How to Polish Concrete

Polishing is similar to traditional terrazzo which is very flat and poured with a special mix to make a more decorative finish. Polished concrete floors can appear bumpy and have little or no aggregate showing.

Keep in mind before considering a project, that flattening and polishing are not necessarily the same thing.

Extra time and diamond wear are needed to shave floors with a concrete grinder until they are flat. They can be ground flat to expose the aggregate like terrazzo, the aggregate can be partially exposed, or the grinding can expose just the fine sands at the surface. It normally takes skill and experience to control the process of polishing floors. The quality of the floor installation plays an important role in the quality of the finish. Special aggregates such as colored pebbles, metals, and glass can be added to the wet concrete mix to enhance the final appearance.

What most people don't realize is there are ten to fifteen steps involved in polishing concrete floors which is time consuming and can be rather expensive.

It typically takes many passes with a grinder to finish a highly polished floor. As a general rule, the diamond grit size is doubled for each pass. The process usually begins with very coarse, 16 or 32 grit size diamonds, followed with 60 grit diamonds and 120 grit. The process then switches to using diamond resin pads, starting with 50 grit. The passes with resin pads may include 100, 200, 400, 800, 1500 and finally 3000 grit. That's ten separate grinding passes, but there are two other important steps.

• Hardening the surface

  Concrete densifiers or hardeners are applied to the floor before the third or fourth pass. They seep into the pores and react with the free lime to create a gel that fills the pores and capillaries. The gel eventually hardens adding strength and durability and eliminating the need for a coating or sealer.

• Filling holes

  The first pass with the grinder removes the top layer and exposes millions of tiny air holes. These should be filled before the hardening process to prevent unsightly imperfections in the final finsih. The holes can be filled with an acrylic adhesive mixed with the grinding dust or cement powder. The paste is applied to the floor with a hand trowel. A faster method is to spray the acrylic adhesive ahead of the grinder, letting the diamonds work it into the holes on the third or fourth pass.

Super Flat Versus Slightly Bumpy Finish

Some floor grinders will produce a very flat surface by shaving down the high spots while others will follow the contour of the floor. If exposing the aggregate isn't the goal, then only the finer resin pads need to be used, which allow for movement to follow the contours of the surface. This will result in a polished floor that doesn't reveal the pattern and texture of the aggregates in the concrete mix. Obviously this is faster and cheaper but still achieves the high gloss finish.

Problems with Exposing Aggregates

Exposing the aggregates can create a problem if the concrete mix was poured unevenly or finished poorly. Boot marks and knee board marks may show because the aggregates were pushed down. Eliminating this will require furthur grinding which only adds time and cost. The possibilty must be considered before beginning the project.

Single Head or Multi-head Grinders

In the past, terrazzo finishes were done with single head floor grinders until three-head planetary machines came along. Planetary means each head turns while the turntable that houses the heads turns as well, in either the same or opposite direction. Some grinders can vary the directions of both the turntable and the heads and some can vary the speeds.

The planetary heads follow the contours better than the single and twin head grinders. They also work faster with less effort because you don't have to move the machine side to side and in circular motions. Single head machines must be moved in a circular motions to avoid grinding lips or shoulders.

Edging

Edges are finished separate from the rest of the floor because the large machines can't grind all the way to the wall. A nine inch angle grinder with a diamond wheel is most commonly used. A dust extraction shroud can be attached to capture the dust. The sit-down-to-use edge grinders are easier to control versus stand up grinders, while kneel-to-use grinders are easy to handle but can be exhausting. After two or three passes, a special dust extraction shroud with a corner feature is used with a seven or five inch grinder to polish with resin pads and finish in the corners.

Knowing how to polish concrete is a skilled trade. Experience is required to know when to change discs and pads. It's not usually recommended as a do-it-yourself project. Although I don't like to discourage people from trying something themselves, the equipment is expensive to rent and the diamond pads are only available for purchase.