Review of The Long-Term Pavement Performance (LTPP)
Backcalculation ResultsChapter 5. LTPP Backcalculation Database Screening
Methodology
LTPP DATA SOURCE USED IN THIS
STUDY
The complete computed parameter tables of backcalculated
pavement layer data plus the supporting tables (Release 16.0—July
2003 Upload) were used and screened in this study.
LTPP Data Tables Used
The following tables from the LTPP database were used in either
computing the forwardcalculated moduli or screening the
backcalculation values:
-
EXPERIMENT_SECTION—Stores current experiment information that is
driven by maintenance and rehabilitation (M and R) activities.
-
TST_L05B—Table containing section representative layer thickness
and descriptions for all constructions.
-
MON_DEFL_FLX_BAKCAL_BASIN—Deflection basin parameters used for
the pavement backcalculation computations using MODCOMP v4.2
computer program.
-
MON_DEFL_FLX_BAKCAL_LAYER—Layer structure and material inputs
for the pavement backcalculation computations using MODCOMP v4.2
computer program.
-
MON_DEFL_FLX_BAKCAL_POINT—Interpreted results of backcalculated
elastic layer moduli from FWD measurements for flexible and rigid
pavement structures using MODCOMP v4.2 computer program.
-
MON_DEFL_FLX_BAKCAL_SECT—Summary of results presented in
MON_DEFL_FLX_BAKCAL_POINT by section and test date. Results in
MON_DEFL_FLX_BAKCAL_POINT with greater than 2 percent ERROR_RMSE
were excluded from summary statistics.
-
MON_DEFL_FLX_NMODEL_POINT—Interpreted results of nonlinear
backcalculated from FWD measurements for flexible and rigid
pavement structures using MODCOMP v4.2 computer program.
-
MON_DEFL_FLX_NMODEL_SECT—Summary of results presented in
MON_DEFL_FLX_NMODEL_POINT by section and test date. Results in
MON_DEFL_FLX_NMODEL_POINT with greater than 2 percent ERROR_RMSE
were excluded from summary statistics.
-
MON_DEFL_FWDCHECK_CMNTS—Comments generated during use of
FWDCHECK for basic analysis of the deflection data (not part of the
backcalculation process, but listed here as a possible information
source even though it is only partially populated).
-
MON_DEFL_LOC_INFO—Test point specific condition data for
Dynatest FWD.
-
MON_DEFL_MASTER—FWD data collection general site measurement
information.
-
MON_DEFL_RGD_BAKCAL_BASIN—Deflection basin parameters used in
backcalculation of rigid pavement structures.
-
MON_DEFL_RGD_BAKCAL_LAYER—Pavement structure input parameters
used in the backcalculation of elastic layer moduli and pavement
structural material properties.
-
MON_DEFL_RGD_BAKCAL_POINT—Interpreted elastic layer moduli and
pavement structural material properties from backcalculation and
direct computations of FWD measurements of rigid pavement
structures using version 2.2 of ERESBACK.
-
MON_DEFL_RGD_BAKCAL_SECT—Test section statistics for interpreted
elastic layer moduli and pavement structural material properties
from backcalculation and direct computations of FWD measurements
for rigid pavement structures for each measurement pass.
LTPP Backcalculation Tables Screened
All the LTPP tables containing backcalculated moduli were
screened using the forwardcalculated values. Table 11 gives a list
of these tables, as well as the number of records contained and
screened in each computed parameter table in the LTPP database.
Table 11. LTPP backcalculation tables and number of
records screened.
