Control Joints in Concrete Slab

April 21, 2020

Control joints are provided in a concrete slab to reduce cracks formed due to shrinkage. The specifications of the control joints such as the depth, timing and place of the cut play an important role in the efficiency of the control joint.

The time when the concrete slab is saw-cut to make control joints is an important aspect for the efficient use of control joint. When the concrete is sawed with the help of blade before it has reached its initial setting time, the aggregates may strew out of the concrete slab and leave a messy, weakened edge along the joint. This effect is known as Ravelling. Sawing too late can result in uncontrolled cracking as the concrete contracts during curing.

It is recommended to place the control joints in the centre of the slab dividing the concrete into equal segments. The spacing of the joints is in the range of 24 to 36 times the thickness of the concrete slab.

The factors that determine the location of control joints are:

Continuity: The joints must be continuous.

Square Pattern: They must form segments in square patterns.

Reinforcement: The ideal location of placing the control joints is where the steel reinforcement is less or not found.

Size of the segment: The square segments formed in the hot climatic conditions are larger than conventional segments due to the fast setting of concrete in a big area.

The thumb rule used to determine the depth of the control joint is to cut the joints one-quarter to one-third the slab thickness. If the joint is too deep, aggregate interlocking will not be sufficient to transfer loads. If the saw cut is too shallow, random cracking might occur.

The locations where the joints are to be made are calculated and marked using a chalk-line. It must be made sure that all the segments created are of the same size and the cuts are in a straight line.

--------------------------------------------------------------------------------------------------------