Comparison between Falling Weight Deflectometer (FWD) and Benkelman Beam Deflection Basin Parameters and back-calculation of layer moduli

Abstract

Many years of experience exist in South Africa on the use and interpretation of Benkelman beam type of road surface deflection measurements with the Road Surface Deflectometer (RSD), as well as depth deflection measurements with the use of the Multi-depth Deflectometer (MDD). These measurements are mainly associated with slow moving (creep or static) loads. The development however, of the impulse load deflection technology, which better reflects deflections ar higher speeds (dynamic loading), allows comparative studies between creep and fast moving loads on pavements. This comparison is needed in order to better understand the effect of load duration on resilient behaviour of pavement systems. This study concentrates on a comparison between the deflection bowls (and its parameters) measured with the Impulse Deflectometer (IDM) and the Road Surface Deflectometer (RSD). The correlations of the deflection basin on granular pavements were higher than on cemented and asphalt base pavement structures. The IDM (impulse load) generally gives lower deflections than the RSD (creep load). It is believed, and it can be shown with rigorous theoretical analysis that the pavements dynamic characteristics, ie damping and inertia are the main reason for this behaviour. Further, poor correlations were found between the RSD and IDM back-calculated effective elastic moduli on asphalt and cemented base pavements. A better correlation, however, was found on granular base pavements. Although more detailed studies and refinement are necessary on this subject, the results presented here are viewed as a practical guide and are aimed to bridge the gap between creep load and impulse load technology on pavement structures in South Africa.