Reinforced concrete floor decks have become increasingly popular in homes over the last decade. In part, this is because consumers' interest in their advantages (strength, rigidity, durability, sound attenuation, and suitability for in-floor radiant heating and decorative concrete finishes) has steadily increased. The growth in homes with concrete walls is a second factor because these walls provide adequate support for the heavier concrete floor without extra measures.Many of the floor systems used are adapted from commercial construction. However, some newer systems have been developed with lighter residential construction in mind. This has decreased costs and also contributed to concrete floors' popularity.
Composite steel bar joists
For more than 30 years, a system based on welded steel bar joists has been popular because of its flexibility and efficient use of labor and materials. The proprietary component is a steel bar joist, which includes a deformed steel top chord that extends vertically upward about 2 inches, L-shaped steel bottom chords, and a web of steel bars welded to the chords. The wide spacing of the bars leaves the joist webs mostly open. The contractor orders the joists to length and in the proper depth. Spans of 40 feet are routine, and spans up to 65 feet are possible with deep joists and a thicker concrete cover.
The crew sets the joists on the walls with a spacing of slightly over 4 feet on center. Special "roll bars" go between the joists to hold them steady and support plywood on top. Standard sheets of plywood are set on top of the roll bars, filling the space between joists. The top chord of each joist protrudes above the plywood. The crew drapes a layer of welded wire mesh over these top chords and casts about 3 inches of concrete on top. The concrete encases the top flanges and the mesh creating a composite structure. Both of these types of embedded steel reinforce the concrete.
After curing, workers remove the roll bars and plywood from below. Utilities can run relatively free through the joists. To create a finished ceiling, a hat channel can be wired to the bottom of the joists and wallboard screwed to the hat channel.
Composite cold-formed steel joist
A newer version of the composite bar joist system uses joists made of a single piece of cold-formed steel instead of heavy steel pieces welded together. Besides the different joist, installation is nearly identical. The main flanges of the joists are punched with large holes for utility pass-through. This system can have its advantages: It may be easier to cut and alter the joists in the field, the cold-formed steel provides a more familiar product for residential crews, and it may be less expensive in shorter spans.
In the 1990s, vendors released deck forms made of foam with embedded light-gauge steel joists. Concrete floors created with these forms tend to be more expensive than the composite steel joist floors, however, the forms are easy to cut and modify in the field. Finishing ceilings with this system is easy-simply attach wallboard to the bottom of the steel joists. The insulating value of the foam may also be useful in maintaining different heating/cooling zones.
The deck forms consist of 2-foot-wide sections. The supplier may cut these to specified length before shipping to the job. Each section contains two steel joists running lengthwise, embedded entirely in the foam. Deep depressions in the foam surface between the joists, also running the length of the section, form beam pockets.
The sections are cut to fit within the walls and are supported by a line of bracing installed underneath at least every 6 feet. Rebar is set on chairs in the beam pockets and lapped with rebar in the walls. Mesh is set on chairs and placed over the entire floor area 1 to 2 inches above the top of the forms. Concrete is cast on top to a depth of about 3 inches over the top of the forms, encasing the rebar in the beam pockets and the mesh. The bracing comes out after curing.Some brands of forms have preformed chases that run the length of each section, where the trades can run utility lines. Alternatively, chases can be cut into the foam.