Form sheathing--the part of the form that is next to the concrete--can be made of many different materials: lumber or plywood; glass fiber reinforced or unreinforced plastics; steel and aluminum; reconstituted wood-base panels; glass fiber reinforced concrete. This article concentrates on the wood and wood-base panels, showing the range of products available, and some factors to consider in choosing the best one for your job.
TYPES OF PLYWOOD AVAILABLE FOR FORMING
The four principal types of plywood used for formwork are: sanded, non-overlaid B-B concrete forming grade; medium density overlaid (MDO); high density overlaid (HDO); and imported overlaid birch. Of plywoods used for forming, the sanded B-B forming grade plywood is the least expensive. It generally offers the fewest reuses, absorbs more release agent and imparts more of its own texture to the concrete surface. High density overlaid plywood may have overlay on one or both sides. The cellulose fiber sheet or sheets of the overlay must have at least 45 percent resin solids to be considered high density (HDO), while the medium density overlay (MDO) must have 17 to 22 percent resin solids. Imported birch plywood, generally the strongest of the plywoods with the heaviest overlay, also has a phenolic material in the overlay. This is the most expensive of the plywoods, and potentially the most durable if handled properly.
RECONSTITUTED WOOD-BASE PANELS
There are several groups of reconstituted wood materials. Hardboard is a fibrous-felted board usually made from wood fibers which have been consolidated under heat and pressure to a density of more than 30 pounds per cubic foot. Particleboard normally refers to a product made up of smaller particles bonded with urea or phenolic resin. Fiberboard is sometimes considered a type of particleboard, but here the wood materials are usually broken down and combined so that some of the individual fibers are linked in a natural wood-like bond. Waferboard is a structural panel made from large thin wafers of wood, mixed with waterproof phenolic resin glue, interleaved together in thick mats and bonded under heat and pressure.