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Arrow Innovations - Light Weight Deflectometer

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Portable device simplifies pavement testing

The light weight deflectometer (LWD) is a lighter and faster portable nondestructive testing (NDT) device used for pavement testing and evaluations. Similar to its heavyweight counterparts–the falling weight deflectometer (FWD) and heavy weight deflectometer (HWD)–the LWD determines the strength and stiffness of pavement material by measuring the material's response under the impact of a load with a known magnitude and dropped from a known height. Weighing nearly 48 pounds and using a drop weight of 22 pounds, the LWD can be carried and operated by one person, which makes it a cost-effective and practical tool for rapid field testing.

Originally developed in Denmark, FWDs have been used routinely for more than four decades to evaluate the condition of roadways. The FWD consists of an impact loading mechanism and a series of sensors to measure vertical surface deflections at the load location and at specified offsets from the load. The entire system is trailer mounted. The loading component delivers a transient load of 1,500 to 27,000 pounds, and the sensors measure the surface deflection at the specified locations. The measured deflections are an indicator of a pavement's overall structural capacity and its elastic properties. The FWD has been used in rehabilitations, designs, forensics, routine maintenance, and roadway management, and in determining seasonal load restrictions and overload permits.

The LWD, sometimes referred to as a portable FWD, also measures the response to dropping a load on a surface. Because the LWD uses a lower load level than the FWD, it is used primarily on materials such as soils, foundations, and granular layers (materials in which particles are not strongly cemented with binding materials such as cement or asphalt). Such materials require a relatively

lower level of load to record a measurable displacement, which is within the range of the LWD's design and testing capacity. To a limited extent, it also has been used on asphalt surfaces. The LWD is battery operated and has an electronic interface to a handheld personal digital assistant via a wireless Bluetooth connection so field technicians can use it without connecting to a power source or additional computer. Its sophisticated electronics are protected in a dustproof and splashproof case for use in harsh construction environments.

The software to process load and deflection readings from the LWD is integrated with the system to get real-time test data. The benefit of obtaining real-time data is that errors can be identified onsite so corrective measures can be taken before additional layers are placed or the pavement is opened to traffic. Results obtained from the LWD also provide data for sophisticated analytical models that predict how well the pavement will perform in the long term. Test results can be used to assess the quality of construction and its impact on the life expectancy of the road so appropriate budget allocations can be planned for maintenance or rehabilitation activities.

The LWD and similar devices are manufactured in Denmark, Germany, and other areas of Europe and marketed in the United States. Several state and federal highway agencies have initiated projects to evaluate the applicability of LWD technology during pavement construction or repair, and for routine surveys and testing. The ASTM International—an international organization that produces technical standards for materials, products, systems and services–recently developed a standard test protocol for measuring deflection response of soils, granular rock beds and asphalt layers.


Mike Moravec
LTPP Field Engineer
Office of Pavement Technology

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Updated: 04/04/2011

United States Department of Transportation - Federal Highway Administration