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.
Fig: Air Entrained Concrete
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:
Where fc = specified characteristic strength
M = the margin
a = percentage by volume of entrained air.