Because of shrinkage, cracks are developed in a concrete slab. To decrease these cracks, control joints play an important role. The specifications of the control joints like the depth, timing and place cut are vital factors to enhance the functionality of the control joint.

Control joints cut are mainly based on the following technical specifications :-

Timing of the Control Joint Cut

The concrete slab is saw-cut to produce control joints and the time required for this purpose is a crucial factor for applying the control joint efficiently. Various factors like concrete mix, weather conditions, hardness of concrete and types of machines utilized for developing the control joints decide the time required to cut the concrete slab.

Delay in sawing leads to uncontrolled cracking since the concrete contracts throughout curing.

In hot weather conditions, sawing can be started 4 hours after the concrete is poured. In cold weather conditions, sawing can be started 12 hours after the concrete is poured.

It is recommended to create trial cuts to examine for ravelling. Saw-cutting should be commenced once the ravelling ends all through these trial cuts.

Position of Control Joint

The control joints should be arranged in the centre of the slab to segregate the concrete into equivalent parts. The spacing of the joints is provided as 24 to 36 times the thickness of the concrete slab.

Depth of the Control Joint

The thumb rule generally applied to find out the depth of the control joint is to cut the joints one-quarter to one-third the slab thickness. When the thickness of concrete slab is 150mm, the depth of the joint should be 40mm to 50mm.

When the joint is adequately deep, aggregate interlocking will be insufficient to transmit the loads. When the saw cut is extremely shallow, random cracking might happen.