Minimum concrete strength requirement: The minimum 28-day cube compressive strength of concrete typically specified for prestressed concrete members is closest to which value?

Introduction / Context:

Prestressed concrete relies on a high-quality concrete matrix to limit prestress losses, control cracking, and safely transfer high compressive stresses from tendons/strands. Therefore, minimum characteristic strengths are higher than for ordinary reinforced concrete.

Given Data / Assumptions:

• Question refers to standard practice minimum 28-day cube strength for prestressed members.
• Working-stress era units: kg/cm² (1 MPa ≈ 10.2 kg/cm²).

Concept / Approach:

Common practice specifies concrete not less than about M40 (≈ 40 MPa) for prestressed members, especially pretensioned elements, to ensure adequate bond and early-age transfer. In kg/cm², 40 MPa ≈ 400 kg/cm², which aligns with the typical minimum strength cited in design guides and examinations.

Step-by-Step Solution:

Verification / Alternative check:

Many codes allow higher grades (e.g., M45, M50) and stipulate minimum transfer strengths at the time of prestress application (often ≥ 70% of f_ck). The exam-style minimum for the 28-day cube strength typically rounds to 400 kg/cm².

Why Other Options Are Wrong:

• 50–250 kg/cm²: Too low for prestressed members; would risk bond failure and excessive losses.
• 350 kg/cm²: Used in some reinforced concrete, but below the common PSC baseline of ≈400 kg/cm².

Common Pitfalls:

• Confusing minimum 28-day strength with minimum transfer strength (at jacking/release), which is an early-age requirement.

Final Answer:

400 kg/cm²