Wet concrete is heavy and can’t support its own weight. Without formwork, and the structures we build to support the forms until the concrete can stand up for itself, concrete would be a big, gray blob. Like a surly teenager, it must be molded into an adult. To support the journey to maturity, forms must be strong and designed to keep the concrete where we want it. Formwork is a mold, but the formwork system is much more: a structure of its own that represents 35% to 60% of total concrete construction cost. Knowing how to design and build that formwork structure for economy, quality, and safety is often the difference between a profitable project and a failure.
Safely designing and erecting a formwork system, then placing concrete and stripping the forms, is the majority of the concrete work on multifloor projects. Deciding what forms to use, the plastic concrete characteristics, concrete placement rates, consolidation techniques, and form bracing and shoring are all balanced to get the most efficient and safe project plan.
“The formula is driven by each job’s specific requirements,” says Phil Diekemper, senior vice president of North American Operations for Ceco Concrete Construction in Kansas City, Mo., a nationwide design-assist structural formwork and frame contractor. “Some people want a simple pour-rate formula, but there’s no perfect formula and there are always trade-offs in terms of speed, performance criteria, and productivity. Those determine type of formwork used and required bracing and shoring. It’s all economically driven.” However, he emphasizes, there are no trade-offs on safety, which is an integral part of the formula. “We don’t do safety willy-nilly. There’s a site-specific safety plan; it’s a full commitment.”
Designing the forms
Not long ago, formwork even for large structures was primarily plywood and structural timber. Today, modular systems are much more common.
“The world’s changed over the last 20 or 30 years,” says Diekemper. “The new systems are more ergonomic, lighter, simpler to use, more productive, and far safer.”
Systems for larger structures are designed by a structural engineer, sometimes hired by the contractor but more often someone who works for the manufacturer. Manufacturers provide a complete and detailed design whether forms are leased or rented. “Typically, we provide the design,” says Francisco Gonzalez, associate product manager at Peri Formwork Systems Inc. in Elkridge, Md. “Some customers, though, have their own team of designers with experience with our formwork and can do the design internally. In those cases, their engineers ask for information to verify the designs.”
Andy Pearson, engineering assurance manager for EFCO Corp. Concrete Formwork and Shoring in Des Moines, Iowa, says every drawing is verified by a senior engineer to assure safety. “Every design gets two sets of eyes,” he says. “Then we call out the required strength for what the forms will bear on, whether that’s the ground or previously placed concrete. Onsite inspection is then typically done by an independent inspector.”
Formwork designers apply various safety factors to account for unknown construction loads, wind, and minor deviations from the design and code requirements. “Safety factors for temporary structures are higher than for permanent structures,” says Pearson. “The safety factors allow us to be safe even when everything isn’t perfectly plumb.”
“It’s the contractor’s responsibility to make sure the safety factors are being followed,” says Scott Fisk, president of Meva Formwork Systems Inc. in Springfield, Ohio. “That allows us to protect us from ourselves. When there’s a failure it’s usually because someone exceeded the safety factors.”
Drawings must be carefully followed. Trying to cut corners to save set-up time can result in unsafe working conditions and unnecessary downtime. “One common mistake is not following the engineering drawings for assembling and setting up the formwork,” says Michael Schaeffer, vice president of North and South American sales for Doka USA Ltd. in Little Ferry, N.J. “Not following instructions letter-by-letter can lead to accidents.”
Erecting and moving forms
The various types of forms require different attention to safety. Many of the newer handset systems are light enough to be placed by a single worker. “Our hand-set forms have aluminum frames so that even larger panels are easy for one worker to lift,” says Fisk. “This helps prevent back issues.”
All manufacturers have multiple built-in safety features that allow deck forms to be erected from below or that have integrated working platforms and guard rails. The clamps are easily hand set and all parts are attached so there’s no danger of them falling. Integrated tie-offs are always within arm’s reach. Many of today’s deck formwork systems come with drop-head shores so form panels can be safely removed without pulling the shores. Jump forms or self-climbing forms and flying deck panel forms, though, are a different animal.
“The rail-guided climbing forms never leave the wall, so they’re quite safe,” says Fisk.” Diekemper agrees: “Without the safety procedures we establish for these forms, it would be a less safe situation since there can be full exposure. We make it safe by designing safety into the process. But these systems are expensive to assemble so there’s that fixed cost. But it’s safer because we don’t have to take the wall panels down to the ground.”
Another factor to consider is that today more vertical work is taking place on very tight urban sites with little or no setback. “It’s sidewalk to sidewalk,” Diekemper says. “There’s no space for assembling forms or laying down rebar. This also drives us to use the self-climbing forms.”
