An Important Tool for Highway Management
Research is a part of life at the Cornell Local Roads Program. Our primary research focus is on low-volume roads and the materials used to build them. The information used to develop the New York State Department of Transportation (NYSDOT) Specification Section 667: Local Road Gravel Surface, Base, and Subbase Courses came from some research done here at Cornell. One of the research devices we own is a Falling Weight Deflectometer (FWD).
What is an FWD?
A Falling Weight Deflectometer is an important research and design tool that mimics the loading of heavy traffic and measures the response of pavement to that load.
During use, the FWD picks up and drops its set of weights from various heights onto the pavement by either hydraulic or mechanical means. The weights land on rubber springs that transfer a force to a plate resting on the pavement. The maximum force applied is typically 16,000 pounds, but much larger loads are achievable. Some FWDs can even mimic the loads of the new A380 Airbus!
The impact of the weights causes the pavement to deflect, closely approximating how a pavement deflects when a truck passes over. The applied load generates a deflection basin with the deflections becoming smaller further from the load plate. A series of sensors measure the pavement deflection. Specialized computer hardware and software record the load and deflection data.
The FWD that Cornell owns has nine sensors, with the first one resting directly under the load plate and the ninth one typically six feet from the center of the load. The locations are adjustable.
Why use an FWD?
FWDs have uses in a variety of pavement analysis techniques. We use our FWD for research. Typically, an FWD is used in network level pavement management or project level pavement design.
Network level analysis
An FWD can test an entire road network. The information gathered can help determine repair and maintenance priorities across that road network. Spacing the testing out to 500-foot intervals in rural areas and 100-foot intervals in urban areas provides enough detail about the road network.
FWD testing can detect weak sections of pavement. Some of the more complex pavementmanagement systems use FWDs or similar devices to measure the change in response to a given load. In 2002 we did a demonstration at the Highway School on Landon Road in Caroline. We stabilized a section of the road with portland cement. The figure shows the center deflection from an FWD test in April prior to the work, and in July, one month after the work was completed. It is very easy to see where the road strength changed.
The same data can be used for pavement design. This requires a more detailed analysis using the backcalculation technique and software called MODCOMP.
You must know the actual thickness of each layer of the road. You will calculate the strain at critical points and an estimate of the remaining life of the pavement structure can then be determined which promotes good decision making about future maintenance and repair activities for that road.
The FWD is a versatile and important tool that can be useful in the management of an entire road network for the purpose of maintenance and repair planning. FWDs are also useful to help design a pavement structure or repair plan through a detailed analysis of a specific section of road.