A comparative evaluation of the structural capacity of cracked and uncracked flexible pavements using mechanistic empirical methods based on deflection measurements by the falling weight deflectometre (fwd) and a traffic speed deflectometre device (tsdd)

Abstract

The FWD has been used for the past three decades in South Africa to emulate a loaded wheel while taking measurement of vertical deflection of the road surface at various offsets from the load. The measured deflection bowls have been used in conjunction with the pavement profile to determine the layer stiffness moduli and ultimately the structural capacity. The FWD test measures the surface deflection via a static vertical impact load. This is contradictory to reality where wheel loads are imposed in a dynamic rolling motion while the pavement exhibits visco-elastic behaviour due to vertical and horizontal stresses. The TSDD is a dynamic loading device that uses Doppler laser technology to calculate deflections using measured horizontal travelling velocity and vertical surface displacement velocity. It could therefore be said that this loading mechanism more accurately simulates actual traffic loading compared to the FWD. The study involves two 100m sections of road P21-1 in Kwa-Zulu Natal, one section was recently rehabilitated and is in a good condition while the other adjacent section is in a state of advanced fatigue. The FWD and TSDD measurements were performed in the outer wheel path at one metre intervals and the continuous TSDD deflection measurements were performed at various traveling speeds. This study aims to investigate the difference between the deflection measurements obtained from these two devices when the structural capacity of the pavement is calculated using mechanistic empirical evaluation methods. Also included in this paper is a comparison of the structural capacities of a 36.5 km section of Provincial Road P21, based on FWD and TSDD measurements.