By Ted McIntyre
Why precast concrete has an increasing role to play
When traditional wood and steel construction has consistently provided a successful and predictable outcome for most residential builders, it takes a compelling argument to encourage one to break the mould. So it’s no small irony that the mould itself—in the form of precast concrete—is changing the minds of some builders.
Not that it’s exactly new technology. Precast concrete dates to the Roman Empire, where engineers designed forms and moulds for construction use. English engineer John Alexander Brodie was credited with the first official precast concrete design in 1905, although the format wasn’t widely employed until the 1950s. Today, however, builders are discovering the advantages of combining a modular approach with precast technology.
“Accelerated Building Construction (ABC) is the generic term used for an almost exclusive use of modular building components in developing a wide variety of structures, especially housing,” notes Ken King of Hanscomb Ltd., a Professional Quantity Surveyor, Royal Chartered Surveyor and Certified Technologist. “It offers significant savings in construction time with a fraction of the on-site workforce while, in some notable cases, offering cost savings over traditional construction methods. And the technologies and methodologies regarding quality, fit and finish of buildings completed under the ABC format have now improved to the point that they often exceed industry standards.
“In British Columbia, we’ve see a dramatic increase of ABC using wood and steel modules for housing. Costs are still at a 10-25% premium for these systems specifically, but time savings are approximately 50% over traditional construction,” says King, who is also a regular instructor at Vancouver’s Langara College and the University of Alberta on a range of construction and related subjects. “The real excitement with ABC, though, is in the ongoing development of the precast concrete module for residential construction.
Both structure and exterior wall systems are now available in advanced precast concrete elements with an impressive array of finishes and features. Recently, in Fort McMurray, nine separate condominium projects, each 150,000 square feet, were constructed using the ABC approach by an on-site crew of six workers. Each building was assembled in 55 calendar days at approximately two-thirds the cost of cast-in-place concrete construction!
“I remember seeing passersby at one of those projects actually stopping to watch the building go up, saying, ‘This is really cool!’” King notes. “People don’t usually do that—they usually just walk past a construction site, because, well, what is there to normally watch? But with the modular precast concrete approach, construction never really stops.”
All architectural and structural building components are fabricated in a Canadian Precast Concrete Quality Assurance-certified environment prior to being installed in their final location. Apart from the usual precision of the design and fit of each part, thanks to the controlled environment, the process ensures that the mix, design, curing and tolerances of the concrete panels themselves are strictly monitored and consistent. There’s also the ability to install windows in the factory setting and test them to ensure a perfect seal.
As for the product itself, precast concrete consists of roughly 41% stone, 26% sand, 12% cement, 15% water and 6% air (the latter of which helps with freeze/thaw resistance specific to air-entrained concrete), and weighs in at 150 pounds per cubic foot. Whether slabs, hollowcore planks or double-Ts (a load-bearing structure that resembles two T-beams connected side by side) precast is capable of withstanding high loads even with long spans, thanks in part to being prestressed during the manufacturing process.
That attribute allows for a very versatile design. And with precast stair cores (replete with railings) also precast in-house, builders can erect an entire building with access to all floors as they go—a considerable safety advantage when compared to ladders or man-hoists for moving crews from one storey to the next.
THE ADVANTAGES
There’s also the beneficial thermal mass effect, since precast concrete absorbs outside and inside heat and slowly releases it. That delays the onset of peak heating or cooling loads, reducing energy consumption and peak demand. That, in turn, allows for downsized HVAC systems and a smaller initial investment. It also reduces temperature fluctuations within a building, enhancing occupant comfort. Other advantages include resistance to fire, impact, insects and mould, the ability to produce very high R-values, long-term durability, all-weather installation, no VOCs, almost endless options for colours, textures and finishes, as well as a much smaller footprint on site—both physically and environmentally.
The latter helps keep the neighbours happy, explains Joshua Fede of precast concrete manufacturer and installer Coreslab Structures, a family-run business founded in Burlington in 1975 but now with 17 locations across North America. “We like to call it ‘Big Boy Lego,’” Fede says. “There is less assembly on site, and the components are already finished, so there’s far less waste and our site impact is very minimal. Whereas for a cast-in-place building, if they are pouring concrete, they only have a certain amount of time to start finishing it. They might be there at 2 a.m. trying to finish off the floors, while we’re done at 4 p.m. and not interrupting neighbours while they’re getting home from work or trying to sleep.”
