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Publication Number:  FHWA-HRT-15-036    Date:  December 2015
Publication Number: FHWA-HRT-15-036
Date: December 2015

 

Long-Term Pavement Performance Program Determination of In-Place Elastic Layer Modulus: Backcalculation Methodology and Procedures

Chapter 5. Backcalculation Results

This chapter provides a review of the procedures for post-processing the results and determining their accuracy as well as the CPTs used to store the backcalculated elastic layer moduli and other data.

LTPP Data Source

As stated in chapter 4, the deflection data were obtained from SDR 27.0 and included data from all LTPP test sections and all days of deflection testing. The dataset contained both GPS and SPS experimental sections and represented all pavement design alternatives included in the LTPP database.

Basic Facts about the Backcalculation Process

There were a total of 7,771 test days, of which 4,534 sections had an HMA surface, and 2,237 sections had a PCC surface. All LTPP sections were grouped by State code, Strategic Highway Research Program (SHRP) ID, and construction number to facilitate the pre-processing described in chapter 4.

Number of GPS and SPS Test Sections

The number of sections by each LTPP experiment included 1,744 sections with an HMA surface and 1,008 sections with a PCC surface. The surface type changed in 381 of the test sections. For example, a JPCP with a PCC surface that was eventually overlaid with HMA was counted in two categories or experiments. Therefore, there were a total of 3,133 unique sections in all States combined, as listed in table 9. Table 10 lists the total number of deflection basins and those basins falling within each of the four categories. As shown, the LTPP quality control processes have resulted in nearly 97 percent of the basins falling within the typical and type 2 categories, which the authors consider excellent. As such, 5,847,770 deflection basins (typical and type 2) were used in the backcalculation process.

The number of deflection basins analyzed and included in the backcalculation process are listed in table 11. The table also lists the percentage of deflection basins that were found to be acceptable after the initial evaluation with EVERCALC© using an expanded range of the upper and lower modulus limits and for the final results. As shown, about 75 percent of all deflection basins were considered acceptable after the second phase analysis and defined as accept or atypical, while about 85 percent of the deflection basins resulted in acceptable values from EVERCALC© and MODCOMP©.

Table 12 lists the percentage of deflection basins defined as acceptable on a State-by-State basis. The percentage of acceptable deflection basins increased to about 85 percent after the fourth phase analysis using MODCOMP© for those basins considered unacceptable from the second and third phase analyses. Table 13 summarizes the percentage of deflection basins defined as acceptable on a State-by-State basis for the PCC-surfaced pavements from the best fit method.

Table 9. Summary of LTPP data used in the backcalculation analyses by experiment.
HMA Surface PCC Surface
LTPP Experiment Number of Sections LTPP Experiment Number of Sections
GPS 1 233 GPS 3 133
GPS 2 145 GPS 4 69
GPS 6A 63 GPS 5 85
GPS 6B 131 GPS 9 26
GPS 6C 31 SPS 1 1
GPS 6D 24 SPS 2 205
GPS 6S 162 SPS 4 220
GPS 7A 35 SPS 6 170
GPS 7B 56 SPS 7 39
GPS 7C 18 SPS 8 16
GPS 7D 3 SPS 9C 7
GPS 7F 3 SPS 9J 37
GPS 7S 27    
SPS 1 245    
SPS 2 2    
SPS 3 445    
SPS 5 204    
SPS 6 123    
SPS 7 1    
SPS 8 37    
SPS 9C 7    
SPS 9J 38    
SPS 9N 50    
SPS 9O 42    
Total GPS sections 931 Total GPS sections 313
Total SPS sections 1,194 Total SPS sections 695
Grand Total 2,125 Grand Total 1,008

Note: Blank cells indicate no additional LTPP experiment within the pavement type.

