A gasket and mandrel assembly, including a gasket having an annular main body portion with an anchor flange and a pipe sealing portion. The gasket is foldable between a folded position in which the pipe sealing portion is disposed substantially within the main body portion and an unfolded position in which the pipe sealing portion extends outwardly of the main body portion along its longitudinal axis. A pair of mandrels are respectively fitted on opposite sides of the folded gasket to enclose most of the gasket for casting the gasket within a wall in a concrete structure. At least one of the mandrels includes an annular outer surface aligned parallel or substantially parallel with the longitudinal axis of the gasket. Removal of the mandrels results in the cast opening in the concrete structure having a circular inner surface which is disposed parallel or substantially parallel to the longitudinal axis of the gasket and the resulting opening such that, if the gasket should fail, the gasket may be removed from the opening and an expansion-type gasket may be sealingly compressed against the opening surface with an expansion band assembly.