JOINTS IN LIQUID RETAINING CONCRETE STRUCTURES

1. MOVEMENT JOINTS:

There are three types of movement joints.
(i) Contraction Joint: It is a movement joint with deliberate discontinuity without initial gap between the concrete on either side of the joint . The purpose of this joint is to accommodate contraction of the concrete. The joint is shown in Fig. 1 (a).

Fig. 1(a)

A contraction joint may be either complete contraction joint or partial contraction joint. A complete contraction joint is one in which both steel and concrete are interrupted and a partial contraction joint is one in which only the concrete is interrupted, the reinforcing steel running through as shown in Fig. 1(b) .

Fig. 1

(ii) Expansion Joint: It is a joint with complete discontinuity in both reinforcing steel and concrete and it is to accommodate either expansion or contraction of the structure. A typical expansion joint is shown in Fig.2.

Fig. 2.

This type of 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.
(iii) Sliding Joint: It is a joint with complete discontinuity in both reinforcement and concrete and with special provision to facilitate movement in plane of the joint . A typical joint is shown in Fig. 3.

Fig. 3.

This type of joint is provided between wall and floor in some cylindrical tank designs.
2. CONTRACTION JOINTS
This type of joint is provided for convenience in construct ion. Arrangement is made to achieve subsequent continuity without relative movement. One application of these joints is between successive lifts in a reservoir wall. A typical joint is shown in Fig. 4.

The number of joints should be as small as possible and these joints should be kept from possibility of percolation of water.

3. TEMPORARY JOINTS

A gap is sometimes left temporarily between the concrete of adjoining parts of a structure which after a suitable interval and before the structure is put to use, is filled with mortar or concrete completely as in Fig.5(a) or as shown in Fig.5 (b) and (c) with suitable jointing materials. In the first case width of the gap should be sufficient to al low the sides to be prepared before filling.

Fig: 5 (a)

Fig. 5(b)

Fig. 5(c)

SPACING OF JOINTS IN LIQUID RETAINING CONCRETE STRUCTURES:

Unless alternative effective means are taken to avoid cracks by al lowing for the additional stresses that may be induced by temperature or shrinkage changes or by unequal settlement, movement joints should be provided at the following spacing:-
(a) In reinforced concrete floors, movement joints should be spaced at not more than 7.5m apart in two direct ions at right angles. The wall and floor joints should be in line except where sliding joints occur at the base of the wall in which correspondence is not so important .
(b)For floors with only nominal percentage of reinforcement (smaller than the minimum specified) the concrete floor should be cast in panels with sides not more than 4.5m.
(c) In concrete walls, the movement joints should normally be placed at a maximum spacing of 7.5m. In reinforced walls and 6m in unreinforced walls. The maximum length desirable between vertical movement joints will depend upon the tensile strength of the walls, and may be increased by sui table reinforcement . When a sliding layer is placed at the foundation of a wall , the length of the wall that can be kept free of cracks depends on the capacity of wall sect ion to resist the friction induced at the plane of sliding.
Approximately the wall has to stand the effect of a force at the place of sliding equal to weight of half the length of wall multiplied by the co-efficient of friction.
(d)Amongst the movement joints in floors and walls as mentioned above expansion joints should normally be provided at a spacing of not more than 30m between successive expansion joints or between the end of the structure and the next expansion joint ; al l other joints being of the construct ion type.
(e) When, however, the temperature changes to be accommodated are abnormal or occur more frequently than usual as in the case of storage of warm liquids or in uninsulated roof slabs, a smaller spacing than 30m should be adopted that is greater proportion of movement joints should be of the expansion type). When the range of temperature is small, for example, in certain covered structures, or where restraint is small , for example, in certain elevated structures none of the movement joints provided in small structures up to 45mlength need be of the expansion type. Where sliding joints are provided between the walls and either the floor or roof, the provision of movement joints in each element can be considered independently.