Another attractive element is the risk and liability reduction due to far fewer trades on site and less coordination required. They’re all part of the intangibles of the long-term financial equation, Fede explains. “When we do presentations to builders, one of the things we stress is the total cost of ownership. If you compare our system to a different system, the initial number is going to be higher, but you are not comparing apples to apples. You have to consider that our total precast buildings are a fully completed system along with the intangible soft costs such as time saved on site and ease of managing the project. When you factor it all in, we know that our number and building solution is extremely competitive.
“If you are looking at a two-storey wood structure, it’ll be hard for us to be cost-competitive. But if you start getting into a higher (or longer) building where there is a lot of repetition and economies of scale, that is where precast starts to shine. But even if we are on par with a typical conventional building, you still have a full concrete building versus other structure options that are; highly combustible during construction, and costly to ensure it meets fire codes post-construction”.
Provincial housing policies are also a driving force in precast’s popularity, says Fede. “A lot of home builders that typically do single detached homes or towns are slowly coming more into mid-rise, with recent mandates of increasing density in new surveys. They’re approaching us to do total precast jobs or even just hollowcore floors on their multi-level buildings.”
A short supply of trades has also fed momentum, says Jason Stubbe, sales manager at Stubbe’s Precast in Harley, Ontario. “Many builders are having trouble finding a labour force and are looking for ways to speed things up.”
How fast? “The rule of thumb is that we can do 10,000 square feet of floor and the supporting walls in five working days,” says Stubbe, whose company worked on The Onyx at The Barrelyards in Waterloo: four buildings—two total precast, two cast-in-place. The developer started the precast buildings roughly six months after the other two, but both precast structures were finished six months before the cast-in-place projects, essentially shaving a year off a three-year construction schedule.
For Starward Homes, which is past the midway mark of its first precast concrete venture of Scenic Trails condominiums on Hamilton mountain, the primary lure was convenience.
Scenic Trails By Starward Homes.
“It was the single-source coordination,” says Starward president Brandon Campbell, whose company is working with Coreslab Structures on the four-storey, 144-unit project. “Coreslab is handling the structural component, the floors, as well as the facade. So you’re essentially putting the responsibility for the erection of the structure on one provider, instead of coordinating multiple trades. If something’s not fitting, it’s up to the precast supplier to find the solution. So there’s a cost certainty on a huge portion of your building. “We also liked the speed of construction and the foreseeable long-term durability of it—going with an all-concrete building, which comes with a certain noise attenuation premium,” Campbell says. “We had some consultants tell us that there’s a preferred market price advantage of a concrete building over a wood building.”
But making sure all your ducks are in a row beforehand is critical, Campbell explains. “In order to have the most success, you have to make the decision early and agree to a time schedule. And you have to be prepared to stick to it. If we have a drawback on a building and our schedule slips due to pre-sales or our ability to get a permit and get into the ground, the precaster could now be working on another project. So you might have to wait for their production and installation window to catch up. It’s one consideration you might not have in other construction formats.”
PRECAST’S MANY FACES
Campbell, whose company is in the early stages of another precast concrete condo project at 153 Wilson St. West in Hamilton, also appreciates the bounty of facade design options. “There’s actually quite of lot in terms of the masonry materials or the form-liners you can use,” he says. “And Scenic Trails is an example—it’s a very good-looking building.”
“Precast’s aesthetic versatility means you can match it to virtually any colour, form or texture, meaning easy integration and historic compatibility within a neighbourhood,” notes M. E. Hachborn Engineering President Malcolm Hachborn, who boasts over 30 years of structural design experience with concrete, steel and wood structures. “There are thousands of options using form-liners, from rib patterns to aged lumber to stonework, or you can attach granite, limestone, marble, porcelain tile, etc. to the face for additional aesthetics.”
The key for making those moulds cost-effective is repetition, Hachborn explains. “A typical form-liner will cost $20 a square foot, so if you get one use out of it, it’s pretty expensive. But if you get 100 uses out it, it’s 20 cents a square foot, which is pretty economical.”
Hachborn also notes the recent development of the product. “Wall panels continue to evolve with the introduction of new types of insulation (such as in double-wythe insulated wall panels, a sandwich of concrete layers with insulation in between). And with the increased modularization of adding windows to the panels, the entire wall system or building envelope can be installed as one piece.”
