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Title: Should Slabs Rest On or Be Attached To Foundations?

Author(s): Bruce A. Suprenant and Ward R. Malisch

Publication: Concrete International

Volume: 32

Issue: 11

Appears on pages(s): 29-31

Keywords: isolation joint, slab-on-ground, elevation control, cracking


Date: 11/1/2010

Isolation joints are the most common detail for slabs-on-ground that abut foundations. In theory, isolation joints allow the slab to move up or down without restraint when soil settlement or expansion is uniform. Because there is no restraint to movement of these floating slabs, no stress is generated in the concrete and no cracking is expected. In practice, soil movement is often nonuniform and differential soil settlement or expansion creates stress in the slab that can lead to cracking. Soil movement can also lead to slab elevation changes that cause problems other than cracking. When soil settlement is a concern, the specifier must decide whether the slab should float to reduce the possibility of restraint and cracking or whether the slab should be supported on or attached to the foundation wall to maintain elevation control.