testing Details

The deflection data were downloaded during the summer of 2001 from LTPP database table MON_DEFL_DROP_DATA.  Information about sensor locations was obtained from database tables MON_DEFL_LOC_INFO and MON_DEFL_DEV_SENSORS (June 2001 release).  For rigid pavements in the LTPP program, the following types of deflection tests are conducted:

  1. Center slab tests (J1 and C1 tests).
  2. Corner tests (J2 and C2 tests).
  3. Midpanel at the pavement edge tests (J3 and C3 tests).
  4. LTE of joints/cracks tests (J4, J5, C4, and C5 tests).

For this study, only the LTE test data were used.  For the LTE testing, the FWD load is applied at one side of the joint or crack and the deflections are measured at both sides of the joint or crack.  The LTE testing with the load plate placed at the leave side of the joint requires deflection sensors placed at 0 and 305 mm (0 and 12 inches) from the center of the load plate.  The LTE testing with the load plate placed at the approach side of the joint requires deflection sensors placed at 0 and -305 mm (0 and -12 inches) from the center of the load plate.

The load sequence, as stored, for rigid pavement testing is as follows:

Drop Height No. of Drops Target Load, kN Acceptable Range, kN
2
3
4
4
4
4
40.0
53.3
71.1
36.0 to 44.0
48.1 to 58.7
64.1 to 78.3

For JCP, LTE tests are performed along the midlane path at each tested slab, and the test locations are designated as J4 for loads placed at the leave slab and J5 for loads placed at the approach slab.  The number of panels can vary from as few as 9 or 10 to as many as 35 or more on a 152.4-m (500-ft)-long section.  Regardless of the total number of panels present, no more than 20 panels are tested at one section.  For the CRCP, deflection basin tests are also performed along the midlane path at spacing of about 7.6 m (25 ft) and on both sides of a crack.  The test locations are designated as C4 and C5 for leave and approach panel loading, respectively.  For CRCP, two adjacent transverse cracks are typically at a spacing of 0.3 to 2.5 m (1 ft to 8 ft).  Tests are performed at 20 effective panels.

Deflection Data Assessment

A total of 850,791 raw deflection basins were extracted from the LTPP database for 581 JCP sections and 116 CRCP sections, as shown in table 3.  The extracted data were examined to ensure their consistency and reasonableness, and some data were excluded from the analysis.  The reasons for data rejection were:

Two percent of the basins (17,214 basins) were eliminated from the analysis.  LTEs were calculated for the remaining 833,577 basins, but 13,181 of them were identified as questionable and excluded from calculation of representative statistical indexes for joint and section LTEs.  The reason for the rejection was inconsistency with other measurements for the same time of testing, joint, load plate location, and load level.

The following procedure was used to examine the consistency of the FWD measurements.  First, for each time of testing, location, and FWD load and level, the average loaded and unloaded deflections and applied pressure were calculated.  After that, each FWD basin was tested on its deviation from the mean values.  The basin was rejected if at least one of the following conditions was violated:

Equation 14 The falling weight deflectometer sensor deflection on the loaded side of the crack/joint in microns is greater than the sum total of 0.99 times the mean falling weight deflectometer sensor deflection on the loaded side of the crack/joint for the same time for testing, joint location, and falling weight deflectometer load level in microns minus 2, and less than the sum total of 2 plus 1.01 times the mean falling weight deflectometer sensor deflection on the loaded side of the crack/joint for the same time of testing, joint location, and falling weight deflectometer load level in microns.                                               (14)

Equation 15 The falling weight deflectometer sensor deflections on the loaded side of the crack/joint in microns is greater than the sum total of 0.99 times the mean falling weight deflectometer sensor deflection on the loaded side of the crack/joint for the same falling weight deflectometer, joint location, and falling weight deflectometer load level in microns minus 2, and less than the sum total of 2 plus 1.01 times the mean falling weight deflectometer sensor deflection on the loaded side of the crack/joint for the same falling weight deflectometer, joint location, and falling weight deflectometer load level in microns.                                                 (15)

Equation 16 The total falling weight deflectometer load level is greater than the sum total of 0.98 times the mean falling weight deflectometer load for the same falling weight deflectometer pass, joint location, and falling weight deflectometer load level in kilo Newtons minus 2.5, and less than the sum total of 2.5 plus 1.02 times the mean falling weight deflectometer load for the same falling weight deflectometer pass, joint location, and falling weight deflectometer load level in kilo Newtons.                                                 (16)

where:

-          FWD sensor deflection on the loaded side of the crack/joint, microns = FWD sensor deflection on the loaded side of the crack/joint, microns.

