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Joints In Concrete Structures

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Joints In Concrete Structures

 

Following are the two types of joints which are to be provided in concrete structures:

(1) Construction joints

(2) Expansion and contraction joints.

(1) Construction Joints: The construction joints are provided at locations where the construction is stopped either at the end of day or for any other reason. The provision of a construction joint becomes necessary to ensure proper bond between the old work and the new one.

If the construction activity is arranged in such a way that the work is stopped at expansion or contraction joint will not arise. However it is often difficult to complete the concreting for large work in one operation. It then becomes necessary to take special measures to achieve perfect continuity between the previously laid concrete that has already set and hardened and the new concrete.

The construction joints may be horizontal or vertical. For an inclined or curve member, the joint should be at right angle to the axis of the member.

It is necessary to determine the locations of the construction joints well in advance from the view point of structural stability.

Following points should be kept in view in case of the construction joints:


    (i) The columns should be filled with concrete to a level few centimeters below its junction with the lowest soffit of the beam or bottom of the hauriching, if any. The operation of concreting above the construction joint should be taken up after an interval of at least 4 hours.


    (ii) The construction joints should be located along or near the planes of the least bending moment and shear force.

    (iii) In case of T-beams or L-beams, the ribs should be filled with concrete first and then the slabs forming the flanges can be filled upto the centre of the rib. If a construction joint between slab and beam becomes

    (v) For slabs supported on two sides, the construction joints should be vertical and paralled to the main reinforcement. Alternatively, the construction joint can also be provided at the middle of span at right angles to the main reinforcement. For two way slabs, the construction joint can be provided near the middle of the either span.

     (vi)For R.C.C. walls, the location of horizontal construction joints is governed by the convenience in placing the formwork and the ease of access for compaction of concreting. The continuity at the joint is achieved by the formation of key. The beveled edged planks are placed on the inner face of the wall shuttering and they are removed before the pouring of concrete above the joint.

     (vii) The curing of horizontal construction joint should be suspended a few hours before new concreting is started. If the new concrete is to be laid within 48 hours, the surface of construction joint should be thoroughly cleaned with wire brush and water and treated with cement mortar of the same proportion as that of concrete. However, if the new concrete is to be laid after 48 hours or more, the surface of the construction joint should first be roughened by chiseling. It should then be cleaned of scum, loose aggregates and other foreign matter, wetted and a coat of cement mortar of the same proportion as that of the concrete is applied before placing new concrete.

(2) Expansion and contraction joints: These joints are provided in all the concrete structures of length exceeding 12 m, mainly for two purposes:

 
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