Plywood understood: how to select and use structural wood-forming panels to get the results you...

Concrete formwork can represent a significant chunk of the budget for a concrete structure. Successful and cost-effective form development must consider both engineering requirements and architectural intentions. Structural wood-forming panels can fulfill a wide variety of forming needs.

Versatile and economical, job-built wood forms can be used for projects ranging from small residential walls to large domed structures. Mass-produced, prefabricated plywood forming systems such as ganged forms, slipforms, jump forms, and flying truss systems can be used to save material and labor costs.

Evaluating formwork design issues and choosing between job-built and prefabricated formwork, however, are not the only important considerations. When selecting structural wood forming panels, contractors must also decide which plywood type, class, grade, overlay, and texture will impart the best, most economical concrete finish for the job.

Plywood types, classes, and grades

A typical plywood concrete forming panel consists of a substrate of five, seven, or nine cross-laminated plies, or layers, of wood veneer joined with waterproof adhesive under heat and pressure. The standard 4x8-foot panel is available in nominal thicknesses of 15/32, 1/2, 19/32, 5/8, 25/32, and 3/4 inch. It is strongest parallel to the face grain (normally in the 8-foot direction).

Building codes do not include grading or rating information for forming panels. Specifications for the manufacture of plywood forming panels are detailed in a commercial standard titled "Voluntary Product Standard PS 1-95," published by the National Institute of Standards and Technology. This standard outlines the species and grade of the wood veneers allowed as well as the type of waterproof adhesives used to ensure that the panels will meet expected strength and durability requirements.

When specifying plywood panels for formwork, you can be assured of their structural properties if you specify plywood that bears the mark of an approved inspection agency. One such organization in the plywood industry that provides a high-quality auditing program for manufacturers is APA--The Engineered Wood Association. Structural wood panels bearing the APA trademark have undergone this audit and meet the specifications of the Voluntary Product Standard. APA trademarked panels account for about 70% of the industry's plywood.

While almost all exterior-type APA trademarked panels can be used for concrete formwork, the plywood industry manufactures a product called Plyform that is recommended for most general concrete forming applications. Plyform is exterior-type plywood limited to certain wood species and veneer grades to ensure high performance. The term Plyform applies only to those panels that bear the APA trademark.

Plyform is available in two basic classes: Plyform Class I and Plyform Class II. Each of these may be ordered with an overlay to add stability, repel foreign substances from the surface, or provide a smoother, more durable forming surface. Structural I Plyform is a variation of Plyform Class I that provides greater strength to resist concrete pressures.

Plywood is manufactured from over 70 species of wood, classified into five groups, with Group 1 being the strongest. Group 1 includes such species as Douglas fir, Southern yellow pine, beech, and birch. Group 2 includes cedar, cypress, hemlock, and yellow poplar. The faces and backs of plywood panels are covered with veneers that are classified into grades depending on the presence of knots and other surface defects. Common grades for exterior formwork panels include A, B, and C, with A having the fewest defects. B grade, for example, has a smooth sanded surface with some minor splits allowed and tight knots, shims, and sled or router repairs 1 inch across or smaller permitted. The Grade-Use Guide for Concrete Forms (left) summarizes the characteristics of commonly used plywood concrete forming panels.

Plyform Class I has Group 1 faces for high strength and stiffness. It may be ordered with a High-Density Overlay (HDO) or a Medium-Density Overlay (MDO) for a smooth, uniform concrete finish and/or increased reuses.

Structural I Plyform is made with Group 1 wood species throughout. All other factors being equal, it will support greater loads than Class I Plyform both along and across the panel. It is specifically designed for engineered applications and is recommended where the face grain is parallel to supports.

Plyform Class II is a slightly more economical plywood that differs from Class I in the types of wood used in its manufacture. Class II Plyform may have Group 2 faces but still provides adequate strength for most forming applications. Class II may not be as readily available as Class I plywood in all areas.

B-B Plyform is one of the most widely used Plyform panels for concrete formwork. It is made with B-grade veneer face and back and is available as Structural I, Class I, or Class II. The panels are sanded on both sides and treated with a release agent at the mill (they are called mill-oiled) unless otherwise specified. Even with the mill treatment, it is recommended that you also treat the panels prior to the first pour. This ensures good coverage and helps season the panels to aid in stripping and protecting the panel face. It is also important to apply an edge sealer before the first pour. Plyform panels can be ordered edge-sealed from the mill. The common terminology for mill-oiled and edge-sealed is B-B Plyform O&ES. If properly handled, B-B Plyform can be reused five to ten times.

