4.1 This test method covers the determination of surface deflections as a result of the application of an impulse load. The resulting deflections are measured at the center of the applied load and may also be measured at various distances away from the load. Deflections may be either correlated directly to pavement performance or used to determine in-situ material characteristics of the pavement layers. Some uses of data include quality control and quality assurance of compacted layers, structural evaluation of load carrying capacity, and determination of thickness requirements for highway and airfield pavements (see Guide D4695).

Note 1: Since pavement and subgrade materials may be stress dependent, care must be taken when analyzing LWD test data on unbound materials so that the applied stress will closely match the stress value applied by the design wheel load at the pavement surface.

Note 2: The volume of the pavement and subgrade materials affected by the load is a function of the magnitude of the load. Therefore, care must be taken when analyzing the results, since the data obtained by the LWD may be obtained from a smaller volume of the unbound materials than under the influence of a heavy moving wheel load at the pavement surface.