Falling weight deflectometer

A falling weight deflectometer (FWD) is a testing device used by civil engineers to evaluate the physical properties of pavement. FWD data is primarily used to estimate pavement structural capacity for 1) overlay design and 2) to determine if a pavement is being overloaded. Use includes (but is not limited to) highways, local roads, airport pavements, harbor areas and railway tracks. The machine is usually contained within a trailer that can be towed to a location by another vehicle. It can also be built on a pickup truck or inside a mini van.

The FWD is designed to impart a load pulse to the pavement surface which simulates the load produced by a rolling vehicle wheel. The load is produced by dropping a large weight, and transmitted to the pavement through a circular load plate - typically 300 mm diameter on roads and 450 mm on airports. A load cell mounted on top of the load plate measures the load imparted to the pavement surface. The load plate can be solid or segmented. The advantage of a segmented load plate is that it adopts to the shape of the pavement, giving an even distribution of the load on uneven surfaces. Typically, the load for road testing is about 50 kN giving about 700 kPa pressure under the load plate.

Load impact system

There are two different types of load impact systems; single-mass (e.g. Dynatest, Carl Bro, PaveTesting) and double-mass (KUAB).^[1][2] In a single-mass system, a weight is dropped onto a single buffer connected to a load plate, which rests on the surface being tested. The load force is transferred through the plate, and the plate creates a deflection that simulates a wheel load. In the double-mass system, the weight drops onto a double-buffer system, which includes a first buffer, a second weight, and a second buffer. The double-mass system essentially produces a longer loading duration that more precisely represents a wheel load. The double-mass system has higher reproducibility and gives a more accurate result on pavements built on soft soils. The single-mass system will seriously overestimate the capacity of pavements built on soft soils. However, single-mass FWDs are smaller, cheaper and faster. Low-cost FWD:s for the Indian market are currently (2015) being developed independently by PaveTesting and KUAB.

Deflection sensors

Deflection sensors (geophones; force-balance seismometers) mounted radially from the center of the load plate measure the deformation of the pavement in response to the load. Some typical offsets are 0mm, 200mm, 300mm, 450mm, 600mm, 900mm, 1200mm 1500mm. The deflections measured at these sensors are termed D0, D200, D300 etc. The advantages of seismometers compared to geophones are built-in calibration devices and higher range (5 mm vs 2 mm). Geophones are more sensitive to disturbance immediately before the impact since the initial error is integrated. Geophones however are much cheaper than seismometers. Dynatest, Carl Bro, Jils and PaveTesting use geophones while KUAB have seismometers in their standard FWD's and geophones in their low-cost models.

Analysis

FWD data is most often used to calculate stiffness-related parameters of a pavement structure. The process of calculating the elastic moduli of individual layers in a multi-layer system (e.g. asphalt concrete on top of a base course on top of the subgrade) based on surface deflections is known as "backcalculation", as there is no closed-form solution. Instead, initial moduli are assumed, surface deflections calculated, and then the moduli are adjusted in an iterative fashion to converge on the measured deflections. This process is computationally intensive although quick on modern computers. It can give quite misleading results and requires an experienced analyst. Commonly used backcalculation software are BAKFAA (Federal Aviation Administration), Elmod (Dynatest), PVD (KUAB), MichBack (Michigan DOT) and Modulus (TxDOT).

Many analysts use simplified methods to calculate related parameters that are empirical in nature. The most common is maximum deflection under the centre of the load plate (D0) which is related to empirical measures such as the Benkelman Beam deflection (after minor adjustment for differences in the two devices). Historically some used the radius of curvature (D0-D200) but this is out of favour now because it is clear that the steel loading plate of 300mm diameter affects the shape of the deflection bowl between the centre (D0) and the D200 sensor at 200mm. However this means that a lot of useful information about the shape of the deflected bowl is wasted. Horak and Emery have published indices that use this information: BLI=D0-D300 and gives an indication of the basecourse performance, MLI = D300-D600 and gives an indication of the subbase performance, and LLI=D600-D900 and gives an indication of subgrade performance. These and other similar indices are known as shape factors. The FWD data can also be very useful in helping the engineer divide the length of the pavement into homogeneous sections.

FWD data can also be used to calculate the degree of load transfer between adjacent concrete slabs, and to detect voids under slabs.

Other models

A Light Weight Deflectometer (LWD) is a portable falling weight deflectometer. It is used primarily to test insitu base and subgrade moduli during construction. Light Weight Deflectometer (LWD) is quicker than the isotope measuring method and requires no reference measurements. The equipment has no radioactive sources requiring safety courses and can be operated by one operator, allowing for the analysis of collected data and printing out of data files on site.^[3]

A Heavy Weight Deflectometer (HWD) is a falling weight deflectometer that uses higher loads, used primarily for testing airport pavements. Maximum load for HWD:s are typically around 300 kN (Dynatest, Carl Bro and PaveTesting) and 600 kN (KUAB).

A Rolling Weight Deflectometer (RWD) is a deflectometer that can gather data at a much higher speed (as high as 55 mph) than the FWD. It is a specially designed tractor-trailer with laser measuring devices mounted on a beam under the trailer. Another advantage of the RWD over the FWD is that it can gather continuous deflection data as opposed to discrete deflection data collected by the FWD. RWD development has been carried out independently by Applied Research Associates (ARA) since 2005 and KUAB Sweden since 1991.^[4]

The test materials are described in ASTM D 4694, and the test method is defined in ASTM D 4695.

References