Name of Backcalculation Table for Screening |
Number of Data Records |
Number of Records Screened |
% Records Screened |
MON_DEFL_FLX_BAKCAL_POINT |
1,645,615 |
1,088,679 |
66.2% |
MON_DEFL_FLX_BAKCAL_SECT |
44,544 |
25,564 |
57.4% |
MON_DEFL_FLX_NMODEL_POINT |
181,051 |
130,805 |
72.2% |
MON_DEFL_FLX_NMODEL_SECT |
3,852 |
2,754 |
71.5% |
MON_DEFL_RGD_BAKCAL_POINT |
37,246 |
36,989 |
99.3% |
MON_DEFL_RGD_BAKCAL_SECT |
1,189 |
1,183 |
99.5% |
The first four lines of Table 11 are the MODCOMP data records,
while the last two lines are the two-layer rigid pavement data
using the dense liquid and elastic solid methodology. Almost all of
the unscreened data from Table 11 resulted from the use of a rigid
or stiff layer at some depth in the flexible section data tables
(for both the flexible and some of the rigid section data) created
by MODCOMP or an assumed layer modulus for any other structural
layer when MODCOMP was used. These moduli were assumed, or fixed,
and were therefore not screened.
In addition, with data migration and other issues, the records
in tables *_BAKCAL_LAYER, *_BAKCAL_BASIN, and *_BAKCAL_POINT are no
longer directly connectable with one another. Since data from these
tables (plus TST_L05B and others) were used in the screening
process, a very limited number (for example, <1 percent of the
total number of records in the rigid backcalculated data tables
from the last two lines of Table 11) of records that could not be
connected were left unscreened.
Since the completion of the backcalculation project (in 1999),
the LTPP program underwent a major change in the way the
CONSTRUCTION_NO field is assigned. As a result, the CONSTRUCTION_NO
values in the backcalculation tables do not directly correlate with
the CONSTRUCTION_NO values in other key LTPP tables such as the
EXPERIMENT_SECTION and TST_L05B tables. For this study, the
forwardcalculation method used the deflection basins in the
MON_DEFL_FLX_BAKCAL_BASIN table and the MON_DEFL_RGD_BAKCAL_BASIN
table (which use the former CONSTRUCTION_NO assignments) while
using the pavement structure data (material type and layer
thicknesses) from the TST_L05B table (which uses the current
CONSTRUCTION_NO assignments). To ensure the proper connection of
the corresponding records, the following fields were compared to
ensure the proper assignment of the CONSTRUCTION_NO values:
EXPERIMENT_SECTION.CN_ASSIGN_DATE,
MON_DEFL_FLX_BAKCAL_LAYER.CN_REF_DATE, and MON_DEFL_RGD_BAKCAL
_LAYER.CN_REF_DATE. This process was successful in almost all
cases, as indicated above.
KEY STEPS FOR SCREENING THE LTPP BACKCALCULATION DATA
The following steps were taken to screen backcalculated
moduli:
- Obtain the FWD deflection basin data.
- Determine the forwardcalculation layer structures.
- Conduct forwardcalculation for each basin with layer structure
identified.
- Compute section level summary statistics of forwardcalculation
parameters.
- Correspond (connect) the forwardcalculated moduli with the
backcalculated moduli.
- Screen the backcalculated moduli using the corresponding
forwardcalculated moduli.
Each step is discussed in detail below.
Step 1. Obtain the FWD
Deflection Basins for Forwardcalculation
The objective of this study was to screen the backcalculated
moduli in the LTPP database and use the same deflection basins that
were used to derive these backcalculated moduli. As a result, the
deflection basins from the tables MON_DEFL_FLX_BAKCAL_BASIN and
MON_DEFL_RGD _BAKCAL_BASIN were extracted and subsequently used in
the forwardcalculation computations.
Step 2. Determine the Pavement
Layer Structures for Forwardcalculation
As presented in chapter 4, elastic
moduli for up to three layers are forwardcalculated for each
deflection basin. Then the pavement systems were divided or
combined into a two- or three-layer structure, as follows:
- Surface (bound) layer (AC or PCC).
- Base layer (unbound or granular).
- Subgrade; depth to apparent stiff layer calculated from the
deflection basin.
For rigid pavements, the uppermost base layer below the PCC slab
was considered the base layer.
For flexible pavements, the layer system is more complex, and
more than three layers were used in backcalculation for many
flexible pavement sections. The researchers applied engineering
judgment to assign each of the backcalculation layers to correspond
to a forwardcalculation layer, according to the scheme shown in
Table 12.
Step 3. Conduct Forwardcalculation
Forwardcalculation is carried out using the method discussed in
chapter 3 for each of the two or three
forwardcalculation layers (i.e., the bound surface course, base if
present, and subgrade).
