Polished concrete is strong, durable and low maintenance; it can have an earthy texture of stone with a polish equal to granite and here is how it is done.
Polished concrete is similar to traditional polished terrazzo which is very flat and poured as a special mix to make the finish more successful. Polished concrete floors can also look almost bumpy and have little or no aggregate showing. Flatness and polishing are not necessarily the same thing which is a good point to clear up before agreeing to your project.
It costs extra time and diamond wear to cut floors with a concrete grinder until they are flat. They can either be ground flat to fully expose the aggregate like terrazzo, or the aggregate can be partially exposed, or the concrete grinding can expose no more than the fine sands at the surface. It often takes skill and experience to control the process of polished concrete floors and the quality of the concreter’s original laying work is also a contributing factor in the quality of the finish.
Where the aggregate is a feature special materials can be added to the wet concrete mix such as coloured pebbles, metals and glass to enhance the final appearance.
Most people are unaware that there are ten to fifteen steps for polished concrete floors which takes a long time to complete and can be quite expensive, but over the normal life of a floor it is often the lowest cost option because there is little maintenance and costly coverings replacement is not required.
Typically it takes many grinding passes to finish a highly polished floor although there are other ways as explained later. The general rule is to double the diamond grit size under the concrete grinder for each pass so a contractor might start with very coarse, 16/20 grit size diamonds, then use 30/40 & 50/60 followed by 120, then start again with a 50 grit diamond resin pad instead of a metal segment. Using the resin pads the steps may be 100, then 200, 400, 800, 1500 and finally 3000 grit. That would be ten separate grinding passes which does not count the two other essential steps.
Two more essential steps:
1. Hardening the surface
Polished concrete floors usually have the surface hardened with a chemical before the second, third or fourth grinding pass. The chemical soaks into the floor to a few millimeters (up to half an inch) and causes a chemical reaction to take place that fills the capillaries and makes the floor harder and easier to polish to a high finish. These floors are very strong and durable without having a surface coating. Even this process may be completed twice.
2. Filling holes
After the first concrete grinder pass removes the top layer of concrete paste it will expose millions of tiny air holes. If these are not filled before the hardening process then the final polished concrete floor will show these unsightly imperfections. The holes are usually filled with an acrylic adhesive mixed with either the grinding dust or cement powder or preferably both which is hand scraped across the floor using a trowel. The preferred way to fill the holes is to spray the acrylic adhesive ahead of the grinder so that the diamonds mix it into the holes with the grinding dust on the third or fourth pass at around 120 grit, but it may be done earlier as well.
Very flat or slightly bumpy polished?
Some floor grinder machines are designed to produce a very flat floor by cutting off the high spots while some others are designed to follow the contours of the floor more. If the surface is to be polished without necessarily exposing the aggregate then only the finer resin pads need to be used on a concrete grinder that allows for 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 aggregates within the concrete which is faster and less expensive. Hardening can still be an advantage to help the durability of the surface and to produce a final gloss. Situp Grinders and Situp Polishers must be used to match the edges with the main floor.
Problems with exposed aggregate
Sometimes the finish of exposed aggregate can be uneven if the mix of concrete was poured unevenly or finished off poorly. Boot marks or kneeling board marks can appear suddenly because they have pushed the aggregate down further which might require grinding down another two or three millimeters which will cost more than was quoted. Grinding this far may not be what the customer wanted either so it can be a risk without any way of telling beforehand.
Single head or multiple head machines
Original terrazzo grinding was done with single head floor grinders until the production of three-head planetary machines. Planetary means that each head turns one direction while the turntable that houses the heads turns independently in either the same direction or the opposite direction. Some grinders can vary the direction of both the turntable and the heads and some can vary the speed of each. There are also multiple head planetary machines with four or more heads some of which may turn in the opposite direction for extra stability.
The planetary heads can follow the contours better than single or twin head grinders and are faster to use with less effort due to eliminating 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.
As with wood floors the edges must be finished separately to the main floor area because the large machines may bump and damage the walls if they are used too closely.
For the first grinding passes a nine inch angle grinder is used having a diamond wheel attached with a dust extraction shroud fitted to remove the dust. The sit-down-to-use edge grinders are more controllable than stand up grinders while kneel-to-use grinders have good control, but are exhausting.
After the first two or three cuts a seven inch polisher is used to polish with resin pads and once again the sit-down-to-use polisher system is faster and easier than kneeling to polish.
Simple polished concrete floors “look”
The number of processes for polishing 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 the 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 coat with a sealer. To remove all the grinding marks it is better to use a ceramic bond wheel. Polyurethanes are very hard sealers with a high gloss and can be purchased as UV stable to stop yellowing (usually double the price of non UV stable urethane), clear epoxy sealers are not as smooth and will chalk and deteriorate if exposed to sunlight and acrylic sealers have a much shorter lifespan due to their poor resistance to wear. Two coats are necessary to provide a high gloss level.
Summing up there are different ways to achieve “polished concrete”
- The first is to cut the floor with coarse diamonds until all the high spots have been removed to produce a flat floor and then use all of the 12 to 15 steps to eventually polish it or as it is sometimes called, hone it. This is the true polishing system and a very flat floor.
- A variation of the above is to cut perhaps half of the height of the higher peaks by starting with a medium coarse diamond grit and proceed through to polishing using a machine that can ride up and over small rises which is a little less expensive and does not show the aggregate as much.
- A third alternative way of producing a honed surface is to start with relatively fine diamonds and simply polish to 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 and is the method preferred by the big box stores such as Bunnings, Masters & Home Depot.
- Lastly, a “polished look” can be obtained by grinding with coarse diamonds and then fine diamonds before coating over with a clear sealer.
Only truly polished floors will retain their gloss with little maintenance because all coatings scratch with wear and lose their high shine. Special buffing pads on a standard floor polisher that contain very fine diamonds can be used to maintain cleanliness and the high gloss levels of true polished concrete. Janitors can be provided with these and keep the floors in great condition without special treatment.
Some hire companies provide the grinding equipment for do-it-yourself concrete polishing and have all the pads for the purpose. This may not be so economical though because the pads may be only half worn when you finish depending on the area involved compared to a contractor who will continue to use the diamond tooling on the next job.