An exposed concrete slab can be an attractive finished flooring surface. Usually when a slab will be used as a finished floor it must be troweled to a smoother finish than a slab slated for use as a subfloor. Smooth-surfaced concrete requires the expertise of an experienced trades — person. When it is first poured, the concrete will have more water than it needs to complete the setting reaction. Some of this extra water will come to the surface. If the concrete is finished before the extra water has fully migrated upward, it will become trapped in the upper layer of the concrete, resulting in a weakened surface that will release concrete dust into the indoor environment. The window of opportunity for properly smoothing the surface is short. If too much time has elapsed, the surface will have set and will remain rough.
For exposed finished slabs, control joint locations must be carefully planned for their esthetics, and often more joints are added to minimize random cracking. Color is often added, either mixed into the wet concrete or applied after the concrete sets. Because the slab is usually poured near the beginning of the construction, it must be kept covered during the course of construction to ensure that it is not damaged or stained. All of these factors will add to the price of the concrete work, but the process will still be cost-effective when the price of covering the concrete with another material is considered.
Both the coloring agents and the surface treatment must be selected carefully to achieve a healthy finish. One process uses muriatic acid to stain floors. Although the finished floor will not outgas, muriatic acid is a highly toxic chemical that requires protective equipment for the installers, and the wash-off is considered hazardous waste that must be properly disposed of.
More benign methods for staining or coloring exposed slabs may be specified:
• Do not use aniline-based coloring agents.
• Use only high-quality mineral pigments such as Chromix Admixture and Litho — chrome Color Hardener or Davis Colors. Verify with the manufacturer that the selected color is free of chromium and other heavy metals.
• PureColor is a two-stage stain formulation of pure mineral ions and oxygen catalyst for concrete surfaces. Colors can be custom mixed.
• A solution of iron sulfate (fertilizer) reacts with the concrete, producing yellow,
orange, red, and brown stain hues depending on the chemical composition of the concrete.
Acceptable sealers are listed in the next section.
