Q.: We will be using some high strength concrete and it has been suggested that we make 4x8-inch test cylinders because the capacity of the testing machine is only 250,000 pounds.
This machine would break a 6x12-inch cylinder whose strength is about 8800 psi or less, but not one whose strength is higher. Would this method be satisfactory?
A.: It is not recommended. True, the machine could break cylinders of strengths almost as high as 20,000 psi if the cylinders were only 4x8 inches. However, these smaller specimens have greater variation in test results than standard 6x12-inch cylinders, particularly at high concrete strengths. This means that, for the same concrete, 4x8-inch cylinder test results would more often fall below the required strength than tests made in a bigger machine of 6x12-inch cylinders of the same concrete. As a result you would have to boost the design strength in order to be sure your strength is adequate. (This is explained in ACI 214, Recommended Practice for Evaluation of Strength Test Results of Concrete.) It would wind up costing more money for concrete. It would be best for you to find a laboratory that has a testing machine of greater capacity, such as 400,000 pounds.
There is disagreement about why there should be more variation in high-strength 4x8-inch cylinders than 6x12-inch. It may be related to either or both of the following:
- the greater effect that the maximum aggregate size has on a small cylinder
- the relatively greater effect on the small cylinder if there is a small error in centering it in the test machine.