This paper describes a nondestructive pavement testing system called the multidepth deflectometer (MDD), which is used to measure depth deflection profiles of pavements. Effective elastic moduli of multilayered pavement structures can be backcalculated from these measurements. This system was developed in South Africa during the late 1970s to enhance the extensive program of full-scale accelerated testing with the aid of the heavy vehicle simulator (HVS).
The MDD consists of a series of up to six linear voltage differential transducers (LVDTs) installed vertically into the pavement at preselected depths in a relatively small-diameter hole. Normally the MDDs are anchored at a depth of approximately 2 m below the surface of the pavement. Resilient depth deflections are measured with the MDD in association with a moving load.
Case studies of measured deflections and backcalculated effective elastic moduli are given for tests on an asphalt base, a granular base, a cemented base, and a concrete base pavement section. The results are interpreted and discussed in terms of the behavioral characteristics and pavement responses, including permanent deformations. It was found that: 1. Asphalt bases may become granular after extensive accelerated testing. 2. Crushed stone bases may become stronger as a result of trafficking. 3. Lightly cemented bases crush near the surface, thus weakening the layer. 4. One concrete pavement debonded from its subbase after 190 000 load repetitions.