Comparison of Two Falling Weight Deflectometer Devices, Dynatest 8000 and KUAB 2M-FWD

This paper presents the results of a detailed comparison conducted by the authors between two falling weight deflectometers, the Dynatest Model 8000 and the KUAB two-mass falling weight deflectometer (2M-FWD). The Dynatest Model FWD has been operating in the United States for several years. It is based upon the principal of dropping a mass on a single buffer system connected to a rigid plate resting on the pavement surface. The 2M-FWD is a newly improved version of the KUAB falling weight deflectometer which recently has been introduced into the United States. It is based upon the principal of dropping a mass on a double buffer system separated by a second mass which is connected to a segmented plate resting on the pavement surface. The addition of the second mass and buffer results in a consistent half sine curve shaped load pulse with a longer duration of loading. The two devices also differ in the type of sensor used to measure the surface deflection during loading. The Dynatest device uses geophones which measure deflection up to 2 mm, while the 2M-FWD uses LVDTs which are capable of measuring surface deflections up to 5 mm. Other hardware differences between the two devices also are discussed in the paper. A testing program was initiated to obtain comparative deflection data from a variety of pavement types including flexible and rigid pavements over weak and strong foundations. In addition to the comparison of actual deflection data, layer moduli are computed and compared for each device. Cores from representative test locations were obtained to verify in-field pavement thicknesses.