General Contractor Practice Exam

At what percentage of the concrete's maximum compressive strength is it deemed safe to post-tension?

1. 50%

2. 65%

3. 75%

4. 85%

The correct answer is: 75%

Post-tensioning is a technique used in concrete construction to improve structural performance and efficiency. The safety threshold for post-tensioning is closely related to the concrete's compressive strength because the tension in the tendons needs to be applied to concrete that has reached a certain level of strength to avoid damage or failure. When concrete has reached about 70% to 75% of its maximum compressive strength, it is generally considered safe to initiate post-tensioning activities. This is because, at this point, the concrete has gained sufficient strength to withstand the additional forces introduced by the tendons without risk of cracking or other failure modes. Choosing to begin post-tensioning at 75% provides a margin of safety, ensuring that the concrete can effectively support the induced stresses from the tensioning process. This level is widely accepted in the industry to ensure the durability and stability of the structure. The other percentages do not align with standard practices for post-tensioning because they either present a risk of insufficient strength at lower percentages or could delay construction at higher percentages where the concrete may already be too hard to effectively tension.