An apparatus and method to install contraction joints in concrete slabs. The method uses a device, which inserts a strip of plastic or paper tape into freshly laced concrete, as the device is driven across a concrete slab. The tape is folded immediately prior to being inserted into freshly placed concrete, and functions to weaken the slab at the contraction joint location for the purpose of causing a crack to form in the slab. Plastic tape can be used, which functions as a moisture vapor retarder, and thereby, protects floor coverings from moisture damage, such as mold and mildew, at contraction joint locations.