Selecting Lumber and Plywood for Concrete Forming

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.

Douglas Fir (DF) is a good choice for dimensional lumber to craft formwork frames. But in recent years, this strong and durable wood has found a new use. Many architects use this wood for board-form concrete. It’s an architectural finish that showcases the beauty of real wood. Douglas fir planks are situated horizontally on the framework. When the lumber is stripped, wood features have been transferred into the harden concrete. 

 

Special Lumber

On select projects, contractors may be asked to use two types of specialized lumber near their casting areas. Plans might call for a structural wood detail to remain in direct contact with the hardened concrete. These areas could be near footings, foundations, or cast stairs, on which something needs to be nailed.  Pressure treated lumber contains a quarter of a pound of preservative per cubic foot, which is sufficient to mitigate rot or decay. 

On many interior applications, projects require fire-treated wood. This lumber is treated to resist combustion which delays fire and smoke spread and this lumber bears a stamp with the identification mark or name of the approved inspection agency that tested the wood, and information from the wood’s quality tests appears, such as the flame spread and smoke-developed index (SDI) value. 

Engineered lumber forming panels are often a better choice for forming concrete structures to comply with many of the ACI surface classifications and have a very high strength-to-weight ratio. These are wood products specifically designed for concrete and are factory treated to resist moisture intrusion.  These products are engineered for straight and consistent performance and are made to resist bowing, twisting, and shrinking. They often have surface treatments that reduce the quantity of release agents and thus, are less likely to allow concrete build up. They can also provide two additional cost benefits. Their inherent strength allows contractors to use less bracing, saving time and material. And because of their durability, contractors can reuse these panel on multiple placements. 

 

 

Plywood

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.

Lumber Cheat Sheet

 

Formwork Accessories

When selecting lumber and formwork for commercial applications, be sure to consider adding important supplies. 

• More and more projects incorporate various curves and radiuses, which has created the need for highly flexible forms. Contractors often use masonite boards or flexible plastic forming products to craft curves and radiuses within the formwork. They are very durable, flexible, and leave a smooth surface following stripping. 
  
  
• When choosing formwork opt for either wood or plastic shims as these materials will not mar the formwork’s face. When using furring strips to level forms, be sure to purchase high quality wood that will not absorb any bled water. 
  
  
• When constructing formwork supports, be sure to use the proper fasteners that are designed to withstand the forces from the formwork pressure. 
  
  
• And when sealing gaps at form joints, select a high-quality caulking material that is water resistant, but easily removeable following stripping.