Abstract: A retaining member for retaining spaced rigid members within a concrete matrix. A form for shaping the concrete matrix includes a base, a planar shaping surface for supporting the concrete matrix during its initial setting period, and a supporting member located on the rear side of the shaping surface. The retaining member includes a compressible hollow cylinder, preferably made of a foamed polymeric material. Portions of the retaining member are associated with the form and the supporting member, holding the retaining member in place to retain each rigid member in a parallel position to the other rigid members.