Contraction joints are planes, usually vertical separating concrete in a structure or a pavement placed at a designed location. Contraction joint is placed in such a way that it interferes least with the performance of the structure as well as prevents formation of objectionable shrinkage cracks elsewhere in concrete.
Why It Is Required ?
Concrete is subjected to shrinkage due to loss of moisture in it at the time of drying, which results in the formation of cracks in the structure. To avoid these cracks, contraction joints are sometimes formed in concrete structures such as roads, retaining walls, floor and lining of tunnels and canals. The interval of these joints is from 5 m to 10 m.
Contraction joints may not be necessary if sufficient reinforcement is provided to distribute the shrinkage cracks uniformly throughout the structure so that the width of each crack will be negligible.
What Is Control Joints ?
Contraction joints are also called control joints. Control joints are the joints which allow horizontal movement of the slabs. If no joints were used, random cracking in the slab would occur when the tensile stress exceeds the tensile strength of concrete. These joints also eliminate random cracking due to thermal volume changes.
In general, contraction joints are made purposely by creating a vertical plane of weakness in the slab. The cracks, if any, can thus occur only at this weak ended plane rather than at random location.