What is a Light Weight Deflectometer (LWD)?
The light weight deflectometer (LWD) is a portable device that drops a weight on a plate to measure the stiffness of soil.
How it works
The LWD is an impulse load device. A weight falls freely and hits a set of rubber springs, and the springs pass the load to a plate on the ground. Sensors measure the resulting deflection. The size of the load depends on the mass of the weight and the height it falls from. ASTM E2583 is the test method for the LWD. It covers paved and unpaved surfaces, and the deflection is measured at the center of the applied load.
What it tests
FAA AC 150/5320-6G describes the LWD as a portable version of the falling weight deflectometer (FWD). The load cell limits it to unbound materials, such as aggregate base layers and soil subgrade, and to thin asphalt layers. The calculated stiffness of each layer helps quality control of the base, subbase and subgrade during construction. The FWD and the heavy weight deflectometer (HWD) give the stiffness of a whole pavement structure.
Use on airfields
The AC lists the FWD, the HWD and the LWD as the deflection equipment for non-destructive testing of airport pavements. The AC says that the FWD and the LWD may give an impulse load that is adequate for thinner pavements that serve general aviation aircraft.
Frequently Asked Questions
- How is the LWD different from the falling weight deflectometer?
- The LWD is a portable version of the falling weight deflectometer (FWD). Its load cell limits it to unbound materials such as aggregate base layers and soil subgrade, or thin asphalt layers. The FWD and the heavy weight deflectometer (HWD) give the stiffness of the whole pavement structure. The HWD uses loads of nearly 70 kips.
- Which standard covers the LWD?
- ASTM E2583 is the test method for measuring deflections with a light weight deflectometer. FAA AC 150/5320-6G refers to it in Appendix C. ASTM E2835 covers a portable impulse plate load test device and is listed next to it in the same table.
- Does the LWD measure the whole pavement structure?
- No. FAA AC 150/5320-6G says the LWD data give the stiffness of single layers. The device works on unbound materials and thin asphalt layers only. The FWD or HWD measures the stiffness of the whole structure.
References
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
- astm-e2583
- ASTM E2583-07(2025), Standard Test Method for Measuring Deflections with a Light Weight Deflectometer (LWD) ASTM International
This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.