Steel reinforcing bar around openings is a critical structural element of cast-in-place concrete walls, whether they are built with removable forms or insulating concrete forms (ICFs). Correct placement of the rebar helps prevent the concrete around openings from cracking from structural loads or shrinkage.
The International Residential Code specifies reinforcement schedules around openings in houses built with cast-in-place walls when the opening exceeds 2 feet in width. The exact requirements depend on the forces acting on the structure, calling for a minimum of one vertical bar along each side of each opening, within 12 inches of the edge. The bar diameter is equal to the diameter used for the other vertical reinforcement in the wall.
Horizontal reinforcement is required across the top and bottom of each opening that is 2 feet or more in width. The horizontal bar over the top creates a structural lintel. In all cases these horizontal bars must extend at least 24 inches beyond either side of the opening. The minimum reinforcement is one #4 bar with its center between 1½ inches and 2½ inches from the edge of the opening to ensure adequate concrete cover.
For wider openings and greater loading conditions, bar size may increase, two bars may be required, and S-shaped stirrups may be necessary between the top and bottom bar. With two bars, one is placed above the other. Depending on the loads placed on the lintel, a depth of anywhere from 8 inches to 24 inches may be needed. The center of the top horizontal bar must be between 1½ inches and 2½ inches from the top of the lintel. Any stirrups required are shaped from a minimum #3 rebar. The spacing of the stirrups is determined by the depth of the lintel and must not exceed half of the lintel depth (D) minus the bottom concrete cover.
Some builders also have added a short diagonal bar at each corner. These provide extra protection against cracks forming at the corners, but usually are not required.
All bars must have concrete cover in accordance with ACI 318. Typically this is 1½ inches for bars in concrete exposed to weather but can be reduced to ¾ inches when not exposed or where the forms stay in place. They must be held securely in position to avoid shifting during concrete placement. Usually this is achieved by wiring or zip tying them to the form ties.
Where vertical bars are dropped into completed formwork, they are threaded into various pre-installed holding devices below and tied at the top. This may be done with a “collar,” (a short length of PVC pipe threaded over a dowel), threading the verticals between offset horizontal bars, or into an indentation in the concrete below that is alongside each dowel. Because some officials object to a collar resting on the foundation below, it may be lifted and wired to the dowel a foot or more up instead.
— Pieter VanderWerf is president of Building Works Inc. (www.buildingworks.com), a consulting firm that helps companies introduce new construction products. The Portland Cement Association and the Concrete Foundations Association provided information.