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Pouring a Structural Slab, continued

Placing the Panels

The panels were delivered in full, 24-foot lengths. But to fit inside the wall forms and leave the ICF cells exposed for filling, they had to be shortened. If not for the integral steel beams, this could be done with a hot wire. But it wasn't much more difficult to cut them with a recip saw fitted with an aggressive hacksaw blade. Once all the forms were trimmed, we stood on the strongbacks and staging planks to set the first panels in place, then moved onto the last panel placed to complete the installation. We had all the panels placed within an hour.

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The tongue-in-groove flanges snugged up to one another easily, although some of the panels had some overall sideways curvature, probably caused during shipping. We placed all the panels, then forced the curves out by wedging and blocking at opposite ends of the deck. To prevent the panels from springing and separating, we screwed 1x cleats to the underside, using the integral Z-beam flanges for attachment.

Our site supervisor, Al Wood, beveled the panel ends to ease the concrete over the deck-to-wall transition.

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The foam panel flanges are fragile and withstood some slight damage in shipping and handling. We used expanding foam to rebond a few breaks and fill small gaps and seams.

It's important to close the chases at the panel ends so the concrete doesn't go wandering off where it doesn't belong. Reddi-Deck offers proprietary foam plugs to cap the ends of the 4 3/4-inch-diameter utility chases. The smaller, 1 1/2-inch chases are easily closed with a squirt of expanding foam.

Steel Reinforcement

Horizontal, free-spanning concrete calls for careful scrutiny to make sure it'll perform properly under varying design parameters. Rebar gives tensile strength to the cast joists; without it, the floor would simply collapse under its own weight. The rebar must be installed on the tension side of a beam to be effective — in this case, near the underside of the joists. We placed it on plastic chairs set on the bottom of the form channels. The chairs are designed to suspend the rebar at the proper depth to ensure full embedment in the concrete. The manufacturer's estimating schedule specified two pieces of #5 (5/8-inch-diameter) rebar per joist for the proposed 24-foot span. I thought that would be too cumbersome to place and tie within the relatively narrow joist forms. Instead, I wanted to use a single length of #8 (1-inch-diameter) rebar in each joist. My engineer calculated the loading on a single joist, assuming a vehicle weight of 1,000 pounds per wheel and an 11-foot wheel base for his load analysis. The #8 bar was more than adequate.

Stress relief. I had planned to tie the horizontal rebar in the joist channels to the vertical rod in the wall forms. However, my engineer was concerned that that would create a moment connection that could transfer undesirable stresses to the lower section of the wall. In the event of floor movement, expansion, or contraction, he wanted any likely cracking to occur right at the floor-to-wall junction, instead, so we cut the vertical rods flush with the top of the wall forms and used independent, L-shaped horizontal #4 connecting rods to tie the floor to the walls. These connecting rods extended 3 feet onto the deck and rested on 3-inch-high wire chairs, with a 2-foot leg hanging down into the wall cells.

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We united the connecting rods and the horizontal #8 rod with a series of #2 rebar stirrups, placed 5 inches on-center and tied to the connecting rod with wire twists. We also ran horizontal rods around the entire deck perimeter for a bond beam effect.

To reinforce the concrete slab, we capped the panels with 6-inch wire mesh, placed above the forms on wire chairs.

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To allow use of a chain fall or hoist in the lower garage bay, I also installed a rugged attachment point in the ceiling near the overhead door. I used standard, L-shaped foundation anchor bolts, punched through the bottom of the form inside a scooped-out access hole. I tied each bolt to a length of rebar that engaged an adjacent joist form. The access hole penetrated a utility chase, which I capped with expanding foam to keep out the concrete.

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The Pour

My flatwork sub, Will Daniels, placed a 5,000-psi mix in a continuous pour to the prescribed 4-inch floor thickness over the forms. Will noted that the foam panels felt sturdier underfoot than any all-steel pan system he'd worked on. We were able to pull the mix truck up near the top of the wall and chute the mix around the floor. This was really convenient, because, ordinarily, the cellular configuration of the wall forms requires the mix to be pumped. The deck pan and the running gap around its edge enabled us to distribute the concrete to the perimeter without the added cost and hassle of pumping.

Bad vibes. We used a cordless concrete vibrator to move and settle the mix. I like the cordless tool because it's less aggressive than a typical corded vibrator, providing just enough temporary flow and plasticity to settle the mix without overpressurizing the ICFs. Nonetheless, it pays to be cautious. Al Wood kept an eye on the underside of the system as we poured. A good thing, too, because he noticed the 2-inch foam spacers we'd installed to help slope the deck pan begin to bulge and move off the blocks at the middle of the 36-foot-long wall. I backed off vibrating in that area, and we avoided a blowout. In hindsight, we could have screwed a retaining cleat to the underside of the deck forms, attaching it to the integral steel furring flanges, to prevent the spacers from moving.

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We installed a wood screed strip, set by laser line, along the inside face of the perimeter forms to control the finish floor line. The strips were diagonally ripped from 1x4 mahogany decking and installed square edge up, leaving a beveled slab edge when removed.

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Strength in unity. The resulting floor system is a seamless, organic whole; the joists support the floor slab, while the slab contributes its thickness to the joist depth and distributes dead and live loads across the joists. The system provides the maximum strength of a reinforced concrete deck using a minimum of material. Although the induced camber was intended to offset the anticipated 3/4-inch dead-load deflection, the floor retains a slightly crowned profile. Should deflection occur under live loading or due to settling over time, the resulting stress may cause cracking at the deck and wall junction. However, that would in no way compromise the strength or integrity of either system.

The final cost of the slab was $16.60 per square foot, including steel, concrete, the forms, the falsework, and labor.

Todd LaBarge
is a residential contractor and structural engineer in Harwich, Mass.