In post-tensioned concrete, when is the steel tensioned?

• A. Before pouring the concrete.
• B. While the concrete is still wet.
• C. After the concrete has set and reached the specified strength.
• D. Only in emergency repairs.

Answer: (C)

In post-tensioned concrete, the steel is tensioned after the concrete has set and reached the specified strength. The tendons run through ducts in the formwork, concrete is poured and cured, and only then are hydraulic jacks used to tension the steel and anchor it at the ends. This timing is essential because the concrete must be strong enough to resist the compressive forces the prestress creates and to effectively transfer that force into the member. Tensioning too early would place high stresses on concrete that isn’t yet capable of carrying them, risking cracks or crushing. After tensioning, the prestress is transferred to the concrete, often with anchorage blocks and sometimes grout for protection.