Air-entrained Concrete Advantages

Air-entrained Concrete Advantages

Entrained air produces discrete cavities in the cement paste and therefore reduces passage of capillary water. It improves the workabillty because the air bubbles kept special by the surface tension, act as fine aggregate of low surface friction and considerable elasticity. Air entrainment of concrete reduces segregation, bleeding, permeability and formation of laitance. But if lowers the density of the concrete then it is liable to shrinkage on drying. Air entrained concrete is frost resistant. This type of concrete is mainly recommended for used in dams, bridges , tunnels and for roads in extremely cold countries and sulfate bearing soils.
There are some disadvantages along with the advantages of air-entrained concrete. For every 1% increase in air, the compressive strength of concrete will be reduced by 2-6%.


Air entrainment is the deliberate creation of bubbles of air in concrete. The number of such air cells per cubic foot of concrete can be measured in the billions. This is done by addition of agents that entrain air and are generally chemicals. The air bubbles are produced when the concrete is mixed and continue to be a part of the concrete even after it hardens. Normal concrete will have small capillaries produced by the evaporation of the water in the concrete. These capillaries are invaded by water from the environment and the freezing of this water can cause a lot of stress in concrete because of the expansion in volume that accompanies the freezing. This causes further cracks and continued deterioration in the concrete. Air entrained concrete allows the expansion to take place without causing any further stresses, as the air bubbles are capable of being compressed. The bubbles also contribute to the workability of the concrete.

Air-Entrained Concrete

Air-Entrained Concrete

Air-Entrained concrete is defined as the air intentionally incorporated by means of a suitable agent. other Definition is 
Air-Entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes.
Air-entrained concrete is produced using air-entraining portland cement, or by the introduction of air-entraining agents






Air-Entrained agents are:

• Animal and vegetable fats 
• oils and their fatty acids.
• Natural wood resins, which react with lime in cement to form a soluble resinate.
• Wetting agents such as alkali salts of sulfate
• sulphonated organic compound.

The air-entraining agents can be dispensed either as an admixture, (a material added to the mix when the ingredients are fed into the concrete mixer) or else as an addition to the cement, in which the agent is interground with the cement in fixed proportion. The air entraining agents represent between 0.005 to 0.05 percent of the weight of the cement. But to facilitate the dispensing operation, a solution of the agent in water is usually made up.

EFFECT OF ENTRAINED AIR ON CONCRETE STRENGTH

Air Entrained Concrete

Compressive strength and workability are two important aspects of concrete. Compressive strength of concrete is inversely proportional to workability of concrete. When workability of concrete increases, its compressive strength decreases. To achieve the required compressive strength of concrete, its workability is limited. But workability cannot be decreased beyond certain limit so that concrete can be effectively placed and compacted at site.


To maintain the desired compressive strength and workability of concrete together, generally in the case of higher strength concrete, admixtures are used. Air entraining agent is one such concrete admixture to increase the workability without affecting much reduction in compressive strength.
But use of concrete admixture in concrete shall be considered in mix design so that desired compressive strength is achieved with required amount of admixture and workability.

In general, the compressive strength of concrete is reduced by use of air entrained admixture. The amount of reduction in strength depends on many factors such as mix proportions, type and grading of concrete, cement and actual air-entraining agent.

Normal reduction in strength of concrete by use of air-entrained admixtures varies from 3 to 7%. This variation of strength shall be considered in the mix design of concrete. Suitably, trial mix designs should be done to find exact variation of strength with the use of this admixture and suitable corrections should be done in mix design to ensure desired strength.

Typically it may be assumed that a loss of 5% in compressive strength of concrete occurs due to each 1% by volume of entrained air in the concrete mix.

In order to estimate water-cement ratio required for an air entrained concrete, an allowance for strength reduction is incorporated in the mix design and higher target mean strength is assumed.

The appropriate higher target mean strength for an air entrained mix is given by:
target mean strength of concrete

Where 
fc = specified characteristic strength
M = the margin
a = percentage by volume of entrained air.