What is the general difference between the FWD and instrumented Benkelman Beam?

The FWD applies an impulse to the road surface by a specially designed loading system which represents the dynamic short term loading of a heavy wheel load. For example, the Dynatest FWD produces an impact load of 25-30 ms duration, and a peak force of up to 120 kN (adjustable). The deflection bowl response of the pavement is measured with a set of 7-9 precision geophones at varying distances from the loading plate. The Benkelman Beam, instrumented for automatic recording of the full bowl shape, measures responses under a slower and variable loading time. As the wheel load is positioned close to the point of maximum deflection during set up, the effective load duration is longer at close offsets than at the more distant points.

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 1, below). 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 deflection as defined by:

Beam/FWD ratio = 1.1 x (FWD deflection in mm)0.4

This relationship is only intended for use with simplified overlay design methods, which require Benkelman Beam deflections (AUSTROADS, 1992; Chapter 10). For determining the elastic properties of the existing pavement for mechanistic design of rehabilitation treatments, deflection bowls measured with either a Benkelman Beam or an FWD with no adjustment are used. When back-analysing the deflection bowls, the user informs the program of the loading geometry that was used to obtain the deflections. However conversion between FWD and Benkelman Beam deflections is unnecessary when mechanistic analysis is adopted for pavement rehabilitation design.

What is the accuracy of the FWD deflections?

Because no reference point (or support) is needed for the FWD deflection bowl measurement, the deflections can be measured with high accuracy. Ullidtz (1998) indicates a typical accuracy of 0.5%