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Construction
Procedures and specifications for construction
of storage and recharge tanks are explained below. There are
a lot of similarities in the construction steps of both storage
and recharge structures.
I. Masonry Tanks:
- When building brick walls for
water tanks, both horizontal and vertical joints are filled
with mortar of a ration of 1:4. For obtaining maximum strength,
lay out a circle of bricks or blocks on the foundation without
mortar, with such spacing that no brick or block is cut
to fit into the circle. A proper foundation of cement concrete
will also have to be provided.
- Each brick or block should be
dipped in water to saturate and make it waterproof. Thereafter
these bricks are laid upon the cement mortar.
- Bricks and blocks in walls should
be sprinkled with water just before plastering for bonding.
The right way to apply plaster is to throw a thin coat of
mortar (1:3) on to the inner wall and then a thin coat of
1:4 mortar on the outer wall while the first coat settles,
and so on until the required thickness of plaster is reached.
The surface of each coat of plaster, except the final one,
is made rough to make sure there is good bonding between
coats.
- For simplicity and maximum strength,
walls built of burnt bricks, or blocks made from compressed
sandy soil and cement, rubble stones and concrete are reinforced
after they have been built to their final height.
- Plaster on the walls of water
tanks must not be allowed to dry or be exposed to sunshine
for the first three weeks. The process of keeping the mortar
wet is called curing. Covering the walls with polythene
sheeting or plastic sacks, which must be properly secured
against the walls using the sisal strings, does this.
Water is poured between the wall and the sacks or polythene
morning and evening for three weeks. The external wall can
be made weather proof (if the tank is above the ground level)
with two coats made of 1 part cement to 10 parts lime.
II. Reinforced Cement
Concrete Tank (RCC)
Reinforced concrete tanks can be built above or below the ground.
Concrete is durable and long-lasting, but is subject to cracking.
An advantage of concrete cisterns is their ability to decrease
the corrosiveness of rainwater by allowing the dissolution of
calcium carbonate from the walls and floors. Each tank must
have an overflow system for situations when excess water enters
the tank. The overflow can be connected to the drainage system.
Design and construction of reinforced
cement concrete tanks shall comply with the requirements of
IS 3370 (part-I)-1965 and IS 456-1964. Accordingly the mix
of cement concrete shall not be leaner than 1:2:4 ( 1 part
cement: 2 parts of coarse sand and 4 parts of stone aggregates
of 20 mm nominal size)
When
constructing water tanks it is essential to adhere to a few
basic yet critical rules with respect to correct mixtures and
applications of concrete and mortar. These include:
- Mixing cement, aggregate and
water properly, and not adding too much water
- Applying the mortar or concrete
within a maximum of half an hour of mixing
- Curing cement work properly by
keeping it moist and under shade for at least three weeks
after its application.
III. Ferro Cement
Jars:
Ferrocement consists of a thin sheet of cement mortar which
is reinforced with a cage made of wire mesh and steel bars.
Because ferrocement is structurally more effectient than masonry,
the thickness of the walls of the container are as low as 10
to 15 mm. Ferrocement components can be casted in any shape
using suitable moulds. The technology is extremely simple to
implement, and even semi-skilled workpersons can learn it with
ease. Ferrocement requires only a few easily available materials
- cement, sand, galvanized iron (GI) wire mesh, and mild steel
(MS) bars - in small amounts compared to masonry and RCC.
a. Pot shaped container:
The process of construction of a pot shaped ferro cement container
is quite simple. The only materials required are hessian cloth,
chaff (waste from agricultural produce), GI wire mesh, MS bars,
cement and sand.
Preparation of mould: The
hessian cloth is first stiched into a sack resembling the
shape of a container. It is then filled with chaff that is
compacted in layers. Dry leaves or dry grass can also be used
in place of chaff. Once the sack is filled with the filler
material, it is beaten into the required shape by a wooden
bat.
Laying
of reinforcement: A GI wire mesh (22-26 guage -
see table) is tied around the mould leaving sockets at suitable
locations for inlet, over flow and cleaning pipes. Tying 6
mm diameter MS bars at wide intervals both horizontally and
vertically strengthens the reinforcement cage.
