Investigation of Backcalculated Moduli Using Deflections Obtained at Various Locations in a Pavement Structure

Abstract

The interpretation and use of deflection measurements on the prepared surfaces of the various pavement layers during construction are examined. Measurements were obtained from four asphalt concrete pavement test sections, two with unbound aggregate base and two with bituminous-treated base over an untreated aggregate base. Deflection basin measurements using a falling weight deflectometer were performed on the prepared surfaces of the subgrade, base layers, and asphalt concrete layers. The elastic moduli of each layer were computed using the EVERCALC backcalculation program. The primary finding from this investigation is that deflection measurements on the subgrade and base layers during construction can be used to control construction uniformity and provide checks on mechanistic-based pavement design assumptions. Also, subgrade uniformity has a profound impact on the entire pavement structure and subgrade variations affect total deflections and computed layer moduli of all successive layers. The backcalculated modulus is directly related to the stress state in the layer. For unbound aggregate bases, the backcalculated elastic modulus decreases with a decrease in the bulk stress, and for fine-grained subgrade soil, the backcalculated elastic modulus increased with a decrease in the deviator stress. As expected, a higher and more variable root-mean-square basin fit error value was obtained for measurements on unbound material as compared with measurements on bound material surfaces.