North Carolina Building Contractor License Practice Exam

At what age should the strength tests be conducted for concrete cylinders?

1. 21 days

2. 28 days

3. 14 days

4. 30 days

The correct answer is: 28 days

The correct age for conducting strength tests on concrete cylinders is 28 days. This is a widely accepted standard in concrete testing because the majority of the strength gain occurs during the first 28 days after the concrete is poured. At this point, the concrete has typically reached its intended compressive strength, which is crucial for determining the performance and durability of the material in construction applications. Many specifications and guidelines, including those from ASTM (American Society for Testing and Materials), emphasize the importance of this 28-day curing period. While some initial strength assessment can be made at earlier ages like 7 days or 14 days, the 28-day mark is recognized as the definitive point for evaluating the overall performance of the concrete mix. Conducting strength tests too early, such as at 14 days, may not provide an accurate representation of the concrete’s long-term capabilities. Therefore, testing at 28 days is critical for ensuring the structure will be safe, durable, and able to withstand the loads it will encounter during its service life.