In selecting a forming material, the first consideration is whether the material is absorbent or nonabsorbent. Absorbent forms remove water from the concrete and reduce the water-cement ratio in the concrete surface. The lower the water-cement ratio, the darker the concrete. If the absorption is not uniform, the concrete surface will vary in color. Nonabsorbent forms prevent moisture loss and result in finishes that are uniform in color but lighter than those cast in absorbent forms. B-B plywood, which is a construction-grade oil-impregnated plywood, does not produce acceptable architectural concrete finishes because of its nonuniform absorptive capacity. Unpiled B-B grade plywood may be used for architectural concrete with certain modifications. The form surface can be made impervious to moisture absorption by the application of wood sealers or special coatings. Regardless of whether wood sealer or special coating are applied, B-B grade plywood should be tested by casting test sections to evaluate the effectiveness of the sealer. Another type of plywood that is effective for architectural concrete work is produced at the mill with an applied plastic overlay. A further nonabsorbent material is plastic-coated birch plywood produced in Finland. Where economically feasible, shop built steel forming systems are an ideal selection for achieving high quality architectural concrete finishes. The use of steel forms minimizes the jointing problems encountered in plywood systems. However, using steel units produced by different rolling techniques can produce a slight variation in color and texture, which may show up in an as-cast finish. Fiber-glass reinforced plastic is being used increasingly as a forming material for architectural concrete. Like steel, it eliminates jointing problems common to plywood, but it is lighter in weight than steel.