Q.: We produce ready mixed concrete for many jobs requiring documentation that the proposed concrete proportions will produce average strengths specified for the job. The documentation includes determination of a standard deviation based on at least 30 consecutive field strength tests. Can we use 4x8-inch cylinder results for this determination? The smaller cylinders are easier to handle, and for high-strength concrete they don't require a large testing machine.
A.: We don't advise using the smaller cylinders for this purpose. ASTM's standard practice for making and curing concrete test specimens in the field (ASTM C 31-90) seems to prohibit smaller cylinders. In Section 5.1 the practice states that unless required by the project specifications, cylinders smaller than 6x12 inches shall not be made in the field.
There's another good reason for basing the standard deviation calculation on 6x12-inch cylinders. Smaller cylinders generally yield higher strengths but the test results also are more variable. This would increase the calculated standard deviation causing the required average strength (and cement costs) to be higher.