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 (Figure 7). We had all the panels
placed within an hour.
7.The 24-foot-long panels
were shortened to bear only on the inner edge of the
foundation forms. A recip saw was used to cut through
the integral steel Z-beams. The entire 24x36-foot deck
was covered within an hour.
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 (Figure 8).
Figure 8.Form edges are eased to reduce the
likelihood of a stress crack forming at the transition from
horizontal to vertical concrete. The site supervisor
approximated the panels' factory longitudinal bevel across the
end cuts, using a reciprocating saw.
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.
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 (Figure 9).
Figure 9.To eliminate a moment connection, steel
rebar connecting the floor system to the walls was installed
independent of the wall rebar, providing for possible movement
in the floor to be released at the top of the wall (left). The
right-angle connecting rods were tied to the #8 longitudinal
rod with #2 rebar stirrups, placed 5 inches on-center over a
3-foot run (right).
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
To reinforce the concrete slab, we capped the panels with
6-inch wire mesh, placed above the forms on wire chairs (Figure
Figure 10.Welded 6-inch-square steel fabric was
placed over wire chairs and wire-tied to the perimeter rebar to
unify the floor system.
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 (Figure 11). The access
hole penetrated a utility chase, which I capped with expanding
foam to keep out the concrete.
Figure 11.To provide for a chain fall or hoist, the
author installed a pair of foundation anchor bolts through one
of the floor forms. The bolts are set in a cavity scooped out
of the deck form and tied to rebar extending back to two
adjacent joists. The bolts will be used to attach a rugged
bracket to the ceiling below.
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
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 (Figure 12). 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.
Figure 12.The pressure generated by vibrating and
compacting the concrete began to force pieces of retrofitted
foam blocking out of place, despite the use of foam adhesive.
Backing off on the vibration averted a blowout.
We installed a wood screed strip, set by laser line, along the
inside face of the perimeter forms to control the finish floor
line (Figure 13). The strips were diagonally ripped from 1x4
mahogany decking and installed square edge up, leaving a
beveled slab edge when removed.
Figure 13.A triangular wood screed strip attached
to the wall form guides the thickness and slope of the floor
and provides a beveled edge to the finished slab (top). At
bottom are two views of the project after
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
Todd LaBargeis a residential contractor and
structural engineer in Harwich, Mass.