Proper Construction of Concrete Control Joints
Cracking in concrete slabs is unavoidable. Contraction joints in concrete slabs are placed to control cracking. Installing joints can dictate where a crack occurs to help maintain the appearance of a concrete slab.
Causes of Concrete Cracking
Concrete is very strong in compression but is weak in tension (concrete’s tensile strength is only about 10% of its compressive strength). Therefore, even moderate tensile forces will cause concrete to crack.
When wet concrete is placed in forms, the hydration of the cement utilizes water in mixture. This consumption of water and evaporation of any excess water in the mixture causes concrete to contract by shrinkage. As the concrete contracts, it develops internal tensile forces. Concrete placed on a soil base also develops tensile stresses as it shrinks and the sand base causes friction. These tensile forces will cause newly placed concrete to develop contraction cracks.
Cracking in concrete slabs can also occur for several other reasons including:
- Structures that are placed in slabs
- Settlement of the soil base under a slab
- Changes in slab thickness
Structures placed in concrete slabs can concentrate stresses producing random cracks. Also, settlement of a structure or the soil base under a concrete slab will produce bending in the slab. Bending will cause tensile stresses in a portion of a slab, resulting in a crack. Moreover, changes in slab thicknesses tend to concentrate stresses where the slab transitions to the thinner section.
Contraction joints for controlling cracking in concrete flatwork should be installed as soon as possible. Tooled joints should be installed during the concrete finishing process but not so early that the joint closes or heals following tooling. Delaying the installation of tooled joints will make it difficult to get the proper depth and it may affect the joint appearance.
Sawcut joints should be installed as soon as the saw can be placed on the concrete but not so early that the saw causes tear-out. This is usually within 24 hours after placing the concrete. If sawing the joints is delayed, uncontrolled contraction cracking may occur before the joint can be installed.
To be effective, contraction joints must be ¼ to ⅓ of the slab thickness. Therefore, the joint must be 1 to 1½ inches deep in a 4 inch thick slab. If it is not installed to this depth, the slab may crack in a random pattern.
Contraction joint spacing should be no more than 24 times the slab thickness of exterior flatwork (no more than 8 feet for a 4 inch thick slab). Joints in interior slabs should be placed at no more than 30 times the slab thickness. Joints for thick slabs should be no more than 15 feet apart outdoors and 20 feet indoors.
The appearance of concrete flatwork can be negatively affected by random shrinkage cracking that will continue to grow and widen with time. Placing properly constructed control joints can encourage contraction cracking to follow a predetermined pattern.