For plain (unreinforced) concrete in sea water or in structures directly exposed to sea water (severe to very severe exposure), which minimum grade should be specified?

Introduction / Context:
Durability in marine environments is governed by chloride ingress, wetting–drying cycles, and sulphate attack. Codes prescribe minimum grades to achieve lower water–cement ratios and denser, more impermeable concrete, especially for plain concrete lacking crack control from reinforcement.

Given Data / Assumptions:

• Environment: sea water exposure or direct sea spray (severe to very severe).
• Material: plain (unreinforced) concrete.
• Objective: choose the minimum grade that is typically mandated for durability.

Concept / Approach:

Higher concrete grades correlate with lower permeability and better durability. For severe to very severe exposures (typical of marine conditions), plain concrete usually has a minimum grade requirement around M 20 to limit ingress and surface deterioration.

Step-by-Step Solution:

Verification / Alternative check:

Practical specifications often combine minimum grade with maximum w/c ratio (≈0.45–0.50) and minimum cement content to achieve required durability performance in marine zones.

Why Other Options Are Wrong:

• M 10/M 15: Too low for marine durability; higher permeability.
• M 30: Overly high for plain concrete minimum; may be specified for special performance but not as the base minimum.
• M 12.5: Not a standard grade for structural durability prescriptions.

Common Pitfalls:

• Assuming the same minimum grade for both reinforced and plain concrete; RC often requires even higher grades.
• Ignoring cover, w/c ratio, and curing, which are also critical for durability.

Final Answer:

M 20