A small percentage of the FWD load-deflection data has been
associated with nonstandard sensor positions. These nonstandard
positions were adjusted to their actual positions before applying
the forwardcalculation procedures.
Table 13 gives an outline of the deflection
data where nonstandard sensor positions were considered, provided
there were any backcalculated data in the database associated with
these data (seldom the case).
Table 12. Assignment of backcalculated layers for
forwardcalculation of flexible pavements.
Layer Structure Information from the TST_L05B or
Backcalculation Tables |
Assigned Forwardcalculated Layer |
Layer Description |
Layer Type |
LTPP Code |
Material Type |
Overlay |
AC |
1 |
Hot mixed, hot laid AC, dense graded |
Asphalt concrete layer |
Overlay |
AC |
13 |
Recycled AC, hot laid, central plant mix |
Asphalt concrete layer |
Overlay |
AC |
16 |
Recycled AC, heater scarification/recompaction |
Asphalt concrete layer |
Seal Coat |
AC |
20 |
Other |
Asphalt concrete layer |
Seal Coat |
AC |
71 |
Chip seal |
Asphalt concrete layer |
Seal Coat |
AC |
72 |
Slurry seal |
Asphalt concrete layer |
Seal Coat |
AC |
73 |
Fog seal |
Asphalt concrete layer |
Seal Coat |
AC |
82 |
Sand seal |
Asphalt concrete layer |
AC Layer Below Surface (Binder Course) |
AC |
1 |
Hot mixed, hot laid AC, dense graded |
Asphalt concrete layer |
AC Layer Below Surface (Binder Course) |
AC |
13 |
Recycled AC, hot laid, central plant mix |
Asphalt concrete layer |
Base Layer |
AC |
319 |
HMAC |
Asphalt concrete layer |
Base Layer |
AC |
321 |
Asphalt treated mixture |
Asphalt concrete layer |
Interlayer |
AC |
71 |
Chip seal |
Asphalt concrete layer |
Interlayer |
AC |
75 |
Nonwoven geotextile |
Asphalt concrete layer |
Interlayer |
AC |
77 |
Stress absorbing membrane interlayer |
Asphalt concrete layer |
Interlayer |
AC |
78 |
Dense graded asphalt concrete interlayer |
Asphalt concrete layer |
Friction Course |
AC |
2 |
Hot mixed, hot laid AC, open graded |
Asphalt concrete layer |
Friction Course |
AC |
71 |
Chip seal |
Asphalt concrete layer |
Surface Treatment |
AC |
9 |
Plant mix (emulsified asphalt) material, cold laid |
Asphalt concrete layer |
Surface Treatment |
AC |
11 |
Single surface treatment |
Asphalt concrete layer |
Surface Treatment |
AC |
71 |
Chip seal |
Asphalt concrete layer |
Surface Treatment |
AC |
82 |
Sand seal |
Asphalt concrete layer |
Interlayer |
EF |
74 |
Woven geotextile |
Subgrade |
Interlayer |
EF |
75 |
Nonwoven geotextile |
Subgrade |
Base Layer |
GB |
302 |
Gravel (uncrushed) |
Granular base layer |
Base Layer |
GB |
303 |
Crushed stone |
Granular base layer |
Base Layer |
GB |
304 |
Crushed gravel |
Granular base layer |
Base Layer |
GB |
306 |
Sand |
Granular base layer |
Base Layer |
GB |
309 |
Fine-grained soils |
Granular base layer |
Base Layer |
GB |
310 |
Other (specify if possible) |
Granular base layer |
Base Layer |
GB |
337 |
Limerock, caliche |
Granular base layer |
Base Layer |
GB |
307 |
Soil-aggregate mixture (predominantly fine-grained) |
Subgrade |
Base Layer |
GB |
308 |
Soil-aggregate mixture (predominantly coarse-grained) |
Subgrade |
Subbase Layer |
GB |
308 |
Soil-aggregate mixture (predominantly coarse-grained) |
Subgrade |
Subbase Layer |
