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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-03-093
Date: August 2006
Study of Long-Term Pavement Performance (LTPP): Pavement Deflections
Chapter 7. Suggested Computed Parameters and FWD Testing Protocols
The Initial Working Data Files for the Project
As mentioned near the beginning of this report, the originally provided database files were reorganized to facilitate further analyses. Specifically, day files were created under directories organized by region and test section; these files consisted of all the data for each date of test at each LTPP section. These day files were changed in accord with the findings of this study to include the deletions, changes, and notes reported above. All of these files were written to a computed parameter data CD, which consisted of normalized and averaged FWD load-deflection data, with data errors and anomalies identified, changed, or deleted from the files as appropriate.
The Modified (Computed Parameter) Individual Data Files
As mentioned previously, each day file was identified by a six-character date (yymmdd). After each date, in the seventh character of the file name, the letter “n” or “m” was employed. The letter “n” stands for normalized, while “m” stands for modified after normalization. Finally, each normalized and/or modified day file has the extension “*.txt” since these are comma-delimited files. Occasionally, “u” or “x” was used instead of the usual “n” or “m.” These two letters denote that a note only is present, with no data. Such notations are further explained below.
The “n” files denote those day files that have been screened for inconsistent basin type load-deflection errors. They have been normalized to the target load levels for each drop height, while each line or record represents the average of the deflection readings for a given test point and drop height. The field for drop sequence number has been replaced with a field indicating the number of drops (1, 2, 3, or 4) used to create the averages. The “n” files have no other changes or notes attached. The “m” files denote that some additional change or changes have taken place. In all these cases, there is a note in the last record of the “m” file that explains the changes or deletions (in some cases) or warns the user of potential problems (in other cases). These pre-autumn 1998 data files presently exist on a single data CD (approximately 100 megabytes).
However, it was decided that these files would not be incorporated into the LTPP database. Therefore, no computed parameters of pavement deflections exist at this time in the LTPP database. It is, however, still possible that the “n” files could eventually be consolidated into database tables so that they are available for general use.
FWD Test Protocol Recommendations
Based on the findings of this report and other considerations, the following LTPP FWD testing procedure changes were recommended in 1999:
Further, an analysis was conducted to develop a procedure to determine if more frequent LTPP testing should be conducted as a pavement ages and exhibits more distress. The approach was to use a representative measure of the deflection basin, adjust this measure for pavement temperature, and determine whether definitive trends in the selected deflection measure exist.
The following specific analysis approach was used:
Figure 5. Equation. AREA.
A list was developed of GPS–1 and GPS–2 test sections that had at least four different times (nonseasonal) of deflection testing between 1989 and 1998. Ten of these test sections were randomly selected for further analysis.
The AREA values were determined for each F3 test point (wheelpath testing at 7.6-m (25-ft) test intervals).
The average middepth temperatures during the approximately 1 hour of wheelpath testing were established, for each test visit and for each of the 10 sections.
The basin adjustment factor (TAF) for the AREA parameter was established using the procedure described in a previous FHWA study, Temperature Predictions and Adjustment Factors for Asphalt Pavement.(2) Although the TAF is a function of the AC layer thickness and the latitude of the test section, it typically ranges in almost a linear manner from about 0.90 at 0 ºC (32 ºF) middepth temperature to about 1.1 at 40 ºC (104 ºF) middepth temperature, using the reference middepth temperature of 20 ºC (68 ºF). The temperature adjusted AREA is then given by the equation in figure 6:
Figure 6. Equation. AREA (@ 20 ºC).
All AREA values determined in step 3 were adjusted to account for the middepth pavement temperature as shown in step 5. The adjusted AREA values are listed in table 8.
A review of the results shown in table 8 indicates no definitive trends in average AREA values with time for the ten sections used in the analysis. Therefore, no specific recommendations can be provided at this time for considering changes in the frequency of deflection testing. Also, no acceptable procedures presently exist to adjust the maximum deflections for temperature at the time of test, although this may be possible in the near future. Finally, it may be necessary to include sections exhibiting significant cracking and/or rutting distress in the study sample, since it is possible that the deflections do not change appreciably until some pavement distress ensues.
The recommendations shown above were reported in 1998. Most (but not all) of these recommendations have since been implemented in connection with FWD upgrades, annual equipment servicing, and other events, and the FWD deflection testing protocols have been changed accordingly, where appropriate.