Overview & Structure
IS 3370 provides guidelines for the design of reinforced and plain concrete structures used to store liquids (primarily water, but also sewage, oil, etc.). Unlike ordinary buildings, these structures must remain watertight under pressure and temperature variations. Cl. 1
| Part | Title | Focus |
|---|---|---|
| Part 1 : 1965 | General Requirements | Materials, general design principles, safety factors. |
| Part 2 : 1965 | Reinforced Concrete | Details for reinforced members, stress limits, reinforcement calculation. |
| Part 3 : 1967 | Crack Widths | Limit state method specifically for crack width control in liquid retaining structures. |
| Part 4 : 1967 | Joints | Expansion joints, construction joints, movement joints, and waterproofing details. |
Materials & Permeability
The quality of concrete is paramount. Permeable concrete allows water ingress and egress. Part 1, Cl. 5
Design Methods
IS 3370 allows design by both the Working Stress Method (WSM) (traditional, conservative) and the Limit State Method (LSM) (modern). Part 2 / Part 3
Working Stress Method (WSM)
This method assumes concrete remains elastic and calculates stresses under service loads. Safety is provided by keeping stresses well below yield/ultimate limits.
| Stress Type | Permissible Stress (N/mm²) for M20-M25 | Condition |
|---|---|---|
| Tension in Steel ($f_{st}$) | 130 (Fe250) / 150 (Fe415) | Direct Tension |
| Tension in Steel ($f_{st}$) | 115 (Fe250) / 130 (Fe415) | Flexural Tension |
| Compression in Concrete ($\sigma_{cbc}$) | 0.33 × $f_{ck}$ (e.g., 5 N/mm² for M15) | Bending |
| Shear Stress | As per IS 456 | — |
Limit State Method (LSM)
Under LSM, the primary concern is the Crack Width Limit State. The section is designed for ultimate strength first, then checked for crack widths under service loads.
- Predicted crack width $\le$ 0.2 mm (general) or $\le$ 0.1 mm (severe exposure).
- Uses partial safety factors for materials (usually 1.15 for steel, 1.5 for concrete).
Crack Width Limits
This is the core of IS 3370 Part 3. Cracks are inevitable in tension zones, but their width must be restricted to prevent leakage and corrosion. Part 3
Exposure Condition
- Mild: Protected from weather (interior walls).
- Moderate: Covered structure, some moisture.
- Severe: Direct rain, splash zone, cyclic wet-dry.
- Very Severe: Marine environment, constant immersion in aggressive water.
Max Crack Width
- General: 0.2 mm
- Severe Exposure: 0.1 mm
- Very Severe: 0.05 mm (often requires prestressing or special lining).
To achieve this, designers must provide:
- High Quantity of Steel: Even if strength calculations require less, minimum reinforcement must be provided to distribute cracks finely.
- Small Bar Diameter: Use many small bars (e.g., 10mm) instead of few large bars (e.g., 20mm) to reduce crack spacing.
- Close Spacing: Reinforcement distributed closely on both faces.
Tank Types & Support
Tanks are classified by their support system and location of the liquid level.
Rectangular: Easier construction, cheaper formwork, but corners induce bending moments.
Detailing Rules
Strict detailing is required to ensure continuity and watertightness. Part 2, Cl. 8
- Minimum Reinforcement:
Vertical/Hoop bars: $\ge 0.24\%$ of gross concrete area for Fe250.
For Fe415/Fe500: $\ge 0.35\%$ to $0.40\%$. - Bar Diameter: Preferably $\le 20$ mm to control crack width.
- Spacing: Horizontal bars spacing $\le 3 \times$ wall thickness or 300 mm (whichever is less). Vertical bars $\le 200$ mm.
- Clear Cover: Min 45 mm (if using mild environment) or 50-75 mm for severe exposure. Use dense cover blocks.
- Wall Thickness: Minimum 150 mm for cast-in-situ. 200 mm is preferred for easier vibration and durability.
- Continuous Walls: If possible, pour walls continuously without horizontal construction joints. If joints are necessary, use waterstops.
Construction Requirements
Even perfect design fails with poor construction. IS 3370 emphasizes execution. Part 4
Concreting
- Continuity: Pour walls in one continuous operation if possible. Avoid cold joints.
- Vibration: Adequate compaction is vital to remove entrapped air pockets (which cause leaks). Do not over-vibrate (causes segregation).
- Waterproofing: Apply a plaster finish (1:3 cement mortar) or waterproofing coating on the inner face after curing. Some codes require internal painting with epoxy or bitumen.
Curing
Curing is the single most important step for watertightness. Concrete must be kept moist for at least 14 days (preferably 21 days).
- Wet hessian cloth or sand layer on walls.
- Water ponding on roof slabs.
- No drying shrinkage allowed during early stages.
Filling Procedure
Never fill the tank immediately after demolding.
1. Allow concrete to cure fully (28 days).
2. Fill slowly in stages (e.g., 1/3 height per day).
3. Inspect for leaks after each stage.
4. If leakage is detected, drain and repair before proceeding.
- Using loose soil backfill against walls (causes uneven pressure).
- Inadequate curing leading to surface crazing and leaks.
- Omitting waterstops at construction joints.
- Too few vertical bars (spacing > 300mm).
Related Codes
IS 3370 works in tandem with other general and specific codes: