The Falling Weight Deflectometer (FWD) was conceived at the Laboratoire Centrales Pontes et Chausses (LCPC) in France but was developed in Denmark into a sophisticated non-destructive test method for evaluating pavement properties. FWDs vary in detail depending on manufacturer and model type. The majority of FWDs (and all those currently in use in the UK) are trailer-mounted devices, towed behind a vehicle.

General Operation

The FWD generates a load pulse by dropping a weight onto a damped spring system mounted onto a circular loading plate. The spring, smoothes the load of the falling weight to produce an almost sinusoidal pulse. The mass, drop height and plate diameter can all be adjusted to achieve the desired impact stress magnitude. The load pulse has a similar, but by no means identical, shape to that generated by a moving vehicle. The resultant deflection bowl of the pavement is accurately determined from measurement of peak deflections at the centre of the loading plate and at several (normally seven) radial positions by a series of geophones, usually referred to as d1 (central) to d7 (outermost). The actual load applied to the pavement is partially dependent on the FWD-pavement interaction and is therefore measured by a load cell placed on the surface of the plate.

Types

Most FWDs typically have a loading range of 30 to 120kN. Some manufacturers have developed deflectometers that are capable of applying greater loads, typically up to 240kN. Very similar in operation and appearance to FWDs, these devices are often referred to as Heavy Weight Deflectometers (HWD). On the vast majority of pavements, provided the machines have taken part in and passed the UK FWD correlation trials (organised by TRL on behalf of HA), the results from FWDs and HWDs are very similar to one another and may be used interchangeably. Normally, testing is undertaken using a 300mm diameter plate with a test load of 50kN (flexible) or at 75kN or 100kN (rigid).

Measurements

Information on the stiffnesses of the component pavement layers can be derived from the shape of the deflection bowl because different parts of this profile are influenced by different pavement layers. The central deflection (d1) gives an indication of the overall pavement stiffness. The outer deflection measurements (eg d6 and d7) give an indication of the subgrade condition. The difference between deflection measured at two points close to the load (eg d1-d4) is mainly dependent on the stiffness of the upper bound layers. Deflection and deflection-difference plots are also useful for showing relative differences in the condition of the layers, and enable delineation of the pavement into sections with similar behaviour, giving an indication of where structural weakness may be present.

Applications

There are many ways of analysing FWD measurements and these can produce relatively consistent results for layer stiffness, but there is no standard approach for estimating residual life or overlay thicknesses. Due to this, the FWD is primarily used for the following:

Flexible pavements:

• Layer stiffness determination.

Rigid pavements:

• Layer stiffness determination.
• Condition assessment - On concrete and concrete covered pavements, the relative degree of load transfer at joints and cracks can be assessed by loading the slab on one side whilst deflections are measured at each side of the joint or crack.

Temperature

The elastic stiffness of bituminous materials is highly dependent on temperature and therefore it is essential to measure temperature at the time of testing (usually at depths of 40mm and/or 100mm). For the FWD, any bituminous layer stiffnesses will normally need to be adjusted to 20oC to allow comparison with reference stiffness values. However, since the adjustment process can introduce significant uncertainty in the results, it is preferable to test at temperatures as close as possible to 20oC.

Temperature can also significantly affect the stiffness of concrete materials. At low temperatures, the widths of cracks and joints are greatest and the degree of movement on loading is more severe. Therefore, the effective stiffness of a cracked concrete layer can often increase as the temperature increases. For this reason, deflections on flexible-composite pavements are not normally adjusted for temperature as the reduction in stiffness of bituminous material with increasing temperature is offset by the increase in the lean concrete stiffness (and vice versa at low temperatures). Furthermore, load transfer testing should normally be undertaken when the joints (or cracks) are open (below 15oC).

TRL FWD Facilities

TRL currently have 2 FWD's each towed by a four wheel drive Jeep.

For more information contact:

Kevin Green
Lead Laboratory Technician
Telephone: +44 (0)1344 770903
Email: kgreen@trl.co.uk