Abstract: A fastener is disclosed for affixing void-forming members to underlying concrete slabs for use in composite slab construction. Each fastener comprises a stem having a cross limb with a barbed end for insertion into a foam member, a stabilizer bar transversely projecting from one side of the stem intermediate its ends, and bottom barbs. Upon insertion of the barbs of the cross limb into a foam member and projection of the bottom barbs of the stem into a concrete slab, the foam member is thus secured thereto. Several fasteners spaced around the periphery of the member are employed. The concrete slab with void-forming members installed is then overlaid with a poured concrete cap, thereby forming a unitary composite construction.