Long-Term Pavement Performance Program Manual for Falling Weight Deflectometer Measurements

Chapter 1. Background

A falling weight deflectometer (FWD) is a device designed to simulate deflection of a pavement surface caused by a fast-moving truck. The FWD generates a load pulse by dropping a weight. This load pulse is transmitted to the pavement through a 300-millimeter (mm) diameter circular load plate.

The load pulse generated by the FWD momentarily deforms the pavement under the load plate into a dish or bowl shape. Envisioned from a side view, the shape of the deformed pavement surface is a deflection basin.

Based on the force imparted to the pavement and the shape of the deflection basin, it is possible to estimate the stiffness of the pavement by using various computational methods. If the thickness of the individual layers is also known, the stiffness of those layers can also be calculated.

In addition, an FWD can be used to determine the degree of interlock between adjacent slabs of a portland cement concrete (PCC) pavement. This degree of interlock is generally known as "load transfer efficiency" or LTE. Measurement of the LTE is obtained by placing the FWD load plate tangent to one side of the joint to be evaluated. A load pulse is then generated, and the deflections at equal distances on either side of the joint are measured. In a perfectly efficient joint, these deflections are equal. For most joints, the deflection on the unloaded slab is less than the deflection on the loaded slab.

An FWD has two types of primary measurement devices. The first is a load cell, located directly above the load plate, and it measures the force imparted to the pavement. The second is a deflection sensor, also known as a "deflector." The FWDs operated by the Long-Term Pavement Performance (LTPP) Program use geophones as the deflection sensors, although there are other types of deflection sensors used in other FWD designs. The LTPP FWDs have nine deflection sensors placed at fixed distances from the load plate to measure the shape of the deflection basin.

In addition to the primary measurement devices, the LTPP FWDs have two additional types of measurement devices. The first is the distance measurement instrument (DMI), a high-accuracy odometer that measures the distance the FWD has traveled along a roadway. The second type is a temperature sensor. The LTPP FWDs have two categories of temperature sensors, an air temperature sensor and an infrared surface-temperature sensor. The data from these two temperature sensors, combined with data from nearby weather stations, are useful for estimating the temperature of various materials in the pavement structure.

Knowing the temperature of the materials in the pavement structure is critical. For example, asphalt is hard and brittle at very low temperatures and soft and ductile at very high temperatures; therefore, the stiffness calculated from FWD data for these materials must be corrected for these temperature effects. In addition, the LTE between two PCC slabs varies as the slabs expand and contract or warp resulting from a difference in temperature between their top and bottom surfaces.

LTPP FWD operators are also required to perform manual measurements during FWD testing. These measurements consist of subsurface temperature measurements taken with a hand-held probe and joint width measurements taken during load-transfer testing.

LTPP FWD operators also must comment on nonequipment-related conditions encountered during testing that might reasonably be expected to cause anomalous measurements. These conditions usually are cracks or other pavement surface distresses.

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