North Carolina Building Contractor License Practice Exam

A strength test for concrete shall be the average of how many cylinders tested at 28 days?

1. 3

2. 5

3. 10

4. 7

The correct answer is: 5

The average strength test for concrete at 28 days is typically based on the results of five cylinders. This standard is established to ensure accuracy and reliability in measuring the compressive strength of concrete. Testing five cylinders allows for a better representation of the concrete batch's overall performance, as it helps to account for any variability in material properties, mixing, handling, and curing processes. Testing only three cylinders could yield results that do not adequately represent the concrete's strength due to the smaller sample size. Meanwhile, testing ten or more cylinders may be excessive for routine quality control, although it could be appropriate for large-scale projects or research purposes. Using five cylinders strikes a balance between practicality and statistical significance, ensuring that the average strength value reflects the true characteristics of the concrete being evaluated.