Table 10. Summary of LTPP deflection basin data.
State/ Canadian Province/ Territory Number of Drops Percentage of Total Drops Percentage of Typical and Type 2
Type 1 Type 2 Type 3 Typical Total Type 1 Type 2 Type 3 Typical
AL 593 56,826 3,326 150,435 211,180 0.28 26.91 1.57 71.24 98.14
AK 8 6703 20 17,856 24,587 0.03 27.26 0.08 72.62 99.89
AZ 982 117,565 5,720 253,549 377,816 0.26 31.12 1.51 67.11 98.23
AR 1,221 33,772 6,271 44,550 85,814 1.42 39.35 7.31 51.91 91.27
CA 2,566 72,207 5,384 129,688 209,845 1.22 34.41 2.57 61.80 96.21
CO 590 40,960 1,671 89,608 132,829 0.44 30.84 1.26 67.46 98.30
CT 87 13,727 1,350 43,544 58,708 0.15 23.38 2.30 74.17 97.55
DE 231 23,371 1,437 41,918 66,957 0.34 34.90 2.15 62.60 97.51
DC 0 311 2 711 1,024 0.00 30.37 0.20 69.43 99.80
FL 319 33,062 6,038 102,937 142,356 0.22 23.22 4.24 72.31 95.53
GA 813 53,464 7,195 99,894 161,366 0.50 33.13 4.46 61.91 95.04
HI 1 4,768 58 11,328 16,155 0.01 29.51 0.36 70.12 99.63
ID 2,104 35,038 1,321 45,974 84,437 2.49 41.50 1.56 54.45 95.94
IL 1,804 42,460 7,052 43,692 95,008 1.90 44.69 7.42 45.99 90.68
IN 1,384 56,244 13,013 54,283 124,924 1.11 45.02 10.42 43.45 88.48
IA 2,117 43,359 6,820 52,852 105,148 2.01 41.24 6.49 50.26 91.50
KS 2,122 47,191 3,343 80,159 132,815 1.60 35.53 2.52 60.35 95.89
KY 150 8,066 685 16,291 25,192 0.60 32.02 2.72 64.67 96.69
LA 242 7,165 1,241 10,932 19,580 1.24 36.59 6.34 55.83 92.43
ME 584 17,903 2,770 63,596 84,853 0.69 21.10 3.26 74.95 96.05
MD 300 44,640 2,009 61,514 108,463 0.28 41.16 1.85 56.71 97.87
MA 55 6,993 247 40,637 47,932 0.11 14.59 0.52 84.78 99.37
MI 775 27,481 3,224 66,084 97,564 0.79 28.17 3.30 67.73 95.90
MN 1,502 67,018 6,828 198,733 274,081 0.55 24.45 2.49 72.51 96.96
MS 615 42,521 1,112 101,452 145,700 0.42 29.18 0.76 69.63 98.81
MO 1,467 49,275 3,194 76,189 130,125 1.13 37.87 2.45 58.55 96.42
MT 455 38,957 2,782 139,478 181,672 0.25 21.44 1.53 76.77 98.22
NE 1,317 25,403 1,562 73,746 102,028 1.29 24.90 1.53 72.28 97.18
NV 1,403 27,078 894 75,115 104,490 1.34 25.91 0.86 71.89 97.80
NH 31 6,642 257 29,166 36,096 0.09 18.40 0.71 80.80 99.20
NJ 129 29,857 692 82,542 113,220 0.11 26.37 0.61 72.90 99.27
NM 39 17,194 551 90,288 108,072 0.04 15.91 0.51 83.54 99.45
NY 133 10,847 359 77,420 88,759 0.15 12.22 0.40 87.22 99.45
NC 1,067 40,461 2,584 91,811 135,923 0.79 29.77 1.90 67.55 97.31
ND 462 8,184 322 8,015 16,983 2.72 48.19 1.90 47.19 95.38
OH 2,072 40,984 4,895 62,117 110,068 1.88 37.24 4.45 56.44 93.67
OK 661 40,762 1,114 97,209 139,746 0.47 29.17 0.80 69.56 98.73
OR 1,257 10,349 2,505 9,067 23,178 5.42 44.65 10.81 39.12 83.77
PA 1,446 37,136 5,584 49,688 93,854 1.54 39.57 5.95 52.94 92.51
RI 1 4,035 295 1,701 6,032 0.02 66.89 4.89 28.20 95.09
SC 21 8,906 522 6,623 16,072 0.13 55.41 3.25 41.21 96.62
SD 704 33,400 2,553 100,325 136,982 0.51 24.38 1.86 73.24 97.62
TN 78 26,402 1,137 54,864 82,481 0.09 32.01 1.38 66.52 98.53
TX 1,381 113,126 8,176 425,696 548,379 0.25 20.63 1.49 77.63 98.26
UT 599 22,078 725 64,515 87,917 0.68 25.11 0.82 73.38 98.49
VT 310 13,392 1,314 53,877 68,893 0.45 19.44 1.91 78.20 97.64
VA 156 26,417 923 126,862 154,358 0.10 17.11 0.60 82.19 99.30
WA 635 22,207 1,348 60,124 84,314 0.75 26.34 1.60 71.31 97.65
WV 144 7,307 410 6,347 14,208 1.01 51.43 2.89 44.67 96.10
WI 1,237 37,348 6,628 46,604 91,817 1.35 40.68 7.22 50.76 91.43
WY 295 35,151 772 54,942 91,160 0.32 38.56 0.85 60.27 98.83
PR 31 2,384 10 4,942 7,367 0.42 32.36 0.14 67.08 99.44
AB 1 8,119 51 44,672 52,843 0.00 15.36 0.10 84.54 99.90
BC 0 1,228 2 16,096 17,326 0.00 7.09 0.01 92.90 99.99
MB 504 27,998 8,294 98,541 135,337 0.37 20.69 6.13 72.81 93.50
NB 0 2,771 154 10,640 13,565 0.00 20.43 1.14 78.44 98.86
NF 0 697 34 8,010 8,741 0.00 7.97 0.39 91.64 99.61
NS 0 109 1 3,761 3,871 0.00 2.82 0.03 97.16 99.97
ON 175 9,415 1,184 56,362 67,136 0.26 14.02 1.76 83.95 97.98
PE 0 1,235 0 9,597 10,832 0.00 11.40 0.00 88.60 100.00
QB 265 10,933 508 34,926 46,632 0.57 23.45 1.09 74.90 98.34
SA 125 17,359 5,470 55,714 78,668 0.16 22.07 6.95 70.82 92.89
Total 39,766 1,717,961 155,943 4,129,809 6,043,479 0.66 28.43 2.58 68.33 96.76

