CONCRETE WATER TANK CONSTRUCTION
Residential water tanks and overhead water tanks are very crucial since the people can obtain an uninterrupted 24 x 7 hours water supply for drinking, cooking, gardening, car washing, and washing clothes in residential homes and multi storied apartments. Concrete water tanks are robust and long-lasting and come in different types of shapes like rectangular and circular. The main ingredients of concrete water tank are reinforced cement concrete, ferrocement, geopiers or other suitable materials which have the strength to resist both the load of the water and tank as well as the uplift forces as a result of overturning. The design of reinforced concrete water tank is created on the basis of IS 3370: 2009 (Parts I – IV). The design varies on the location of tanks, i.e. overhead, on ground or underground water tanks.
Basis of Concrete Water Tank Design While designing the RCC water tank, consideration should be given on adequate resistance strength against cracking to get rid of leakage. The design is made on the basis of the following assumptions :•
Prior to bending, plain section should be plain after bending
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Both concrete and steel should be properly elastic and modular ratio value should contain a value provided in IS 456- Table 21.
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While computing the stresses, both for flexural and direct tension or combination thereof concerning resistance to cracking, the whole section of concrete along with the cover as well as reinforcement.
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The concrete tensile strength should not be considered throughout the strength calculation.
Allowable Stress On Concrete For Resisting Cracking There should not be any leakage in water tank. For this purpose, M 20 grade and greater concrete should be used, and concrete near water face should be such that no crack takes place. The wall thickness of the water should be designed in such a manner that the stress on the concrete is smaller. In members with thickness lower than 225mm and in contact with liquid on one side, these allowable stresses in bending apply also to the face remote from the liquid.
Allowable Stress In Steel The stress in steel should not be permitted to surpass the following values under several positions to stop cracking of concrete. •
When steel is provided adjacent to the face of the members in contact with liquid 115 N/mm2 for mild steel Bars and 150 N/mm2 for high strength deformed bars.
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When steel is provided on the face far from the liquid for members 225 mm or more in thickness then allowable stress in steel should be 125 N/ mm2 for mild steel bars and 190 N/ mm2 for high strength deformed bars.
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If steel is provided on face far from liquid for members lower than 225 mm in thickness similar to former.
Floors of Reinforced Concrete Water Tank Movement Joints Movement joints should be placed compliant with IS 3770 (part I)
Floor of tanks rest on support •
It should be designed for bending moments as a result of dead load and water load.
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While designing the floor of multi-cell water tank, special attention should be given.
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If the walls and floor are attached firmly, then moment at the junction along with other transferred loads should be taken into consideration in floor design.
Concrete Water Tank Walls Provision Of Joints Sliding joints should be applied in the following situations: •
When the walls are desired to be extended or contracted discretely from the floor.
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To resist moments at the base of the wall due to fixity to the floor.
RCC Water Tank Roof •
To get rid of sympathetic cracking, make sure that movement joints in the roof correspond with those in the walls, when the roof and walls are monolithic.
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When the tank is used for the purpose of preserving the water for domestic purpose, the roof should be made water-resistant.
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It can be obtained by restricting the stresses as for the rest of the tank, or by applying the covering of the waterproof membrane or by creating the slopes to retain sufficient drainage.
Minimum Reinforcement For RCC Water Tank •
Minimum reinforcement for 199mm thick sections should be 0.3 % of the area of concrete section that is decreased linearly to 0.2% for 450 mm thick sections.
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The floor slab for tank that is situated on ground the minimum reinforcement should not be lower than 0.3% of the gross sectional area of the floor slab.
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When the thickness of the section (wall, floor or roof slab of the tank) remains 225 mm and over two layers of reinforcing steel should be provided, one near each of the section to form the minimum reinforcement requirements.
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