Polished concrete is strong, durable and low-maintenance, while having that earthy texture of stone 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 to no aggregate. Flatness and polish are not necessarily the same thing, which is a good point to clarify before accepting your project.
It costs 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 the fine grits on the surface. Skill and experience are often needed to control the polished concrete flooring process, and the quality of the 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 are time consuming to complete and can be quite expensive.
Typically, many grinding passes are required to finish a highly polished floor, although there are other ways as explained below. The general 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 size diamonds, then use 60 grit diamonds followed by 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 not counting 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 pass of grinding. The chemical penetrates the floor 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. Filling holes
After the first pass of the concrete grinder removes the top layer of concrete paste, millions of tiny air holes will be exposed. If not filled before the hardening process, the final polished concrete floor will show these unsightly blemishes. The holes are usually filled with a type of acrylic tile adhesive mixed with emery 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 blend into the holes with the grinding powder on the third or fourth pass at about 120 grit. This method is faster and more the powder matches the color of the floor to hide holes better than using powdered cement.
Very flat or slightly polished?
Some floor sanders are designed to produce a very flat floor by cutting 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 the 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 the aggregate has been pushed down, which may require another two or three millimeters (a quarter inch) of grinding, which will cost more than quoted. Grinding this far may not be what the customer wanted either, so it may be a risk.
Single or multi-head machines
The original polishing of terrazzo was done with single-head floor grinders until the production of three-head planetary machines. Planetary means that each head rotates in one direction while the turntable that houses the heads rotates independently in the same or opposite directions. Some grinders can vary the direction of both the turntable and heads and some can vary the speed of each. There are also multi-headed planetary machines with four or more heads.
Planetary heads can follow contours better than single or dual 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 the lips or shoulders.
border
As with hardwood floors, the 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 blade attached with a dust extraction shroud installed to remove dust. Seated edge grinders are more controllable than standing grinders, while kneeling grinders have good control, but are tiring.
After the first two or three cuts, a different dust extraction shroud with a corner feature is used with a 7-inch polisher or a 5-inch multi-speed grinder to buff 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 a similar appearance by grinding and then coating with a clear sealer. The first step is to grind with 30/40 grit diamonds to expose the aggregate, then fill all small air holes as described above (in 10 to 15 steps…) before a second grind with 60/80 grit or 80/ 100 grit diamonds.
This will produce a smooth enough surface to cover with a sealer. Polyurethanes are very hard sealers with a high gloss and can be purchased as UV stable to prevent yellowing (usually double the price of non-UV stable urethanes), clear epoxy sealers are not so smooth and will chalk and deteriorate if exposed to sunlight and acrylic sealers. they have a much shorter useful life due to their poor resistance to wear. Two coats are required to provide a high level of gloss.
Different ways to achieve “polished concrete”
The first is to cut the floor with coarse diamonds until all high spots have been removed to produce a flat floor and then use the 12 to 15 steps to finally sand it down, or as it is sometimes called, hone it. This is the true polishing system.
A variation on the above is to cut perhaps half the height of the taller peaks starting with a medium coarse diamond grit and continue grinding using a machine that can climb up and over small elevations.
A third alternate way to produce a polished surface is to start with relatively fine diamonds and simply grind to the top of the surface after hardening without removing much of the top cement paste. It is easiest to do this when the floor has been laid flat and smooth.
Finally, a “polished look” can be achieved by polishing with coarse diamonds and then fine diamonds before coating with a clear sealer.
Only truly polished floors will retain their shine with little maintenance because all coatings scratch with use and lose their high gloss. Special buffing pads can be used on a standard floor buffer that contain very fine diamonds to maintain the cleanliness and high gloss levels of true polished concrete. Janitors can count on these to keep floors in excellent condition without special treatment.
Some rental companies provide the do-it-yourself grinding kit for the concrete and have all the pads for that purpose. However, this may not be as economical because the pads may only be half worn when finished, depending on the area involved.