PR = Puerto Rico; AB = Alberta; BC = British Columbia; MB = Manitoba; NB = New Brunswick; NF = Newfoundland; NS = Nova Scotia; ON = Ontario;
PE = Prince Edward Island; QB = Quebec; and SA = Saskatchewan.

Table 11. Number of deflection basins analyzed and percentage of those deflection basins considered acceptable.
EVERCALC© Analysis Total Drops Analyzeda Accept Results Atypical Results Atypical and Accept Results Error Drops Percentage Atypical and Accept Results
Run 1 using typical material-specific ranges for each layer 5,694,207 2,573,025 1,327,896a 3,900,921 1,793,286 68.5
Run 2 using wider range for cases where more than 25 percent of the run 1 results hit the limits 5,611,563 1,853,484 2,377,648b 4,390,049 1,380,431 75.4
Final Results (EVERCALC© and MODCOMP©) 5,662,494 1,817,186 2,494,628 4,311,814 1,350,680 76.1

Note: Different values are reported for the total number of basins analyzed because the total number accounts for basins that may have been dropped or added for the individual runs for basins that were borderline between typical and type 2 versus types 1 and 3.

aThe atypical results in run 1 represent basins with backcalculated moduli that converge to the lower or upper limits of the specified typical range. These may include sections that may produce results outside the atypical range if the range is expanded. Therefore, these basins may include basins that can be categorized as errors if the range is expanded.

bThe atypical results in run 2 represent basins with backcalculated moduli that converge within the lower or upper limits of the specified atypical range (i.e., these basins may not be categorized as errors).

