a control joint for forming and controlling concrete structures such as floors and tilt - up walls . the joint has a base and an upstanding web with an off - set section and is preferably extruded from plastic . in alternate embodiments , the joint has a base and a web with a removable top cap for forming a chamber or recess in the completed product . a reusable , reinforced form for structures such as stem walls is also disclosed which form can be cut at spaced - apart locations to the desired length .