Q.: A new slab on grade in a parking garage was found to have 12 percent entrained air, rather than the 6 percent that had been specified. We don't question the accuracy of the air test but we want to know how this air content will affect the strength, durability and integrity of the job.
A.: As a general rule of thumb, the compressive strength of concrete will be lowered by 3 to 5 percent for each 1 percent of entrained air. Air entrained mixes are proportioned with this in mind. A mix with an unexpected excess of 6 percent entrained air could have a compressive strength 18 to 30 percent below design strength. Abrasion resistance and general durability will also be markedly lower than expected.
Short of replacing the slab, the only corrective measure would be to cast a topping over it. The type of topping and the thickness would be determined by performance requirements and how much increase in floor elevation would be acceptable.