There are two types of concrete- concrete that has cracked, and concrete that will crack.
Cracking in concrete is inevitable, the best the contractor can do is try to control the cracking. There are two basic strategies to control cracking. One method is to provide steel reinforcement in the slab which holds random cracks tight. And the most widely used method is to place joints.
The three basic types of joints in slabs-on-ground are:
- Isolation joints
- Contraction joints
- Construction joints
Isolation joints separate slabs from fixed objects such as walls and columns. Isolation joints in slabs-on-ground permit horizontal and vertical movement between the slab and any walls, columns, or footing that the slab is in contact with. Isolation joints are also called expansion joints.
To minimize random cracks, contraction joints are used to create straight-line planes of weakness in the slab. As the slab shrinks, the joints open slightly and cracks occur at the predetermined locations instead of randomly over the slab. Contraction joints can be established using jointing tools, inserting joint forming strips while the concrete is still plastic, or sawing the concrete after it has been finished.
Construction joints are placed in a slab where concrete placement stops for the day. Typically, doweled construction joints are used in pavements and industrial floors that carry heavy wheeled traffic. When transferring loads across the joint, the dowels help hold the two sides at the same elevation.