For years, judging the quality of the as-cast concrete was really a guessing game. Often “good enough” translated to “only a few bugholes” and walls that weren’t too curvy. The subcontractors who followed the concrete placement were expected to patch any honeycombed areas and adjust their framing work to the dimensions left after formwork removal.
But in the last few years, project documents have more detailed specifications outlining the acceptance criteria for as-cast concrete surfaces due to the publication of the Guide to Formed Concrete Surfaces (ACI 347.3R-13). This document was the industry’s attempt to provide guidance for contractors to agree on realistic expectations regarding the quality of concrete surfaces on commercial projects. For example, concrete surface levels for individual parts of the structure may be specified to reflect project needs and budget.
The ACI document defines the various measurable quality properties for each formed concrete element, such as surface texture, surface void ratio, color, flatness, and joints. While the proper combination of concrete mix design, placement techniques, and curing are important, the sturdy supporting lumber and formwork face material are also a determinant for final quality. Contractors can align their investment in formwork lumber and panels to match the expectations of the project. One thing is for certain, the potential cost of fixing a poor-quality formed surface will exceed the initial cost of quality forming lumber.
Plywood panels produce smooth surfaces, can be used repeatedly, and offer excellent stiffness that minimizes deflection during pouring. Manufacturers offer several surface textures to meet all levels of ACI quality classifications. There are special textures available for casting attractive and unusual concrete patterns and the thinner panels can be easily bent for custom curved forms and liners.
The most basic plywoods are ACX, BCX, and CDX. The first letter of the classifications depicts the quality of one of the panel faces. A represents prime, top-quality wood. It’s consistent in color, grain, and appearance. The letter B is like grade A but allows small variations such as burls, knots, and streaks. The letter C represents that there are obvious defects, patches and color variations. CDX is often the most common used material for single-use applications when the concrete finish is not an important factor. The grading system is in place mostly for aesthetics, but it also refers to core layers, voids and manufacturing issues that might go unnoticed.
BBOES (B -face, B-back, oiled, and edge sealed) panels are specially designed natural wood surface panels for the concrete forming industry when a non-overlaid surface is desired. These panels feature two key upgrades from CDX that are focused on increasing service life. Panels are oiled during the manufacturing process allowing contractors to easily strip forms. Also, panel edges are sealed at the factory to reduce moisture absorption. BBOES are typically made from either Southern Yellow Pine (SYP) or Douglas Fir (DF). The two limitations for BBOES products are that they will leave grain and patch transfer on the finished concrete. And in most applications, forms can be only used four to eight times.
An MDO panel is a premium, medium density overlaid plywood. It differs from BBOES plywood by leaving a matte or flat finish on the concrete surface. The form surface is treated with a heat-bonded, resin-fiber overlay that leaves a smooth, durable surface. They are well-suited for commercial wall work. The panels are durable, reliable, offering multiple reuses and the smooth surfaces require less maintenance between placements. The surface resists abrasion from stiff, sand-rich concrete mixes. And since they are normally factory-treated with a release agent, they only require a light application of releasing agent prior to placement.
MDO panel manufacturers often offer two options. MDO 1 step panels produce concrete surfaces that are smooth but with some grain and patch transfer. They have a service life of about 8 to 12 placements. MDO 2 step panels provide a smoother finish. The result is a smooth matte finish that can handle 12 to 15 concrete placements.
HDO panels are high density laminates designed for long-life and high-quality work. Manufacturers offer a few surface options that can be matched to any job specification.
HDO panels are often specified when the smoothest possible concrete finishes are desired and are especially good for high-gloss, architectural finishes. These panels offer excellent cost-to-pour ratio. The hard, semi-opaque surface of thermosetting phenolic resin-impregnated material forms a durable, continuous bond with the plywood that resists wear from aggressive mixes. A key reason for long-life and improved quality is that both sides of a HDO panel are treated to be moisture resistant which allows 20 to 50 reuses, and with proper care can last for up to 200 placements or more. The abrasion-resistant surface should be treated with a release agent prior to its first use and between each pour to preserve the surface and facilitate easy stripping.
Phenolic plywood is excellent for some concrete formwork projects. The panel’s surfaces, soaked in phenolic resin, are extremely stable, smooth, and highly water resistant. These qualities help contractors to cast a smooth surface that is easily released during stripping. The plywood core is generally made of a strong wood, such as birch, that provides strong resistance to foam pressure. While durable for many applications, most contractors use phenolic panels only for special placements that require high quality finishes on small-pour projects. They may not provide the same service life as the HDO, mostly due to its lack of total waterproofing; but for short-term uses phenolic plywood is excellent.
