Abstract: A waterstop/mechanical seal for use with an expansion board providing a watertight seal between adjacent concrete sections. The waterstop/mechanical seal comprises a continuous diaphragm element for covering one end area of the expansion board when positioned between the adjacent concrete sections. The waterstop/mechanical seal includes a top portion adapted to lie in general co-planar relationship with an upper surface of the adjacent concrete sections. Spaced sidewalls are integrally connected to and extend a predetermined distance downwardly from the top portion along an outside surface adjacent to one end area of the expansion board. Each of the sidewalls include at least one arm projecting laterally outwardly and spaced below the top portion, which arms are provided with a plurality of waterstop means for channeling liquid away from the expansion board.