Many highway agencies use falling weight deflectometers (fwds) for pavement evaluation. The primary function of the fwd is to measure a deflection basin due to a load imparted to the pavement. Deflection basins measured in the field are used to backcalculate modulus profiles of pavement sections. Accurate determination of deflection basins in the field is critical. Velocity transducers (geophones) areused to determine the deflections, and load cells are used to measure the applied loads. The accuracy of deflections and loads obtainedfrom these devices is of much concern, and a system is needed to calibrate them. A recently developed absolute calibration system is described. The system consists of two well-calibrated geophones, threeload cells, a signal-conditioning unit, a loading plate, an analog-to-digital (a/d) board, and a computer. Software was developed to control the a/d board and to reduce the data. Geophones were selected for the calibration of the fwd sensors after an evaluation of the accuracy and precision of five candidate sensors. An aluminum loading plate was constructed to allow mounting of the load cells in line with and below the fwd load cell. The calibration curves of all load cells were traceable to the national institute of standards and technology (formerly national bureau of standards). The effects on the calibration factors of load cells of using the aluminum plate were small. The signal-conditioning unit was developed to condition the signals before acquisition. The system was used to calibrate an fwd device. The system was effective. A calibration process is proposed on the basis of an analysis of the data collected. The calibration system is cost-effectve, accurate, and rugged, and it can be used by highway agencies to calibrate their fwd devices. This paper appears in transportation research record no. 1293, Backcalculation of pavement moduli 1991 .
Samenvatting