Civil engineering materials: In which type of concrete are steel tendons (wires or strands) pretensioned before any superimposed service load is applied to the member?

Introduction / Context:
Structural concrete systems are categorized by how internal steel is used to resist tension. Understanding the difference between reinforced, prestressed, and other terms is crucial for design, detailing, and inspection. This question targets the concept of prestressing, specifically pretensioning before service loads.

Given Data / Assumptions:

• We are discussing concrete members containing steel tendons.
• Pretensioning occurs before superimposed loads (live or additional dead loads) are applied.
• Focus is on the timing and method of inducing internal compressive stress in concrete.

Concept / Approach:
Prestressed concrete uses high-strength steel tendons to introduce compressive stress into concrete, improving crack control and deflection performance. There are two main methods: pretensioning and post-tensioning. In pretensioning, tendons are tensioned against an external abutment, concrete is cast and allowed to gain strength, and then tendons are released, transferring force to the concrete. This all happens before the member is subjected to superimposed loads.

Step-by-Step Solution:

Verification / Alternative check:
Contrast with reinforced concrete: in conventional reinforced (non-prestressed) members, steel is not tensioned beforehand; it only resists tensile stress after loading. Contrast with post-tensioned: tendons are stressed after concrete hardens but typically on-site with anchorages.

Why Other Options Are Wrong:

• Aggregate: Refers to the granular constituents of concrete, not a system of prestress.
• Reinforced: Steel bars resist tension after loading; no intentional precompression is introduced.
• Preloaded: Not a standard term for concrete systems; does not denote tendon tensioning practice.

Common Pitfalls:
Confusing pretensioned with post-tensioned methods; both are prestressed, but only pretensioned fits the sequence described.

Final Answer:
Prestressed