Cracking Concrete

Concrete, like other construction materials, contracts and expands with changes in moisture and temperature, and deflects depending on load and support conditions. Cracks can occur when provisions to accommodate these movements are not made in design and construction. Some forms of common cracks are:

• Plastic shrinkage cracks
• Cracks due to improper jointing
• Cracks due to continuous external restraint
  Example: Cast-in-place wall restrained along bottom edge of footing
• Cracks due to lack of isolation joints
• D-Cracks from freezing and thawing
• Craze Cracks
• Settlement cracks

Most random cracks that appear at an early age, although unsightly, rarely affect the structural integrity or the service life of concrete. Two exceptions are:

• D-cracks, which occur due to freeze-thaw deterioration of some types of porous aggregate in concrete. These cracks initiate at joints at the bottom of exterior slabs and typically appear at later ages
• Cracking due to alkali aggregate reactions will lead to long-term structural damage

All concrete has a tendency to crack and it is not possible to produce completely crack-free concrete. However, cracking can be reduced and controlled if the following basic concreting practices are followed.

• Design the members to handle all anticipated loads
• Provide proper contraction and isolation joints
• In slab on grade work, prepare a stable uniformly compacted subgrade

• Place and finish according to recommended and established practices
• Protect and cure the concrete properly