For a reinforced concrete member totally immersed in sea water (marine exposure), by how much should the normal nominal cover to reinforcement be further increased to improve durability?

Introduction / Context:
Marine environments accelerate corrosion due to chlorides and wetting–drying cycles. To protect reinforcement, codes require additional nominal cover for concrete members directly exposed to sea water or fully immersed. This extra cover delays chloride ingress and extends service life.

Given Data / Assumptions:

• Exposure condition: totally immersed in sea water.
• Base nominal cover is known for the member type and grade.
• We need the additional increment to be added to the normal cover.

Concept / Approach:

For immersion in sea water, detailing provisions typically increase the normal cover by a fixed increment to enhance durability without excessively enlarging sections. A practical rule is to add 10 mm to the normal cover under total immersion conditions.

Step-by-Step Solution:

Verification / Alternative check:

Project specifications often combine increased cover with low water–cement ratio, supplementary cementitious materials, and surface sealers as part of a durability strategy in marine structures.

Why Other Options Are Wrong:

• 20–50 mm: Excessive as a blanket increment; cover must also remain constructible and consistent with bar spacing and compaction.

Common Pitfalls:

• Failing to adjust bar chairs and spacers, resulting in less cover than specified.
• Ignoring workmanship; honeycombing near cover negates the benefit of added thickness.

Final Answer:

10 mm