Strength test results of concrete cylinders are used as the basis of acceptance of ready mixed concrete when a strength requirement is specified. Cylinders are molded from a sample of fresh concrete, cured in standard conditions and tested at an age indicated in the specification, usually at 28 days. For strength test results to be reliable, procedures must be in accordance with ASTM standards. The average strength of a set of two 6×12 in. (150×300 mm) or three 4×8 in. (100×200 mm) cylinders made from the same concrete sample constitutes one test result. In some cases cylinders are tested at 7 days to get an early indication of the potential strength, but these test results are not to be used to determine the acceptability of the concrete. Cylinders used for acceptance of concrete should not be confused with field-cured cylinders. Tests of field-cured cylinders are used to evaluate whether the in-place concrete has been properly cured and protected, to estimate the early-age strength in the structure to strip forms or for post-tensioning, and to continue construction activity.
The ACI Building Code, ACI 318, and the Specification for Structural Concrete, ACI 301, recognize that when mixtures are proportioned to meet the requirements of the standards, strength test results will fail to comply with acceptance criteria about one time in 100 tests due to normal variability.
The strength acceptance criteria used are:
No single test is lower than (0.9 × specified strength) when the specified strength is greater than 5000 psi (35 MPa).
An example of these strength acceptance criteria is provided in the table. If the strength test results fail either condition (a) or (b), steps must be taken to increase the strength of the concrete. If the results fail condition (b), an investigation should be made to ensure structural adequacy of that portion of the structure.
The major reasons for low compressive strength tests are:
High air content in the concrete (and test specimens); and
Errors in production and unanticipated factors during delivery.
It is important that procedures are conducted in accordance with ASTM standards. Almost all deficiencies in handling and testing cylinders will result in a lower measured strength. All violations add up to cause significant reductions in measured strength. The more significant factors are: improperly finished surfaces; initial curing over 80°F (27°C); frozen cylinders; extra days in the field; damage during transportation; delay in stripping molds and curing at the lab; improper capping; and insufficient care in breaking cylinders.
The laboratory should be held responsible for deficiencies in its procedures. Field-testing technicians and laboratory personnel should be certified; construction workers untrained in concrete testing must not make and handle cylinders. ACI 318 requires laboratories to conform to ASTM C1077 for their quality system and personnel qualifications. Laboratories should be inspected by the Cement and Concrete Reference Laboratory (CCRL) laboratory inspection or an equivalent program. Field testing personnel must have a current ACI Grade I Field Testing Technician certification or equivalent. Laboratory personnel should have the ACI Grade I and II Laboratory Testing Technician and/or the ACI Strength Testing Certification, or equivalent.
If the deficiency justifies investigation, first verify testing accuracy and then compare the structural requirements with the measured strength. If testing is deficient or if strength is greater than actually needed in that portion of the structure, there is little point in investigating the in-place strength. However, if procedures conform to the standards and the strength as specified is required for the structural capacity of the member in question, further investigation of the in-place concrete may be required (CIP 10).
