Are cracks in a garage or house slab a foundation problem?
In short: Most cracks in a garage floor or house slab come from concrete shrinking as it dries and do not affect structural integrity. A crack becomes a foundation question when one side is higher than the other, when it keeps widening, or when it lines up with other signs such as sloping floors or brick cracks.
What does this sign look like?
Cracks show in the exposed concrete of a garage, porch or patio, or telegraph through floor coverings as cracked tile or a ridge under carpet. They may be hairline, open, or offset with one edge higher than the other. This sign applies to slab-on-grade, post-tensioned slabs, basement floors and exterior flatwork.
Why do concrete slabs crack?
Drying shrinkage
Concrete shrinks as it cures and dries, by about 0.0002 to 0.0008 inch per inch 2. Over a 20-foot slab, that works out to roughly 0.05 to 0.19 inch of shortening, which is why slabs crack even when nothing is wrong with the soil. Inspection guidance cites lab tests finding that synthetic fibers reduce shrinkage cracking by more than 80 percent 3. This look-alike is covered in material shrinkage and thermal cracking.
Soil movement under the slab
Swelling soils frequently cause distress in floor slabs 5. Building officials in expansive-soil areas list cracks in garage slabs, walkways and driveways among the effects of foundation movement 6. Soil can also be lost from under a slab: underground water from leaking mains can scour soil and displace footings 3, and in Oklahoma's karst areas, sinkholes and caverns in soluble rock can cause land settlement 7. Related mechanisms include center heave, erosion and washout voids and karst subsidence.
Post-tensioned slab issues
A post-tensioned slab has steel cables (tendons) stretched after the concrete hardens to squeeze the slab together. Design guidance calls for a minimum residual average prestress of 100 psi, continuous post-tensioned beams, and confirming that each tendon reaches its specified elongation and load during stressing 4. A slab whose tendons lose tension behaves differently; see post-tension tendon failure.
How is a slab crack read?
| Feature | More consistent with |
|---|---|
| Hairline, no offset, not growing | Drying shrinkage |
| Clean edges, recently appeared | Recent movement 3 |
| Dirty or previously filled | Possibly inactive 3 |
| One side higher than the other | Soil movement under the slab |
| Crack that lines up with sloping floors or brick cracks | Possible foundation movement 1 |
Engineering guidance notes that most concrete cracks do not compromise structural integrity, so their physical characteristics are evaluated rather than their presence alone 1. An offset (one side higher) shows that the two pieces of slab have moved relative to each other, which a shrinkage crack does not do on its own.
How is it documented?
- Width and offset at several points along the crack.
- Change over time with a crack monitor; cyclical movement may take six months or more to measure 3.
- Floor elevation survey, the engineering step beyond a visual first impression 1. See elevation surveys.
Repair guidance describes epoxy injection for cracks of about 1/32 inch and larger 1.
Related
- Material Shrinkage & Thermal Cracking (Mimic)
- Post-Tension Tendon Failure
- Floor Tile Cracks in a Line
- Heaved or Settled Flatwork
- Slab on Grade
Common questions
Are cracks in a concrete slab normal?
Many are. Concrete shrinks as it dries, and engineering guidance notes that most slab cracks do not compromise structural integrity. Their physical characteristics, such as offset, pattern and whether they are growing, are what get evaluated.
What does it mean if one side of a slab crack is higher than the other?
An offset means the concrete on one side has moved up or down relative to the other. That points to soil movement, settlement or heave under the slab rather than simple drying shrinkage.
Can slab cracks be repaired by injection?
Engineering repair guidance describes epoxy injection for cracks of about 1/32 inch and wider. Injection restores the crack but does not address soil movement that caused it.
Are post-tensioned slabs different?
Yes. Post-tensioned slabs contain steel tendons stretched tight after the concrete hardens to hold the slab in compression. Design guidance calls for a minimum residual average prestress of 100 psi and continuous post-tensioned beams.
Sources
- Guidelines for the Evaluation and Repair of Residential Foundations, Version 3 Texas Section, American Society of Civil Engineers
- Volume Changes – Analysis and Effects of Movement (Technical Note 18) Brick Industry Association
- Inspecting Foundation Walls and Piers International Association of Certified Home Inspectors (InterNACHI)
- Recommended Practice for the Design of Residential Foundations, Version 2 Texas Section, American Society of Civil Engineers
- Damage to Foundations from Expansive Soils Missouri University of Science and Technology
- County Expansive Soils Policy & Procedures Roanoke County Development Services
- Mapping of Karst Terranes in Oklahoma to Identify Potential Environmental and Hydrologic Problems AAPG Search and Discovery (Oklahoma Geological Survey author)
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