Water Tanks

A water tank is used to store water to tide over the daily requirements. In general, water tanks can be classified under three heads:

(i) tanks resting on ground

(ii) elevated tanks supported on staging, and

(iii) underground tanks.


From the shape point of view, water tanks may be of several types, such as

(i) circular tanks

(ii) rectangular tanks

(iii) spherical tanks

(iv) Intze tanks and

(v) circular tanks with conical bottoms.

In the construction of concrete structures for the storage of water and other liquids, the imperviousness of concrete is most essential. The permeability of any uniform and thoroughly compacted concrete of given mix proportions is mainly dependent on the water-cement ratio.

The increase in water-cement ratio results in increase in the permeability. The decrease in water-cement ratio will therefore be desirable to decrease the permeability, but very much reduced water-cement ratio may cause compaction difficulties and prove to be harmful also.

For a given mix made with particular materials, there is a lower limit to the water-cement ratio which can be used economically on any job. It is essential to select a richness of mix compatible with available aggregates, whose particle shape and grading have an important bearing on workability, which must be suited to the means of compaction selected. Efficient compaction preferably by vibration is essential. It .is desirable to specify cement content sufficiently high to ensure that thorough compaction is obtainable while maintaining a sufficiently low water-cement ratio. The quantity of cement should not be less than 330 kg/m3 of concrete. It should also be less than 530 kg/m3 of concrete to keep the shrinkage low.

In thicker sections, where a reduction in cement content might be desirable to restrict the temperature rise due to cement hydration, lower cement content is usually permissible.

It is usual to use rich mix like M 30 grade in most of the water tanks.

Design of liquid retaining structure has to be based on the avoidance of cracking in the concrete having regard to its tensile strength. It has to be ensured in its design that concrete does not crack on its water face. Cracking may also result from the restraint to shrinkage, free expansion and contraction of concrete due to temperature and shrinkage and swelling due to moisture effects. Correct placing of reinforcement, use of small sized bars and use of deformed bars lead to a diffused distribution of cracks. The risk of cracking due to overall temperature and shrinkage effects may be minimized by limiting the changes in moisture content and temperature to which the structure as a whole is subjected. Cracks can be prevented by avoiding the use of thick timber shuttering which prevent the easy escape of heat of hydration from the concrete mass. The risk of cracking can also be minimized by reducing the restraints on the free expansion or contraction of the structure.

For long walls or slabs founded at or below the ground level, restraints can be minimized by founding the structure· on a flat layer of concrete with interposition of sliding layer of some material to break the bond and facilitate movement. However, it should be recognized that common and more serious causes of leakage in practice, other than cracking, are defects such as segregation and honey combing and in particular all joints are potential source of leakage.

GENERAL DESIGN REQUIREMENTS ACCORDING TO INDIAN STANDARD

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

Plain Concrete Structures: Plain concrete members of reinforced concrete liquid structures may be designed against structural failure by allowing tension in plain concrete as per the permissible limits for tension in bending specified in IS : 456 – 2000 (i.e. permissible stress in tension in bending may be taken to be the same as permissible stress in shear, q measured as inclined tension). This will automatically take care of failure due to cracking. However, nominal reinforcement in accordance with the requirements of IS: 456 shall be provided for plain concrete structural members.

Permissible Stresses in concrete

a) For resistance to cracking: Indian Standard Code IS: 456-2000 does not specify the permissible stresses in concrete for its resistance to cracking. However, its earlier version (IS: 456-1964) included the permissible stresses in direct tension, bending tension and shear. These values are given in Table below. The permissible tensile stresses due to bending apply to the face of the member in contact with the liquid. In members with thickness less than 225 mm and in contact with the liquid on one side, these permissible stresses.

b) For strength calculations: In strength calculations the usual permissible stresses, in accordance with IS: 456-2000 are used. Where the calculated shear stress in concrete above exceeds the permissible value, reinforcement acting in conjunction with diagonal compression in concrete shall be provided to take the whole of the shear.

Permissible Stresses in Steel Reinforcement

a) For resistance to cracking: When steel and concrete are assumed to act together for checking the tensile stresses in concrete for avoidance of cracking the tensile stresses in steel will be limited by the requirement that the permissible tensile stress in concrete is not exceeded so that tensile stresses in steel shall be ‘equal to the product of modular ratio of steel and concrete, and the corresponding allowable tensile stress in concrete.

b) For strength calculations: Though the Indian Standard Code IS: 456 had its fourth revision in 2000, the corresponding Codes IS: 3370 (Part I, II, III and IV) for concrete structures for the storage of liquids have not been revised since 1965. The main Code on concrete-IS: 456 is in SI units. However, the fourth reprint (May 1982) of IS: 3370 (Part 11)-1965 incorporates the amendment regarding the permissible stresses in steel reinforcement. The revised values of permissible stresses are given in Table. Converted into SI units, using the approximation 10 kg/cm2 = 1 N/mm2.

Stresses due to drying shrinkage or temperature change:
(i) Stresses due to drying shrinkage or temperature change may be ignored provided

a) The permissible stresses specified for concrete and steel respectively are not exceeded.

b) Adequate precautions are taken to avoid cracking of concrete during the construction period and until the reservoir is put into use.

c) The recommendations as regards the provision of joint and for suitable sliding layer are complied with, or the reservoir is to be used only for the storage of water or aqueous liquids at or near ambient temperature and the circumstances are such that the concrete will never dry out.

(ii) Shrinkage stresses may, however, be required to be calculated in special case, when a shrinkage coefficient of 300 x 10-6 may be assumed.

