Cracks in Basement Floor

Old 06-09-03, 09:17 AM
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Cracks in Basement Floor

I have recently purchased a “brand new one-story with full basement home”. Only the front of the house and a side are underground. My concern is that I have noticed cracks in the basement/garage floor.
Are Basement Floor Cracks common?
What could cause this?
Should I get this repaired immediately?
What other factors if any should I look into that could be causing the cracks in the flooring and would these cracks be affecting anything else?

Thanks!! I really need some expert advice. If need any more details please please ask
Old 06-09-03, 08:32 PM
Doug Aleshire's Avatar
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These are just some thoughts on why concrete cracks... You live in a warm and humid climate so the exact reason could be one or more things. I would call the contractor back if these are excessive. Bear in mind concrete can and will crack but to what degree is acceptable is debateable. Contact you local building official if you have questions

Excess water in the mix

Concrete does not require much water to achieve maximum strength. But a wide majority of concrete used in residential work has too much water added to the concrete on the job site. This water is added to make the concrete easier to install. This excess water also greatly reduces the strength of the concrete.

Shrinkage is a main cause of cracking. As concrete hardens and dries it shrinks. This is due to the evaporation of excess mixing water. The wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 1/2 inch per 100 feet. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces.

Rapid Drying of the concrete

Also, rapid drying of the slab will significantly increase the possibility of cracking. The chemical reaction, which causes concrete to go from the liquid or plastic state to a solid state, requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete.

You can make sure that the necessary water is available for this reaction by adequately curing the slab.

Lack of control joints.

Control joints help concrete crack where you want it to. The joints should be ¼ of the depth of the slab and no more than 2-3 times (in feet) of the thickness of the concrete (in inches). So 4"concrete should have joints 8-12' apart.

Improper strength concrete poured on the job

Concrete is available in many different strengths. Verify what strength the concrete you are pouring should be poured at.

Improper Backfilling

If excavation was done below the finished grade, in excess, and backfilling was done without proper tamping, these areas can settle causing voids and cracking will occur. The sub grade must be prepared according to your soil conditions. Some flatwork can be poured right on native grade. In other areas 6"of base fill is required along with steel rebar installed in the slab. This can cause settling which can effect the wood structure on up to the roof.

Pouring concrete on frozen ground

Cracks more than 1/8" are not a major concern but larger ones are patchable.

Hope this helps!

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