In prestressed concrete design, which material combination is most appropriate for achieving the intended precompression and crack control in members?

Introduction / Context:
Prestressed concrete relies on precompression introduced by tensioned steel so that service loads do not cause tensile cracking. Selecting the correct combination of concrete and steel grades is fundamental to achieve adequate transfer of prestress, limit losses, and ensure durability under service conditions.

Given Data / Assumptions:

• Member is prestressed prior to (pretensioning) or after (post-tensioning) casting.
• Goal is to keep concrete largely in compression under service load.
• Materials must resist high initial stresses and long-term losses.

Concept / Approach:
High-strength concrete provides higher compressive capacity, improved bond, reduced creep and shrinkage (thereby reducing losses), and better cracking resistance. High tensile steel (prestressing strands or wires) sustains high initial stress with low relaxation to produce the required precompression. The synergy of both is the foundation of prestressed systems.

Step-by-Step Solution:
Choose concrete with characteristic strength adequate for transfer and service (often ≥ 40 MPa) to resist compressive stress and reduce time-dependent losses.Use high tensile, low relaxation steel to anchor the prestress effectively while limiting long-term stress reduction.Detail anchorage, duct profile, and cover to maintain intended force and durability.

Verification / Alternative check:
Service stress checks confirm that with high-strength concrete and high tensile steel, tensile stresses remain limited or negative (compression) across critical sections, validating material choice.

Why Other Options Are Wrong:

• Low- or high-strength concrete alone cannot provide prestress; steel is needed.
• Low-strength concrete increases creep/shrinkage losses.
• Low tensile steel cannot carry required prestress safely.

Common Pitfalls:
Underestimating relaxation of steel, using concrete that has not reached transfer strength, and neglecting environmental exposure leading to durability issues.

Final Answer:
High-strength concrete with high tensile steel.