(iii) When the shrinkage stresses are allowed, the permissible stresses, tensile stresses in concrete (direct and bending) as given in Table 21.1 may be increased by 33 ~ percent.

(iv) Where reservoirs are protected with an internal impermeable lining, consideration should be given to the possibility of concrete eventually drying out. Unless it is established on the basis of tests or experience that the lining has adequate crack bridging properties, allowance for the increased effect of drying shrinkage should be made in the design.

Steel Reinforcement

a) Minimum reinforcement :

(i) The minimum reinforcement in walls, floors and roofs in each of the two directions at right angles shall have an area of 0.3 percent of the concrete section in that direction for sections upto 100 mm thickness. For sections of thickness greater than 100 mm and less than 450 mm the minimum reinforcement in each of the two directions shall be linearly reduced from 0.3 percent for 100 mm thick section to 0.2 percent for 450 mm, minimum reinforcement in each of the two directions shall be kept at 0.2 percent. In concrete sections of thickness 225 mm or greater, two layers of reinforcing bars shall be placed one near each face of the section to make up the minimum reinforcement specified above.

(ii) In special circumstances, floor slabs resting directly on the ground may be constructed with percentage of reinforcement less than that specified above. In no case the percentage of reinforcement in any member be less than 0.15 % of the concrete section.

b) Minimum cover to reinforcement :

(i) For liquid faces of parts of members either in contact with the liquid or enclosing the space above the liquid (such as inner faces of slab), the minimum cover to all reinforcement should be 25 mm or the diameter of the main bar, whichever is greater. In the presence of sea water and soils and water of corrosive character the cover should be increased by 12 mm but this additional cover shall not be taken into account for design calculations.

(ii) For faces away from the liquid and for parts of the structure neither in contact with the liquid on any face nor enclosing the face above the liquid, the cover should be the same as provided for other reinforced concrete sections.

JOINTS IN WATER TANKS

The various types of joints may be categorized under three heads:

(a) Movement joints

(b) Constructions joints

(c) Temporary open joints.

a) Movement joints: These require the incorporation of special materials in order to maintain water-tightness while accommodating relative movement between the sides of the joints. All movement joints are essentially flexible joints. Movement joints are of three types

(i) Contraction joint

(ii) Expansion joint

(iii) Sliding joint.


(i) Contraction joint: A contraction joint is a typical movement joint which accommodates the contraction of the concrete. The joint may be either a complete contraction joint in which there is discontinuity of both concrete and steel, or it may be partial contraction joint in which there is discontinuity of concrete but the reinforcements run through the joint. In both cases, no initial gap is kept at the joint, but only discontinuity is given during construction. In the former type, a water bar is inserted while in the later type, the mouth of the joint is filled with joint sealing compound and then strip painted. A water bar is a pre-formed strip of impermeable material (such as a metal, polyvinyl chloride or rubber). Joint sealing compounds are impermeable ductile materials which are required to provide a water-tight seal by adhesion to the concrete throughout the range of joint movement. The commonly used materials are based on asphalt, bitumen, or coal tar pitch with or without fillers such as limestone or slate dust, asbestos fibre, chopped hemp, rubber or other suitable material. This are usually applied after construction or just before the reservoir is put into service by pouring in the hot or cold state, by trowelling or gunning or as preformed strips ironed into position.

(ii) Expansion joint: It is a movement joint with complete discontinuity in both reinforcement and concrete, and is intended to accommodate either expansion or contraction of the structure. In general such a joint requires the provision of an initial gap between the adjoining parts of a structure which by closing or opening accommodates the expansion or contraction of the structure. The initial gap is filled with joint filler. Joint fillers are usually compressible sheet or strip materials used as spacers. They are fixed to the face of the first placed concrete and against which the second placed concrete is cast. With an initial gap of 30 mm, the maximum expansion or contraction that the filler materials may allow may be of the order of 10 mm. Joint fillers, as at present available cannot by themselves function as water-tight expansion joints. But they can only be relied upon as spacers to provide the gap in an expansion joint when the gap is bridged by a water bar.

(iii) Sliding joint: Sliding joint is a movement joint with complete discontinuity in both reinforcement and concrete at which special provision is made to facilitate relative movement.

(iv) Construction joints: A construction joint is a joint in the concrete introduced for convenience in construction at which special measures are taken to achieve subsequent continuity without provision for further relative movement. It is, therefore, a rigid joint in contrast to a movement joint which is a flexible joint. Fig. shows a typical construction joint between successive lifts in a reservoir wall. The position and arrangement of all construction joints should be predetermined by the engineer. Consideration should be given to limiting the number of such joints and to keeping them free’ from possibility of percolation in a manner similar to contraction joints.


(v) Temporary open joints: A temporary open joint is a gap temporarily left parts of a structure which after a suitable interval and before the structure is put into use, is filled with mortar or concrete completely as provided below, with the inclusion of suitable jointing material. In the former case the width of gap should be sufficient to allow the sides to be prepared before filling. Where measures are taken for example, by the inclusion of suitable joining materials to maintain the water-tightness of the concrete subsequent to the filling of the joint, this type of joint may be regarded as being equivalent to a contraction joint (partial or complete) as defined.

When water is filled in circular tank, the hydrostatic water pressure will try to increase its diameter at any section. However, this increase in the diameter all along the height of the tank will depend upon the nature of the joint at the junction B of the wall and bottom slab. If the joint at B is flexible (i.e. sliding joint), it will be free to move outward to a position B1. The hydrostatic pressure at A is zero, and hence there will be no change in the diameter at A. The hydrostatic pressure at B will be maximum, resulting in the maximum increase in diameter there, and hence maximum movement at B if the joint is flexible.