Considering what form substrate what form substrate material and what surface finish will be best for an application depends on the surface finish specified for the concrete, the number of form reuses the project requires, the strength and deflection characteristics of the substrate, price, and perhaps the thickness tolerances of a product—especially important for architectural concrete work.
Choices for form substrates and surface finishes range from inexpensive CDX plywood that serves as both the substrate and the surface finish, to more expensive aluminum, steel, or plastic resin composite materials with exotic finished surfaces for multiple reuses.The latter may be the bargain compared to plywood when considering the per placement cost versus the original purchase cost.
Which substrate and surface finish to use is driven by project requirements. If the forms will be reused 100 times, form panels should be purchased to meet the requirement. Refacing a form assembly halfway through a project causes delays and is expensive. Conversely, it isn't cost productive to invest in materials that can get 100 uses for a job that will only require 20.
Plywood, steel, aluminum, and composite materials are used for both vertical and horizontal forming systems as a substrate material. Located behind these panels are an array of strong-backs, whalers, and trusses that also are made from wood, steel, or aluminum.
Wood substrates. For single-use applications, CDX plywood is probably the cheapest substrate material when the concrete finish doesn't matter. For example, the back side of a unique retaining wall section not in public view might be a good application for a single-use form. But be aware that CDX is designed for other purposes, has only 5 plys, and isn't rated for the pressures that concrete can impose. For repetitive work, better grades of plywood are necessary. They don't absorb as much moisture and help to reduce the concrete build up (laitance build) on the casting surface. These can be pre-oiled panels or substrate plywood panels with waterproof surface finishes.
Most plywood for concrete forming is 7 ply, though there is some plywood available with more laminations. Although some 5-ply sheets are available, most contractors prefer 7 ply because they are stronger and have less deflection. Sheets are manufactured by either a one-or two-step process. Manufacturing a one-step sheet involves placing the overlay material and all the veneers with waterproof and boil proof glue in a hot press. The resulting panel has thickness tolerances of ± 1/32 inch. Two-step panel processing involves gluing the veneers in a press, repairing panel faces with poly-patch compounds, sanding the surfaces smooth to a tight tolerance, and then bonding the overlay surface to the panel in a hot press. These panels have thickness tolerances of ± 0.005 to 0.006 inch with no transfer of grain lines or knot holes to the concrete, making them a good choice for high-end, architectural concrete finishes.
The first place deterioration starts on a wood panel is along the edges because water can easily enter and cause swelling. Manufacturers of overlay panels typically seal the edges with polymer products so every cut you make also should be sealed. Here are some of the wood substrate classifications available.
B-face B-back, oiled, and edge sealed (BBOES). Plywood panel surfaces are ranked as “A,” “B,” or “C.” Surfaces marked as “B” permit 20 to 25 knot holes and exposed grain on a 4x8-foot sheet. “Oiled” refers to the panel being oiled during the manufacturing process to allow for easier release from the concrete. “Edge sealed” means the panel edges are sealed at the factory to reduce moisture absorption. BBOES is typically made from either Southern Yellow Pine (SYP) or Douglas Fir (DF). SYP panels are less expensive than DF and usually are heavier. DF panels typically are better quality than SYP because moisture doesn't cause them to swell or move as much and they are almost always the panels used for overlay surfaces. All BBOES products will exhibit grain and patch transfer to the finished concrete. You can expect four to eight uses and the application of release agents is required before each concrete placement.
Hardwood panels. Hardwood veneers on top of DF panels offer blemish-free surfaces with less moisture absorption. The wood for these veneers includes maple, oak, Brazilian, and Indonesian hardwoods. Used as overlay panels, they are cleaner and offer better adhesion for overlay resins.
Steel substrates. The typical substrate thickness of a steel panel is 3/16 to ¼ inch, but can be as little as 14 gauge for handset panels. With a life expectancy of several decades, or even a lifetime, deflection of these panels is minimal and create very smooth concrete surfaces. When using steel forms, designers usually are concerned more about panel joint and tie-hole locations than finish. Steel deck forms feature a steel frame, a plywood substrate, and a resin surface finish. Form manufacturers generally require thin-film, reactive-type form release compounds for good form release.
Aluminum substrates. Like steel, the aluminum in a panel serves as both the substrate and the finished surface. Standard substrate thicknesses range from 0.094 to 0.120 inch, but they can be more for gang form wall panels where greater head pressures are anticipated. The panels are much lighter than steel, are built to have little deflection under load, and can provide very smooth concrete finishes. When they are properly cared for, you can achieve more than 2000 uses. New forms must be “seasoned” (some manufacturers preseason them) to avoid a reaction between the aluminum and the alkali of the cement that results in a disfigured surface.
Material that is factory applied to any given substrate is referred to as an overlay. Overlay panels are alkali resistant and minimize labor costs by reducing clean up time between placements. They are more resistant to moisture penetration—increasing resistance to cracking or “checking” as well—extending the life of the form. They also produce smooth concrete finishes.
There are two categories of form surfaces with a resin impregnated paper: medium-density overlays (MDO) and high-density overlays (HDO). MDOs consist of a large family of phenolic resin impregnated overlays designed for use on engineered wood substrates. Easy field use combined with the ability to produce superior matt or architectural finishes make them a favorite choice for concrete contractors.