Revised Standard for Plywood and Overlays

Concrete International, Jun 2007 by Zylkowski, Steve, Gibson, Dave

What you need to know about U.S. Voluntary Product Standard PS 1-07

As a high-performance, renewable building material, plywood has been an ideal choice for concrete formwork for several decades. U.S. Voluntary Product Standard PS 1 establishes minimum requirements for plywood used in construction and industrial applications. It was originally developed in 1966 and provides manufacturing specifications such as species, veneer thicknesses, and grades as well as performance requirements for certain end uses. APA-The Engineered Wood Association (formerly the American Plywood Association), in coordination with the Office of Standards Services of the National Institute of Standards and Technology (NIST), recently announced revisions to PS 1-95, "Construction and Industrial Plywood."1 These revisions were approved by the PS 1 Standing Committee, comprising a balanced membership of plywood producers, users, researchers, and regulatory staff.

This article highlights the changes in the PS 1-07 standard, "Structural Plywood,"2 that pertain to overlaid plywood panels for concrete formwork. With the new standard, end users will be able to more accurately specify concrete forming panels according to their primary application and desired performance.

WHY ARE THE REVISIONS IMPORTANT?

The previous standard didn't differentiate between different types of high-density overlay (HDO) and mediumdensity overlay (MDO) applications. Just as the substrate must match the application, the prescriptive definitions for these products must be driven by the demands of the intended application (such as industrial, general use, or concrete forming). PS 1-95 had also become outdated due to improvements in overlay technology that include the capability to tailor overlays to specific uses. As a result, the older standards and descriptions were being used or specified on new formwork projects without regard to the end user's cost and performance needs or the availability of newer, high-performance products.

WHAT'S NEW?

The plywood grades available under PS 1-07 are shown in Table 1. To recognize that overlays are designed and manufactured to meet specific user requirements, HDO grades are now differentiated into HDO-Concrete Form and HDO-Industrial, and MDO grades are now differentiated into MDO-Concrete Form and MDO-General.

Along with the new plywood grades, several changes and additions to the prescriptive definitions for overlays were also included in the new standard. The general requirements for overlays have remained mostly the same as in the previous standard. Overlaid plywood is available with one or both sides overlaid. If only the face is overlaid (or when a backer sheet is applied), the back face veneer is required to be C or better. The surface of overlaid plywood is required to be smooth or uniformly textured, but some evidence of underlying grain is permitted.

HDO-Industrial plywood is typically used for highway signs or industrial applications and is manufactured with thermosetting phenolic resin impregnated cellulosefiber sheet or sheets bonded to the working face. The requirements for HDO-Industrial plywood in PS 1-07 remain basically the same as in PS 1-95 for HDO plywood. The overlay layer must consist of a sheet or sheets containing not less than 45% phenolic resin content, based on the volatile-free weight of fiber and resin. The thickness of resin-impregnated materials for each working face can't be less than 0.30 mm (0.012 in.) thick before pressing and must weigh not less than 290 g/m^sup 2^ (60 lb/ 1000 ft^sup 2^) in the ready-to-use condition. An additional requirement applies when the overlay layer is composed of an MDO cushion sheet with one or more HDO cap sheets. In this case, the MDO sheet can't have less than 34% resin content, based on the volatile-free weight of resin and fiber, exclusive of the glueline.

The requirements for HDO-Concrete Form plywood are more stringent than they were in the previous standard. The overlay layer sheet or sheets must have a slightly higher phenolic resin content of not less than 52%, based on the volatile-free weight of fiber and resin. The thickness and weight requirements for the overlay are the same as for HDO-Industrial plywood, as are the requirements for the MDO cushion sheet when HDO cap sheets are used.

The requirements for an overlay sheet or sheets on MDO-General plywood have also increased. These sheets must have not less than 27% phenolic resin content, and the thickness of resin-impregnated materials for each working face must not be less than 0.30 mm (0.012 in.) thick after pressing and weigh not less than 280 g/m^sup 2^ (58 lb/1000 ft^sup 2^) including both resin and fiber, but exclusive of glueline. The resin-treated facing on the finished product must also be suitable for painting.

MDO-Concrete Form plywood overlay layer sheets must contain a slightly higher phenolic resin content of not less than 34% and must meet the same thickness and weight requirements as MDO-General plywood. Unless otherwise agreed upon between the buyer and the seller, MDO-Concrete Form plywood must be treated with a mill-applied release agent.