When selecting wood structural forming panels, contractors must decide among a multitude of factors to choose the concrete forms that will produce the best, most economical concrete finish for the job.
San Diego’s 504,000-square-foot library is a good example. It features a graceful concrete gravity arch that’s 46 feet tall, 70 feet wide at the base and tapers to 37 feet wide at the top, with a total volume of 194 yards. Built in three lifts using a combination of PERI VARIO wall forms, GRV radius forms, and multi-flex shoring, the soaring arch supports each of the fourth through eighth floor slabs directly above it.
To create the quilted random plywood pattern, contractor Morley Construction coordinated every seam and tie location. The 232 plywood panels were pre-cut, numbered, and organized in order of assembly and finally installed on the forms in the order and orientation specified on the concrete shop drawings.
All column, wall, and beam forms were double-sheeted with 3/4-inch plywood substrates and 3/4-inch plywood with a film fascia. All column forms and most other architectural formwork were back-screwed, mitered whenever possible, and installed with a minimal use of ties (spaced 5 feet vertically and 4 feet horizontally).
As this project demonstrates, contractors must carefully evaluate and choose wood structural concrete forming panels to properly construct job-built forms, use prefabricated forming systems, and get the desired results.
Here’s a closer look at three key variables to consider when selecting which type of concrete form is right for your project:
1. Plywood tolerances. Structural plywood is an engineered product, manufactured to exacting tolerances under U.S. Voluntary Product Standard PS 1-09. A tolerance of plus 0.0 inch and minus 1/16 inch is allowed on the specified width and length. Sanded Plyform panels are manufactured with a thickness tolerance of plus or minus 1/64 inch of the specified panel thickness for Performance Categories of 3/4 and less, and plus or minus 3 percent for Performance Categories greater than 3/4.
Overlaid Plyform panels have a plus or minus tolerance of 1/32 inch for all Performance Categories of 13/16 and less. Panels with greater Performance Categories have a tolerance of 5 percent.
For squareness, the product standard requires panels to be square within 1/64 inch per nominal foot of length when measured corner to corner along the diagonal for panels 4 feet and greater in length.
For edge straightness, panels must be manufactured so that a straight line drawn from one corner to an adjacent corner falls within 1/16 inch of the panel edge.
These tolerances, along with consistent levels of quality required to meet standards set by APA – The Engineered Wood Association, help minimize the time and labor required in building forms. Good construction practices dictate an awareness of the tolerances at the jobsite.
In an extreme case, two sanded panels with a Performance Category of 3/4, both within manufacturing tolerances, could form a joint with a 1/32 inch variation in surface level from panel to panel. Realignment of panels and shimming are quick, easy solutions.
2. Plywood Grades. Plyform is manufactured with exterior bond classification plywood and is limited to certain wood species and veneer grades to assure high performance. Products bearing this specific identification are available in two basic grades: Plyform Class I and Structural I Plyform. Each may be ordered with a High or Medium Density Overlaid surface on one or both sides.
Plyform Class I: Class I Plyform has Group 1 faces for high strength and stiffness.
Structural I Plyform: This concrete forming panel is made with Group 1 wood species throughout— the strongest. All other factors being equal, it will support the highest loads both along and across the panel. It is specifically designed for engineered applications and is recommended where face grain is parallel to supports.
B-B and B-C Plyform : Nonoverlaid Plyform is usually made with B grade veneer face and B or C grade veneer back, and referred to as B-B Plyform or B-C Plyform. These panels are available as Structural I, or Class I. The panels are sanded on both sides and treated with a release agent at the mill (called “mill oiled”), unless otherwise specified.
Unless the mill treatment is reasonably fresh when the panels are first used, the plywood may require another treatment of release agent. It also is important to apply a top-quality edge sealer before the first pour. Plyform panels can be ordered edge-sealed from the mill. Five to ten reuses of B-B and B-C Plyform are common.
HDO Plyform (HDO—Concrete Form): This Plyform panel meets the same general specifications as Plyform Structural I or Class I. All classes of HDO Plyform have a hard, semi-opaque surface of thermosetting phenolic resin-impregnated material that forms a durable, continuous bond with the plywood. 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.
HDO Plyform is most often specified when the smoothest possible concrete finishes are desired because the panel has a hard, smooth surface. It can impart a nearly polished concrete surface. Both sides of HDO panels are moisture resistant but cannot always be used to form concrete with equal effectiveness unless specifically made for that purpose. Scratches and dents in the backs caused by fastening the panels to the supports may make the use of both sides impractical.
Various grades of HDO Plyform may be available; check with your supplier. With reasonable care, HDO Plyform will normally produce 20 to 50 reuses or more. Some concrete-forming specialists achieve 200 or more reuses with good results.
MDO Plyform (MDO – Concrete Form): Special proprietary MDO Plyform is available for concrete forming. Regular MDO or MDO-General is intended for use as a paint surface and should not be used for concrete forming. Panels are typically overlaid on only one side, although they can be produced with MDO on both sides.
MDO Plyform is normally factory-treated with a release agent and edge-sealed to protect the edges from water absorption. 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. MDO Plyform panels create a matte or flat finish on the concrete surface.
Related Grades: Additional plywood grades specifically designed for concrete forming include special overlay panels and proprietary panels. These panels are designed to produce a smooth, uniform concrete surface. Some proprietary panels are made of Group 1 wood species only, and have thicker face and back veneers than those normally used. These provide greater parallel-to-face grain strength and stiffness for the panel. Faces may be specially treated or coated with a release agent. Check with the manufacturer for design specifications and surface treatment recommendations.
3. Special Textures. Plywood is manufactured in many surface textures, ranging from polished HDO plywood to patterned board-and-batten siding panels. When working with these special panels and with field-applied patterns, virtually any texture can be created.
Textured plywood having an Exterior bond classification usually is applied in two ways in formwork design: (1) as a liner requiring plywood backing so that the liner delivers texture, but contributes little to the structure of the formwork, or (2) as the basic forming panel.
In the second case, the best reports come from projects where the number of pours required is limited, because the textured surface can increase necessary stripping forces and, therefore, the possibility of panel damage in the stripping process. Film coatings, such as lacquer, polyurethane or epoxy, can be used with a release agent to make stripping easier.
For more information on selecting the best form panel for the project, download the APA Concrete Forming Design/Construction Guide.