Multi-Depth Deflectometers (MDD) are pavement sensors used to measure in-situ elastic deformation and/or permanent deformations in the various pavement layers of a test section. The BDI MDD system is a series of parallel rods in a 50 or 75 mm diameter hole in the test section. The parallel rods are anchored at various depths inside the MDD tube wall. Each rod, starting from the bottom hydraulic anchor, passes up to the reference head where linear potentiometers measure rod (depth anchor) movements relative to the road surface.

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