Table 12. Deflection basins analyzed and percentage classified as acceptable drops using EVERCALC© and MODCOMP© for all pavements.
STATE_ CODE TOTAL_DROPS ERROR_DROPS ATYPICAL_DROPS ACCEPT_DROPS Percent Acceptable
1 199,465 28,341 99,850 71,274 85.8
2 24,559 6,188 6,018 12,353 74.8
4 367,159 116,309 162,913 87,937 68.3
5 77,904 14,206 30,161 33,537 81.8
6 193,500 60,991 75,684 56,825 68.5
8 130,421 49,733 47,512 33,176 61.9
9 57,271 6,129 17,565 33,577 89.3
10 65,289 9,282 36,820 19,187 85.8
11 1,022 160 319 543 84.3
12 135,999 50,662 48,522 36,815 62.7
13 153,065 16,886 69,767 66,412 89.0
15 16,096 3,727 6,307 6,062 76.8
16 80,804 28,423 33,432 18,949 64.8
17 86,152 22,671 34,200 29,281 73.7
18 93,376 24,438 39,795 29,143 73.8
19 95,867 28,030 40,923 26,914 70.8
20 126,318 38,013 46,290 42,015 69.9
21 24,357 8,725 9,355 6,277 64.2
22 18,097 1,324 4,621 12,152 92.7
23 81,499 6,151 33,610 41,738 92.5
24 106,138 27,664 49,703 28,771 73.9
25 44,976 4,671 21,745 18,560 89.6
26 93,392 15,832 34,056 43,504 83.0
27 264,185 54,432 112,931 96,822 79.4
28 143,797 31,183 72,221 40,393 78.3
29 117,624 44,556 51,913 21,155 62.1
30 178,393 39,040 91,324 48,029 78.1
31 98,629 18,671 40,554 39,404 81.1
32 101,504 17,931 56,182 27,391 82.3
33 35,808 2,775 12,998 20,035 92.3
34 112,399 12,285 65,259 34,855 89.1
35 107,482 20,493 45,650 41,339 80.9
36 88,267 8,243 58,641 21,383 90.7
37 132,159 32,663 56,570 42,926 75.3
38 16,199 4,218 6,972 5,009 74.0
39 103,092 24,359 37,449 41,284 76.4
40 136,754 25,706 56,493 54,555 81.2
41 19,416 5,817 8,703 4,896 70.0
42 86,759 28,033 47,666 11,060 67.7
44 5,736 1,488 3,825 423 74.1
45 15,529 7,544 5,710 2,275 51.4
46 130,246 21,574 48,238 60,434 83.4
47 81,234 31,811 32,462 16,961 60.8
48 534,862 134,664 219,904 180,294 74.8
49 86,480 29,619 48,924 7,937 65.8
50 67,199 4,830 34,503 27,866 92.8
51 153,279 32,257 89,569 31,453 79.0
53 81,825 23,132 37,514 21,179 71.7
54 12,316 6,630 3,970 1,716 46.2
55 82,468 20,558 40,304 21,606 75.1
56 90,093 29,562 42,538 17,993 67.2
72 7,326 4,178 2,912 236 43.0
81 52,791 21,534 26,103 5,154 59.2
82 17,324 1,682 4,952 10,690 90.3
83 126,539 21,726 44,798 60,015 82.8
84 13,411 4,480 8,562 369 66.6
85 8,164 1,871 2,147 4,146 77.1
86 3,870 180 1,176 2,514 95.3
87 65,777 7,945 20,217 37,615 87.9
88 10,832 4,454 5,606 772 58.9
89 45,250 13,087 19,373 12,790 71.1
90 73,073 27,001 32,359 13,713 63.0

Note: Not all agencies are included within this table because of missing data that were needed for the backcalculation process for the LTPP sections, or there were no LTPP test sections within that agency.