GS |
302 |
Gravel (uncrushed) |
Granular base layer |
Subbase Layer |
GS |
303 |
Crushed stone |
Granular base layer |
Subbase Layer |
GS |
304 |
Crushed gravel |
Granular base layer |
Base Layer |
GS |
308 |
Soil-aggregate mixture (predominantly coarse-grained) |
Subgrade |
Subbase Layer |
GS |
306 |
Sand |
Subgrade |
Subbase Layer |
GS |
307 |
Soil-aggregate mixture (predominantly fine-grained) |
Subgrade |
Subbase Layer |
GS |
308 |
Soil-aggregate mixture (predominantly coarse-grained) |
Subgrade |
Subbase Layer |
GS |
309 |
Fine-grained soils |
Subgrade |
Subbase Layer |
GS |
310 |
Other (specify if possible) |
Subgrade |
Subbase Layer |
GS |
338 |
Lime-treated soil |
Subgrade |
Embankment Layer |
GS |
102 |
Fine-grained soils: lean inorganic clay |
Subgrade |
Embankment Layer |
GS |
131 |
Fine-grained soils: silty clay |
Subgrade |
Base Layer |
TB |
319 |
HMAC |
Asphalt concrete layer |
Base Layer |
TB |
320 |
Sand asphalt |
Asphalt concrete layer |
Base Layer |
TB |
321 |
Asphalt treated mixture |
Asphalt concrete layer |
Base Layer |
TB |
324 |
Dense graded, cold laid, mixed in place |
Asphalt concrete layer |
Base Layer |
TB |
328 |
Recycled asphalt concrete, plant mix, hot laid |
Asphalt concrete layer |
Base Layer |
TB |
334 |
Lean concrete |
Asphalt concrete layer |
Base Layer |
TB |
325 |
Open graded, hot laid, central plant mix |
Granular base layer |
Base Layer |
TB |
327 |
Open graded, cold laid, mixed in place |
Granular base layer |
Base Layer |
TB |
331 |
Cement aggregate mixture |
Granular base layer |
Base Layer |
TB |
350 |
Other |
Granular base layer |
Base Layer |
TB |
333 |
Cement-treated soil |
Subgrade |
Base Layer |
TB |
339 |
Soil cement |
Subgrade |
Base Layer |
TB |
340 |
Pozzolanic-aggregate mixture |
Subgrade |
AC Layer Below Surface (Binder Course) |
TS |
1 |
Hot mixed, hot laid AC, dense graded |
Asphalt concrete layer |
Subbase Layer |
TS |
320 |
Sand asphalt |
Asphalt concrete layer |
Subbase Layer |
TS |
325 |
Open graded, hot laid, central plant mix |
Granular base layer |
Subbase Layer |
TS |
331 |
Cement aggregate mixture |
Granular base layer |
Subbase Layer |
TS |
338 |
Lime-treated soil |
Subgrade |
Subbase Layer |
TS |
339 |
Soil cement |
Subgrade |
Table 13. FWD- and time-specific sensor positioning
anomalies in the LTPP database.
Region & FWD S/N |
Dates Affected (inclusive) |
Actual Sensor Positions (inches) |
Approx. # of Test Dates |
Reg. 1–s/n129 |
3 Nov ’95?14 Apr ’96 |
0, 8, 12, 18, 24, 36 & 48 |
21 |
Reg. 1–s/n129 |
15 Apr ’97?21 May ’97 |
0, 8, 12, 18, 24, 36 & 48 |
12 |
Reg. 2–s/n061* |
4 Aug ’89?10 Aug ’89 |
0, 8, 12, 24, 30.5, 48 & 72 |
4 |
Reg. 2–s/n130 |
25 Aug ’94?7 Sep ’94 |
0, 8, 12, 18, 36, 48 & 60 |
16 |
Reg. 3–s/n075 |
17 Jan ’90?22 Jan ’90 |
0, 8, 12, 18, 30, 42 & 66 |
4 |
Reg. 3–s/n132 |
29 Jul ’96?25 Oct ’96 |
0, 8, 12, 18, 24, 36 & 48 |
29 |
Reg. 4–s/n061* |
<26 Feb ’89?8 Sep ’89 |
0, 8, 12, 24, 30.5, 48 & 72 |
97 |
Reg. 4–s/n061 |
17 Jul ’95?31 Oct ’95 |
0, 8, 12, 18, 24, 36 & 48 |
65 |
Reg. 4–s/n131 |
<24 May ’ 94?30 Apr ’96 |
0, 8, 12, 18, 24, 36 & 48 |
191 |
Reg.4–s/n13110 |
16 Dec ’97?20 Jan ’98 |
0, 8, 18, 24, 36, 48 & 60 |
8 |
* Same FWD and overlapping
dates—LTPP field tests conducted in two different LTPP Regions.
Step 4. Compute Section Level Summary Statistics of
Forwardcalculated Parameters
For the section level data, to identify and remove outliers for
a given section (and test date), the researchers used the so-called
interquartile range (IQR) as outlined below.
Pavement engineers typically use two standard deviations as
criteria for identifying outliers. The shortcoming of this approach
is that neither the population mean nor the population standard
deviation is available when evaluating the data for outliers.
Instead, the sample mean and sample standard deviation are
generally used, without consideration of the actual distribution.
This technique introduces a bias in identifying the outliers when
using the two standard deviation approach because outliers are
(incorrectly) included in the computation of the sample mean and
the sample standard deviation. In addition, the section standard
deviation on an arithmetic basis is often large enough to compute
negative values on the low end of the section level modulus range.
This calculation results in elimination of few (if any) of the
unreasonably low values and only eliminates the unreasonably high
values. The nonnormal and asymmetric distribution of the modulus
data usually causes this problem. In fact, the distribution of
moduli generally is closer to a log-normal distribution than an
arithmetic normal distribution.
To overcome these difficulties, the IQR method has been used to
identify and remove outliers in the computed point-by-point moduli
for each section (and date of test).(11) The following
provides specifics of this method:
Figure 22. Equation. Interquartile
range.
where: Q1 |
= |
25th quartile of the logs of the moduli |
and Q3 |
= |
75th quartile of the logs of the moduli |
Outliers are defined as values that are outside the range:
(Q1 - 1.5 * IQR, Q3 + 1.5 * IQR).
Data points that were outside of the above range were identified
as outliers and not used to calculate the section means from
forwardcalculation.
Step 5. Correspond the Forwardcalculated Moduli with the
Backcalculated Moduli
After all forwardcalculation is completed, the forwardcalculated
moduli were paired with the backcalculated moduli using their
section IDs, according to table 12.
Step 6. Screen the Backcalculated Moduli Using the
Corresponding Forwardcalculated Moduli
The LTPP database of backcalculated moduli was screened. This
screening took place using the following sequential steps:
- The modulus values in the backcalculation tables
that were assumed or held constant were
not screened—Since these values are presumably a good
educated guess at the actual in situ stiffness, they were not
screened and accordingly were left as is in the database with an
appropriate flag.
- Reasonableness screening—Various
pavement materials are commonly associated with typical or
reasonable moduli, depending on the material type and other
factors. Assuming the backcalculated value was not fixed, a wide
range of feasible modulus values was assigned to the various
material types, and any value outside of these rather liberal
ranges could then be called into question, whether the method of
determining the modulus is back- or forwardcalculation. Table 14 lists these values. During screening of the
database, also including the newly created forwardcalculated
values, these limits were applied before applying the
remaining criteria. In other words, if a given modulus calculation
was outside of the broad ranges indicated in Table
14, then it was either flagged in the case of the existing
tables or noted in the case of forwardcalculated values, and then
not used in further screening routines.
- Correspondence screening using forwardcalculated
values – Provided the data did not become flagged
during Steps 1 or 2 of
the screening process, each backcalculated value was then compared
to the corresponding forwardcalculated value. If the backcalculated
modulus was within a factor of 1.5 of the forwardcalculated value,
then it was assumed to be “acceptable” and not recommended for
flagging (except through the use of a “0,” see Table 15) in the database. If it was outside of this
reasonable range (1.5 times the forwardcalculated value or the
forwardcalculated value divided by 1.5), then the flags shown in
Table 15 were used to indicate that the
existing value in the database was “suspect.” These flagging codes
were used for both the “point” data tables and the “section” data
tables.
