Code of Practice for Concrete Structures
for Storage of Liquids
The primary standard for the design and construction of liquid-retaining concrete structures — overhead tanks, ground-level reservoirs, underground sumps, and water treatment structures. The 2009 revision replaced the 1965 edition and introduced limit state design with explicit crack width control.
📋 2009 Revision: IS 3370:2009 is a significant revision from the 1965 edition. It adopts limit state design (per IS 456), introduces explicit crack width limits, and updates material requirements. The four-part structure is retained: Part 1 (general), Part 2 (RCC), Part 3 (prestressed), Part 4 (design tables).
IS 3370 applies to concrete structures used for the storage of liquids — primarily water, but also other liquids with similar properties. It covers structures at all levels: overhead, ground-level, and underground.
| Part | Title | Coverage |
|---|---|---|
| Part 1 | General Requirements | Materials, workmanship, inspection, testing |
| Part 2 | Reinforced Concrete Structures | RCC design — walls, floors, roofs, joints |
| Part 3 | Prestressed Concrete Structures | Prestressed tanks; no tension in concrete |
| Part 4 | Design Tables | Moment and shear coefficients for standard tank geometries |
| Structure Type | Examples | Key Design Concern |
|---|---|---|
| Overhead Tank | Elevated RCC tank on staging | Seismic forces on staging; sloshing |
| Ground-Level Tank | Service reservoir, sump | Hoop tension in walls; base pressure |
| Underground Tank | Underground sump, cistern | Uplift when empty; earth pressure |
| Reservoir | Large open storage basin | Long wall spans; temperature effects |
Liquid-retaining structures demand denser, less permeable concrete than ordinary RCC. IS 3370:2009 specifies stricter material requirements to ensure water-tightness throughout the structure's service life.
| Parameter | Requirement | Remarks |
|---|---|---|
| Minimum concrete grade | M30 | For all water-retaining surfaces; M25 not permitted |
| Minimum cement content | 320 kg/m³ | Ensures adequate paste for impermeability |
| Maximum w/c ratio | 0.45 | Lower w/c = lower permeability |
| Preferred cement type | PPC or PSC | Portland Pozzolana / Slag cement — reduced permeability, lower heat |
| Admixtures | Integral waterproofing compounds permitted | Must not affect concrete strength or durability |
| Aggregate | Well-graded; max size 20 mm for walls < 200 mm thick | Ensures proper compaction around reinforcement |
M30 is the Absolute Minimum Unlike IS 456 where M20 is the minimum for RCC, IS 3370 mandates M30 as the minimum grade for all water-retaining structures. Using M25 — even with waterproofing admixtures — does not comply with IS 3370:2009.
PPC / PSC Preferred Portland Pozzolana Cement (PPC) and Portland Slag Cement (PSC) are preferred over OPC for liquid-retaining structures. The pozzolanic reaction fills capillary pores, significantly reducing permeability and improving long-term durability.
Crack width control is the most critical design check in IS 3370. Even hairline cracks allow water seepage and lead to reinforcement corrosion. The 2009 revision introduced explicit crack width limits and calculation methods.
| Surface / Exposure | Limiting Crack Width | Remarks |
|---|---|---|
| Water-retaining face (normal) | 0.2 mm | Face in contact with stored liquid |
| Severe exposure / aggressive liquid | 0.1 mm | Sewage, chemicals, coastal environment |
| Non-water-retaining face | 0.3 mm | Per IS 456 general limit |
Crack Width Governs Steel Area In most liquid-retaining structures, the required steel area is governed by the crack width limit — not by the ultimate limit state (strength). This typically results in more steel than a pure strength design would require.
Cover requirements for liquid-retaining structures are more stringent than IS 456 general requirements, reflecting the aggressive environment and the need to protect reinforcement from moisture ingress.
| Surface / Location | Minimum Nominal Cover | Remarks |
|---|---|---|
| Water-retaining face | 45 mm | Face in direct contact with stored liquid |
| Non-water-retaining face | 35 mm | Outer face of tank walls, roof soffit |
| Underground structures | 50 mm | Walls and base slab in contact with soil/groundwater |
| Prestressed concrete | 50 mm (water face) | Additional protection for prestressing tendons |
Cover Tolerance IS 3370 requires that cover be maintained within ±5 mm of the nominal value. Use of proper cover blocks (chairs) is mandatory — tying bars to formwork or using inadequate spacers is not acceptable for water-retaining structures.
