Bubbles in Concrete While Finishing

Air Entraining Agents

• Certain organic or carbon-based compounds help stabilize air bubbles that form in the concrete as it cures. These chemicals are called air-entraining agents. When added to the concrete mix, the agents enable the formation of billions of tiny air bubbles scattered throughout the concrete. The resulting product is about 5 to 8 percent air by volume. It's important to use the correct amount of air entraining agent, so careful supervision by engineers often is required.

Factors

• The amount of air entrained varies with the temperature: the higher the temperature, the less air is entrained. The type of fly ash used in the concrete also affects air entrainment. Sometimes lignosulfonate water reducers are added to concrete as water-reducing additives. If this occurs, less air entraining agent is required because the lignosulfonates possess a small amount of air-entraining capabilities. Calcium chloride also slightly increases the amount of entrained air as well.

Materials

• Other factors that affect air entrainment include the alkali level, texture and particle size of the sand or aggregate used in the concrete mix. Higher alkali levels, for example, generally imply higher air-entrainment levels, and more fine-grained aggregate results in less trapped air. Air-entraining agents fall into one of four chemical categories: wood resin salts, synthetic detergents, petroleum acid salts and fatty acid salts.

Effects

• Ice takes up more space than water, so when water freezes it creates pressure on the surrounding concrete. Helping to reduce the pressure on surrounding material, air entrainment creates evenly distributed microscopic air bubbles in the concrete, which in turn creates space for the water to expand as it freezes. Not only is air-entrained concrete more resistant to frost wedging, it's also somewhat more workable as well. These properties have made it popular in construction.