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.

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.

Concrete formwork is a temporary or permanent mould that is filled with wet concrete. As plywood is used in formwork, the kind of plywood used affects the appearance, rigidity and reuse of the entire concrete structure. 

So, we are going to take a look at the considerations when choosing plywood for formwork and the following:

• Factors to Consider 

• Why Plywood is so Popular in Construction

• Industry Advancements

Types of Plywood for Formwork

Structural Plywood

This is a plywood classified for strength. This kind of plywood has a high bearing quality that sustains weight. It is recommended for use in heavy and major construction, where mechanical support is of utmost importance.

Film-Faced Plywood

The front face of the plywood is treated with a waterproof coated film so that it not only has a smooth surface but is also recyclable. Film-faced plywood is often used in concrete formwork. 

High-Density Overlay (HDO) Plywood

HDO plywood features an impregnated fibre surface containing resin that creates a hard, durable and water-resistant layer, providing a superior concrete surface and improved potential for reusability.

For more information, please visit concrete form plywood.

Medium-Density Overlay (MDO) Plywood

MDO is somewhat in the in-between: The overlay is not hooded off like it is in HDO, but, like HDO, MDO has a lower-density underlayment. MDO looks good and is cheaper than HDO, plus is suited to work where a high-quality finish is desired, but a perfect one is optional.

Factors to Consider

Durability

Reusable living is also dependent upon the durability of the plywood. HDO and MDO types of plywood are more durable and can be used many times.

Finish Quality

The difference in overlay results in a different concrete finish. HDO makes the finish smoother than MDO or ordinary plywood.

Load-Bearing Capacity

Think of the load of the concrete and the forces of pouring. There are greater loads for structural plywood compared to other options.

Environmental Conditions

If the formwork is exposed to the elements, select water-resistant, UV-resistant, and temperature-resistant treated plywood.

Getting concrete formwork correct on the first try will save you money. Therefore, the kind of plywood you need to use will depend on the concrete formwork being applied.

Why Plywood is so Effective in Many Settings

Plywood is one of the most suitable composite materials for formwork applications, in large part due to its ability to perform well in all of the below-stated instances:

Strength and Stability

The cross-laminated structure of each sheet of plywood – multiple thinner layers of wood veneers bonded together with alternate grain directions – provides the material with exceptional strength and dimensional stability.

This also means that it can take on heavy loads and won’t warp or split so that the overall structure can stay resistant to failure over many years.

Cost-Effectiveness

Plywood is much more cost-effective than using solid wood or other engineered wood products such as laminated veneer lumber. Coupled with the highly optimised manufacturing process, plywood saves a lot of building and labour costs, which is reflected in the rapid expansion of housing developments in and around Sydney.

Versatility and Customisation

For instance, plywood is easy to cut. Depending on certain project-specific needs, it can easily be shaped and formed to any size and shape level. It comes in various thicknesses, grades and finishes, so it can easily be adapted to any construction needs.

Environmental Sustainability

More on its environmental impact. Plenty of plywood came from sustainably managed forests. 

Today, plywood may be considered an eco-friendly alternative to some timber applications, especially if sourced from a neurogenic landscape. 

Given the advancing technologies in the timber industry, the majority of plywood in the market comes from areas where wood resources are carefully monitored to ensure sustainability.

Resistance to Moisture and Weather

Coupled with the ability to produce plywood with treatments and coatings to improve resistance to moisture, weather and insects, this is a versatile product for use either internally or externally.

These characteristics mean that ply formwork is ideally suited to formwork. It produces a reliable, durable end product that meets a project’s exact specifications. 

Its quality concrete cast surface, good weight-to-strength ratio, no deflection, and high durability are the main reasons it is widely used in modern-day construction. 

By the mid-20th century, plywood was used almost everywhere in Australia, from plywood formwork shed walls, fencing, portable classrooms for schools and formwork offices for building sites.

Click here to find out how modern-day carpenters in Sydney use formwork ply to upgrade a shop's facade and overall aesthetic appeal.

Industrial Advancements

Plywood saw further innovations after the Second World War, which came alongside broader industrial advances that made huge quantities of plywood more affordable, widely available and versatile. 

Numerous resins were invented in the postwar era, leading to better manufacturing methods. Plywood could now be mass-produced with fewer waste products and higher success rates. 

As well as being used in the booming aircraft industry, plywood could now be utilised by anyone wanting to construct simple modern buildings.

Economic Growth

Led by strong economic growth following the Second World War, Australia’s construction boom created an ever-expanding demand for materials that were cheap, flexible and reliable. Plywood is all of those things.

Technological Innovations

New adhesives and treatments also improved resistance to environmental stresses, rendering plywood formwork suitable for use in the more hostile Australian climate. As well, increasing the strength and load-bearing capacity of plywood enabled its use in concrete formwork.

If you want to learn more, please visit our website plywood x .

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