Why Does New Concrete Crack? What Is Normal on the Tablelands and What Is Not

Short answer: almost every concrete slab cracks somewhere, and most of those cracks are harmless. Concrete shrinks as it cures, and a hairline crack is the slab relieving that stress. What matters is where the crack is, how wide it is, and whether it is moving. Here is how to tell the difference, and what changes on the Tablelands.

The cracks that are normal

Hairline shrinkage cracks, thinner than a credit card and not growing, are part of concrete being concrete. They usually show up in the first few weeks, run in random directions, and do not go all the way through the slab. They are cosmetic. On a driveway or shed floor they are not a structural problem and they do not need repairing.

Cracks that follow the control joints are the slab doing exactly what it was designed to do. Those joints are cut into the surface so that when the slab shrinks, it cracks along a straight line we chose rather than somewhere random. A crack in the bottom of a joint is a success, not a fault.

The cracks worth a phone call

• Wider than about three millimetres, or one you can slide a coin into
• One side of the crack sitting higher than the other, so you can feel a lip with your foot
• A crack that keeps getting longer or wider over weeks, not one that appeared and stopped
• Cracks running through a slab that is holding up something structural, like a house slab, a suspended deck or a retaining wall footing
• A slab that sounds hollow when you tap it, which can mean the ground underneath has moved or washed out

None of those automatically mean the slab is ruined, but they do mean the cause needs to be understood before anyone patches over it.

Why the Tablelands is harder on concrete

Two things up here push slabs toward cracking. The first is the wet season: heavy rain saturates the ground under a slab, and if the base was not compacted properly or the drainage is poor, the ground moves and the slab follows it. The second is the swing between hot dry days and cool nights, which pulls moisture out of the surface faster than the inside can cure and causes surface cracking in the first days after a pour.

That is why the prep under a Tablelands slab matters more than the pour itself. A properly compacted base, drainage that moves water away from the slab rather than under it, and keeping the surface damp for the first few days are the three things that decide whether a slab cracks the harmless way or the expensive way.

What we do to keep cracking under control

• Compact and level the base before a single bag of cement arrives, and sort the drainage first
• Cut control joints at the right spacing for the slab thickness, so the shrinkage has somewhere to go
• Use steel reinforcement sized to the job, which does not stop cracks forming but holds them tight so they stay hairline
• Cure the slab properly for the first days, especially in the dry season, so the surface and the inside harden together
• Tell you up front what to expect, so a hairline crack in week two is not a surprise

Got a crack you are not sure about?

Send us a photo with something for scale, like a coin next to the crack, and tell us how old the slab is and whether the crack is growing. We will tell you straight whether it is normal or whether it needs a look. We work across the Atherton Tablelands including Malanda, Atherton, Yungaburra, Herberton and Millaa Millaa.

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