-          FWD sensor deflection on the loaded side of the crack/joint for the same time of testing, joint location, and FWD load level microns = mean FWD sensor deflection on the loaded side of the crack/joint for the same time of testing, joint location, and FWD load level microns.

-          FWD sensor deflection on the loaded side of the crack/joint, microns = FWD sensor deflection on the loaded side of the crack/joint, microns.

-          FWD sensor deflection on the loaded side of the crack/joint for the same FWD pass, joint location, and FWD load level, microns = mean FWD sensor deflection on the loaded side of the crack/joint for the same FWD pass, joint location, and FWD load level, microns.

-          P and FWD sensor deflection on the loaded side of the crack/joint, microns = total FWD load and mean FWD load for the same FWD pass, joint location, and FWD load level in kN, respectively.  They are defined as follows:

Equation 17 The total falling weight deflectometer load equals pi times the sum total of the falling weight deflectometer plate radius in millimeters squared, times the falling weight deflectometer pressure.                                                              (17)

Equation 18 The mean falling weight deflectometer load for the same falling weight deflectometer pass, joint location, and falling weight deflectometer load level in kilo Newtons equals pi times the falling weight deflectometer plate radius in millimeters squared, times the mean falling weight deflectometer pressure for the same falling weight deflectometer pass, joint location, and falling weight deflectometer load level in kilo Newtons.                                                                  (18)

where:

-          a = FWD plate radius, mm.

-          p and FWD pressure for the same FWD pass, joint location, and FWD load level in kN  = FWD pressure and mean FWD pressure for the same FWD pass, joint location, and FWD load level in kN, respectively.

Only 597 basins were rejected because of high variability in FWD load magnitude measurement.  Significantly more basins were rejected because of variability in loaded or unloaded deflection, as shown in table 2.

Table 2.  Number of basins rejected because of high variability.

Reason for Rejecting Number of Basins

High variability in FWD load magnitude

   597

High variability in loaded deflection

4,293

High variability in unloaded deflection

6,784

High variability in both loaded and unloaded deflection

1,507

Overall, the quality of the deflection data was found to be very high.  More than 96 percent of the deflection basins measured for almost 700 sections were accepted for future analysis. Table 3 shows the distribution of the accepted and rejected basins for each test types.

Table 3.  Availability of deflection data.

Test Type Number of Sections Represented Number of Records Number of Excluded Records

C4 (approach)

 116

  69,025

     847

C5 (leave)

 116

  65,172

     805

J4 (approach)

  581

355,825

  7,243

J5 (leave)

  572

343,555

  4,286

Total

1,385

833,577

13,181

Load Transfer Index Calculation Procedures

From the basins accepted after initial screening, representative LTE parameters were calculated for each deflection basin, for each joint, and for each FWD pass.  Analyses were performed separately for approach and leave tests.  Procedures for calculation of each set of parameters are presented below.

First, for each deflection basin extracted from the LTPP database, deflection LTE is calculated using equation 1.  After that, statistical summaries of LTE for each joint were computed.  This involved the following steps:

Step 1.  Compute mean LTE for joint/crack and FWD load level

For each FWD pass, joint/crack location, and drop height, mean LTE was computed.  Only LTEs from deflection basins that passed the criteria defined by equations (14) through (16) were used for computing these parameters.

Step 2.  Compute LTE crack/joint statistics

For each FWD pass and joint/crack location, the following parameters were computed:

LTEs from all FWD load levels were used to compute these parameters.

