What is the typical method to test concrete compressive strength at 28 days?

• A. Use the slump test to estimate 28-day strength.
• B. Drill cores from field specimens and test in compression.
• C. Prepare and cure cylinders of the concrete, then test them in compression.
• D. Rely on non-destructive testing methods only.

Answer: (C)

The main concept is obtaining a direct, standardized measure of 28-day strength by preparing and curing concrete specimens that are representative of the actual batch. This is done by casting cylinders from the concrete mix, curing them under controlled conditions for 28 days, and testing them in compression. This approach provides a direct, repeatable strength value that can be compared to the project specification.

Slump testing only measures workability, not strength, so it cannot predict 28-day compressive strength. Drilling cores from field specimens is a valid alternative when cylinders aren’t available or when in-place strength is needed, but it introduces more variability and is not the typical routine method for confirming the concrete’s 28-day strength. Non-destructive tests can give estimates, but they do not provide a definitive 28-day compressive strength value for acceptance.