Building Poured-Concrete Homes, continued

Forming Walls

Wall form erection actually begins in the front office with a CAD-generated plan that details the size and placement of every form panel in the wall system. The crew snaps offset wall lines and carefully checks the spans and diagonals for square and parallel. The slabs are generally poured quite level and reasonably square. If we do run into a dimensional discrepancy, we can make adjustments of up to 2 inches without serious problems. But because the roof trusses rest directly on our walls, the layout must be as square, parallel, and accurate as possible.

Most forming systems are based on a 2-foot module; we use a 3-foot modular aluminum form system (Precise Forms, 800/537-0706, www.preciseforms.com), which effectively reduces erection time by about 33%. Using this system, we can start setting panels for a full house at 7:00 a.m. and be ready to pour by 10:00 a.m.

The panels are held together by a series of captive pins that also engage the wall ties. The ties are flat 10-inch-long steel straps with a hole at each end. The pins pass through the tie holes and connect to the next form in line (Figure 5).

Figure 5.Attached captive pins are affixed to the face of each aluminum form on 12-inch centers. The pins pass through flat steel snap-ties before engaging the next form in line. Adjustable guide-ties enable proprietary window bucks to hang on the pins, automatically leveling the bucks in the forms.

The panels are spaced for a 6-inch-thick wall. The outside forms rest on the slab sub's dimensional lumber forms, and the interior forms are set parallel to the offset lines.

At the bottom of the form, a plastic clip gets anchored to the slab with a hand-driven masonry nail to prevent movement during the pour. The clip removes easily during form stripping.

Window bucks. The aluminum bucks we use to block out the window openings are made to fit a variety of typical windows, aluminum or vinyl clad. The bucks have adjustable tie guides that hang on the captive pins and hold the buck level inside the forms. The guides are set to allow for a standard 16-inch-deep concrete header. The buck forms a flared exterior opening and a 7/8-inch stop, recessed about 2 inches in from the interior face. This allows the windows to be installed from the outside, bedded in caulk against the stop (Figure 6).

Figure 6.Windows are installed from the outside of the wall and are pressed into a continuous bead of silicone sealant against a 7/8-inch offset lip cast in the opening. The installation tolerance is tight, typically only about 1/4 inch over the unit dimension, to minimize air infiltration around the window.

Pumping Walls

After the forms are set, braced, and leveled, they're ready for pumping. We use a 3,000-psi concrete mix with 3/8-inch aggregate — limestone in our neck of the woods. Pumping can empty a 10-cubic-yard truck in 15 minutes. The average house takes 30 yards of concrete, so we schedule the trucks to arrive at 15-minute intervals. Even so, we take about 1 1/2 to 2 hours to pump a typical house. We use a "Super-P" mix containing water reducers and plasticizers, which flows well around the bucks and eliminates the need for vibrating. The forms are filled to the top and allowed to mound for about 10 minutes before striking off. The forms are of precise, uniform height and leveled on the slab, making the top of the wall automatically level.

With a block wall, the framers grout their own steel truss anchors, but we wet-set them. We generally use USP TA-22 (22-inch) wall-to-truss connectors (Figure 7). Truss anchors wrap over the top chord and are nailed in accordance with the manufacturer's recommended requirements. The anchors have to be accurately set to work with the 2-foot-on-center truss layout; we're allowed only an 1/8-inch tolerance. We set the anchors according to a layout plan created using MiTek 4.2 truss layout software (MiTek, 314/434-1200, www.mitekinc.com). The anchors are embedded 4 to 6 inches into the top of the wall. We have to replace any off-layout anchors with Tapcon-fastened ties, so we try hard to avoid layout errors.

We can safely strip forms the day after the pour, when the concrete has reached 50% of its design strength. (Concrete reaches full design strength in 28 to 45 days.) The stripping crew removes the pins, strips the forms, and stacks them in special vertical transportation baskets. The wall tie protrusions snap off with a sideways hammer blow. If they're too high to reach, we use an 8-foot stud (Figure 8). The stripping crew polices the site and cleans excess concrete from the slab, leaving it ready for the framing crew. This ends our company's involvement in the house, and we move on to the next site.

Figure 8.Wall ties snap on impact; those out of easy hammer reach fall to a length of 2x4. The forming crew leaves the slab broom-clean for the framers.

Interior Framing

Over the next few days, the framers set the trusses, sheathe and shingle the roof, and stand the interior partitions. Foil-faced 3/4-inch-thick rigid foam paneling is applied to the inside face of the concrete walls and held in place with vertical 1x3 furring strips, 2 feet on-center, nailed to the concrete with powder-actuated fasteners (Figure 9). The electrician cuts the foam away to set 1 1/2-inch-deep outlet boxes directly against the concrete, using a powder-actuated tool (PAF).

Figure 9.Foil-faced 3/4-inch-thick polyisocyanurate foam board is nailed to the interior wall surface and strapped on 2-foot centers with vertical 1x3 furring strips to receive the drywall. The furring and electrical boxes are nailed to the wall with PAFs.

Interior partitions are typically framed with light-gauge steel studs, continuing the termite-resistant construction. Occasionally, SPF 2x4s are used, stood on a bottom plate of pressure-treated pine.

Exterior Finishing

The exterior finish is a one-coat colored stucco skimmed over a bonding agent and textured (Figure 10).

Figure 10.One-coat stucco and standardized decorative detailing provide a simple, attractive, and durable exterior finish.

Because concrete continues to cure for several weeks, we wait 30 to 45 days before bringing in the stucco crew. This delay minimizes the telegraphing of any shrinkage cracks through the stucco finish. Though infrequent and typically only hairline, these cracks are normal. If a crack does appear through the stucco finish, it gets repaired with colorized caulk.