Some concrete producers are cashing in on a market that didn't even exist a few years ago--above-grade walls built with insulating concrete forms (ICFs). But these producers aren't just providing the concrete. They're selling ICF systems and concrete to contractors who do residential and light commercial work.
Benefits spur market growth
In the residential market alone, the number of homes built with above-grade ICF walls doubled from about 2,000 in 1995 to about 4,000 in 1996; the Portland Cement Association's promotional efforts aim for 80,000 above-grade ICF homes by the year 2000. The ICF wall systems' superior insulation, fire and wind protection and soundproofing contribute to the growing demand by homeowners. These same benefits are also attracting business owners.
Mix design and placing equipment
As suggested in ICFA Codes and Standards Committee guidelines, concrete mixes for placement into ICFs should meet requirements of ACI 332, Guide to Residential Cast-In-Place Construction, or ACI 318, Building Code Requirements for Structural Concrete. Slump is usually 4 to 6 inches, the higher value for forms with narrow cavities.
Most mixes require 3/8-inch maximum-size coarse aggregate and a higher sand content, which increases water demand. A mid-range or high-range water reducer can offset some of the water demand, but producers should design mixes to avoid excessive bleeding and segregation. This may include adjusting the ratio of fine to coarse aggregate or increasing the cement, fly ash or fine sand content. If the high-range water reducer dosage is too high, setting time increases, causing higher form pressure that may lead to blowouts.
Placing concrete in above-grade walls requires the right placing equipment--usually a ball-valve or 1-inch-minus small-line pump or a boom pump. For small-line pumps, 2-inch-diameter are easier to handle, helping workers to control concrete flow into form voids. Boom pumps should be equipped with reducers that decrease hose diameter to 2 or 3 inches, slowing concrete flow and minimizing form pressure. It's also important to use two 90-degree elbows at the end of the discharge hose to break the long fall of the concrete.
Educating the customer
Producers who sell insulating concrete forming systems need to educate their customers in two major areas--setting the formwork and placing the concrete.
Formwork setting. Builders should check several items before the pour, including:
All formwork measurements. Are wall openings in the right locations, and do they match plan dimensions? Are walls level, plumb and square?
Proper steel placement. Rebar placement should meet ACI 332 or ACI 318 requirements. Dowels protruding from a concrete footing or foundation wall must be positioned so they're in the center of the ICF wall above. For some systems, the dowels must also be in the center of the correct vertical cavity within the forms. Installers can position the dowels by holding them in place with wire before pouring footings or foundations, pushing them into position in the fresh concrete or drilling holes in green concrete after installing the first course of formwork. The last option allows the most accurate rebar positioning within the center of ICFs' vertical cavities, but also requires an adhesive to bond rebars in the hole. Horizontal rebar is typically spaced at 48-inch intervals.
First-course levelness. On a hardened concrete surface, shim bottom gaps in ICFs and shave uneven top surfaces. When installing forms on a fresh concrete footing, press them about 1/4 to 3/4 inch into the wet concrete, aligning each section's top edge to a stringline (this method is used only for short ICF units such as planks and blocks).
Formwork bracing for proper installation. Most manufacturers recommend bracing at corners (using 2x6s or 2x8s) and at 15- to 20-foot intervals along walls (using 2x4s). Bucks made of 2x10s or 2x12s are essential for door and window support during concrete placement; these also serve as frames. Bracing should remain in place for 24 hours after concrete placement.
Lintels (spans atop openings) for correct placement.
All sleeves and fasteners to be embedded in concrete for proper placement.
Formwork itself for gaps, weak spots, irregular joints or damage.
Pouring the concrete
After the forms are set, contractors also need to know the unique requirements for placing concrete in ICFs.
Pour pattern. Concrete placement should begin under each windowsill until the concrete reaches the bottom of the sill or a height of 3 feet, whichever is lower. Then, starting in a corner, the contractor should move around the wall perimeter in a clockwise direction, placing concrete in 3- foot lifts between windows.
Consolidation. During concrete placement, workers on the ground should consolidate concrete by using hammers to tap 2x4 pieces placed flat against the wall. Some ICF manufacturers recommend using immersion vibrators to consolidate concrete, while others prohibit their use because of possible form damage.
Excess concrete. Workers should check for concrete seepage, removing any with a trowel, and smooth and level concrete at the roof line to eliminate high spots that might interfere with roof placement.
Avoiding blowouts. Many manufacturers say limiting lift depth to 4 feet minimizes the chances of blowouts. Blowouts usually occur not at panel or block joints but at mid-panel, where form walls are weakest.
Filling voids. To detect voids, a worker should tap formwork during concrete placement. A hollow sound in an otherwise solid wall indicates concrete has bridged over a void. Pushing a piece of strapping or rebar up and down into the form dislodges the concrete and fills the void.