When are concrete cylinders typically tested to determine 28-day strength?

• A. 7 days
• B. 14 days
• C. 28 days
• D. 56 days

Answer: (C)

The main idea is that the standard strength used to judge a concrete mix is its 28-day compressive strength. Cylinders are molded from the same batch of concrete, properly cured, and then tested at 28 days to see if they meet the specified strength. This timing reflects when the concrete typically reaches a stable, representative strength under normal curing, giving a reliable basis for acceptance. Early tests at 7 or 14 days show initial strength but can vary more with mix and curing conditions, and waiting to 56 days isn’t needed for standard projects. So testing at 28 days best aligns with standard specifications and provides a dependable measure of the mix’s performance.