ESTIMATION OF PAVING MATERIALS DESIGN MODULI FROM FALLING WEIGHT DEFLECTOMETER MEASUREMENTS

The emergence of mechanistic pavement thickness design procedures or semiempirical design procedures, as contained in the 1986 AASHTO Guide for Design of Pavement Structures, has created a need for methods of evaluating elastic moduli of paving materials and subgrade soils. A study was conducted to develop methods for using falling weight deflectometer (FWD) measurements to determine moduli of in situ pavement materials and to compare FWD-estimated moduli with laboratory-measured values in order to achieve consistent input to thickness design procedures. A three-layer pavement model was used to characterize typical Alabama flexible pavements. Simple procedures were developed to account for seasonal variations and to estimate average or effective moduli values for granular base-subbase and subgrade soils from limited FWD measurements. A procedure for adjusting asphalt-aggregate moduli to standard design temperature (70 deg F) was developed. Laboratory moduli for asphalt-aggregate mixtures measured with indirect tension tests (ASTM D4123) produce moduli that compare well with moduli backcalculated from FWD pavement deflection basin measurements. As expected, characterization of granular base-subbase was most difficult. There were large differences between FWD moduli and laboratory moduli from triaxial testing (AASHTO T274). Although some inconsistencies in input to thickness design procedures may result, FWD moduli are recommended for characterizing in situ granular base-subbase. In general, good agreement was demonstrated between FWD and laboratory (AASHTO T274) moduli for subgrade soils.