This very desirable polished concrete finish is permanent without having a coating on the surface that will eventually wear through. Concrete polishing can be done to the same degree as granite where it is used in showrooms, homes and industrial floors.
Many large commercial premises such as hardware chains use polished concrete that has the surface polished without showing the stones underneath while more colourful, decorative and interesting effects can be obtained by cutting down into the stones and polishing the surface.

In this guide you will learn
 

1. About grinding and polishing concrete using diamonds
2. All the steps involved
3. Problems to watch out for
    

 

Action Steps
The best contacts and resources to help you get it done

How diamonds work to grind concrete
Diamond grinding is the best way to cut into the surface of concrete. The diamonds are mixed with metal powders in a mold and melted together under pressure in a furnace to produce a segment. These blocks or segments are welded onto a metal grinding plate that is driven by a motor and scrapes across the surface of the concrete. The sand in the concrete wears away the metal to expose the diamonds which then cut into the concrete. During this process the diamonds are worn away and fracture, but at the same time the metal is worn a little more to expose more of the diamonds until all the segment is completely worn away and then the diamond grinding disc will have to be replaced. The metal surrounding the diamonds can be soft (for hard concrete) or hard (for soft concrete) and the size of the diamonds can vary from large 16 grit to extremely fine 5,000 grit. The coarse diamonds are used to cut the concrete quickly and fine diamonds are for polishing.

I recommend: For a more detailed explanation of grinding concrete see "Grinding Hard Concrete".
 

Grind the floor to flatten the surface
The first step for polished concrete is to use a "planetary" head concrete grinder that has three or more grinding discs underneath spinning in one direction and a turntable that spins in either the same or the opposite direction to cut the concrete to the desired flatness by removing the higher areas. This can also be done with a single head grinding disc although more skill is required to maintain a flat look without grinding marks which are commonly called "shoulders". Use a coarse diamond grit from 16 to 60 depending on how aggressively you wish to cut. 16 grit would be more common if a coating was being removed and the lower the grit number (more coarse) the more grinding that needs to be performed with higher grit sizes to smooth out the scratches.

I recommend: Larger machines are used for the bulk of the floor and special edging grinders are used for the wall edges and for small rooms. For a comparison of edging machines see "Edge Grinders".
 

Concrete grinding to expose the aggregate
Continue with the concrete grinding process until the desired look is achieved with respect to exposing the aggregate. The floor can be ground to expose the sands near the surface, or to partially show the stones or to cut into the stones to reveal more aggregate than concrete. When laying the concrete if this is a new project various colours can be selected for the aggregate and also for the concrete to enhance the appearance of the ground floor. Even objects like metal and glass can be added for greater visual effect.

I recommend: A good source of polished concrete ideas can be found at "Polished concrete photos".
 

Fill the air holes for a beautiful finish
Many people do not realize that concrete has millions of small and tiny air holes throughout which are revealed when the top paste of the concrete is removed by grinding. These need to be filled for a uniform and dense polished concrete surface finish which can be done by hand using a polymer cement mixture or a polymer similar to tile glue mixed with the grinding dust from the first grind which is applied with a trowel across the entire floor surface working it into the holes without leaving very much on the surface. Some floor grinders have spray devices to wet the floor ahead so that the grinding dust mixes with the polymer as the grinding process proceeds which eliminates the labour of hand troweling. This should be left to dry overnight or for a day before grinding again.

I recommend: If you want to do it yourself obtain some white tile adhesive and mix it with 10 parts of water to prime the surface. Then mix the neat adhesive with either cement or concrete dust until you have a flowing mud consistency which you can trowel across the surface.
 

Grind again then harden the concrete for greater durability
After filling the air holes the floor can be ground again with finer diamonds and then it should be "hardened" or "densified" with a chemical flooded across the floor. Most of these hardeners react with the chemical makeup of concrete to change the structure of it to a harder material than the original. Some require one application and others require two which may include a water flooding process. The object of hardening is to achieve a denser, more uniform polished surface with greater wearing characteristics. This should be left for around 24 hrs before continuing the grinding steps towards polishing.

I recommend: If you harden the floor yourself use a very wide broom to sweep the liquid across the floor evenly walking in one direction all the time.
 

The polishing process
By now the floor will have been ground three or four times doubling the diamond grit size each time which will continue until the desired polish sheen is achieved. Starting with 16 grit, the next would have been 30 grit, then 60grit. With all the air holes filled and the floor hardened all the grinding steps are now performed until it is polished. The metal bond diamonds can continue to be used to 120 grit after which resin bond diamond pads are used. These are pads or buttons of resin with the diamonds mixed into them. The grinding passes will start again at 50 grit, then 100, then 200, 500, 1,000, 2,000 and 5,000. Between each pass the floor and the grinding equipment must be thoroughly cleaned so that the larger grit diamonds do not scratch the floor when a finer diamond pad is used. Edges must also be polished with each grit size and are done before each floor pass. After 5,000 grit the floor should have a magnificent polished appearance that is hard, durable and easy to clean.

I recommend: For more information see "Polished Concrete".
 

Things to look for when considering polishing a floor
Structural cracks that go all the way through a floor are usually caused by movement of the soil underneath. These will always move with the varying moisture content of the soil and the expansion and contraction of the concrete with changes in temperature unless the substrate is stabilised. Small surface cracks due to shrinkage of the concrete will be filled and will remain unnoticed. If the floor is obviously poorly laid you should expect problems like indentations from boots that have been trowelled over later, but show up when the floor has been ground back. Grind your slab early because concrete hardens quickly and becomes slower and more difficult to grind so for a new home it is better to gind, fill and harden, then take it to a low shine in the first week. The polishing work can be finished later when the house has been erected. Do you need to see the aggregate? It is more expensive and time consuming to expose the aggregate.

I recommend: If a floor is in bad shape it is usually better to remove it and lay another floor. Patching a bad slab may not produce the quality of finish that is expected.

 

27/8/09 07:00 AM

 

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