“Our typical insulated wall panel used to be R-20, and we have more than effectively doubled that by introducing our new insulated panels that can easily achieve R-40,” adds Coreslab’s Fede. “So an owner can save a ton of money going with smaller mechanical units at the time of construction. But they will also save during the life of the building because the insulation performs so much better.”
LIMITATIONS
Although there are some engineering limitations to how high you can build with precast concrete, Fede notes that they can “easily go up to 30 storeys. And California has gone up to 40 storeys—and that’s in a seismic zone!”
“The only downside is that compared to more conventional systems, the lead time is a little longer than developers are used to from an engineering and developing aspect, so we have to get involved in the process a little earlier on,” Fede observes. “But the beauty of it is that as the building’s units are being sold and the site is being prepped, we can start producing the building and have it stored in our yard, and once the site is ready we’ll have the whole building ready to assemble on site. Most people understand this, but we like to reiterate and stress that time on site is much more costly than time in the boardroom. So, the time up front will pay dividends once we mobilize.”
While precast doesn’t lend itself to the flashy glazing of Toronto condo towers, it’s ideal for projects like rental buildings,” notes Wayne Harrison, vice-president at Burlington’s KNYMH Architecture Solutions. “We actually have to make them tougher for rental buildings than for condominiums, where owners tend to be more careful with their property,” Harrison says.
For KNYMH, the precast side of the business is bustling, including a condo project for Carriage Gate Homes, another for Hamilton Housing and the recent Village Creek condos in Stoney Creek, a 100% precast project.
But there are also challenges. Although precast concrete is generally accepted as providing superior sound insulation to traditional wood-framed structures, “an eight-inch precast floor is no longer being accepted as sufficient sound barrier,” says Harrison. “It might in fact be, but the suppliers have not yet acquired the paperwork acceptable to acoustic consultants/municipalities that a piece of precast meets the current residential sound requirement. So we’re having to do things on top or beneath precast floors to supplement the soundproofing, like a minimum one-inch concrete topping, or 1 5/8” insulation and drywall beneath, or other sound-deadening techniques.”
And while there’s no question of its firebreak and storm-proofing abilities, a technical requirement for drainage is driving precast supporters batty. “The Ontario Association of Architects has its own insurance firm called Pro-Demnity,” Hachborn explains. “After years of approving the precast two-stage joint system as a rainscreen, they’re now taking a step back and saying that precast walls need to have a rainscreen behind them (or within the system for double-wythe insulated panels). We’ve talked with several building envelope consultants and they all disagree.
“I have done some research on the issue,” Hachborn notes. “What happens is that because you’ve introduced a high-volume cavity inside the panel, to pressure-equalize that cavity, there’s air going in and out, which can bring moisture in with it. If you look at the amount of moisture that would penetrate a 100 sq. ft. area of 4” precast concrete, you’re looking at just 3-4 mm— (not even a teaspoon—over a 24-hour period with a 25 PSI wind load on it! But introducing the cavity and the recommended pressure-equalization venting within the joints in the system will introduce about 28 litres—four pails! It makes absolutely no sense. Concrete is not susceptive to mould or deterioration due to water, so even if you do get moisture in there, the concrete won’t fall apart and there’s no food source for mould, so there are no mould or air quality issues.
“In the meantime, the owner/developer/precaster has to get a building envelope consultant to verify the design and the exterior details so that the architect has no problems with their insurance,” Hachborn continues. “The precast industry is working to educate Pro-Demnity about this area of concern, because the two-stage jointed precast system is a perfect barrier system—similar to the curtainwall system, with the precast being similar to the glass, and the joints being similar to the curtainwall frames). Concrete doesn’t leak; it is the joints that have the potential to leak, so they have to be done correctly.”
Regardless of the advantages or workarounds, changing your construction format admittedly involves a paradigm shift in project planning, says Hachborn. “You put a higher priority on preconstruction, prefabrication and modularization processes,” he says. “You get all of your trades involved early. And you get your precaster involved early, which enables you to appreciate all the benefits of precast—longer spans, lighter systems, etc. For example, if you do hollowcore floor slabs, you might be able to knock off six inches to a foot of height for every floor. So you might potentially be able to put an extra storey on a building while staying within your height limits.”
“The biggest fear for many remains the unknown—not having worked with the product before or not understanding exactly what to expect,” says Fede. “People intuitively know it’s a great option—they just don’t know how to get there,” adds King. “The lights are finally going on.”
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