High-Density Overlay (HDO) panels have resin-impregnated fiber sheets bonded to the face, and in some cases the back also. The weight of the paper and the amount and type of resin used vary depending on the type of finish and the number of intended reuses. Generally speaking, the higher the weight of the fiber sheet used, the more durable the finish or the more uses that should be attained. The panels range in weight designation from 60/60 to 120/120 where the first number designates the weight of the face overlay in pounds per 1000 square feet and the second number designates the weight of the back overlay. The face and back may have different overlay weights (for example, 90/60).

From giant gang forms and complex shapes to simple conventional form components, HDO panels meet the same general specifications as Plyform Structural I, Class I, or Class II. All classes of HDO Plyform have a hard, semiopaque surface of thermoset resin-impregnated material that forms a durable, continuous bond with the plywood. The abrasion-resistant surface should be treated with a high-quality 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 finish to a concrete surface. While both sides of HDO panels are moisture-resistant, they may not form concrete with equal effectiveness because the back may have a lower overlay weight than the face. Scratches and dents in the backs caused by fastening the panels to the supports may make the use of both sides impractical. With reasonable care, HDO Plyform can be reused 20 to 50 times while some specialists achieve good results with 200 or more reuses.

Medium-Density Overlay (MDO) panels are typically intended for use as a paint surface and should not be used for concrete forming. Special proprietary grades of MDO, however, are available for concrete forming. MDO panels are often overlaid on only one side, although they can be produced with MDO on both sides. Proprietary MDO concrete form plywood is normally factory treated with a release agent and edge sealed to protect the edges from water absorption. As with HDO panels, the abrasion-resistant surface should be treated with a high-quality release agent prior to its first use and between each pour to preserve the surface and facilitate stripping. MDO form panels create a matte or flat finish on the concrete surface, which is sometimes specified by architects.

Maintenance and life span

To increase plywood forming panel longevity, regular maintenance--both before and after the placement of concrete--is important. The use of metal bars or pries when stripping plywood forms may damage the panel edge and surface, especially a textured surface. Instead, crews should use wood wedges, tapping gently when necessary.

Soon after being removed, plywood forms should be inspected for wear and cleaned with a hardwood wedge and a stiff fiber brush rather than a metal brush, hammer, or claw hammer. Then they should be repaired, spot primed, refinished, and lightly treated with a form-release agent before reusing.

After crews strip and clean the forms, the panels can be stacked faces together to slow the drying rate and minimize face checking (hairline cracks on the face ply). Plywood panels should be stored out of the sun and rain and covered loosely to allow air Circulation without heat buildup. For transporting, the forms should be carefully piled flat, face-to-face and back-to-back. Proper upkeep and repair can ensure a longer service life and a stronger formwork structure.

How to order

To order Plyform, state the class, number of pieces, width, length, thickness, and grade. For example: "APA Plyform Class I, 100 pcs., 48x96x5/8 BB exterior-type, mill-oiled." Even though concrete form panels are mill-treated with release agents, it is good practice to indicate treatment requirements with the order. When ordering overlaid plywood, specify High-Density Overlay (HDO) or Medium-Density Overlay (MDO) along with piece count, size, and thickness in the order shown above. Special surface requirements, such as the weight designation for overlaid panels, should be stated after the standard form of the order.

For additional information about plywood concrete forming panels and form design, contact APA--The Engineered Wood Association at 253-565-6600, visit www.apawood.org, or circle 7 on the reader service card.

RELATED ARTICLE: Release agents affect durability.

Concrete form-release agents have a much greater impact on the durability of plywood forming panels than many contractors recognize. With the many different form-release agents available, contractors are faced with the difficult task of sorting out which product is best for their specific requirements. While there are many factors to consider when selecting a form-release agent, such as concrete appearance, concrete paintability, environmental issues, employee safety, transportation, and jobsite storage requirements, plywood durability cannot be overlooked.

Form-release agents, which are often referred to as form oils, can be broken down into two basic categories: barrier types and chemically reactive types. Barrier types function by creating a physical barrier between the plywood form and the freshly placed concrete in the same way that butter prevents cookies from sticking to a cookie sheet. Diesel oil, heating oil, recycled motor oil, and lubricating oil are some of the more common ingredients found in barrier-type form-release agents.

