What is a Falling Weight Deflectometer (FWD)?
A falling weight deflectometer (FWD) is a test device that drops a weight on a pavement and measures how far the surface deflects.
How it works
The weight falls onto a set of rubber springs. This gives the pavement an impulse load. The size of the load depends on the mass of the weight and the height of the drop. Engineers change both so that the load is similar to a wheel load on the main gear of an aircraft.
Deflection sensors record the movement of the surface under the load plate and at offsets from it. On airport pavements the usual offsets are 12 inches (30 cm) out to 72 inches (180 cm). The sensor under the plate shows the stiffness of the whole pavement structure. The outermost sensor shows the stiffness of the subgrade.
FWD and HWD
The FWD load is often not adequate for thick airfield pavement. The heavy weight deflectometer (HWD) uses the same principle with a load of nearly 70 kips. The FAA says that an HWD is commonly used in airfield pavement evaluation. A light weight deflectometer (LWD) is a portable version. Its load cell limits it to unbound materials such as base layers and subgrade.
What airfield engineers use it for
The FAA lists these uses of FWD and HWD data:
- Evaluate the load-carrying capacity of an existing pavement and calculate its remaining life.
- Calculate load transfer efficiency of cracks and joints.
- Find voids at the corners and joints of concrete pavement.
- Find the material properties of the layers and the subgrade, for overlay design.
Backcalculation turns the measured deflections into layer properties such as the modulus of elasticity. The FAA software BAKFAA does this. The results depend on the structural model and the software, so they are estimates.
The data also help to calculate the pavement classification rating (PCR). Annex 14 requires that the bearing strength of a pavement is determined. For aircraft above 5 700 kg apron mass, the report uses the ACR-PCR method. Annex 14 does not name a test device.
What it does not show
An FWD does not show the surface condition of a pavement. A pavement condition index (PCI) survey to ASTM D5340 rates the visual condition. Layer thickness needs cores or ground penetrating radar. The FAA notes that a pavement can have a low PCI and still have enough structure for the traffic that uses it. FWD data and PCI data therefore answer different questions.
Standards
ASTM D4694 is the test method for deflections with a falling-weight-type impulse load device. ASTM D4695 is a guide to general pavement deflection measurements. ASTM D5858 is a guide to calculating in situ equivalent elastic moduli with layered elastic theory.
Frequently Asked Questions
- What is the difference between an FWD and an HWD?
- The heavy weight deflectometer (HWD) works like the FWD but applies a larger load, nearly 70 kips (about 310 kN). The FAA says the FWD load is often too low for thick airfield pavement. Airfield engineers commonly use the HWD.
- Does an FWD test damage the pavement?
- No. The FWD is a non-destructive test. It does not remove or destroy pavement material, and it tests the pavement in place. The FAA also lists minimal disruption to traffic as an advantage.
- Does an FWD replace a PCI survey?
- No. The FWD shows the structural condition of the pavement. The PCI survey shows its surface condition. A pavement can have a low PCI and enough structure for the traffic, so engineers use both kinds of data.
References
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
- faa-ac-150-5335-5d
- AC 150/5335-5D, Standardized Method of Reporting Airport Pavement Strength, PCR FAA
- icao-annex-14-vol-i
- Annex 14 to the Convention on International Civil Aviation, Aerodromes, Volume I, 9th edition ICAO
- astm-d4694
- ASTM D4694, Standard Test Method for Deflections with a Falling-Weight-Type Impulse Load Device ASTM International
This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.
