Water Tanks

A water tank is generally used for storing of water to make usage for the daily requirements and chores. Generally, water tanks can be distinguished in three main heads:

  • Tanks rested on ground level
  • Underground tanks
  • Elevating tanks supported through staging

On the basis of shape, water tanks are of different types, such as:

  • Rectangular tanks
  • Spherical tanks
  • Circular tanks
  • Intze tanks
  • Spherical/circular tanks with conical bottoms

During the construction of concrete structure for storing water, or other liquids, the impermeableness of the concrete structure is very important.  Permeability of any uniform and properly firmed concrete of given mixed proportions mostly depends on the cement and water ratio.

The raise in water and cement ratio leads to raise in the permeability. The decrease in water and cement ratio will thus be sought-after to lessen the permeability, however highly lessened water cement ratio may lead to compaction complications and show to be injurious as well.

For a given mixture prepared with selected materials, there is a lesser limit to water and cement ratio which could be used cost-effectively on any job. It is very important to choose a richness of mixture attuned with existing aggregates, whose particle grading and shape play an important bearing on expediency, which should be well-matched to all the means of compaction chosen. Proficient compaction if possible by vibration is crucial. It is enviable to indicate cement content amply high to guarantee that methodical compaction is available while sustaining a satisfactorily low water and cement ratio.  Quantity of the cement cannot be lesser than 330 kg/m3 of concrete. However, it also has to be less than 530 kg/m3 of concrete to keep the shrinkage/contraction lower.

In thicker areas, where a cutback in cement content may be wanted to limit the temperature increase due to the cement hydration, low cement content is usually permitted.

It is normal to make usage of rich mix like M 30 grade in majority of the water tanks.

The initial blueprint/design of liquid preserving structure has to be based on prevention of cracking in the concrete in regards to its tensile strength. We have to make sure that in its design that the concrete will not crack on its water face. Cracking may also occur due to the moderation to shrinkage, free extension, and shrinkage of concrete which happens due to temperature and swelling and shrinkage due to moisture effects. Right placing of back up reinforcement, usage of petite sized bars and usage of distorted bars will lead to diffused distribution of the cracks. The perils of cracking due to general temperature and contraction effects might be diminished by limiting the altering in moisture content and temperature to which the whole structure is subjected. The cracks can be barred by shunning the usage of thick and broad timber shuttering which puts off the simple getaway of heat of hydration from concrete mass. The hazards of cracking can also be diminished by dropping the restraints on free expansion or shrinkage of the structure.

For long slabs or walls established at or underneath the ground level, limitations can be diminished by establishing the structure· on a plane layer of concrete with the inter position of descending layer of some materials to break the bond and encourage movement within. Despite, it ought to be perceived that basic and more serious reasons of spillage, other than splitting or cracking, are deformities, for example honey combing and segregation and specifically all joints are probable sources of spillage.

General Design Requirements in accordance to the Indian Standard

CODE OF PRACTICE (IS: 3370 – Part IT, 1965)

  1. Plain Concrete Structures: Plain solid members from fortified concrete fluid structures might be planned and designed against auxiliary disappointment by permitting strain in plain concrete according to as far as possible for pressure in twisting indicated in IS : 456 – 2000 (i.e. admissible stress in tension in bending might be taken to be the same as allowable stress in shear, q measured as slanted pressure). This will consequently deal with failure because of splitting/cracking. In any case, ostensible reinforcement as per the necessities of IS: 456 might be accommodated plain cement/concrete structural members.
  1. Permissible stresses in concrete: Indian Standard Code IS: 456-2000 does not state the permitted stresses in concrete for its strong resistance to cracking. On the other hand, its previous version (IS: 456-1964) incorporated the permitted stresses in straight tension, shear, and bending tension. These values are shown in the Table below. The permitted tensile stresses because of the bending apply to face of member in contact with fluid. In members with depth less than 225 mm and in contact with fluid on one side, permissible stresses

