Joints allow a concrete element to move independently of other parts of the building or structure. Joints also allow concrete to shrink as it dries, avoiding what is called internal restraint. Internal constraint is created when one part of a slab shrinks more than another or contracts in a different direction. Construction joints are designed to allow displacement between each side (both) of the slab, but they also transfer the bending stresses produced in the slab by external loads at the same time.
Construction joints require precise calculations to determine their location on the surface of an element. Essentially, these should be located where there is the least structural weakening. A construction joint is a type of concrete joint that is used when a new section of concrete is poured adjacent to another section of concrete that has already set. The purpose of a construction joint is to allow some horizontal movement, while being rigid against rotational and vertical movement.
Construction joints prevent premature failure of the concrete structure. The concrete has still cracked, which is normal behavior, but the absence of random cracks in the concrete surface gives the appearance of an uncracked section. Concrete has the property of expanding and contracting due to temperature changes, which results in a change in the volume of the concrete. Construction bonding is essential if the size or length of concrete elements exceeds any specified limit.
Joints must be carefully designed and properly constructed if uncontrolled cracking of concrete floors is to be avoided. It is important to recognize that the use of steel reinforcement in a concrete slab actually increases the chance of random fine cracks occurring in the exposed surface of the concrete. Concrete restoration joints are characterized by preventing crushing and distortion between adjacent elements, caused by compressive forces triggered by the expansion of concrete. Construction joints are generally placed on the concrete slab to determine the extent of the individual placements.
