Cracks In A Concrete Garage Floor

What's happening?

The Concrete Society says drying shrinkage cracks form when concrete loses water and shrinks while something holds it back. They usually appear several months to three or four years after the slab was cast, and thin members with a large surface area, such as slabs, are particularly vulnerable. Concrete near corners and edges is prone to cracking because moisture is lost from adjacent surfaces, and there is no typical pattern, because cracks form wherever shrinkage is restrained. Cracks of this kind are non-structural. The Society adds that some cracks are live, so movement across them can continue with changes in loading, temperature and moisture, and that fine cracks can reappear at filled cracks. A crack that is widening, has a step across it, or runs up the garage wall is a different matter and needs inspection.

What usually causes it?

• Drying shrinkage as the slab loses water, restrained by its edges or the ground
• Thin slab with a large surface area, which is more vulnerable
• Cracks near corners and edges where moisture is lost faster
• Changes in temperature and moisture that keep a crack moving
• Settlement or ground movement under the slab, which is a builder's question

Is it dangerous?

Not usually. A crack with a step across it, or one that runs up an adjoining wall or keeps widening, should be looked at by a builder or surveyor. Trip hazards from raised edges are worth fixing.

Can I fix it myself?

Measure the width of the crack and photograph it next to a ruler with the date, then check again after a few months. The Society says it is prudent to fill cracks over about 1 mm wide where small hard wheels cross them, and that filling is only needed to protect the crack edges, because filling tends to make cracks more visually apparent. A proprietary crack filler is used, to the supplier's instructions. Do not cut out or rout a crack unless the edges are already breaking up.

When should you call a builder?

Call a builder if a crack is widening, has a step across it, has broken edges, or continues up a wall, or if the garage has other signs of movement. They can say whether it is shrinkage or settlement and repair it.

What will the builder actually do?

• Inspect the slab and any walls and door frames for signs of movement
• Measure and monitor cracks to see whether they are live
• Fill cracks that need protecting with a suitable crack filler
• Repair broken or fretting crack edges
• Investigate the ground or drains if the slab is settling

Who fixes this

• Structural repairs
• Groundwork and foundations

Questions people ask

Sources & References

1. Concrete Society: drying shrinkage cracks
2. Concrete Society: remedial repair of cracks in floors