Permissible concrete stresses in calculations relating to resistance to cracking

  1. b) For calculations of strength: In strength calculations, the normal permitted stresses, in agreement with IS: 456-2000 is used. Where the intended shear stress in concrete above goes beyond the permitted value, reinforcement acting in concurrence with sloping compression in concrete will be given to take whole of shear.
  2. Permissible stresses in steel reinforcement
  3. a)Resistance to cracking: When steel and concrete are supposed to act jointly for checking tensile stresses in concrete for prevention of splitting the tensile stresses in steel will be inadequate by the prerequisite that the permitted tensile stress in concrete does not exceed so tensile stresses in steel will be ‘equal to the product of modular ratio of concrete and steel, and the analogous permissible tensile stress in the concrete.
  4. b)For strength calculations:Even though the Indian Standard Code IS: 456 had its 4th revision in 2000, the analogous Codes IS: 3370 (Part I, II, III and IV) for concrete structures for the storing of fluids have not been amended since 1965. The major Code on concrete-IS: 456 is in SI units. But, the 4th reprint done on May 1982 of IS: 3370 (Part 11)-1965 includes the modifications concerning the permitted stresses in steel reinforcement. The modified values of permitted stresses are given in Table. Converted into SI units, with usage of the approximation 10 kg/cm2 = 1 N/mm2

Steel Reinforcement

a) Minimum reinforcement:

(i) The bare minimum reinforcement in walls, floors and roofs in each of the 2 directions at right angles should have the area of 0.3 percent of concrete area in the direction for areas or sections up to 100 mm depth. For areas of depth higher than 100 mm and lesser than 450 mm the least amount reinforcement in each of 2 directions should be linearly diminished from 0.3 percent for 100 mm depth/thick section to 0.2 percent for 450 mm, bare minimum reinforcement in both of the two directions should be kept at 0.2 percent. In concrete sections of depth 225 mm or higher, 2 layers of reinforcing bars should be located one near each face of area to make up the least amount of  reinforcement mentioned above.

    (ii) In particular situations, floor slabs resting straight on the land may be built with percentage of reinforcement lesser than mentioned above. However, in no case percentage of reinforcement in any of the member should be less than 0.15 % of concrete section.

b) Minimum cover to reinforcement :

(i) For fluid faces of parts of members in contact with the fluid or encircling the space above fluid (like internal faces of the slab), the least amount of cover to all reinforcement have to be 25 mm or diameter of main bar, either is greater of them. In the vicinity of nautical environment and soils and water of acidic nature the cover should be augmented by 12 mm but this extra cover should not be taken into consideration for design calculations.

(ii) For faces away from fluid and for parts of construction neither in contact with fluid on any side or encircling the face above the fluid, the cover has to be the same as given for other reinforced concrete areas.

Joints in Water tanks

The different types of joints may be classified under three headings:

(a) Construction joints

(b) Movement joints

(c) Temporary open joints.

Construction joints: It is a joint in concrete brought forward for the convenience in building at which unique actions are taken to attain subsequent permanence without prerequisites for additional relative movement. It is, as a result, a stiff joint in comparison to a movement joint which is elastic joint.

Movement joints: These involve the integration of unique resources in    order to uphold water-tightness while co-operating relative faction amid the faces of the joints. All the movement joints are fundamentally flexible joints. Movement joints are of three types:

  • Contraction joint
  • Expansion joint
  • Sliding joint

Temporary open joints:A temporary open joint is a space momentarily left parts of a formation which after a appropriate period and before formation is put into use, is packed with concrete or mortar wholly with enclosure of appropriate jointing matter. In the previous case the girth of gap ought to be adequate to allow the faces to be primed before filling. Where actions are taken for instance, by the addition of appropriate joining resources to uphold the water-tightness of concrete succeeding to the filling of joint, this type of joint might be considered as being equal to a reduction joint (partial/complete) as clear.

Circular Tank with elastic joint between floor and wall

When liquid is filled in the circular tank, the hydrostatic water pressure will attempt to enhance its diameter at any segment. But, this swell in the span all along height of tank will be determined on the character of the joint at the junction B of the wall and the bottom slab. If the joint at B is elastic, it will be liberated to move away from to the position B1. The hydrostatic pressure at A is 0, and for this reason, there shall not be any change in the diameter at A. The hydrostatic pressure at B will be highest, resulting in highest increase in diameter, and thus maximum movement at B if joint is elastic.