Table 13. Deflection basins analyzed and percentage classified as acceptable drops using the best fit method for PCC-surfaced pavements.d
STATE_CODE TOTAL_DROPS ERROR_DROPS ATYPICAL_DROPS ACCEPT_DROPS Percent Acceptable
1 7,246 1,076 580 5,590 85.2
4 23,495 1,950 1,913 19,632 91.7
5 20,167 2,242 1,976 15,949 88.9
6 22,712 3,455 5,539 13,718 84.8
8 15,798 733 4,040 11,025 95.4
9 960 4 44 912 99.6
10 11,698 449 1,248 10,001 96.2
12 6,793 1,707 777 4,309 74.9
13 14,018 1,985 6,400 5,633 85.8
16 6,103 256 1,357 4,490 95.8
17 8,646 438 211 7,997 94.9
18 11,663 406 6,296 4,961 96.5
19 16,935 482 4,399 12,054 97.2
20 24,563 1,942 3,348 19,273 92.1
21 1,788 229 89 1,470 87.2
22 655 12 174 469 98.2
23 1,603 8 822 773 99.5
26 15,148 955 2,388 11,805 93.7
27 19,250 1,110 4,494 13,646 94.2
28 4,922 892 2,210 1,820 81.9
29 20,662 2,192 1,818 16,652 89.4
31 11,970 576 6,561 4,833 95.2
32 8,772 2,101 2,946 3,725 76.0
34 337 0 0 337 100.0
35 953 131 89 733 86.3
36 5,265 330 426 4,509 93.7
37 17,420 1,202 4,440 11,778 93.1
38 13,673 241 1,200 12,232 98.2
39 21,524 991 4,701 15,832 95.4
40 8,077 565 4,508 3,004 93.0
42 16,876 4,409 1,865 10,602 73.9
45 1,161 12 0 1,149 99.0
46 12,766 771 3,272 8,723 94.0
47 4,146 59 579 3,508 98.6
48 18,083 2,384 11,062 4,637 86.8
49 12,253 1,173 5,004 6,076 90.4
50 433 24 0 409 94.5
51 156 47 8 101 69.9
53 21,151 1,937 5,909 13,305 90.8
54 1,489 312 61 1,116 79.0
55 22,021 2,701 5,139 14,181 87.7
56 1,110 1 331 778 99.9
72 1,892 1,126 154 612 40.5
83 4,096 485 34 3,577 88.2
84 564 1 21 542 99.8
89 8,103 361 681 7,061 95.5

Note: Not all agencies are included within this table because some of the agencies did not have any LTPP rigid pavement test sections.

Hardware Used in Backcalculation Processing

The following subsection provides a summary of the hardware used to backcalculate the results as well as an estimate of the length or amount of computational time to execute the first phase analysis using EVERCALC©.

The first and second phase analyses were performed in two modes using the interface for the cases with a bedrock layer and by running the executable from an MS-DOS prompt for the cases without a bedrock layer. The runs were executed in a batch mode for both cases.

For cases with the bedrock layer, three virtual machines were used, each with 2 GB of random access memory, and running the Windows XP® operating system. These machines had 2.4 Ghz dual core processors. The runs took about 2 weeks of computational time (336 h), with the programmer or user having to monitor the progress of the software’s status. In some cases, the program had to be restarted twice each day.

For cases without the bedrock layer, three virtual machines were used, and they had the same specifications as those described. The analyses took about 1 week of computational time (168 h). These analyses were more stable, but the programmer still had to monitor the program’s status when processing data for States with large data sets (e.g., Arizona, Colorado, and Texas).

Evaluating the success of Backcalculation

The two parameters used to determine whether the results were acceptable were also used as a measure to judge the success of the backcalculation process: (1) the average error or difference between the measured and calculated deflection basin and (2) the percentage of points with acceptable modulus values for all layers. The criteria for determining acceptable and unacceptable backcalculated layer modulus values were discussed earlier. The following list provides a generic definition of success:

For sites with anomalies or high RMSE, the resulting elastic layer moduli were identified as such. MODCOMP© was used to determine a probable reason for the results. These sites or days of testing were flagged for investigation, if still available. If forensics have not been completed and a site has been taken out of service, these were simply identified along with some potential causes for the differences encountered.

Table 14 lists the LTPP test sections and the areas within the test section for which the results were eliminated from the CPT because most of the results were found to be outside one or both of the acceptance criteria. Table 15 lists the error flags and their description used in the CPTs and included in the feedback reports.

Table 14. LTPP test sections identified as errors and eliminated from determining the test section statistics.
Test Section Area Excluded Station Type of Structure
01-4127 110+ Conventional flexible pavement structure
01-4129 -40 Conventional flexible pavement structure with a thin surface
04-1006 140+ Conventional flexible pavement structure
02-9035 80+ Conventional flexible pavement structure with a thin overlay
04-1022 -50 Conventional flexible pavement structure
04-1015 120+ Conventional flexible pavement structure with overlay
06-8534 130+ Conventional flexible pavement structure with a thin surface
30-7088 70+ Conventional flexible pavement structure with overlay
37-2824 -60 Semi-rigid pavement
38-2001 80+ Semi-rigid pavement
40-4088 50+ Semi-rigid pavement
42-1618 -35 Conventional flexible pavement structure with overlay
48-9005 -50 Conventional flexible pavement structure with overlay
53-1002 -40 Conventional flexible pavement structure

