Construction | Centre for Science and Environment


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.
Back to Top

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

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

Source: Action for food Production and United Nations Children's Fund, Rooftop rainwater harvesting systems

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

Source: Action for food Production and United Nations Children's Fund, Rooftop rainwater harvesting systems

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

a) Construction of a new recharge well

 

Step 1: Excavating the earth

Step 2: Making a borehole to facilitate groundwater recharging

Step 3: Providing masonry or RCC walls in the excavated portion and thereafter providing the filter materials.

Step 4: Covering the tank made with a RCC or stone slab provided with a manhole.


Step 1:
Replace top few metres of the cast iron casing pipe of the dried tubewell with a perforated poly Vinyl chloride (PVC) pipe.

Step 2:
Wrap the perforations with a screen-made of either coir screen or closely knit nylon mesh.

 

Follow us on 
 
 
 
 
 
 
 
 
 
 

 
 
 
 
 
 
gobar times