Watch the video on Preventing Foundation Cracks.
Even if a basement or slab-on-grade foundation is placed on correctly prepared soil, an adequate gravel layer, and a properly installed vapor barrier, it will crack. Builders have to deal with three common types of cracks in basement and slab-on-grade foundations—surface cracks, area cracks, and restraint cracks.
Surface cracks
Surface cracking takes place during the first few days of curing. The slab surface cures faster, and therefore shrinks more quickly, than the slab core. This difference in shrinkage rates results in surface cracking. The major causes of surface cracking are an inadequate concrete mix and incorrect curing methods.
Solution:
- Reduce the amount of water in the mix. One effective way to reduce the water content while maintaining workability is to add a plasticizer to the mix.
- Refrain from repeatedly trowelling the slab surface. Trowelling repeatedly could cause too much of the cement paste to rise, moving the aggregate downward in the slab; this causes the slab surface to shrink more quickly.
- Keep the slab surface moist during the first three days of curing by misting it, applying curing blankets to it, applying spray-on curing compounds to it, or covering it with polyethylene sheeting.
Area cracks
Large areas of concrete crack when they expand and contract. In order to control cracking in a basement or slab-on-grade foundation, control joints are installed to divide the slab into smaller areas. The major cause of area cracking is incorrectly spaced control joints.
Solution: Make sure that control joints are spaced correctly. The amount of spacing needed between control joints depends on slab thickness and the composition of the concrete mix. A general rule of thumb is that in any rectangular area of the slab that's defined by a control joint, the longer side shouldn't be more than twice as long as the shorter side. Control joints should be placed approximately 1/4 into the slab thickness.
Restraint cracks
When concrete expands, it needs room to expand without cracking. In a basement slab, the foundation walls can restrain the slab from expanding, causing the slab and basement walls to crack. Expansion joints allow the basement slab to expand without cracking itself or the basement walls. The major cause of restraint cracks is incorrectly installed expansion joints.
Solution: Confirm that the expansion joint material is correctly placed around the perimeter of the foundation walls before placing concrete for the slab.
To learn more about preventing foundation cracks, visit:
www.cement.org
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