Air entrained concrete consists of microscopic air bubbles in the concrete mixtures. Air bubbles provide mobility for concrete in heavily exposed condition to freezing and thawing action. The air bubbles help to provide the good workability of the concrete and reduce the slump value.
Benefits of Air Entrained Concrete
Air entraining chemical component creates bubbles of air in concrete mixture. Air bubbles are generated when the concrete is mixed properly and exist in the concrete even after it hardened condition. Basically concrete mixture will have small capillaries which produced by the evaporation of the water from concrete compound. It may cause capillaries are sucked by water from the environment and the freezing of this water may cause a lot of stress in concrete mixture because of the expansion in volume of concrete that actually accompanies the freezing action. As a result it causes further cracks and continuous deterioration in the concrete mass. Now at this situation Air entrained concrete allow to expansion and build without causing any further stresses, as the air bubbles are capable to compress. These bubbles are also participate to the improvement of workability of the concrete.
Air Entraining Agents
The chemical components which can be used for air entraining are different types. As per records air entraining agent was first used in the 19th century for road construction work , and civil engineers team of that period found that few Vensoul resin had been neutralized or other salts of special wood resins is very effective effective for this purpose. Some synthetic detergent , petroleum mono product of acid salts, and salts from fatty acid and resinous acids included as air entraining agents.
Air Entrained Concrete preparation and Test process
An Air entrained concrete is prepared by adding a chemical agent in the concrete and quantities that each manufacturer will specify with manual. Water dilution of the agent is not preferred until and unless it is specifically recommended by the manufacturer. Portland type cement is the right cement and recommended to use.
From experimental data it was observed that higher concrete casting temperatures will require more amount of the air-entraining agent, and its requires frequent tests of the air content parameter. This concrete needs to be well mixed for at least 1 or 2 minutes properly. Vibration of the concrete may further reduce the air content by 15 to 20 percent.
Concrete entrained air testing is a must for such concrete. Air content of the freshly mixed concrete can be measured by gravimetric, volumetric, or pressure methods. The pressure method is the most commonly used, and in this method a calibrated air meter is used to test a known volume of concrete that is subjected to a defined pressure.
Workability and Strength of Air Entrained Concrete
The concrete slump test it is mandatory and normally slump is about 75 – 80 mm are used for good workability. whenever air entraining agents are used, the slump may reduce to between 25 to 50 mm while also having the same workability. Another advantage of this slump reduction is to reduce the water, to cement ratio in the concrete which can easily increase the strength of the concrete.
The strength of air entrained concrete reduces 5 percent prominently for every 1 percent of entrained air , which can be adjusted by the reduction in the water-to-cement ratio or by reduction in fine aggregates, as per design of structural engineer. This reduction in concrete strength comes from the air/voids caused by the entrained air.
The quantity of entrained air is maintained between 4 and 7 percent for best freezing and thawing resistance though differences may be evolved depending on special conditions.