Contraction Joints:

Concrete is subjected to plastic shrinkage and drying shrinkage due to which concrete shrinks. Stresses are established and cracks are created when shrinkage is restrained. Contraction joints are provided to avoid these cracks. Normally these joints are provided at 5 m to 10 m interval. Contraction joints are also called dummy joints or control joints. These joints will not be needed if a provision is made to take up the shrinkage stresses by reinforcement. Hence, contraction joints are provided in un-reinforced floors and pavements. Contraction joints are made at the time of laying concrete by imbedding a timber plank or batten of sufficient depth and required thickness. This is afterwards removed when the concrete is hardened. A few times, steel plates of needed thickness and width are forced down into the fresh concrete and then removed when the concrete is hardened. Sometimes contraction joints of required width and depth are cut by using a joint sawing machine. It is necessary that groove cut should be filled up with some joint sealing compound to protect the edges of concrete and also to prevent water from being held. The depth of joint shall be about 2 cm. In residential flooring, the conventional contraction joint is omitted by casting the slab in alternate bays, to permit for the complete plastic shrinkage and also for maximum extent of drying shrinkage. It is common practice to place glass-strip or aluminium strip in between the bays to create discontinuity between adjacent bays to prevent the development of continuous cracks.