IS 3370:2009 adopts the Limit State Design framework of IS 456, with an additional serviceability check for crack width. The design must satisfy both the ultimate limit state (strength) and the serviceability limit state (crack width).
| Design Check | Method | Load Factor |
|---|---|---|
| Ultimate Limit State (strength) | Per IS 456 — factored loads | 1.5 (DL + LL) |
| Serviceability — Crack Width | IS 456 Annex F formula | 1.0 (service loads) |
| Prestressed tanks — no tension | Full prestress maintained under all loads | — |
| Direct tension design | Hoop tension = γ × w × r (cylindrical walls) | 1.5 (factored) |
Two-Stage Design Process Step 1: Design for strength using factored loads per IS 456. Step 2: Check crack width at service loads using IS 456 Annex F. If crack width exceeds the limit, increase steel area (not section size) until the crack width criterion is satisfied.
Joints are the most vulnerable locations for leakage. IS 3370 classifies joints by their function and specifies appropriate water stops and sealants for each type.
| Joint Type | Purpose | Water Stop Required | Spacing |
|---|---|---|---|
| Construction Joint | Between successive concrete pours | Yes — PVC or rubber water stop | As required by pour sequence |
| Expansion Joint | Accommodate thermal expansion/contraction | Yes — with flexible sealant | 20 – 30 m spacing |
| Contraction Joint | Control cracking due to shrinkage | Water stop + sealant | 10 – 15 m spacing |
| Movement Joint | Allow differential settlement | Full water stop assembly | At structural discontinuities |
Water Stop Continuity is Critical Water stops must be continuous around the full perimeter of the joint — including corners. Any break, splice failure, or displacement during concreting creates a direct leakage path. Inspect water stop position before every pour.
The roof slab of a liquid-retaining structure must be designed for structural loads and also provide a waterproof cover to prevent contamination of stored water and protect the structure from weathering.
| Parameter | Requirement | Remarks |
|---|---|---|
| Minimum slab thickness | 150 mm | Ensures adequate cover and waterproofing layer |
| Design method | Simply supported or continuous per IS 456 | Continuity reduces mid-span moments |
| Waterproofing layer | Required on top surface | Brick bat coba, IPS, or membrane waterproofing |
| Inspection manhole | Minimum one per compartment | For cleaning and inspection access |
| Ventilation | Required for enclosed tanks | Prevents vacuum/pressure build-up during filling/emptying |
| Slope of roof | Minimum 1:50 for drainage | Prevents ponding on roof surface |
Roof Live Load IS 3370 requires the roof slab to be designed for a minimum live load of 1.5 kN/m² for inaccessible roofs and 3.0 kN/m² for accessible roofs (per IS 875 Part 2). The weight of the waterproofing layer must also be included as dead load.
Every liquid-retaining structure must be tested for water tightness before being put into service. IS 3370 specifies a standardised water tightness test procedure with clear acceptance criteria.
| Test Step | Procedure |
|---|---|
| 1. Preparation | Allow concrete to cure for minimum 28 days; clean interior surfaces |
| 2. Filling | Fill tank with clean water to the design full supply level |
| 3. Absorption period | Allow 7 days for initial absorption into concrete pores (do not measure during this period) |
| 4. Measurement | Record water level at start and end of 24-hour observation period |
| 5. Acceptance | Check drop against acceptance criteria below |
| Acceptance Criterion | Limit | Basis |
|---|---|---|
| Maximum water level drop in 24 hours | 10 mm | Absolute limit |
| Maximum loss as fraction of tank capacity | 1/500 of tank capacity per day | Proportional limit for large tanks |
| Governing criterion | Whichever is less (more stringent) governs | |
Evaporation Correction During the 24-hour test period, evaporation losses must be measured using an open evaporation pan placed near the tank. The measured evaporation is subtracted from the total water level drop to obtain the actual leakage loss.
Failed Test Remediation If the tank fails the water tightness test, the source of leakage must be identified and repaired using approved crystalline waterproofing compounds or epoxy injection. The test must be repeated after repairs and the 7-day absorption period.