Step 3.  Determine joint/crack LTE load dependency index

Using mean LTE for nominal load levels of 40 kN (drop height equal to 2) and 70 kN (drop height equal to 4), LTE load dependency index was identified using the following criteria:

Equation 19 The flag indicating if joint/crack load transfer efficiency equals: 

·	1, if the absolute value of the mean load transfer efficiency for the crack/joint for nominal load levels of 70 kilo Newtons percent minus the mean load transfer efficiency for the crack/joint for nominal load levels of 40 kilo Newtons is less than 5.

·	2, if the mean load transfer efficiency for the crack/joint for nominal load levels of 40 kilo Newtons percent minus the mean load transfer efficiency for the crack/joint for nominal load levels of 70 kilo Newtons percent is less than 5

·	3, if the mean load transfer efficiency for the crack/joint for nominal load levels of 40 kilo Newtons percent minus the mean load transfer efficiency for the crack/joint for nominal load levels of 70 kilo Newtons percent is greater than 5.                         (19)

where:

-          LTE_LOAD_LEVEL_FLAG = a flag indicating if joint/crack LTE depends on FWD load level

= 1, if load level independent

= 2, if LTE increases with load

= 3, if LTE decreases with load

-          LTE_40 = mean LTE for the crack/joint for nominal load levels of 40 kN, percent.

-          LTE_70 = mean LTE for the crack/joint for nominal load levels of 70 kN, percent.

Second, once LTE parameters for each joint were calculated, statistical summaries of LTE for each section were created.  These parameters were calculated for each FWD pass.  This required performing the following steps:

Step 1.  Determine FWD pass number

To compute statistics of joint/crack LTE parameters for the SMP LTPP sections, FWD deflection basins were grouped by FWD passes.  Each FWD test and corresponding LTE parameters received an FWD pass number from 1 to 9.  The following procedure was used:

Step 2.  Compute LTE section statistics for each FWD load level

For each FWD pass and FWD load level, the following parameters were computed:

Step 3.  For each FWD pass, perform t-test for load level dependence

Statistical t-tests were conducted if the LTEs from nominal load levels of 40 kN and 70 kN were statistically different (p-value is less than 0.05).

Step 4.  Compute LTE section statistics for deflection basins from all FWD load levels

For each FWD that pass using LTEs from all joints and load levels, compute the following parameters:

Step 5.  Determine section LTE load dependency index

Using the mean section LTE for nominal load levels of 40 kN and 70 kN, and the results of t-test from step 4, determine LTE load dependency flag for crack/joint:

Equation 20 The flag indicating if section load transfer efficiency depends on the falling weight deflectometer load level equals:

·	1, if the absolute value of the mean load transfer efficiency for the crack/joint for nominal load levels of 40 kilo Newtons minus the mean load transfer efficiency for the crack/joint for nominal load levels of 70 kilo Newtons is less than 5.

·	2, if the mean load transfer efficiency for the crack/joint for nominal load levels of 40 kilo Newtons minus the mean load transfer efficiency for the crack/joint for nominal load levels of 70 kilo Newtons is less than 5 and the falling weight deflectometer pressure is less than 0.05.

·	3, if the mean load transfer efficiency for the crack/joint for nominal load levels of 40 kilo Newtons minus the mean load transfer efficiency for the crack/joint for nominal load levels of 70 kilo Newtons) is greater than 5 and the falling weight deflectometer pressure is less than 0.05.       (20)

where:

-          LTE_SECT_LOAD_LEVEL_FLAG = a flag indicating if section LTE depends on     FWD load level

= 1, if load level independent

= 2, if increases with load

= 3, if decreases with FWD load

-          LTE_40 = mean LTEs for the crack/joint for nominal load levels of 40 kN.

-          LTE_70 = mean LTEs for the crack/joint for nominal load levels of 70 kN.

Results of LTE Analysis

The research team proposes that the results of the basin-by-basin LTE calculation be stored in the LTPP database table MON_DEFL_RGD_LTE_POINT.  Summary values of calculated LTE parameter results for each joint of the GPS and SPS sections should be stored in database table MON_DEFL_RGD_LTE_JOINT.  The results are presented in terms of the mean values, minimum values, maximum values, and load dependency index.  Summary values of calculated LTE parameters for each section should be stored in database table MON_DEFL_RGD_LTE_SECT.

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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). 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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