 

Table 15. Error flags used in the CPTs.
Report No. Flag Subject Situation Recommended Action
1 MISSING_DEFL Missing deflection data Deflection data is missing in the MON_DEFL_DROP_
DATA for this STATE_CODE, SHRP_ID, TEST_
DATE, TEST_TIME, POINT_
LOC, and DROP_NO.
None. The basin was not used in backcalculation analysis.
2 LAYER_THK_
UNAVAILABLE
Missing thickness data Layer thickness data is missing in the TST_L05B table for this STATE_CODE, SHRP_ID, TEST_DATE, and CONSTRUCTION_NO None. The basin was not used in backcalculation analysis.
3 BLACKLISTED_
BASINTYPE
Non-decreasing deflection basin Invalid deflection data for the basin in the MON_DEFL_
DROP_DATA table. The test location has a non-decreasing deflection basin and was characterized as either a type I or a type III basin.
None. The basin was not used in backcalculation analysis.
4 MISSING_TEMP Missing temperature data and depth of temperature measurement Temperature measurement and depth of measurement is missing in the MON_DEFL_TEMP_
DEPTHS and MON_DEFL_
TEMP_VALUES for this SHRP_ID, test_date, and Point_LOC.
None. The basin was used in backcalculation analysis.
5 MISSING_DEPTH_
THKCOR
Missing depth of temperature measurement Missing layer depth Information in the MON_DEFL_TEMP_
DEPTHS table LAYER_TEMP_
DEPTH from the Supplementary Temperature Table).
None. The basin was used in backcalculation analysis.
6 TEMPERATURE_
DEPTH_MISMATCH
Temperature depth mismatch The data for the layer thicknesses from TST_L05B and depth of temperature measurements do not align None. The basin was used in backcalculation analysis.
7 MISSING TEMP_
DEPTH
Missing temperature within the surface layer For the STATE_CODE, SHRP_ID, and TEST_DATE, the temperature within surface layer could not be determined using the temperature depth and temperature values reported in MON_DEFL_TEMP_DEPTHS and MON_DEFL_TEMP_
VALUES tables.
None. The basin was used in backcalculation analysis.
8 INVALID_LTE Invalid LTE Deflection measurement was considered invalid because of missing deflection or sensor spacing. None. The LTE was not reported in the summary tables.
9 LTE_NON_DEC LTE non decreasing Invalid deflection data for the basin in the MON_DEFL_
DROP_DATA table. The test location has a non-decreasing deflection basin.
None. The LTE was not reported in the summary tables.
10 F_OUTPUT_IDD_
MISMATCH
Failure to obtain results for all basins for an identification descriptor designation Results were generated for a subset of the drops for the IDD. The results are written to the drops starting from the first drop; however a mismatch might exist. None. The results are reported for the IDD.
11 F_NO_IDD_MATCH No results were obtained for this IDD No output data are found for an IDD (i.e., POINT_LOC, TEST_DATE, TEST_TIME, SHRP_ID, and STATE (likely that the input data were not fully processed by EVERCALC©). No results reported for IDD.
12 EXCLUDED_FROM_
SUMMARY
Subsection excluded from results summary Subsection of the SHRP_ID showed high variability in calculated modulus values relative to the majority of the section. Subsection had higher error rate. Results excluded from summary.

Note: “Report No.” refers to an integer that defines a specific error flag.

The individual test sections with error numbers 1 through 9 in table 15 were included in unpublished data feedback reports and were submitted to FHWA because these pertain to the data themselves. Error numbers 10–12 were excluded from the unpublished data feedback reports, which were prepared and submitted to LTPP to document and report errors or anomalies found in the database, because these errors only pertain to the results from the backcalculation programs. In addition, error numbers 1–3 were excluded in the datasets for the individual results because of missing critical data such that the backcalculation process could not be performed.

Manual Review of Results

The first level review of results was the post-processing of the backcalculated results. The post-processor groups and determines the number of deflection basins that are found to be acceptable and unacceptable based on the two parameters previously discussed. If the majority of the results were found to be acceptable, that test section and test date were accepted, and the averages, standard deviation, and other information were determined in the post-processing of the data. If a significant amount of the results are unacceptable, the test section and test date went through a second and possibly third level review.

