Polished concrete is strong, durable, and low-maintenance, while having that earthy stone texture with a granite-like polish. This is how you do it.
It is similar to traditional polished terrazzo, which is very flat and is poured as a special mix to make the finish more successful. Polished concrete floors can also look almost uneven and show little or no aggregate. Flatness and polish are not necessarily the same thing, which is a good point to clarify before accepting your project.
It takes extra time and diamond wear to cut floors with a concrete grinder until they are flat. They can be ground flat to fully expose the aggregate such as terrazzo, or the aggregate can be partially exposed, or concrete grinding cannot expose more than fine sands on the surface. Skill and experience are often needed to control the polished concrete floor process, and the quality of the concrete mixer’s original placement work is also a contributing factor to the quality of the finish. When aggregate is a feature, special materials such as colored pebbles, metals, and glass can be added to the wet concrete mix to enhance the final appearance.
Most people don’t know that there are ten to fifteen steps to polished concrete floors, all of which take a long time to complete and can be quite expensive.
It usually takes many passes of grinding to finish a highly polished floor, although there are other ways, as explained later. The rule of thumb is to double the diamond grit size under the concrete grinder for each pass, so a contractor might start with very coarse, 16 or 32 grit diamonds, then use 60 grit diamonds in a row. of 120 and then start over with a 50 grit diamond. resin pad instead of a metal segment. Using the resin pads, the steps can be 100 grit, then 200, 400, 800, 1500, and finally 3000. That would be ten separate grinding passes that don’t count the other two essential steps.
Two more essential steps:
1. Surface hardening
Polished concrete floors are usually surface hardened with a chemical prior to the second, third or fourth grinding pass. The chemical penetrates the floor up to a few millimeters (up to a half inch) and causes a chemical reaction that makes the floor harder and easier to buff to a high-quality finish. These floors are very strong and durable without having a surface coating.
2. Hole filling
After the first pass of the concrete grinder removes the top layer of concrete paste, millions of tiny air holes will be exposed. If they are not filled before the hardening process, the final polished concrete floor will show these unsightly imperfections. The holes are usually filled with a type of acrylic tile adhesive product mixed with grinding powder or cement powder that is hand-scraped across the floor with a trowel. The preferred way to fill the holes is to spray the acrylic adhesive in front of the grinder so that the diamonds are mixed into the holes with the grinding powder on the third or fourth pass at around 120 grit. This method is quicker and the powder matches the color of the floor to hide holes better than when using powdered cement.
Very flat or slightly polished?
Some floor polishing machines are designed to produce a very flat floor by cutting off high spots, while others are designed to more closely follow the contours of the floor. If the surface is to be polished without necessarily exposing the aggregate, then only the finest resin pads should be used on a concrete polisher that allows movement of the pads so they can follow the contours of the surface. This will result in a polished floor without showing the pattern and texture of the exposed aggregate within the concrete, which is faster and less expensive. Hardening can still be an advantage in aiding surface durability and producing a final gloss.
Problems with exposed aggregate
Sometimes the finish of exposed aggregate can be uneven if the concrete mix was poured unevenly or finished poorly. Boot marks or knee board marks can appear suddenly because they’ve pushed the aggregate down, which could require another quarter-inch (2-3 millimeters) polishing, costing more than quoted. Grinding this far may not be what the customer wanted either, so it can be a risk.
Single head or multi head machines
The original polishing of terrazzo was done with single head floor polishers until the production of three head planetary machines. Planetary means that each head rotates in one direction while the turntable housing the heads rotates independently in the same or opposite direction. Some grinders can vary the direction of both the turntable and heads and some can vary the speed of each. There are also multi-head planetary machines with four or more heads.
Planetary heads can follow contours better than single or double head grinders and are quicker to use with less effort because they eliminate the requirement to physically move the grinder from side to side or in a circular motion. Single head terrazzo grinders should be moved in a circular motion to avoid grinding lips or shoulders.
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As with hardwood floors, edges should be finished separately from the main floor area because large machines can hit and damage walls if used too close. For the first few grinding passes, a nine-inch angle grinder is used that has a diamond wheel attached with a dust extraction hood installed to remove dust. Seated edge grinders are more controllable than stand grinders, while knee grinders have good control, but are tiring.
After the first two or three cuts, a different dust extraction hood with a corner function is used with a seven-inch polisher or five-inch multi-speed grinder to polish with resin pads and tighten the corners.
Simple “look” polished concrete floors
The number of processes to polish concrete can be reduced by up to 60% and still achieve an appearance similar to grinding and then coating with a clear sealer. The first step is grinding with 30/40 grit diamonds to expose the aggregate, then filling all the small air holes as described above (in 10 to 15 steps…) before a second grinding with 60/80 grit. 80/ 100 grit diamonds.
This will produce a surface that is smooth enough to cover with a sealer. Polyurethanes are very hard sealants with a high gloss and can be purchased as UV stable to prevent yellowing (typically twice the price of non-UV stable urethanes), epoxy sealers Clear are not as smooth and will chalk and deteriorate if exposed to sunlight and acrylic sealers. they have a much shorter service life due to poor wear resistance. Two coats are required to provide a high gloss level.
Different ways to achieve “polished concrete”
The first is to coarsely diamond cut the floor until all the high spots have been removed to produce a flat floor and then use the 12 to 15 steps to finally grind it, or as it is sometimes called, buff it. This is the true polishing system.
A variation of the above is to cut perhaps half the height of the highest peaks starting with a medium coarse diamond grit and continue polishing using a machine that can go up and over small elevations.
A third alternative way to produce a polished surface is to start with relatively fine diamonds and simply polish to the top of the surface after hardening without removing much of the top cement paste. It is easier to do this when the floor has been laid flat and smooth.
Lastly, a “polished look” can be achieved by polishing with coarse diamonds and then fine diamonds before covering with a clear sealer.
Only truly polished floors will retain their shine with little maintenance because all floor coverings scratch with use and lose their high gloss. Special polishing pads can be used in a standard floor polisher that contain very fine diamonds to maintain the cleanliness and high gloss levels of true polished concrete. Janitors can count on these and keep floors in excellent condition without special treatment.
Some rental companies provide the grinding kit for you to sand the concrete yourself and have all the pads for that purpose. However, this may not be as cheap because the pads may only be half worn when you’re done, depending on the area involved.
