Exposed Aggregate Polished Concrete
Concrete is a mixture of cement, sand, and rocks of various sizes. These ingredients tend to separate by size, so the surface of the slab is mainly cement, with the rock packed a significant distance below. In an exposed aggregate floor the surface of the concrete is heavily ground, removing enough of the slab to consistently expose the large rock in the slab. When it comes to polished concrete floors, exposed aggregate is often the most sought-after installation and by far the most difficult.
Concrete Mix
One of the most important factors for an exposed aggregate floor is the concrete mix. While any slab can be ground to aggregate, you only have true control of the final outcome on a freshly poured slab with a modified mix.
Exposed Aggregate Concrete Mix Considerations
- Concrete Additives
Fly ash and slag can clump in a mix creating porosity. An ideal mix for exposed aggregate should be free of these common additives. - Color
Most cement is tan. If a different cement paste color is desired, the pigment should be added to the mix to modify the color. - Rock type
The type of rock that is exposed is the foundation of this installation. Often granite is used in these installations. For a special appearance, specific colors and sizes can be specified in the mix. - Finishing
The slab should be poured by a highly experienced concrete crew. Anything that impacts how the rock settles into the slab can cause difficulties with this finish. This includes excessive footprints, debris, screed lines, and disturbed rebar.
Cost
The cost of an exposed aggregate concrete slab is dependent on many factors. These include:
- Porosity and hardness of the slab
- Consistency of concrete pour
- Desired level of gloss
- Overall square footage
A typical price range for an exposed aggregate polished concrete floor in 2021 is $9-12 a square foot.
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