The second level review was a more exhaustive review to identify possible causes for the unacceptable results. This included an indepth review of the post-evaluation factors and other LTPP data and layer properties to try to identify the reasons for the high error term or modulus values that fell outside the range of typical values. If the results were still considered unacceptable after the second level review, the test section and test date moved to a
third level review.

The third level review included making additional runs with MODCOMP© based on all available information. If this third level did not result in acceptance, a feedback report was prepared for the test section and test dates. As part of this review level, a decision was made regarding the probable cause for the unaccepted results and the results flagged. The probable cause and recommended resolution were included in the feedback reports. Appendix B lists the test sections that exhibited a high percentage of moderate to high RMSE values and did not meet all of the acceptance criteria.

Post-Processing Evaluation Factors

Many factors can lead to erroneous results in the backcalculation process because they impact the measured deflection basin. Some evaluation factors include the following:

Another evaluation criterion was to compare the backcalculated elastic modulus values with those values measured in the laboratory for PCC, HMA, and unbound materials and soils. LTPP sponsored other projects to determine the dynamic and resilient modulus values that are consistent with the MEPDG methodology and inputs.(1) Dynamic moduli were calculated for the different HMA mixtures from volumetric and component properties and are included in the LTPP database, which represent an undamaged and unaged condition. The backcalculated modulus values, however, represented an in-place composite elastic modulus and were not discrete values, as measured in the laboratory. In addition, the backcalculated modulus values represented any in-place damage and aged conditions. As such, one-to-one correspondence was not expected between the backcalculated elastic modulus values and those calculated from volumetric properties (laboratory-derived dynamic modulus values).

The layer- or mixture-specific dynamic modulus values were combined to determine a composite modulus value using the equivalent modulus concept and compared to the backcalculated composite values. The equivalent modulus concept was used in the FHWA-sponsored study to compare backcalculated values to laboratory-derived values.(45) Use of the equivalent modulus concept to determine the composite laboratory-derived dynamic modulus was found to be of little value in explaining the difference between the laboratory-derived and field-derived values.

Compensating Layer Effects

Identifying compensating layer effects is an important step in the backcalculation process. Compensating layer effects occurs when one layer consistently increases in stiffness while an adjacent layer decreases in stiffness. Compensating layer effects can result in layer modulus values that are inappropriate for the pavement structure, even when the error between the measured and predicted deflection basins is considered acceptable. Characterizing the deflection basins can assist in identifying LTPP test sections that are prone to compensating layer effects. The deflection basins were categorized in the first backcalculation study to identify the sites where this characteristic is likely to occur.(4) The same process was used in this project to reduce the possibility for compensating layer effects on the calculated elastic modulus values. When compensating errors identified for a specific test section, MODOMP was used or the pavement structure was revised.

Discontinuities

Discontinuities in the pavement structure (i.e., cracks, joints, segregation, etc.) have an impact on selected deflection basins, especially if the discontinuity is supporting a sensor platform or located between two sensors near the loading plate. The sensor bar and loading plate are usually located in intact areas, but that is not always possible or the condition noted by the operator. The shape of the deflection basin can distinguish some of these conditions.

As part of the backcalculation process, the distress data were extracted for each site for review when selected areas along the LTPP site continually do not meet the acceptance criteria to explain the high RMSE or surface layer modulus values that deviate significantly from typical laboratory-derived values. These results were flagged as having extensive cracking or distress along the section as part of the post-processing of results.

HMA Stripping and Other Material Defects

Stripping can have a softening effect on HMA mixtures. Some of the forensic investigations that have been completed on the SPS-1 and SPS-2 supplemental sites have identified HMA mixtures that exhibit stripping in selected layers. This softening effect can result in low elastic modulus values backcalculated for that structure. The first LTPP backcalculation study recognized this behavior, and all layers that were found to exhibit stripping were simulated as a separate layer. The error term using this approach was lower in comparison to combining all HMA layers into a composite layer.