Chemically reactive form-release agents contain some type of fatty acid (the active ingredient) that reacts with the free lime in fresh concrete to form a metallic soap that is not soluble in water. This soap becomes the releasing mechanism for the plywood panels, makes the faces of plywood or overlaid plywood panels more water-resistant, and thus helps protect them from the alkalis in fresh concrete. The result may be an increase in plywood durability.

In addition to a fatty acid, chemically reactive release agents also contain some type of diluent, or carrier, which dissolves the fatty acid and facilitates its proper application. These carriers have historically been petroleum derived, but more recently, water has been introduced as an effective and more environmentally friendly carrier.

Both B-B and MDO panels will readily adsorb most chemically reactive release agents. HDO panels, on the other hand, will not. Therefore, it is always a good idea to use a reactive release agent that dries and will not be removed by rain showers. Nondrying reactive release agents generally do not have good rain resistance prior to concrete placement, and they tend to collect dust and will create a slippery work environment when used on horizontal forms.

Chemically reactive release agents typically cost more per unit volume than barrier-type release agents; however, they are usually applied at a considerably lesser rate and almost always have a lower cost per unit area. This benefit, in addition to increased plywood durability, makes chemically reactive release agents a wise choice for plywood and overlaid plywood concrete form panels.

Applying release agents. Although forming panels are typically treated with form-release agents at the mill, careful evaluation before use is still necessary. Unless the mill treatment is reasonably fresh when the panels are first used, they may require another treatment of release agent before the first use. Even MDO should be treated with a chemical release agent prior to first use and between each pour.

For reused panels or new panels not freshly mill-treated, application of a thin film of form-release agent will prolong the life of the plywood forming panel, enhance its release characteristics, and minimize the potential for staining the concrete. For best results, apply the release agent a few days before using the forms.

GRADE-USE GUIDE FOR CONCRETE FORMS *

Use These Terms When
You Specify Plywood     Description

APA                     Specifically manufactured for concrete
B-B PLYFORM             forms. Many reuses. Smooth, solid
Class I & II **         surfaces. Mill-treated unless
                        otherwise specified.

APA                     Hard, semi-opaque resin-fiber
High Density Overlaid   overlay, heat-fused to panel faces.
PLYFORM                 Smooth surface resists abrasion. Up
Class I & II **         to 200 reuses. Light application for
                        releasing agent recommended
                        between pours.

APA                     Especially designed for engineered
STRUCTURAL I            applications. All Group 1 species.
PLYFORM **              Stronger and stiffer than Plyform Class I
                        and II. Recommended for high pressures
                        where face grain is parallel to supports.
                        Also available with High Density
                        Overlay faces.

Special Overlays,       Produces a smooth uniform concrete surface.
proprietary panels      Generally mill treated with form release
and Medium Density      agent. Check with manufacturer for
Overlaid plywood        specifications, proper use, and surface
specifically designed   treatment recommendations for greatest
for concrete            number of reuses.
forming. **

APA                     Sanded panel often used for concrete
B-C EXT                 forming where only one smooth, solid
                        side is required.

                                                   Veneer Grade

Use These Terms When    Typical                       Inner
You Specify Plywood     Trademarks            Faces   Plies       Backs

APA                             APA             B         C         B
B-B PLYFORM               THE ENGINEERED
Class I & II **          WOOD ASSOCIATION

                              PLYFORM
                            B-B CLASS I
                             EXTERIOR
                                000
                              PS-1-56

APA                     HDO * B-B * PLYFORM     B     C-Plugged     B
High Density Overlaid   I * 6060 * EXT-APA
PLYFORM                   * 000 * PS 1-56
Class I & II **

APA                             APA             B       C or        B
STRUCTURAL I              THE ENGINEERED              C-Plugged
PLYFORM **               WOOD ASSOCIATION

                           STRUCTURAL I
                              PLYFORM
                            B-B CLASS I
                             EXTERIOR
                                000
                              PS-1-56

Special Overlays,
proprietary panels
and Medium Density
Overlaid plywood
specifically designed
for concrete
forming. **

APA                             APA             B         C         C
B-C EXT                   THE ENGINEERED
                         WOOD ASSOCIATION

                            B-C GROUP 1
                             EXTERIOR
                                000
                              PS-1-56

* Commonly available in 19/32", 5/8", 23/32"  and 3/4" panel
thicknesses (4" x 8" size).

** Check dealer for availability in your area.

--Bruce Cordova is a market development specialist and Fulton Desler is a senior engineer at APA--The Engineered Wood Association, headquartered in Tacoma, Wash.