When there's a question about in-place strength of concrete as a result of low cylinder test results, cores anywhere from 1 1/2 inches to 4 inches in diameter are taken. Is there any disadvantage in taking the smaller cores?
We think you're better off drilling larger diameter cores. Here's why. Variability in core strength increases as the core diameter decreases. That means small-diameter cores have a greater chance of yielding a core test result that falls below the 0.75 f'c lower limit given in paragraph of "Specifications for Structural Concrete for Buildings" (ACI 301-89).