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The terrace water storage tank is generally constructed using brick masonry, which is placed on an elevated platform to provide a reserve supply of water for daily house activities. The construction of a water storage tank with the use of brick masonry is a low-cost option for storing water.
In this article, we discuss the important specification of construction of masonry water storage tank on a terrace :
Location of Tank
The location of the water storage tank shall follow the below specification-
- It must be near to connections or fitting for which the water is supplied so that long horizontal length of delivery pipe is avoided.
- It should not be visible, as far as possible, from the main road and compound of the building.
- In the case the tank goes out of order, the overflow pipe shall discharge at a convenient and visible point so that it does not damage the building.
- A bottom clearance of not less than 30 cm shall be provided to permit easy clearance of the terrace below the tank.
Size of the Tank
The size of the tank is governed by the requirement of the capacity of water storage. If the requirement is more, and more than one tank is to be provided at one place, a larger tank of combined net capacity may be built with suitable internal partitions to achieve an economical design.
The suggestive sizes of tanks depending on the capacity of water, with a freeboard of 15 cm are given in Table 1 below-
Table 1: Size of the tank with respective water storage capacity.
|Net Capacity(Liters)||Internal size (L X B X h)(cm)|
|300 liters||70 x 70 x 75|
|600 liters||90 x 90 x 90|
|900 liters||105 x 105 x 90|
|1200 liters||120 x 120 x 100|
|1400 liters||120 x 120 x 115|
|1600 liters||120 x 120 x 120|
|1800 liters||130 x 130 x 125|
Construction of Overhead Masonry Water Tank on Terrace
1. Construction of Bottom Slab
- The bottom slab of the tank shall be constructed of reinforced cement concrete 1:1:5:3 (1 cement: 1:5 coarse sand : 3 stone aggregate 20 mm nominal size).
- The bottom slab shall be supported on structurally adequate supports of brick masonry walls / reinforced cement concrete beams / or rolled steel sections and shall be given a mild slope towards the scour pipe outlet.
- The bottom slab thickness shall be 10 cm for sizes given in table 1, and the reinforcement details for the slab shall be as given in figure 2.
- For sizes other than those given above, or a combination of tanks, the structural details shall be supplied by the structural engineer.
- If the slab is supported on brick masonry walls, the top of the walls shall be provided bearing plaster.
- If the slab is supported on the RCC beam, this can be either integral or simply supported as the situation may prevail.
- A slab with simply supported conditions on the beam is preferable. If it is to be simply supported, the beam top shall be rendered with a coat of neat cement and provided with a thick coat of limewash.
2. Construction of Tank Walls
- The walls of the tanks shall be constructed using brick masonry in cement mortar 1:3 (1 cement : 3 coarse sand) of class 7.5N of bricks.
- The first course of brick masonry shall be laid immediately after concreting of the base slab, i.e., when the concrete is still green to form a good bond and avoid leakages or cracks.
- The remaining masonry shall be raised and complete within a week of laying the slab.
3. Installation of Tank Fittings
The fittings recommended for the masonry water storage tank shall be of galvanized iron(GI), and the same shall be assumed in the following specification.
A. Scour Pipe
A 40 mm dia pipe is joined with a 40 mm 'T' pipe to prevent slipping, which shall be embedded inside the wall to serve as a scour pipe. The end of this scour pipe shall be provided with a socket and plug, as shown in figure 2 (Detail A).
B. Delivery Pipe
The delivery pipe shall be of 20 mm dia fixed with 'T' to prevent slipping which shall be fixed at least 30 mm above the bottom slab of the tank to prevent silt at the bottom of the tank entering the pipeline and choking it as shown in figure 2 (Detail B).
C. Overflow Pipe
The overflow pipe shall be of 25 mm, socket at the internal end, and shall be fixed at the freeboard level of the tank at a convenient point to drain out the overflow. The length of the pipe, including the socket, shall be 30 cm.
D. Inlet Pipe
The inlet pipe shall be 20 mm with a 20 mm socket at the internal end and socket or elbow at the outer end as required, which shall be fixed at the freeboard level, as shown in figure 2 (Detail C).
E. Mosquito Proof Coupling
The mosquito-proof coupling of rigid PVC or any other suitable material with sieve No. 725 dia perforations shall be provided to the overflow pipe to check the entry of mosquitos into the tank, as shown in figure 2 (Detail D).
F. Ball Valve
The ball valve shall be of brass with a plastic float of a specified size and pressure. The ball valve shall be securely fixed to the tank in continuation of the inlet pipe and adjusted to limit the level of the water in the tank at 25 mm below the lip of the overflow pipe.
4. Plastering of Tank
- The base slab and the sidewalls shall be plastered from inside with 15 mm thick cement mortar 1:3 (1 cement : 3 fine sand).
- The plastering activity shall be carried out within one week of construction of the side walls.
- The junctions of the wall and the base slab in the interior and those between walls shall be rounded with mortar 1:3 to reduce the water pressure on the junctions.
- All the joints of the inlet, delivery, scour, and overflow pipes shall be made leak proof entirely.
- A coat of neat cement punning shall be applied to the plaster on the surface of the walls and the base slab on the inner side of the tank.
- The same shall be carried out at the top of the tank to serve as bearing plaster for the top side.
- The external wall of the tank shall be plastered with 12 mm thick cement mortar 1:4 (1 cement: 4 fine sand).
4. Curing of RCC and Brickwork
- The curing of RCC, brickwork, and plastering shall be as per general specifications of curing.
- The tank shall be filled with water on the first day for half the capacity and to the full level a day thereafter.
- In order to prevent cracks from developing in concrete due to shrinkage, the water in the tank shall be kept until the tank is put into use.
5. Construction of Top Slab
- The top slab shall be of precast reinforced cement concrete of 1:2:4 mix, 5 cm thick.
- The details of reinforcement shall be as shown in figure 2.
- The size of the top slab shall be such that it projects by 15 mm on all sides beyond the external finished faces of the tank wall.
- The top of the slab shall be rendered smooth while casting.
- While casting the top slab, the frame of the C. I. hinged cover shall be fixed beforehand so that it is embedded in concrete.
6. Testing of Tank
- The tank shall be tested for water tightness at full supply level.
- The requirement of the test shall be deemed to be satisfied, if the external faces show no sign of leakage and remain dry throughout the observation of seven days, after allowing seven days for absorption after filling.
The masonry tanks are generally strong, long-lasting, and keep water cooler due to the insulation value of concrete.
The cost of the masonry tank shall vary country wise depending upon the cost of material and labor. Probable cost per m³ for different countries is mentioned below-
1. US - $20 per m³
2. Kenya - $37 per m³
3. Sri Lanka - $28 per m³
The sizes of the tank for different storage capacity is tabulated below-
The walls of the tanks shall be constructed using brick masonry in cement mortar 1:3 (1 cement : 3 coarse sand) of class 7.5N of bricks.