Table 14. Reasonable ranges for various pavement layers in
the LTPP database.
|
LTPP Code |
Reasonable Range |
MPa |
psi |
min |
max |
min |
Max |
Base Materials |
Asphalt-Treated Mixture, not Permeable Asphalt-Treated Base
(PATB) |
321 |
700 |
25,000 |
101,500 |
3,625,000 |
Gravel, Uncrushed |
302 |
50 |
750 |
7,250 |
108,750 |
Crushed Stone |
303 |
100 |
1,500 |
14,500 |
217,500 |
Crushed Gravel |
304 |
75 |
1,000 |
10,875 |
145,000 |
Sand |
306 |
40 |
500 |
5,800 |
72,500 |
Soil-Aggregate Mixture (predominantly fine-grained) |
307 |
50 |
700 |
7,250 |
101,500 |
Soil-Aggregate Mixture (predominantly coarse-grained) |
308 |
60 |
800 |
8,700 |
116,000 |
Fine Grained Soil or Base |
309 |
35 |
450 |
5,100 |
65,000 |
Hot-Mixed AC |
319 |
700 |
25,000 |
101,500 |
3,625,000 |
Sand Asphalt |
320 |
700 |
25,000 |
101,500 |
3,625,000 |
Dense-Graded, Cold-Laid, Central Plant Mix AC |
323 |
700 |
25,000 |
101,500 |
3,625,000 |
Open-Graded, Hot Laid, Central Plant Mix AC |
325 |
350 |
3,500 |
50,750 |
507,500 |
Cement Aggregate Mixture |
331 |
2,000 |
20,000 |
290,000 |
2,900,000 |
Econocrete |
332 |
3,500 |
35,000 |
507,500 |
5,075,000 |
Lean Concrete |
334 |
4,500 |
45,000 |
652,500 |
6,525,000 |
Soil Cement |
339 |
1,000 |
7,000 |
145,000 |
1,015,000 |
Open-Graded, Cold Laid, In-Place Mix AC |
327 |
200 |
3,000 |
29,000 |
435,000 |
Limerock; Caliche |
337 |
150 |
1,500 |
21,750 |
217,500 |
Other—Treated Base (TB) |
350 |
400 |
8,000 |
58,000 |
1,160,000 |
Bound Surface Courses |
Concrete Surface (uncracked) |
|
10,000 |
70,000 |
1,450,000 |
10,150,000 |
AC Surface (>0 °C–<45 °C, not alligatored) |
|
700 |
25,000 |
101,500 |
3,625,000 |
Unbound Subgrades |
Any unbound type |
|
15 |
650 |
2,175 |
94,250 |
Table 15. Flagging codes used to screen the backcalculated
LTPP database.
Description of the correspondence between the
forwardcalculated and the backcalculated modulus values |
Correspondence code values (flags) |
Ratio between the forwardcalculated and
backcalculated modulus values |
Acceptable |
0 |
2/3 = Ratio = 1.5 |
Marginal |
1 |
1/2 = Ratio = 2 (& not code 0) |
Questionable |
2 |
1/3 = Ratio = 3 (& not codes 0 or 1) |
Unacceptable |
3 |
Ratio < 1/3 or Ratio > 3 |
Incorporation of the Screening Results into the LTPP
Database
All applicable flags associated with the backcalculated moduli
in the LTPP computed parameter database will be submitted to FHWA,
including all records where the backcalculated values were assumed
(fixed) or not reasonable. All remaining data will be submitted
with an appropriate correspondence flag (i.e., 0, 1, 2, or 3—see
Table 15) to be incorporated into the existing backcalculation data
tables, as outlined above.
In addition, tables of all forwardcalculated moduli, both at the
point and section levels, will be given to FHWA as candidates for
incorporation into the LTPP database as computed parameters. These
data will also include the “reasonableness flag” where
applicable.
FHWA-HRT-05-150
|