Control joints are provided in the concrete slab to reduce cracks formed due to shrinkage. The specifications of the control joints such as the depth of the cut, timing and place of cut play an important role in the efficiency of the control joint.
In this article we discuss the technical specifications of how to cut control joints.
1. Timing of the Control Joint Cut
The time at when the concrete slab is saw cut to make control joints is an important aspect of efficient use of control joint. The factors such as concrete mix, weather conditions,
When the concrete is sawed with the help of blade before it has reached it initial setting time can pull out the aggregates out 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.
As the weather condition in which the cuts are made determines the time of making control joints. 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.
The best way to determine whether the slab is ready is to make trial cuts to check for ravelling. Saw cutting should start as soon as the ravelling stops during these trial cuts.
2. Location of Control Joint
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 of the thickness of the concrete slab.
If you are using high shrinkage concrete, you might want to decrease the cut spacing. The factors that determine the location of control joints are,
- The joints must be continuous.
- They must form segments in square patterns.
- The ideal location of placing the control joints is where the steel reinforcement is less or not found.
- The square segments formed in the hot climatic conditions are larger than conventions segments due to the fast setting of concrete in a big area.
3. Depth of the Control Joint
The normal 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 concrete slab laid is of 150mm, the depth of the joint must be 40mm to 50mm.
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.
4. How to Cut the Control Joints
The location of 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 in the cuts are in a straight line.
It is recommended to use machines for making the joints rather than using manual means. If water cutting equipment are used, make sure that the water is running all the way down to the blade.
The machine blade depth must be set to the depth of the control joint and slowly the concrete slab must be sawed following the chalk mark.
The saw cuts must be cleaned and filler material must be filled in the cuts to protect the reinforcement from corrosion and from a watertight joint.
Tips for Making Control Joints
- Do not twist the saw blade.
- Do not let the blade spin in the cut, as this will increase wear on the bond.
- When cutting concrete with heavy rebar, use blades with soft metal segment bonds.
- Always ??use the required PPE (personal protection equipment).