How does a Benkelman Beam reading compare with the FWD central deflection?
As part of ongoing research in New Zealand, Beam/FWD deflection ratios were determined for one unbound granular pavement with thin friction course surfacing and one structural asphaltic pavement (CAPTIF) giving respective Beam/FWD ratios of 1.05 and 1.22. The CAPTIF data from recent research at the University of Canterbury allowed precise positioning of both Beam and FWD, producing a high correlation. After including data from other sources, there appears to be a slight trend for greater Beam/FWD ratios with greater overall deflection (Figure 5). The data presented, supports the conclusions by others that there is no real correlation (even when plotted logarithmically) and that site specific correlations should be carried out. As an interim guide, the following approximations, are suggested : Where deflections are less than 1 mm under a 40 kN FWD impact load, adopt a Beam/FWD ratio of about 1.1. Where deflections exceed 1 mm, the ratio is likely to be in excess of 1.1, and related to d