Other jobsite concerns include access, protected work areas, easy, safe setup, and tie-off points. “Workers must have safe horizontal and vertical access routes,” Schaeffer says. Stair towers provide safe access to various points on the formwork for either tight areas or two-way traffic. “The platforms that support the crew must quickly and easily lock in place and have secure guardrails to protect workers from falls.”
Where workers are exposed to heights, proper fall-arrest is necessary. Formwork systems must have easy spots for tie-offs with 5,000 pounds ultimate load capacity. Some systems have designated tie-offs within 4 feet of each other so one is always within reach.
Shoring and reshoring for deck forms must be carefully designed. “We don’t design re-shores,” says Pearson. “That’s not something we can control. We provide the posts and coordinate with the project engineer but ultimately, re-shores have to be the contractor’s responsibility.”
Formwork must be inspected after each use. “We have two levels of inspection,” Diekemper says. “The stripping crew inspects on every strip, although that’s a bit cursory. Once the form is back in the yard or on the ground, there’s a detailed inspection with safety in mind.” Leased forms are inspected when they go back to the manufacturer. “We clean the faces and inspect the structural components,” says Pearson. Any damage is refurbished or designated as ‘damaged-beyond-repair’ for which the customer is charged.
The pressure a fluid exerts on the container supporting it depends on how high the liquid is from top to bottom. The width of the container – the form – has no effect on the pressure. Full liquid head in a concrete form is how much pressure would be exerted on the forms by the fresh concrete if it were completely fluid. Concrete has a density of 150 pounds per cubic foot, so 4 feet of liquid head exerts 600 pounds per square foot (psf) at the bottom of the form. The higher the concrete while still fluid, the higher the pressure the forms must be able to withstand. That’s why you’ll hear that a form is rated at 600 psf or as high as 2,400 psf for forms used with self-consolidating concrete.
One thing that controls the pressure in the forms is how fast the concrete is being placed, the pour rate. As the concrete begins to set it starts to carry its own weight and the pressure on the forms dissipates. How quickly it sets depends on the mix design and especially on the temperature of the concrete so that all contributes to the allowable pour rate.
Another factor affecting pressure is vibration for consolidation. Vibrating the concrete fluidizes it so air can escape, so by definition it becomes fluid and exerts pressure. Knowing the permissible pour rate, the set time, and design pressure of the forms can help avoid blowouts.
“I won’t say we don’t have blowouts, but it’s more often due to a tie that was left out or a kicker that wasn’t secured than pour rate,” says Diekemper. “The placing crew is in charge of the pour and that foreman was part of the pour plan’s development. He had an opportunity to provide feedback.”
Pearson agrees that an error at the jobsite is more likely the cause of blowouts and usually something unusual is the cause. “A lot of problems happen on Friday at 3:00 p.m. when the guys are in a hurry. Or it happens when there’s a sudden cold snap and the pour rate doesn’t get adjusted to the temperature.”
Peri’s Gonzalez says that failures do occur but almost always because the formwork wasn’t constructed in accordance with the design. “Formwork is designed for a specific pour pressure and due to the nature of the industry, time is one of the main drivers and so sometimes these pressure limits are exceeded.” Another common reason for failure is exceeding the bearing capacity of the soil. “A formwork designer should specify the loads expected in the design drawings.”
Stripping the forms
Forms can be stripped when the concrete has enough strength to support itself and any construction loads that will be applied. How long that takes depends on the mix design and the temperature. One way to optimize formwork removal is with maturity methods. During stripping, access below should be restricted in case tools or bits of aggregate drop off. Perimeter protection systems can help protect people below on tight jobsites.
“Those systems are expensive, but having something fall onto a sidewalk is catastrophic, so we use them when needed,” says Diekemper. “It’s another one of those trade-offs.”
“The transient nature of our workforce is a constant training challenge,” says Diekemper. “We’re focused on safety down to the crew level. We have regular internal audits to try to improve and even surprise audits.”
It might seem that having a safety manager on-site would improve safety, but that’s not necessarily a given. “We sometimes have onsite safety directors if the client wants that, but we make safety a line of responsibility for everyone from the super to the laborer,” says Diekemper. “Sometimes when there’s a safety director onsite, everyone defers to that person and it can actually be detrimental.”
“Optimum safety means optimum productivity,” Schaeffer says. “If a worker feels comfortable and safe, he moves easily from one task to the next and gets the work done quicker. A worker is not as productive if he feels hesitant about moving around the formwork.”