All structural steel is considered satisfactory even if rusted, as long as density requirements are met, except which type?

• A. Reinforcing steel
• B. Pre stressing steel
• C. Welded steel
• D. Stainless steel

Answer: (B)

The important idea here is how corrosion affects different types of steel inside concrete, especially when a component is under high tensile stress. Prestressing steel carries significant tension in prestressed concrete, and its capacity relies on having a full, intact cross-section and a reliable bond with the surrounding concrete. When rust forms on prestressing tendons, it reduces the effective cross-sectional area and creates pitting that can disrupt bonding and transfer of prestress. Even small losses in area or bond strength can meaningfully reduce the prestress and lead to dangerous loss of performance, so rust on prestressing steel is not acceptable, even if other criteria like density are met.

Reinforcing steel, welded steel, and stainless steel have different roles and typically less reliance on maintaining an extremely precise high-tension status. Surface rust on ordinary reinforcing bars or some welded steel members can be tolerated within design and durability considerations because the overall structural capacity and bond with concrete can remain adequate, especially with sufficient cover and proper detailing. Stainless steel is inherently more corrosion-resistant, so rust is rarely a controlling factor there.

So, the exception is the prestressing steel: rusting cannot be tolerated because it directly compromises the tensioned tendon's strength and the effectiveness of prestressing.