Polished concrete flooring has become a popular option for commercial and industrial venues, especially ones that want to minimize maintenance. Polished concrete is extremely durable, can be buffed to various degrees of shininess and can be decorated in different colors and styles. Maintenance requires a daily dry dusting with a microfiber pad and weekly washing with water, perhaps mixed with some pH-neutral floor cleaner, and applied with soft pads. The surface need not be sealed or resealed as long as it is properly prepared and maintained.
Concrete is a mix of Portland cement, water and aggregate, often augmented with admixtures for particular applications. Cement is a mixture of various oxides –calcium, silicon and aluminum — that result from the heating of limestone with clay and grinding the result with gypsum, a source of sulfate. Aggregate can consist of various chunky materials, such as gravel, granite, glass, black basalt mix and crushed limestone, often mixed with sand and other fine materials. The dry materials are mixed with water and the composite goes down wet, curing to a hard surface. Various admixtures and reinforcements change the concrete’s properties, such as color, drying rate or tensile strength. Pouring concrete correctly requires great expertise to avoid air bubbles and to create a perfectly level surface with occasional expansion joints to prevent cracking.
Preparing Concrete for Polishing
After a concrete floor has cured, it receives an application of a liquid concrete densifier prior to polishing. On older floors, any previous sealers or curing compounds should first be removed. The job of the densifier is to harden the porous concrete’s surface, increase its water resistance, improve shine, increase stain resistance and increase its hardness. First-generation densifiers use silicates that react to concrete’s calcium hydroxide, but are caustic and labor-intensive. Second-gen densifiers use lithium silicate colloids containing nano-sized particles that penetrate into the concrete. The material acts as a pozzolan — a cement-like material — that binds to the lime in the concrete to strengthen its surface. Densifiers can do their work in a couple of hours, but will continue to harden the concrete for a month or two.
Polishing the Concrete
In a method similar to sanding wood, the densified concrete is ground with progressively finer grits, not of sandpaper, but of diamond polishing pads. These are metallic pads holding a matrix of diamond segments. Generally the progression ranges from 16 or 30 grit up to as high as 1,500, but a higher level of shine is available by going to a 3,000 grit or higher. Densifier is added between grits, as sometimes is a grouting chemical to fill any cracks or imperfections. Dry grinding raises potentially dangerous dust. Wet grinding with water fixes this problem, though the crew has to dispose of the slurry. Workers where special protective masks when dry grinding. A high polish is achieved using plastic or resin disks with very fine diamond grit. Optionally, the contractor might use a polishing compound with the final grit to remove residues and boost the shine.
The cost of polished concrete compares favorably with those of other types of floors, especially when you factor in the low long-term maintenance costs. Prices per square foot can range roughly from $3 to $12 or more, depending on the ultimate shininess, consistency of look and any special treatments to impart colors or designs. Some design ideas include scoring the surface and creating grids, radial lines, colored bands and special borders. Polished concrete floors are impervious to humidity, are naturally resistant to microbes and allergens, and can radiate heat and store solar energy. They are often used in LEEDS-certified projects.
Old concrete floors that are damaged or stained can receive a polished overlay to cover up the problems. The overlays can be seeded with colored aggregate or receive decorative sawcuts. The process may require sandblasting and grinding the existing floor, especially if residual mastic is present. Installing a polished overlay is a demanding process requiring expertise and tight coordination.