This report is a comprehensive review and evaluation of the
Long-Term Pavement Performance (LTPP) backcalculation data. In this
study, a new approach, called forwardcalculation was developed to
determine layered elastic moduli from in situ load-deflection data
with procedures and results documented. the entire set of
LTPP-computed parameters of backcalculation results was screened
using forwardcalculated moduli.
Although users cannot reject any backcalculated modulus value
merely because it is outside a reasonable or acceptable range, the
forwardcalculated values were, in most cases, more stable on a
section-by-section basis than the backcalculated values in the LTPP
database. The exception was the portion of the backcalculated
database based on slab-on-dense-liquid and slab-on-elastic-solid
theory, where the correspondence between the two rigid pavement
analysis techniques was excellent and both the backcalculated and
forwardcalculated moduli and k-values were very stable.
It is recommended that the backcalculated database be retained
as is, with the addition of the complementary forwardcalculated
dataset and screening flags, so the database user can decide which
method is more suitable to the application.
This report will interest highway agency engineers involved in
pavement analysis, design, construction, and deflection data
collection, as well as researchers who use LTPP load-deflection
data to improve design procedures and standards for constructing
and rehabilitating pavements.
Gary L. Henderson
Office of Infrastructure
Research and Development
Notice
This document is disseminated under the sponsorship of the U.S.
Department of Transportation in the interest of information
exchange. the U.S. Government assumes no liability for the use of
the information contained in this document. This report does not
constitute a standard, specification, or regulation.The U.S.
Government does not endorse products or manufacturers. Trademarks
or manufacturers’ names appear in this report only because they are
considered essential to the objective of the document.
Quality Assurance Statement
The Federal Highway Administration (FHWA) provides high-quality
information to serve Government, industry, and the public in a
manner that promotes public understanding. Standards and policies
are used to ensure and maximize the quality, objectivity, utility,
and integrity of its information. FHWA periodically reviews quality
issues and adjusts its programs and processes to ensure continuous
quality improvement.
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1. Report No.
FHWA-HRT-05-150
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2. Government Accession No.
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3. Recipient's Catalog No.
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4. Title and Subtitle
Review of the Long-Term Pavement Performance Backcalculation
Results—Final Report
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5. Report Date
February 2006
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6. Performing Organization Code
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7. Author(s)
R.N. Stubstad, Y.J. Jiang, M.L. Clevenson, and E.O. Lukanen
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8. Performing Organization Report No.
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9. Performing Organization Name and Address
Applied Research Associates, Inc.
Elkridge, Maryland 21075-7056
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10. Work Unit No. (TRAIS)
C6B
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11. Contract or Grant No.
DTFH61-02-D-00138
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12. Sponsoring Agency Name and Address
Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, Virginia 22101-2296
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13. Type of Report and Period Covered
Final Report
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14. Sponsoring Agency Code
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15. Supplementary Notes
Contracting Officer’s Technical Representative (COTR): Larry
Wiser, HRDI
LTPP Data Analysis Contract.
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16. Abstract
A new approach to determine layered elastic moduli from in situ
load-deflection data was developed. This “forwardcalculation”
approach differs from backcalculation in that modulus estimates
come directly from the load and deflection data using closed-form
formulae rather iteration. the forwardcalculation equations are
used for the subgrade and the bound surface course for both
flexible and rigid pavement falling weight deflectometer (FWD)
data. Intermediate layer moduli are estimated through commonly used
modular ratios between adjacent layers.
The entire LTPP set of backcalculated parameters was screened
using forwardcalculated moduli. Any assumed or fixed modulus value
was left as is and not further screened (e.g., hard bottom).
Further, any back- or forwardcalculated values outside a broad
range of reasonable values were not further screened, but flagged
as unreasonable. Finally, a set of broad range convergence flags (0
= acceptable, 1 = marginal, 2 = questionable, and 3 = unacceptable)
were applied to the backcalculated dataset, depending on how
closely the pairs of back- and forwardcalculated moduli matched.
Since both techniques used identical FWD load-deflection data as
input, the moduli derived from each approach should be reasonably
close to each other (within a factor of 1.5 to qualify as
acceptable, for example).
Although backcalculated values cannot be rejected merely because
they are outside a reasonable or acceptable range, the
complementary forwardcalculated values were usually more stable on
a section-by-section basis. the exception was the portion of the
database based on slab-on-dense-liquid or slab-on-elastic-solid
theory, where the correspondence between the two approaches was
excellent and very stable. therefore, it is recommended that the
backcalculated database be retained as is, with the addition of
checks and flags so the database user can choose the best method,
depending on the application.
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17. Key Words
Pavements, LTPP, FWD, deflection data, elastic modulus,
backcalculation, forwardcalculation, screening.
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18. Distribution Statement
No restrictions. This document is available to the public
through the National Technical Information Service, Springfield, VA
22161.
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19. Security Classif. (of this report)
Unclassified
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20. Security Classif. (of this page)
Unclassified
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21. No. of Pages
97
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22. Price
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—both Cases.