Preparation
of cement mortar for plastering: Cement mortar
of suitable proportion (see table) is prepared, having water
content equal to 0.45 times the volume of cement.
|
Capacity of containerLitres
|
Thickness of the walls
|
Ratio Cement: sand
|
Thickness of GI wire (guage)
|
| 400 |
10 |
1:3 |
26 |
| 600 |
10 |
1:3 |
24 |
| 900 |
12 |
1:2:5 |
24 |
| 1500 |
15 |
1:2:5 |
22 |
Plastering:
The mortar is plastered in two layers along the wall thickness,
the second layer being applied 24 hours after the first. The
ferro cement wall normally has a thickness of 10 to 15 mm,
depending on the volume of the container. The cement mortar
is applied ensuring a minimum clearance (cover) 3 mm between
the reinforcement mesh and the outer surfaces of the wall.
Removal
of mould: The mould
of the container is removed 24 hours after casting of the
wall is completed, by removing the filler material. The container
can be brought into use after 10 days of wet curing.
b. Ferrocement Tank using Skeletal
Cage:
Phases of construction
i Selection of site
ii Marking for circular
foundation:
Choose the diameter of foundation (Df) for required storage
capacity from the table
|
Capacity of storage tank (litres)
|
5,000 and 6,000
|
7,000 and 8,000
|
9,000 and 10,000
|
| Df |
2.40 m |
2.70 m |
3.00 m |
iii Excavation
for foundation
iv Compacting the excavated pit
v Placing cement concrete in foundation:
Prepare Plain Cement Concrete of 1:4:8 mix ( 1 cement: 4 sand:
8 stone aggregate of 40mm size)
vi Erection of mould/ Preparation of elements of skeletal
cage
vii a. Preparation of Elements of Skeletal Cage:
Skeletal cage is an assembly of
4 types of elements (of different shapes) made from mild steel
rods. They are
- 'U' shaped elements
- 'L' shaped elements
- 'ë' shaped elements
- 'O' shaped elements
Dimensions of elements for tank capacities 5,000 litres to
10,000 litres
|
Element
|
No.
|
Dimensions
|
Capacity of Storage Tank (in litres)
|
| |
|
|
5,000 |
6,000 |
7,000 |
8,000 |
9,000 |
10,000 |
| u |
2 |
H |
1.8 |
2.1 |
1.9 |
2.1 |
1.9 |
2.1 |
| |
|
W1 |
2.05 |
2.05 |
2.35 |
2.35 |
2.65 |
2.65 |
| |
4 |
H |
1.8 |
2.1 |
1.9 |
2.1 |
1.9 |
2.1 |
| L |
|
W2 |
0.82 |
0.82 |
0.95 |
0.95 |
1.05 |
1.05 |
| |
8 |
H |
1.8 |
2.1 |
1.9 |
2.1 |
1.9 |
2.1 |
| L |
|
W3 |
0.5 |
0.5 |
0.6 |
0.6 |
0.65 |
0.65 |
| |
|
D1 |
9Nos |
11Nos |
10Nos |
11Nos |
10Nos |
11Nos |
| |
|
|
2.05 |
2.05 |
2.35 |
2.35 |
2.65 |
2.65 |
| |
1 |
D2 |
1.25 |
1.25 |
1.41 |
1.41 |
1.60 |
1.60 |
| |
1 |
D3 |
0.62 |
0.62 |
0.71 |
0.71 |
0.84 |
0.80 |
Notes:
* Refer to above table for dimensions and number
of each of these elements for 5,000, 6,000, 7,000, 8,000, 9,000
and 10,000 litres capacity system
* Use 6 mm diameter rods for preparing 'U', 'L' and
'ë' shaped elements. Use 4 mm diameter GI wire for "O"
shaped elements (Circular rings)
* Straighten, cut and bend the mild steel rods to form
these elements
* Adopt an overlap length of 10 cm in forming the circular
elements.
b. Assembling the elements:
- Place the two 'U' shaped rods
vertically over the foundation, perpendicular to each other
- Place the outer, middle and
inner rings over the two 'U' shaped rods, coinciding with
the circular marking and tie the intersections with binding
wires
- Place and tie 4 'L' shaped elements
on the center marking of each quarter, each rod extending
upto the inner most ring
- Place and tie 8 'ë' shaped
elements on the remaining markings, each element extending
to the middle ring
- Place and tie all the rings of
diameter 'D1" over the vertical reinforcement at a
uniform spacing of 20 cm
- For providing cylindrical shape
to the skeletal cage, fix cross bars at the top of skeletal
cage and ie with ropes, 3-4 vertical rods to wooden pegs
pegged to the ground.
