No one likes to see random cracks in concrete floors. Two of the most common causes of floor cracking are drying shrinkage and contraction due to cooling. Cracking can also occur when floor slabs aren't free to move independently of the building elements they're in contact with. Curling of slabs on grade causes cracking, too.
TYPES OF JOINTS AND HOW THEY WORK
Joints permit concrete to move slightly by creating planes of weakness where cracks can form. Three types of joints are used in concrete floor construction:
- Isolation joints (sometimes called expansion joints) allow movement in both vertical and horizontal directions. They separate or isolate concrete slabs from columns, walls, footings and other points of restraint such as machine foundations and stairwells. No connection should be made across an isolation joint either by reinforcement, keyways or bond.
- Control joints (frequently called contraction joints) allow movement only in the plane of the floor and control cracking caused by restrained forces resulting from drying or cooling. Control joints can be produced in several ways: by saw-cutting, tooling, or inserting plastic strips.
- Construction joints are stopping places for a day's work. They can be made to function as control joints by using keyed bulkheads. If construction joints are located where no movement is wanted, tie bars or welded wire fabric can be used to hold the adjacent slabs together.