Abstract: There is described a one-piece, elongated, hollow plastic section which is useful in the support of reinforcing steel in the casting of concrete structures and the formation of recesses in said structures. The section has a trapezoidal configuration and includes (1) a pair of converging wall portions, (2) a wall portion connecting said converging wall portions at their converging ends and (3) a covering strip portion parallel to said connecting wall portion and connecting the opposite ends of said converging wall portions, said (2) connecting wall portion having areas of reduced thickness formed therein and adapted to be perforated by rigid members whereby said wall portion is self-sealing at the area of perforation, said (3) covering strip portion having reduced thickness areas formed therein adjacent to the converging wall portions whereby said covering strip portion may be manually removed from said trapezoidal cross-section so as to form an elongated, U-shaped plastic device.