Technical Questions

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Air content common limits

Q. Are the limits on air content that have been commonly used (3 to 7% by volume of concrete) considered satisfactory for nearly all purposes?

 

A. In most air-entrained concrete, values between these limits (based on total concrete volume) have been the aim. (ACI 211.1 considers different exposure conditions and makes some allowance for different sizes of coarse aggregate, requiring higher percentages of air where the mortar portion of the mixture is higher.) To keep within the desired range, reliance has been chiefly on adjusting the amount of air entraining admixture. To achieve the generally desired bubble spacing of 0.2 mm using an air-entraining admixture meeting the requirements of ASTM C260, the dosage should be sufficient to produce 9% air in the mortar fraction of the concrete; the portion that would pass through a 4.75 mm (No. 4) sieve.

 

References: SP-1(02); ACI 211.1-91; ASTM C260

Topics in Concrete: Admixture; Concrete Fundamentals; Mixture Proportioning

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