Analysis and evaluation of methods for backcalculation of Mr values.

PDFTech ReportFHWA/LA-9-/263LA 736-17-0103BackcalculationModulus of resilienceDeflectionData collectionComputer programmingLouisianaLouisiana Transportation Research CenterRoberts, F. L.Wedgeworth, J. B.Pumphrey, N. D.Louisiana Tech University. Dept. of Civil EngineeringLouisiana Transportation Research CenterUnited States. Federal Highway AdministrationUS Transportation CollectionUse of the 1986 AASHTO Design Guide requires accurate estimates of the resilient modulus of flexible pavement materials. Traditionally, these properties have been determined from either laboratory testing or by backcalculation from deflection data. Since Louisiana has chosen the backcalculation approach, there was a need to review the available computer programs and determine which were most suitable for use in Louisiana.A literature review was conducted to determine which of the programs worked best for other investigators and to select a small group for trial use in Louisiana. Deflection data was secured on a series of experimental sections constructed from pavement materials which had been previously tested. The programs chosen for study included: BOUSDEF, EVERCALC, MODULUS and WESDEF. Two other programs, MODCOMP2 and ELSDEF, were initially included but technical difficulties necessitated their elimination from the study. some of the programs had different control options, such as MODULUS and EVERCALC; therefore, a total of seven program options were included.Statistical analyses were conducted using ANOVA techniques and the Student-Newman-Keuls, Duncan's Multiple Range and Least Significant Difference tests to determine whether the backcalculated and laboratory determined moduli were significantly different. Overall, these tests showed that MODULUS Step 3 and WESDEF provided the best match between estimated and laboratory measured moduli when all pavement layers were considered. The ability to run multiple deflection basins, automatic temperature corrections for hot mix asphalt moduli and internally developed estimates of layer moduli made MODULUS the choice of project investigators. Additionally, the selection of MODULUS by SHRP for use in the LTPP project ensures that any program improvements will be fully documented and made widely available to the technical community