Any construction project can come with an overwhelming array of options. To keep the EVstudio team on top of the choices, we host industry experts to give us the run-down on their specialties for our Lunch & Learn series. We recently had a presentation on one of the most versatile and popular materials used in construction: polished concrete.
Polished concrete is concrete with a densified surface that has been cut, refined, and polished to achieve a smooth, durable finish. This ubiquitous building material is found in almost everywhere, including commercial and residential buildings, offices, schools, shops, restaurants, and hospitals, and is used on floors, walls, ceilings, counter tops, decorative items, and more.
This blog breaks down some of the basics of polished concrete, giving our clients, industry colleagues, and the curious general public alike a chance to learn more.
How It’s Made
Polished concrete is produced with three overarching steps: Grinding the existing concrete, densifying the material, and refining the surface.
Step 1 is grinding the existing concrete until the surface is flat, or until the desired amount of aggregate is exposed. This step is dependent on the condition of the concrete. While diamond tooling is typically used, very rough surfaces require coarser tools to remove more material to achieve a flat floor. Once the floor is sufficiently flat, the surface is refined until it is smooth.
Step 2 is densifying the concrete by adding densifiers that contain silicates that chemically react with the concrete. This step aims to decrease porosity while making the surface denser and harder. Different types of densifiers penetrate and react differently with the concrete. Without a densifier, a polished concrete floor will be usable for a few years before signs of wear begin to show up. Adding a densifier increases impact- and abrasion-resistance of the concrete by 30-40%.
Step 3 is refining the surface of the densified concrete to achieve the specified reflectivity. After the densifier has completed its chemical reaction, the concrete is honed to remove any scratches, then polished with 400 grit or more. Impregnating sealer may also be applied to fill in small pores in the surface, making the concrete more impenetrable to liquids while increasing the sheen.
The degree of surface finishing is dependent on the use case. For instance, a pool deck is left relatively rough, whereas supermarket floors are polished to a high sheen.
The Concrete Polishing Council has specifications for polished concrete based on two finishing criteria: class and level. Class refers to the amount of aggregate shown in the finished polished concrete, and level refers to the degree of shine.
- Class 0 “Refined”: No material is removed or cut from this concrete. It is mainly cleaned (removing glues, paints, etc.) and adequately refined with 100 grit before a densifier is applied. Because no material is removed, this is the most cost-effective concrete class.
- Class A “Cream”: Less than 1/16 in. of material is removed from this concrete. Very little aggregate is exposed in this finish, it is mainly the surface cream. Class A is a very popular choice of finish; however, the concrete must be as flat and smooth as possible prior to cutting. If the concrete has waves and bumps, cutting can reveal zones with aggregates. Because of this, Class A is difficult to achieve without careful planning and pouring, and this finish might not be possible for preexisting concrete that is not perfectly flat and smooth.
- Class B “Fine Aggregate”: Between 1/16 in. -1/8 in. of material is removed from this concrete. Fine aggregates are exposed in this finish with very little medium aggregate. “Salt and Pepper” is another term that refers to this finish.
- Class C “Medium Aggregate”: 1/8 in. of material is removed from this concrete. Medium aggregates are exposed in this finish with very few large aggregates. This is also the most expensive concrete class.
There are four classes of polished concrete: Level 1, 2, 3, and 4.
- Level 1 “Flat”: This is a flat finish with minimal reflective sheen. Level 1 is typically achieved with 400 grit. This is the cheapest level finish.
- Level 2 “Satin/Matte”: This has a low to medium reflective sheen with diffuse reflections. It is easily cleaned with a microfiber cloth and is very durable, making it a popular finish. Level 2 is typically achieved with 800 grit. Due to the high level of refinement and reflectivity, polished concrete levels 3 and 4 require a good foundation and dense slab, and the concrete needs to be well poured to be very flat. Additional steps, like filling pinholes with resin, may be used to achieve these two highly polished finishes.
- Level 3 “Semi-Polished”: This finish has a medium to high reflective sheen. While objects may be identifiable in the reflections, they are not visually sharp or crisp. Level 3 is typically achieved with 1500 grit.
- Level 4 “Highly Polished”: This is a highly reflective, mirror-like finish. Reflected objects are sharp and crisp. This is the most expensive finish. Because Level 4 requires an extremely flat surface, it is sometimes not possible for certain slabs. Level 4 is typically achieved with 3000 grit or higher.
Now that we’ve covered the nitty gritty on what polished concrete even is, check out our next blog in the series on the pros and cons of selecting polished concrete for your project.