In post-tensioned concrete, at what percentage of full strength must the concrete harden before the tendon is pulled?

In post-tensioned concrete construction, the concrete must reach a specific level of strength before the tendons are tensioned to ensure safety and effectiveness. The correct threshold is that the concrete should achieve at least 70% to 75% of its specified compressive strength prior to the stressing of the tendons. This requirement is in place because concrete continues to gain strength over time, and adequate strength is necessary to support the forces introduced by the tendons without experiencing cracking or other structural failures.

When the concrete is still relatively new and not sufficiently hardened, any application of tension could result in significant damage, as the concrete would not yet be able to bear the loads imposed by the stressed tendons. Therefore, waiting until it reaches 75% of its full potential strength allows for a safe practice where the concrete can effectively interact with the tension introduced by the post-tensioning process.

The other percentages do not provide enough strength safety margin for the post-tensioning process, which could potentially lead to issues in structural integrity.