A cap for a tubular sleeve having a cylindrical wall extending along a longitudinal axis for forming a passage through a concrete structure. The sleeve has radially extending ridges separated into segments with a channel parallel to the longitudinal axis. The channel has a circumferential width W. The ridges spaced apart a distance Z along the longitudinal axis. The sleeve having an inner diameter D. The cap has a cap element and a positioning tab. The cap element has a circular outer periphery with a flat top surface and a flange having a bottom end, and the tab extends inwardly from the bottom end and has a bottom surface that is orthogonal to the longitudinal axis. The tab has an inner surface that is inclined at an acute angle to the bottom surface. The tab has a width smaller than W and an axial length smaller than Z.