Specifying the right plywood for concrete formwork

By Mark Halverson

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Concrete formwork may represent close to half the cost of a concrete structure. For cost-effective building, contractors select forming panels that will stand up to the job and to multiple uses.

Member manufacturers of APA−The Engineered Wood Association produce a variety of plywood form panel products—Plyform—designed for most general concrete-forming applications. Usually pretreated with a release agent to facilitate stripping, these panels are available in many grades from basic B-B to products with overlays (i.e. Medium-density Overlay [MDO]) and High-density Overlay [HDO]) that add stability, resist abrasions, and provide a smoother and more durable forming surface.

Each producer of overlaid concrete-forming panels offers proprietary products offering a variety of different features and benefits.

Choosing panels
When planning multiple reuses, it is best to use plywood with more durable construction. All Plyform panels have more solid and stronger veneers than traditional sheathing panels, offered in a range of durability. These panels are rated as Class I, which means they are manufactured with the strongest and stiffest veneers. The grades of veneer used in Plyform panels are indicated in the APA trademark.

MDO and HDO overlaid panels have resin-impregnated cellulose fiber sheets laminated to their faces, and can have either single- or double-sided faces. With careful handling and maintenance, any double-faced forming panel, from B-B Plyform to the overlaid panels, can be poured against on both sides.

HDO forming panels have a greater amount of resin in the overlay; their surfaces are harder and more impervious to water and impacts than MDO forming panels. A greater number of uses and smoother finish on the surface of the concrete can be expected when using HDO Plyform.

Forming panels with MDO faces are often only overlaid on one side, but may have pourable faces on both sides of the panels. While MDO surfaces are not as smooth and durable as HDO panels, a contractor should expect to get more pours from them than the panels without overlay or other surface treatment. The finish of the concrete from an MDO surface will be softer and not as slick as that formed against HDO panels.

There are many different types of overlays and treatments for Plyform that can be used to maximize durability. B-B and B-C panels can be made more durable and resistant to moisture with a coating such as polyurethane or other type of liquid surface treatment. One of the newer high-performance overlays is phenolic surface film.

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It is important to remember not all MDO and HDO overlaid panels (e.g. some General and Industrial products, respectively) are designed for concrete formwork. Care should be taken when specifying and purchasing concrete-forming panels; one can look for the APA trademarks clearly identifying the panel grade and type.

How many pours can you get from a concrete form panel?
The question of how many pours are achievable with a single concrete form panel depends not only on the product itself, but also its care and maintenance. Nearly all types of plywood panels can be used in concrete-forming, from common roof and wall sheathing to sanded panels. APA Plyform panels, however, are specifically designed to withstand the wear-and-tear of repeated concrete pours, as well as to provide a smoother and more consistent finish to the concrete’s surface.

Many factors can affect the number of pours anticipated from concrete-forming plywood, including:

• desired finish on the concrete;
• durability of the panel’s surface, including overlays;
• types of concrete formulation; and
• which release agents are used.

The most critical factor is the handling and care of the form panels.

For example, because panels absorb water, and the increase in moisture content usually causes the panels to expand and move, it is important to adequately fasten the form panels to the framing and to leave space between the panels that allows for their flatwise movement within the formwork.

With reasonable maintenance, more than five pours can be expected from a B-B grade Plyform, while HDO panels can withstand up to 20 to 50 reuses, and MDO Plyform somewhere in between. In fact, with proper care, some proprietary concrete-forming panels can achieve more than 100 reuses.

Caring for the panels 101
One can optimize the life and performance of forming panels if they are adequately maintained. Here are some commonsense considerations:

• apply a release agent prior to every pour;
• only use non-ferrous wedges and tools (e.g. wood or softer plastics) to remove excess concrete from the faces and edges of the panels;
• keep the panels stored face to face to protect cleaned surfaces;
• protect vulnerable corners and edges of panels during removal and handling;
• if form panels are cut, reseal the ends and edges to protect them from excessive moisture; and
• consider admixtures in concrete formulations when choosing a type of forming panel, understanding that overlaid panels provide greater resistance to chemicals.

To determine the optimal panel for an application, design/construction professionals can visit PerformancePanels.com to see lists of manufacturers, access product specifications, and read APA’s Concrete Forming Design/Construction Guide.

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I've been thinking about the concrete board idea and wanted to ask everyone if they think we should consider making that the standard installation method?

Can you post your thoughts on the trade-offs between using concrete board (and leaving it) and plywood (and dropping it out) -- in terms of time, energy and cost. I don't think there is a "quality" issue, as both methods are equally stable.

As DMUN noted, it's too bad you can't buy hollow terracotta beams in the states.

Let me know what you think.
James 1 vote for plywood

O.K., I haven't built my oven yet. I have, however, used concrete board in a lot of bathrooms. Perhaps it's paranoia, but I don't think the 1/2" board will stand up to time and exposure like real poured concrete 4" thick. It will probably last a long time and I'm just goofy, though I have seen crumbling and deterioration. It will essentially be a shim, so I would worry about it. It will bond with the slab and this may be its saving feature. I'll build mine with removable wood. Being a newie, I look forward to other opinions and you guys changing my mind. I'm not being clear

My fault. I can see that I wasn't clear on this.

You still pour the 3 1/2" concrete with a 1/2" rebar grid set on 12" centers, and set a full course of concrete blocks across angle iron for the stand. The only question is whether it is faster, easier and cheaper to use the concrete board or plywood as the form to hold in the concrete while it sets and cures.

Sorry about that.

Yep, no way concrete board could hold up an oven.

James In favor of concrete board, it is cheaper than heavy plywood and if you place the board just inside the block walls (rather than on them) it is not performing any structural function (it is just hanging on by itself, not as a shim), other than perhaps being more waterproof than poured concrete (which is not really a function in this location). If I had an unneeded piece of plywood that was substantial enough I'd use it to reduce the bracing needed, but after the form function is done I'd have to dispose of it. I really think both are reasonable options depending on what is readily available for the builder. I gotcha. It's not going on the block, so it's not between block and hearth and thus does nothing but save you the time of removing it. I'm starting to like it. My bad in thinking you also used it to stop the block openings.
James: I understood one would still pour, but I was suggesting the pour would be best if directly in contact with the block and not "shimmed" by
concrete board. Apparently this is just what was done. That I did not understand.

Thanks to both of you, Chris. I agree cement board

I used Durock cement board on my pour and I am very happy I did.

I placed the board directly on the blocks, rough side up, overlapping the opening 3 inches or so. You could very well use it to cover the holes in the blocks, but I did not want to go thru that much trouble to exactly cut the board, after all it was a time saver for me. The sheets of Durock was $10 less than equivalent ply-wood, so $$ wise it is not much difference. But time savings, I think would be a great deal. All I had to do was cut 3 2x4's the length of the opening, close enough so that they were held in by friction when placed in the opeing, flush with the top of the block. I then measured and cut each of the 9 legs, and taped them to the braces to temp hold them. Lay the durock on top, tape the seems and you are done. I had no problems with sag, for my 4" (22 bags) of concrete + 2.5" of vermiculite concrete.

This was my experience anywway.