Concrete that reflects light is entrancing people and contractors are polishing it to find the shine. Kitchen countertops, interior floors, and concrete furnishings are candidates. Polishing saves removal cost, corrects problem concrete, and beautifies concrete slabs. Placing concrete, adding color, seeding semiprecious stones, and polishing all create stone-like surfaces that save money. The main variables that affect the polishing process and equipment needed are the condition and function of the concrete, the artistic effect, and the desired reflectivity.
Polished concrete is a systematic mechanical abrasion of the concrete surface; a process of up to eight steps using diamonds embedded in a matrix of metal or resin. The polishing process begins with an aggressive grind using 50- to 80-grit diamond pads, usually mounted on planetary head grinders, which use a large planetary disc traveling in one direction with three or four smaller discs mounted on it traveling in the opposite direction. The smaller discs hold the diamond pads.
Each successive step approximately doubles the grit rating-80 to 160 grit, 800 to 1500 grit-to achieve polished finishes as high as a 3000 grit. Each pass removes surface layers and the scratches of the previous pass, increasing smoothness and shine. The desired reflectivity determines how many passes are necessary. The polishing process can be done either wet or dry. With the arrival of dust control equipment and HEPA filters, indoor dry grinding has become more popular.
Grinding, honing, and polishing are the three levels of abrasion. The process is similar whether one is polishing countertops, furnishings, or floors, although the equipment and technique differs. Moving from heavy industrial floor polishers to working inside homes requires a smaller machine. Hand-held polishers and hand-held pads are useful for smaller work.
The first grind is to flatten the surface and remove the high spots. "The first pass on a floor should not be blindly done," says Brad Padgett, CEO, Concrete Polishing Technology, Norris, Tenn. "If the desired flatness isn't achieved in those first grinds, it will not be gained in the passes thereafter." The first grind can expose fines or expose aggregates depending on the grit.
Honing, using a finer grit, smoothes the concrete removing the scratches left by the preceding grind. The surface achieves a soft glow or stone-like appearance.
Polishing the concrete is done using 800- to 3000-grit pads. "Do not skip grits polishing concrete," says Jeffrey Girard, president, Concrete Countertop Institute, Raleigh, Tenn. "Each grit is designed for one purpose only, to remove the scratches from the preceding grind." A 3000-grit pad will not remove the scratches left from a 50-grit pad.
Achieving the shine
Highly polished and abrasion-resistant concrete has the most reflectivity because it is truly flat at the micro level. Typically one would begin with 80-grit diamonds embedded in a metal matrix. The initial grind does the major work of flattening and removing imperfections. "Plan to spend 40% to 60% of the time on the initial grind," recommends Padgett. The subsequent runs remove scratches but will not correct elevation. After the 80-grit grind there is still a feeling of texture to the concrete. The slab does not shine because light is hitting the slab and refracting; there are too many highs and lows. Proceeding through the grits, 800 to 1500 to 3000, the light begins to reflect instead of refract. As reflection increases so does the abrasion resistance because the highs and lows of the surface are minimized. "Really good reflection means the surface is really flat," says Padgett.