Figure 34 illustrates this point for an LTPP site where deflection testing was done prior to any forensic investigation. The pavement structure was an HMA overlay of an existing HMA pavement. The backcalculated modulus reported for the in-place dense-graded base mixture beneath the HMA overlay was similar to that of a good quality aggregate base material. That modulus value was believed to be incorrect and rejected. After coring operations, the low modulus value was found to be representative of moisture-induced damage or stripping. Similar examples are applicable for the unbound layers (e.g., high water content resulting in very low modulus values to base layers with an excessive amount of fines or low water content (a dry condition) resulting in very high modulus values representative of a bound layer).

Figure 34. Photo. Cores recovered where moisture damage occurred, decreasing the in-place modulus of the layer/mixture (Texas SPS-5 project). This photo shows two cores recovered from a Texas Specific Pavement Studies (SPS)-5 project where moisture damage occurred, decreasing the in-place modulus of the layer/mixture. Both of the cores show the hot mix asphalt (HMA) overlay layers are intact, but the existing lower layers were recovered in pieces or disintegrated during the wet coring operations, suggesting moisture damage or stripping in the lower HMA layers.

Figure 34. Photo. Cores recovered where moisture damage occurred, decreasing the in-place modulus of the layer/mixture (Texas SPS-5 project).

Loss of Bond between Layers

Most backcalculation studies and software packages assume adjacent layers are fully bonded. However, forensic investigations have shown that there were SPS-1 and -5 test sections where bond was lost with time. Some of these sections have been reconstructed or taken out of the LTPP monitoring cycle because the distress increased significantly.

The bond between adjacent bound layers can affect the deflection basin, but the effect is usually considered minor. Based purely on the deflection basins, it is difficult to determine if bonded or full slip between layers should be assumed. If adjacent layers become debonded, the distresses usually increase at a significant rate. Simulating this condition becomes difficult because of the limit on the number of layers that can be backcalculated, and most of the commonly used programs have the bond between layers fixed to full bond. If this condition is expected from the results of the backcalculation process through low modulus values, the results are flagged as such.

Variable or Perched Water Table Depth

The water content of unbound materials has a significant effect on the elastic properties of that layer. This was recognized during the first backcalculation study, so boring logs and water content were used to determine if this condition was exhibited at each LTPP site. Where this condition was found, two runs were made—one where the subgrade was combined as one layer and another where the subgrade was divided into two layers where the water table or nearly saturated conditions existed. The error between the measured and calculated deflection basins was almost always lower by simulating the depth of saturation. In most cases, the backcalculated elastic modulus values were also higher for the layer above the water table depth than the values below the water table depth. In some cases, these depths were seasonal.

Variable Depth of an Apparent Rigid Layer

The depth to bedrock can be determined from the shoulder borings that were drilled at all (or nearly all) of the LTPP test sections. However, a saturated or soft (weak) layer overlying a dry, overconsolidated clay layer can behave as a very stiff layer; all of the energy is being absorbed in the soft, weak layer. Similar to the comments relative to saturated layers, the boring logs and physical properties were used to identify these conditions. Where these conditions were found, two runs were made, one with and one without a rigid layer at the interface between the wet and dry layers. In most cases, the error between the measured and calculated deflection basins was less by simulating a rigid or stiff layer at that location or dividing the subgrade into two layers at that depth.

Warping and Curling of PCC Slabs

Curling of JPCP slabs can occur over a day’s test program and significantly impact the measured deflection basin. It this condition is not recognized, it will result in lower PCC elastic modulus values not representative of the in-place material.

CPTs

The CPTs were designed to store the results from the backcalculation process and assist in interpreting the results. One set of tables stores the EVERCALC© and MODCOMP© results, while a second set of tables stores the best fit results. The data in these sets of tables include the following:

The data included in the CPTs are similar to those designed and prepared from the first round of backcalculation results in 2002 and include, as a minimum, the following:(4)

A user’s manual and source code, Backcalculation Procedures and User Guide for Software Programs and Utility Tools, was prepared for all software packages and tools that were modified, revised, or developed as a part of this study.(56) In addition, source codes were prepared for each of the pre- and post-processing utility tools used in this round of backcalculation. Source codes were not prepared for any program that was already available in the public domain and was not changed in any manner.

Note: Bolding indicates the critical value.

 

 

 

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