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 .