United States Department of Transportation
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Review of the long-term pavement performance backcalculation results : final report
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2006-02-01
[PDF-819.55 KB]
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OCLC Number:70824448
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Edition:Final Report
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NTL Classification:NTL-HIGHWAY/ROAD TRANSPORTATION-HIGHWAY/ROAD TRANSPORTATION;NTL-HIGHWAY/ROAD TRANSPORTATION-Materials;NTL-HIGHWAY/ROAD TRANSPORTATION-Pavement Management and Performance;NTL-REFERENCES AND DIRECTORIES-Statistics;
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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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