As per Indian Standard code (IS-456), what should be the minimum clear cover provided during the design of a water tank during moderate exposure conditions?
30 mm
Understanding Minimum Clear Cover in Water Tanks
Clear cover in reinforced concrete structures refers to the distance between the surface of the concrete and the nearest surface of the reinforcing bar. Providing adequate clear cover is crucial for protecting the steel reinforcement from corrosion and ensuring the durability and fire resistance of the structure. For water tanks, which are retaining liquids and are subjected to specific environmental conditions, the requirement for clear cover is particularly important.
IS 456 Provisions for Concrete Cover
The Indian Standard code IS 456:2000 provides guidelines for the design and construction of plain and reinforced concrete structures. This code specifies minimum clear cover requirements based on the type of structural element (like beams, columns, slabs, footings, walls, etc.) and the exposure conditions it will face throughout its service life.
Exposure Conditions as per IS 456
IS 456 categorizes exposure conditions into different levels to account for the environmental severity that can affect the concrete and reinforcement. These conditions include Mild, Moderate, Severe, Very Severe, and Extreme. Each condition corresponds to a different level of risk for deterioration, primarily corrosion of reinforcement.
- Mild: Concrete surfaces protected against weather or aggressive conditions.
- Moderate: Concrete surfaces exposed to severe rain, alternate wetting and drying, freezing while wet, or completely immersed in water.
- Severe: Concrete surfaces exposed to coastal environment, severe freezing and thawing, or corrosive fumes.
- Very Severe: Concrete surfaces exposed to sea water spray, corrosive ground, or severe abrasion.
- Extreme: Concrete surfaces exposed to tidal zone, severe chemical attack.
The question specifies moderate exposure conditions for the water tank design.
Minimum Clear Cover for Water Tanks (IS 456, Table 16)
Table 16 of IS 456:2000 provides minimum clear cover requirements for different members and exposure conditions. While the table primarily lists requirements for beams, columns, slabs, etc., structures like water tanks, which retain liquids, need special consideration for crack control and durability. IS 3370, the code for concrete structures for the storage of liquids, should also be consulted, but IS 456 provides the fundamental cover requirements based on exposure.
For members like walls and slabs in a water tank, which are critical for preventing leakage and ensuring durability under continuous water exposure (which falls under Moderate exposure), the minimum cover specified for Moderate conditions in IS 456 is relevant.
Referring to IS 456, the minimum clear cover requirements according to exposure conditions for general structural members are:
| Exposure Condition | Minimum Clear Cover |
|---|---|
| Mild | 20 mm |
| Moderate | 30 mm |
| Severe | 45 mm |
| Very Severe | 50 mm |
| Extreme | 75 mm |
Based on this table, for moderate exposure conditions, the minimum clear cover specified is 30 mm.
Water tanks are typically designed considering moderate or more severe exposure conditions due to the presence of water. Under moderate exposure, a clear cover of 30 mm is required as per IS 456 for maintaining durability and protecting the reinforcement.
Conclusion
For the design of a water tank under moderate exposure conditions, the minimum clear cover to the reinforcement, as specified by IS 456, is 30 mm. This cover is essential for preventing corrosion of the steel bars due to the continuous presence of water and moisture associated with a water tank.
Revision Table: IS 456 Clear Cover Basics
| Aspect | Key Point |
|---|---|
| Purpose of Clear Cover | Protects reinforcement from corrosion, ensures durability and fire resistance. |
| Governing Code (India) | IS 456:2000 (and IS 3370 for liquid retaining structures). |
| Factors Affecting Cover | Type of member, exposure condition. |
| Moderate Exposure Definition | Exposed to severe rain, alternate wetting & drying, completely immersed in water. |
| Min Cover (Moderate Exposure) | 30 mm (as per IS 456 Table 16). |
Additional Information: Designing Liquid Retaining Structures
While IS 456 provides general guidelines, IS 3370 specifically deals with concrete structures for the storage of liquids (water tanks). IS 3370 focuses on design for preventing leakage by controlling crack widths. It specifies permissible crack widths for different types of liquids and exposure conditions. Although IS 3370 emphasizes crack control design, the fundamental requirement for minimum clear cover based on environmental exposure, as given in IS 456, still applies to ensure the long-term durability of the concrete itself and protection of the steel against corrosion, which is a primary concern in water tanks.
Important Questions from Introduction
In the following table, the left column contains the names of standard graph algorithms and the right column contains the time complexities of the algorithms. Here, n and m are number of vertices and edges, respectively. Match each algorithm with its time complexity.
List I List II Standard graph algorithms Time complexities A. Bellman‐Ford algorithm I. O(m*log n) B. Kruskal’s algorithm II. O(n 3) C. Floyd‐Warshall algorithm III. O(n*m) D. Topological sorting IV. O(n + m) Choose the correct answer from the options given below :
How many cards must be selected from a standard deck of 52 cards to guarantee that at least three hearts are present among them?
Match List 1 with List 2 and choose the correct answer from the code given below:
List I
(Graph Algorithm)
List II
(Time Complexity)
a) Dijkstra’s algorithm
i) Θ(E log E)
b) Kruskal’s algorithm
ii) Θ(V 3)
c) Floyd-Warshall algorithm
iii) Θ(V 2)
d) Topological sorting
iv) Θ(V + E)
Where V and E are the number of vertices and edges in graph respectively.
The solution of recurrence relation: T(n)=2T(sqrt(n)) + lg(n) is
Modulus of elasticity of concrete, E is calculated using:
