Final answer:
Concrete control joints are added after the surface sheen disappears, which provides a balance between being firm enough to form clean joints and still soft enough to cut properly.
Step-by-step explanation:
Concrete control joints should be put into the moist concrete after the sheen disappears. This timing ensures that the concrete is firm enough to hold the joint without collapsing, but not so hard that cutting would be difficult.
These joints are necessary to control where the concrete cracks as it contracts during the curing process. Cutting them too early, when the water is still on top or immediately after brooming, could result in a messy joint that is not effective.
If you wait too long after the surface loses its sheen, the concrete could become too hard, leading to uncontrolled cracking.
Concrete control joints are put into the moist concrete after brooming. Brooming is typically done to create a rough surface texture that prevents slipping. Once the brooming is complete, control joints can be placed. It is important to install control joints as soon as possible to help prevent cracking of the concrete as it cures.