For a reinforced concrete (R.C.) water tank designed with M 25 concrete, what is the permissible bending tensile stress in concrete for the vertical wall to control cracking under service conditions?

Introduction / Context:
Water-retaining structures are designed for crack control under service loads because leakage and durability are critical. Codes specify permissible tensile stresses in concrete (in bending and direct tension) that are lower than ordinary building limits to minimize crack widths, especially on the liquid face of walls and slabs.

Given Data / Assumptions:

• Concrete grade: M 25 (characteristic compressive strength fck = 25 N/mm2).
• Member: vertical wall of an R.C. water tank.
• We seek permissible bending tensile stress for service design of liquid-retaining structures.

Concept / Approach:

For liquid-retaining structures (e.g., per IS guidelines for water tanks), permissible tensile stresses in concrete are intentionally restricted to limit crack initiation/width. Typical values for M 25 are about 1.8 N/mm2 in bending tension (on the liquid face) and even lower in direct tension. Higher values like 8.5 N/mm2 would be incompatible with crack control requirements and are not used for serviceability checks in water tanks.

Step-by-Step Solution:

Verification / Alternative check:

Design examples for liquid-retaining walls consistently adopt about 1.8 N/mm2 bending tensile limit for M 25 to satisfy leakage and durability criteria under sustained hydrostatic pressure.

Why Other Options Are Wrong:

• 8.5 N/mm2, 2.5 N/mm2: Too high for liquid face service conditions; risk of cracking.
• 6.0 N/mm3: Wrong units; tensile stress should be N/mm2.
• 0.8 N/mm2: Overly conservative for M 25 bending tension.

Common Pitfalls:

• Using general building code tensile limits instead of water-retaining criteria.
• Ignoring differential temperature/shrinkage which also influences cracking.

Final Answer:

1.8 N/mm2