North Carolina Building Contractor License Practice Exam

At what point is a strength test for concrete done?

1. 7 days

2. 14 days

3. 28 days

4. 30 days

The correct answer is: 28 days

The strength test for concrete is typically conducted at 28 days because this timeframe is considered the standard period for concrete to reach its design strength. During the curing process, concrete undergoes hydration, which allows it to develop strength over time. While initial strength gains can be measured earlier (for instance, at 7 days or 14 days), the standard reference for assessing whether the concrete has achieved the intended compressive strength is at 28 days. This duration is established based on the average behavior of concrete mixtures, allowing for variability between different mixes, environmental conditions, and curing practices. Testing at this point ensures a reliable measure of strength for structural and safety assessments before the material is placed under significant loads.