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My First ICF Foundation - Continued

With the first ICF course in place, we made preparations to pour the slab. I covered the crushed stone and the exposed interior portion of the footing with 1-inch-thick extruded polystyrene, topped with a 6-mil poly vapor barrier. We then poured a 4-inch slab with fibermesh in the mix (Figure 4).


Figure 4. The first course of block acts as a form for the basement slab. The finished slab provided a level work surface, since the foundation is built from the inside.

Carpenters, Not Form Setters

Engineering tables provided by the manufacturer specced 5/8-inch (#5) vertical rebar every 16 inches, to provide the necessary lateral resistance. I chucked a 5/8-inch carbide-tipped bit into a rotary hammer, drilled a 2-inch-deep hole in the footing in the center of each core of block, and then enlarged the hole with an 11/16-inch drill bit chucked into a 1/2-inch drill. This gave us some wiggle room to set the rebar.

The manufacturer strongly suggested that I use carpenters rather than concrete workers to stack the blocks, because the skills required to place the blocks are more compatible with a carpenter's background than a concrete worker's. After working with the blocks, I agree with him. (While writing this article, I heard several stories about ICF walls built by concrete workers — walls so out-of-plumb that they had to be shimmed out before interior wall finishes could be applied.)

Therm-O-Wall blocks make tongue-and-groove connections with each other, so I squirted a 1/2-inch bead of aerosol urethane into each groove before pressing the next block into place (Figure 5).


Figure 5. Aerosol urethane foam cements the tongue-and-groove connections between blocks. Note the plastic webs, which hold the sides of the blocks together and provide support for horizontal rebar.

When a block had to be cut to length, we formed a tongue or groove as needed to join with the abutting block. Sometimes these fits were sloppy, so we shot in extra foam. Where we thought we might have a weak joint, we reinforced the area by screwing 1x4 strapping into the plastic tees.

Number 5 horizontal rebar was cradled in the first course of 16-inch-high block (Figure 6) and clipped into place with Snap-Lok rebar fasteners, 4 feet on-center. We inserted the horizontal rebar every four courses, as well as in the top course, overlapping the ends 24 inches.


Figure 6. A preshaped wire wraps under the plastic web and over the rebar to keep the steel in place.

For window openings, we scabbed together pressure-treated 2-by material equal in width to the ICFs. The manufacturer insisted on 1/4x2x2 angle iron at each horizontal corner — four lengths per opening — to give the window bucks additional strength. But with small windows, this was overkill. Since we were essentially casting a lintel in place, I'm confident that two pieces of rebar spaced apart and placed above the buck would have been sufficient.

We set four courses and then erected 2x6 commercial-weight steel studs as wall braces, approximately 7 feet on-center. The studs were also screwed to the plastic tees. Next we hung Lite-Form scaffold brackets from the steel studs; this way, the wall braces doubled as staging for laying more courses as well as for pouring the concrete. At the top of each steel stud, we fastened a diagonal 2x4 kicker attached to a turnbuckle brace. Turnbuckle braces are available from masonry distributors and some ICF manufacturers. Each turnbuckle brace has a steel plate at the bottom, designed to be fastened to the slab with concrete screws. The turnbuckle attached to the kicker provides the necessary adjustment to plumb the walls and keep them aligned.