A concrete block basement wall has a long horizontal crack at about mid-height, and the wall bows inward along the crack. What is the most likely cause?
- Normal shrinkage of the mortar as it cured
- Lateral pressure from the soil outside the wall
- Uniform settlement of the footing under the wall
- Thermal expansion of the floor framing above
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B. Lateral pressure from the soil outside the wall
Soil, water and frost pressing against the outside of the wall bend it inward, and a block wall cracks along a mortar joint near mid-height where bending is greatest. Shrinkage produces small, usually vertical cracks with no bowing.
Reference: Structural systems: foundation wall crack patterns (lateral pressure)
Stair-step cracks follow the mortar joints near one corner of a block foundation, and the cracks are wider at the top than at the bottom. What do they most likely indicate?
- Shrinkage of the concrete blocks after laying
- Differential settlement of the foundation
- Inward pressure from saturated backfill soil
- Corrosion of the steel reinforcing in the wall
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B. Differential settlement of the foundation
Step cracking that tapers in width shows one part of the foundation has moved down relative to the rest, usually a corner settling. Soil pressure typically produces horizontal cracking and bowing rather than tapered step cracks at a corner.
Reference: Structural systems: foundation wall crack patterns (settlement)
A poured concrete foundation wall has several thin vertical cracks of even width with no offset between the two sides. What is the most likely cause?
- Shrinkage of the concrete as it cured
- Overloading of the wall by the framing
- Pressure from expansive clay backfill
- Heaving of the footing from frost action
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A. Shrinkage of the concrete as it cured
Concrete shrinks as it cures, and hairline vertical cracks of uniform width with no displacement are the typical result. Cracks from structural movement usually taper, are wider, or show the two faces offset from each other.
Reference: Structural systems: shrinkage cracks in poured concrete