Concrete Forms Defined

Revised Voluntary Standards Reflect Panel Improvements

The new grades will allow manufacturers to tailor overlays to specific end users. The amount of resins in the grades will vary, and forming contractors should use a panel specifically made for concrete forming.

By Debra Wood

With new concrete forming products coming on the market and contractors adopting methods that speed construction, the National Institute of Standards and Technology has issued revised voluntary plywood standards that reflect improvements in overlaid panels used in concrete forming.

Calling the old standards outdated, Dave Gibson, technical director for North America for Dynea, a company in Finland that produces overlays, says, “The biggest difference is we are describing what is being used today.”     

The standard divides High Density Overlay grades into HDO-Concrete Form and HDO-Industrial, and Medium Density Overlay grades into MDO-Concrete Form and MDO-General. The new grades will allow manufacturers to tailor overlays to specific end users. Gibson says the amount of resins in the grades will vary, and that forming contractors should use a panel specifically made for concrete forming.

“It clarified that there is a concrete- and a sign-grade MDO,” adds Ken Pratt, technical director for Olympic Panel Products in Shelton, Wash. Steve Zylkowski, director of the quality services division for APA – The Engineered Wood Association, says he expects contractors will begin seeing the new grades later this year. The organization independently tests and certifies that members’ products meet standards.

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Forming panels

Plain plywood panels, without an overlay, are commonly used in forming, when the surface does not matter and a wood grain will be visible on the finished product. Gibson says MDO gives the surface a mat finish and HDO a glossy appearance.

“The higher the grade of plywood or the thicker the overlay,  the better finish on the concrete,” says Joe Sandoval,  Peri Formwork Systems south central regional manager in Fort Worth. Overlays are created bonding sheets of paper impregnated with phenolic resin,  an adhesive agent.

During manufacturing, an MDO panel receives one resin-impregnated overlay. An HDO panel has multiple overlays. Phenolic surface film panels, a third type of overlaid panel, use a hardwood with a phenolic surface film applied to the top, not the paper. Overlays are fused under pressure and heat to resist water, chemicals and abrasion.

Chuck Baldwin, district manager of Mid-South Lumber Co. in Dallas, indicates MDO panels cost about $7 per sheet more than a BB panel, a plain plywood panel, an HDO panel costs about $10 per sheet more than an MDO panel and a phenolic resin panel costs about $8 more than an HDO.

“The advantage is durability,” says Jeff Erson, manager of the Baker Engineering Group in Houston. “Overlaid panels prevent water intrusion, and they also provide a harder wearing surface.” 

Manufacturers label panels by weight per 1,000 sq ft of surface area. Historically, higher weights equaled better performance, but Pratt says with products imported from all over the world, it’s harder to tell because European panels show weight in grams per sq meter.

Plain plywood panels typically can be used only a few times before they must be discarded, Gibson says. He adds that an MDO will last five to 10 times and an HDO 20 to 50 times.

Doug Karn, United Forming district manager for Texas in Houston, says that using an HDO can end up saving money, if the contractor gets enough reuses out of it. Pratt says rather than focusing on the per-panel price, contractors should consider the cost per pour. They also should consider finishing costs.

Except for the additional expense associated with an overlay, Erson and Karn could not think of any drawbacks to working with overlaid panels. Even so, Pratt says few firms in Texas use the higher-end panels due to cost.

“In Texas, in some higher-end commercial jobs, where they want a good-looking concrete surface, they will use HDOs, but if they are just looking for general-use concrete surface for a garage, they will go with an MDO,” Gibson adds.

United Forming used a HDO panel while building the Austin City Hall, completed in 2004 by Hensel Phelps Construction Co. of Austin; at several University of Texas parking garages; the 14-story Walker at Main Garage in downtown Houston; and at parking garages at Bush Intercontinental Airport.

And Karn says the company is using phenolic resin panels on a 22-story, 192-unit condominium in Austin, called the Shore.  General Contractor Harvey Cleary Builders of Austin is building the project for High Street Rainey, a subsidiary of Trammell Crow Co. of  Dallas. Karn estimates plywood costs for the slabs came to approximately 21 cents per gross sq ft of slab. If the company had used MDO or HDO, the cost would have been 28 cents to 30 cents per sq  ft. Plain panels were not appropriate for the project due to the  exposed slabs.

Selecting the right panel

Industry experts recommend considering the project, how many times the contractor wants to reuse the panel and the concrete mix before deciding on the panel.

“Everybody wants something inexpensive,” Gibson says. How the panel is used affects the number of times it can be reused. Baker Engineering Group’s Erson says a contractor will obtain more reuses on a vertical application, such as a wall or column, than on a deck, because there is no foot traffic and the form is set after the reinforcing steel is placed. Therefore, the steel does not gouge into the panel and rip the overlay.

Olympic Panel Products’ Pratt says that using HDO and a flying-table method of forming on a 40- to 50-story building, a contractor might only have to replace the panels two or three times. The crane moves the forms from floor to floor. When employing the flying-table method, the panels are screwed or permanently nailed into place and not pulled out of the table. Therefore they last longer.

“One reason panels [normally] don’t last long is you are constantly nailing them to lumber,” Pratt adds. “Nailing and ripping them out, that mechanically destroys the plywood.”

The Peri Skydeck employs an aluminum slab-forming system with high-end HDO panels. The shores stay behind to support the newly poured concrete, while workers move the Skydeck panel to the next pour. Sandoval says that the system decreases labor costs on the jobsite and allows for faster cycling of equipment.  

“You don’t have to wait for the strength of the concrete to get to the point it can support itself,” Sandoval says. “It leaves the slab supported, which enables you to cycle faster.”

Urban Concrete Contractors of San Antonio, Texas, used the Peri Skydeck system on the Great Wolf Lodge at Grapevine,  as a subcontractor to Turner Construction Co. of Dallas. The 450,000-sq-ft themed indoor waterpark resort topped out in March.

Concrete mix also affects panel longevity.

“Concrete has changed radically,” Pratt says. “A lot of contractors don’t understand how much it has changed.” There is greater use of super plasticizers and self-consolidating concrete.

Pratt adds that high-strength concrete with a 10,000 psi or higher is harsh on panels. A super plasticizer increases the pH, and the higher alkalinity breaks down the overlay on plywood. Self-consolidating concrete also takes a toll on overlaid panels, Pratt says.

“It’s hot and liquid, and it creates pressure against the forming panel,” he adds. “Between the heat and pressure, it tends to break it down.”

To address these problems, Dynea is developing an overlay with greater resistance to high alkalinity. And Olympia is beta testing a melamine-phenolic blend of barrier film to deal with more aggressive concrete mixes, such as those used in Florida with super plasticizers. Pratt also is trying to devise a method for benchmarking alkalinity resistance.

Pratt advises contractors to discuss the project and the type of concrete with manufacturers to determine the best product for that specific purpose.

Useful Sources:

National Institute of Standards and Technology Voluntary Product Standards Program
http://ts.nist.gov/Standards/Conformity/vps.cfm