Backcalculation of Dynamic Modulus from Falling Weight Deflectometer Data

US Transportation Collection2021PDFTech ReportKim, Y RichardZeng, ZheLee, KangjinNorth Carolina State University. Department of Civil, Construction, and Environmental EngineeringNorth Carolina Department of Transportation. Research and Analysis GroupUnited States. Department of Transportation. Federal Highway AdministrationNorth Carolina Department of Transportation. Research and Development UnitNorth CarolinaUnited StatesAsphalt concrete pavementsBackcalculationDynamic modulus of elasticityFalling weight deflectometersRehabilitation (Maintenance)Witczak\u2019s predictive equationAASHTOWare Pavement ME Design (PMED)FHWA/NC/2017-03Final Report, August 2016 \u2013 July 2020The Mechanistic-Empirical Pavement Design Guide (Pavement ME Guide) and its related software, AASHTOWare Pavement ME Design, have been widely used in the asphalt pavement community. The Guide proposes three analysis levels for highway agencies to use for rehabilitation design. However, as these methods were developed in the early 2000s, certain aspects were not considered sufficiently in terms of their accuracy and efficiency. Over time, researchers have offered various suggestions to modify the theories and guidelines in the Pavement ME Guide, and from a practical point of view, some parts of the Guide still need to be improved, such as how best to perform backcalculations and characterize multilayered existing asphalt concrete (AC) pavement. To this end, this study focused specifically on improving methods to characterize damaged existing AC layers. Based on North Carolina (NC) 96 highway data, North Carolina State University researchers evaluated and compared the accuracy of the three levels in the Pavement ME Guide. Through tests and analysis of field cores, this report provides specific guidelines for the North Carolina Department of Transportation (NCDOT) to select representative layers in multilayered AC pavements. The research team compared predicted damaged dynamic modulus mastercurves to laboratory-measured values. The results show that Level 1 should always be the first choice for highway agencies. Nonetheless, if Level 2 or 3 needs to be applied, then the transfer function that relates the damage factor to the percentage of bottom-up cracking needs to be calibrated first to ensure that the results are consistent with those of Level 1. Although sometimes the in situ modulus of the granular layer is measured during the backcalculation process, the modulus value is not recommended to be fixed in the program in order to give the algorithm enough freedom to satisfy the root mean square error criterion. Regarding the selection of the representative layer, the total core, not the thickest layer, should be used for laboratory evaluation. The NCDOT can thus save time and effort without sacrificing the accuracy of rehabilitated pavement performance predictions.127