Why your concrete is cracking - and which cracks matter

One of the most common calls we get is "my concrete is cracking - what should I do?". The honest answer surprises most people: all concrete cracks. It is in the nature of the material. The question is whether the cracks you are seeing are normal, or whether something needs attention.

Why concrete cracks at all

Concrete is poured wet and cures dry. As it loses moisture, it shrinks - typically by about 0.05% (roughly 0.5 mm per metre). On a 5-metre driveway, that's 2.5 mm of length the slab "wants" to give back. Since the slab is held at the edges by friction with the base, the shrinkage forces have to go somewhere.

The job of a good concreter is to control where those forces release. That is what crack-control joints (the deliberate grooves cut every 3-4 metres) are for. Cracks form in the joints, hidden in the design, and the visible slab stays apparently uncracked.

If joints are skipped, spaced too far apart, or cut too shallow, the slab cracks where it wants to - usually in a roughly straight line across the surface.

Cracks that don't matter

Hairline shrinkage cracks: typically 0.1-0.3 mm wide, running short distances, often in irregular patterns. Especially common on the surface of newly-poured slabs in hot or windy weather. These are cosmetic only and don't affect strength or longevity. They tend to be more visible on dark or coloured concrete.

Crack-control joint cracks: the saw-cut grooves are designed to crack. If you peer into the joint, you'll often see a clean crack at the bottom. That's the system working as designed.

Tooled joint cracks: cracks at the construction joints between separate pours. Normal - those joints are where pours meet and movement is expected.

Cracks that warrant attention

Wide cracks (over 1-2 mm) running across slab panels: probably means joint spacing was wrong, or the slab wasn't reinforced properly. Worth getting checked. Often can be repaired with crack-injection but the underlying cause should be understood first.

Cracks with one side higher than the other: differential settlement. Means the base under the slab is sinking on one side. Real problem - the fix is usually to lift, level and re-bed the slab (or sometimes saw out the bad section and re-pour it).

Diagonal cracks at corners: re-entrant corner cracks. Caused by stress concentration at inside corners (eg where a slab wraps around a column or a building). Should have had a diagonal reinforcement bar at that corner - in the wrong direction without one, cracks are predictable.

Pattern cracks (map-like cracking) on the surface: usually plastic shrinkage cracking from drying out too fast during the cure. Cosmetic on most slabs but can lead to surface spalling later. Sealing helps.

What to do about it

If your concrete is under a workmanship warranty (we offer 2 years; statutory warranty in most states is 1-2 years for non-structural and 6 for structural), call the original concreter. If they're reputable they'll come out and look. The honest answer might be "that's normal" - in which case you've lost nothing but had peace of mind.

If they're not responsive or the slab is out of warranty, get an independent concreter (or a structural engineer for serious cases) to assess. Most "scary" cracks turn out to be cosmetic. A few turn out to be serious. Knowing which is which is worth the assessment fee.

Prevention - if you're getting a slab poured

• Make sure crack-control joints are specified and at the right spacing (every 3-4 m)
• Make sure reinforcement is sized to the slab and properly elevated on bar chairs
• Make sure the slab is properly cured (curing compound, wet hessian, or repeated misting for at least 3 days)
• Avoid pouring in extreme weather - over 35°C or under 5°C

None of this is glamorous. All of it is what separates concrete that lasts thirty years from concrete that needs replacing in seven.