Adding Entrained Air to Outdoor Countertops

Question:
Do I need to do anything special to my concrete for an outdoor kitchen countertop to prevent problems during the winter?

Answer:
Using an air-entraining agent in your concrete is the most effective way to combat freeze-thaw issues. Most concrete countertop mixes are cement rich and have lower water-cement ratios as compared to ordinary concrete. Both of these characteristics enhance freeze-thaw resistance, but adding an air entrainer will significantly reduce the likelihood of problems due to freezing weather.

I recommend an entrained air content of around 5% to 6%. Most outdoor concrete countertops are not exposed to deicers, continuous moisture or other severe exposure conditions, so the air content does not need to be very high. But concrete countertops are high-value items, so minor freeze-thaw damage that might go unnoticed on a driveway would be considered unacceptable in a countertop. Using an air-entraining agent could be looked at as very cheap insurance.

Air-entraining agents create a non-interconnected matrix of very fine air bubbles. The typical sizes of entrained air bubbles range from 10 to 100 m (0.01 to 0.1 mm), generally too small to be visible. Entrained air differs from entrapped air, which is simply air that gets whipped into the concrete during mixing and offers little freeze-thaw resistance. The size (1 mm in diameter or larger) and distribution of entrapped air bubbles are far different from entrained air.

Adding entrained air does change the properties of concrete somewhat. It enhances workability, but does make the concrete more sticky and therefore harder to finish. It also lowers water demand and significantly reduces bleeding. It doesn’t affect the color of the concrete.