Stress cracks due to applied loads are settlement cracks and can be avoided by having a proper subbase, slab design, and proper concrete mix designed for the load.

Common Causes: Use of High Slump concrete (over 4 inch) which increases shrinkage. Low strength mix. Control joints too far apart. Isolation and control joints omitted. Insufficient depth of control joints. Inadequate or improper curing Poor subgrade preparation Improper placement of reinforcement

Prevention: Minimizing cracks requires proper placement of control joints and proper curing to hold the moisture in the concrete long enough to get adequate strength gain before the moisture leaves and the concrete shrinks. All concrete has a tendency to shrink and it is not possible to consistently produce completely crack free concrete. However, cracking can be reduced and controlled if the following basic safeguards are observed:

• The subgrade beneath the slab should be adequately compacted by rolling, vibrating or tamping.
• The subgrade should be sloped for proper drainage.
• If a vapor barrier is used, cover it with a layer of 2 inches of compacted damp sand.
• Thoroughly dampen the subgrade just prior to placing the concrete.
• Use low-slump concrete ( not over 4 inch.)
• Reduce rapid evaporation of surface moisture, especially in hot weather. Cover the concrete with wet burlap or poly sheeting between finishing operations in hot weather.
• Install proper control joints. Closer spacing will be necessary for thin slabs (4 inches or less.)
• Isolation joints should be provided whenever restriction to freedom of either vertical or horizontal movement is anticipated.
• Start curing as soon as possible.
• Cover cured slab for at least 5 days