Abstract: Concrete blocks with an open cell structure extending throughout the block are assembled in substantially parallel columns and rows to from a mat structure capable of articulating over underwater structures. The blocks are assembled by flexible cables that pass through the center of each block through perpendicular apertures, which are located on different horizontal planes to prevent intersection of the cables. A removable abrasive resistant pad is placed on one surface of the blocks to prevent the concrete block from causing damage to the structure. The pad is comprised of a solid base with an opening matching the opening on the block. It also has a plurality of upwardly extending post which serve as a contact point between the pad and the underwater structure and at least one self-locking dowel extending downward, which are inserted into dowel holes on one surface of the block, after the concrete has hardened. The entire pad is one homogeneous material.