Heavy Weight Deflectometer (HWD)
The heavy falling weight deflectometer (HWD) is a device used to evaluate the load carrying capacity of flexible and rigid pavements. By dropping a known weight from a known height onto a surface and measuring the resultant deflections of the surface, the material modulus can be determined using backcalculation techniques. This can be employed for both quality control and quality assurance (QC/QA) and structural evaluation surveys of multiple layers in a pavement system. Due to its heavyweight load and testing method, the HWD is able to simulate the effect of a moving wheel load and is ideal for evaluation of airport pavements.
The system consists of a falling weight, a guide system, a load plate, deflection sensors, a load cell, and a data processing and storage system. The HWD itself is mounted on a trailer towed by a truck in which the data processing and storage system are installed. The load plate is segmented to provide uniform pressure distribution to the pavement and has eight deflection sensors mounted to it. One sensor is mounted at the center of the plate, one is mounted 12 inches in front of the plate along the path of the tow vehicle and trailer, and six are mounted behind the plate at 12-inch increments.
The HWD performs a plate-bearing test, where a load pulse is generated on a plate and transmitted to the surface. The operator positions the truck and trailer so the loading plate is positioned above the test point and then lowers the plate onto the surface. The objectives of the testing dictate the position of the deflection sensors in relation to the test point on the surface. The falling weight is raised to a specified height and released to impact the base plate with the desired force. This load pulse is transmitted through the plate to the surface. A load cell mounted on the plate measures the force imparted while the deflection sensors measure the deflection of the surface at the specified points. The peak load and deflections, along with their time histories, are recorded and used to calculate the stiffnesses of each layer in the pavement system. Multiple loading sequences with different impact loads can be performed at each test point, as required by the objectives of the testing. The HWD can also be used to quantify the joint load transfer efficiency for jointed rigid pavements.
HWD Testing at an Airport Test Section (Click to Zoom)
The HWD is a towed device that can be operated by the driver with an adequate video system for test point alignment or with a spotter outside the truck and is capable of relatively fast data acquisition. The flexibility of multiple impact loads and the ability to position the deflection sensor array in variable orientations and locations with respect to the test point allow the collection of data for different research objectives for a single test location. The HWD is ideal for testing in the field or laboratory test beds.
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