IN COMMERCIAL CONSTRUCTION, THE USE OF SITE-POURED, TILT-UP concrete walls is growing at record speed. But residential builders, for the most part, haven't joined the tilt-up revolution. It's still primarily a commercial construction tool.
“It's their mind-set,” says Sauter. “First of all, you need a crane on the site to raise the panels, and that can run $500 bucks an hour. To make it work, they have to be doing a very large—10,000-square-foot or larger—house or a subdivision. And they have to have room for the forms, which need a flat surface because they are poured on site.”
However, Sauter says, it doesn't have to be that way. The trouble is that no residential builders have yet spent the time and energy necessary to develop a fast, low-cost, alternative method for using tilt-up. He thinks it's just a matter of time before somebody figures out the problem, cleans up the process, and creates a residential revolution.
“All of the concepts are there for residential,” Sauter says. “You can do insulated sandwich walls or lay conduit for electrical—that's not a problem. What's missing is the logistics of doing smaller projects. And foundation contractors—the crews most likely to do the work—don't have a prescriptive guide to follow.”
That guide, if it's ever written, could open a new world for builders. In the meantime, here's a primer on the key principles of tilt-up construction, extracted from the “Tilt-Up Construction and Engineering Manual,” available from the TCAWeb site. Maybe your firm will be the one to tweak this guidance and create a manual for residential tilt-up.
Planning the layout for form construction of tilt-up walls is a critical early step in the erection process (see illustration above). The latest techniques call for the tilt-up walls to rest on plastic shims on continuous concrete footing. The shims are about 1 ½ inches thick and a foot or so long. They sit under the panel seams so that grout can be pressed into the gap all the way along the bottom of the panels (see illustration below). The floor slab typically is poured after the walls are erected, forming a monolithic link with the walls.
1. PLAN AND PREPARE FOOTERS Planning the layout for form construction of tilt-up walls is a critical early step in the erection process (see illustration above). The latest techniques call for the tilt-up walls to rest on plastic shims on continuous concrete footing. The shims are about 1 ½ inches thick and a foot or so long. They sit under the panel seams so that grout can be pressed into the gap all the way along the bottom of the panels (see illustration below). The floor slab typically is poured after the walls are erected, forming a monolithic link with the walls.
2. MAKE THE FORMS Creating the tilt-up form is a fairly low-tech process. You can build one using wood forms braced with wood cleats on the exterior at proper intervals. For window and door openings, the cleats go on the inside. Rebar is specified in much the same way as any slab on grade. Most residential walls also will include chamfered indentations for electrical wires or for architectural variety. These can be created with pieces of lumber or other material pushed into the top of the wet concrete.
3. ADD BOND BREAKER After the forms are all built, it's important to apply “bond breaker” to all areas that will come in contact with wet concrete. This product creates a nonstick surface and comes in brand names such as Form-Aid. The TCA suggests applying bond breaker to the outer perimeter and tops of the forms so that excess concrete will fall away—and the forms can be used again.
4. PUMP AWAY Tilt-up panel concrete must have a compressive strength of at least 3,000 pounds per square inch, with maximum aggregate size of 1 inch, and a slump of 4 to 5 inches. One major time-saver is the use of pumps and conveyers instead of manpower to place the concrete in the forms. The TCA estimates that this equipment costs an additional $2,000 per day to rent, although that amount is often recouped on commercial sites by time saved.
5. TILT AND BRACE This is where your handiwork is finally put to the test. Once the forms are removed, the walls are ready to be tilted into place. Before lifting, however, the experts go through a checklist that includes specifying the pick-up points where the crane will be attached; ensuring an adequate supply of temporary bracing (see illustration at right); getting access to the site for the crane; and making sure you have all necessary crew standing by to set braces, shim, and apply grout. Many crew managers hold safety meetings on the day of a big lift to remind everyone of potential risks.
BEST FACE FORWARD One of the objections to tilt-up concrete walls has been the lack of a range of finishes. But in recent years, several types of finishes have come into wide use, from colored stucco to a 2-inch-thick, embedded concrete block system called Chameleon Cast Wall Systems, from a company by the same name based in Brookline, Mass. And builders won't have any trouble seeing the uses for traditional-looking embedded thin brick (shown here). The industry reports that the growth of thin brick averages 30 percent to 50 percent every year over the previous few years.
TO LEARN MORE ABOUT TILT-UP CONCRETE WALLS, VISIT OUR WEB SITE AT WWW.BUILDERONLINE.COM, CLICK ON “THE MAGAZINES” TAB, AND THEN CLICK ON “BUILDER ARTICLE LINKS.”