c. Tying of mesh over skeletal cage:
Select the reinforcement mesh that suits the capacity of the
tank from the table below:
|
Capacity of Tank (Lt)
|
5,000 & 6,000
|
7,000 & 8,000
|
9,000 & 10,000
|
| Specification of wire
mesh |
Chicken wire mesh of
22 gauge and 12 mm (1/2") opening |
Chicken wire mesh of
20 gauge and 25 mm (1") opening |
Chicken wire mesh of
20 gauge and 25 mm (1") opening |
Note: Woven wire mesh of rectangular
opening, of same specifications mentioned above, can be used
if chicken wire mesh is not available
Wrap two layers of selected mesh, one layer on the outer side
and one layer on the inner side of the skeletal cage. Tie
the mesh with binding wire to the skeletal cage at all intersections
of elements
Provide a tucking length of 30 cm (1 foot) at the base
Project the mesh 10 cm above the top of the skeletal cage
Cut the skeletal cage and insert pipe fixtures such as overflow
pipe, drain pipe and tap at appropriate places as given in
table
| Over flow pipe |
10 cm below the top of
cage |
| Drain pipe |
5 cm above the foundation |
| Tap |
10 cm above the foundation |
viii. Plastering the tank's outside wall
- Prepare cement slurry (cement
mixed with water) and add anti-rust agent (chrometrioxide
tablets)
- Apply one coat of cement slurry
(mix of cement and water) over the mesh using a painting
brush
- Prepare cement mortar of depending
on capacity of tank
- Apply the first coat of cement
mortar on the outer surface at a thickness of 1 cm. Care
has to be taken to fill the space between the two layers
completely. This could be done by using a GI sheet, slightly
curved in shape to be held close to the skeletal cage from
inside by a person, while cement mortar is applied by another
from outside
- Leave 10 cm of mesh projected
above the cage unplastered in order to join the skeletal
dome to the tank
- After two hours, apply a second
coat of mortar of a thickness of 1 cm.
ix.
Plastering the tank's inside wall
- After two hours of outside plastering,
apply cement slurry to the inner surface of the tank wall
- Prepare cement mortar of 1: 3
mix and add waterproof compound in liquid form
- Apply first coat of cement mortar
of 1 cm thickness on the inner surface, starting from bottom
of the tank moving laterally and progressing towards the
top
- After two hours, apply second
coat of mortar to attain a total wall thickness of 2 cm
- Apply cement slurry as final
coat on outer and inner surfaces of tank and smoothen using
coir brush
x.
Removal of mould
xi.
Casting of tank floor:
- Sprinkle cement slurry over
the foundation concrete
- Prepare plain cement concrete
of 1:2:4 mix ( 1 cement: 2 sand: 4 stone aggregate of 12
mm size), pour it over the base and compact to a thickness
of 50 mm (2 inch)
- Finish the floor base using cement
mortar keeping the slope towards the drain pipe
- Finish the wall and base joints
(inner and outer) with cement mortar
- Twelve hours after setting the
tank floor, add waterproof compound (liquid form) with cement
slurry and apply it over inside surface of the tank and
smoothen with coir brush
xii. Curing the tank
- Cure the tank for 14 days by
pouring water thrice a day or covering the tank with wet
gunny bags
- In coastal areas, after curing
for 14 days, apply rust proof paint over the outer surface
of tank wall
xiii.
Construction of roof for the tank
- An assembly of mild steel elements
is prepared as a skeletal frame for the roof. Chicken wire
mesh is tied over it and plastered in cement mortar
- The roof is provided with two
openings. One is an opening of diameter 35 cm for accommodating
the filter container. Another is a manhole with a 60 cm
opening. The opening for the filter will be on one side
of the roof. The manhole is provided at the centre of dome
Recharge well
a) Construction of a new recharge
well
b) Conversion of a dried up tube well
into a recharge well
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