When are molded concrete cylinders tested for final acceptance and payment?

Molded concrete cylinders are tested for final acceptance and payment primarily at 28 days after casting. This period is significant because it typically allows the concrete to reach its designed strength, which is crucial for assessing the quality and durability of the mix used in construction. Concrete strength gains are most pronounced in the first 28 days, and standards set by various organizations, such as the American Concrete Institute (ACI), recommend this time frame for testing.

The 28-day testing point aligns with industry practices that reflect the time required for standard curing conditions, permitting the test results to be a reliable indicator of the concrete's performance in service. As a result, this time frame signifies a balance between practicality and accurate assessment of the material's structural integrity.

Testing at earlier or later intervals, as suggested by the other options, may lead to potential inaccuracies in the assessment of the concrete's performance strength and may not align with standard acceptance criteria. Therefore, 28 days is the widely recognized and accepted period for final testing of molded concrete cylinders.