There are only a few reasons concrete cracks: it contracts while curing, the soil below it shifts, it's loaded beyond what it was built for, or nothing was done to control where shrinkage would show. Most of the cracks that worry homeowners turn out to be harmless, while a handful point to something serious. Telling them apart is the important part, and a few Illawarra conditions make some kinds of cracking more common.
Below we cover the main causes, how to interpret what you're seeing in your own concrete, and what separates a slab with a few fine lines from one that breaks up.
Why concrete cracks
Concrete has huge compressive strength and very little tensile strength. Press down on it and it can take a massive load; stretch it or take away the support beneath it and it splits without much fuss. Almost all cracking traces back to one of the following:
- Drying shrinkage: as new concrete cures it gives up water and contracts a touch. That's behind most hairline cracks, and it can't be prevented, only directed.
- Movement in the ground: when the base settles, gets washed away or was never compacted well, the slab is left bridging a void and snaps under its own weight. Saturated or badly drained soil makes this the leading cause of serious cracking.
- Overloading: a drive designed for cars can give way under a concrete truck, a removal truck or a heavy trailer, typically at the edges where support is thinnest.
- Missing or badly placed joints: with no planned line of weakness, the slab cracks wherever stress peaks, quite often right across the middle of a bay.
- Mix and curing faults: a wet, sloppy mix, or a surface that dries out too fast in hot sun and wind, produces weaker concrete prone to crazing and early shrinkage cracks.
- Water and rusting steel: rain seeping into cracks scours the base and keeps the slab damp, and once moisture reaches the reinforcement it corrodes and swells, more quickly in salty sea air. The result is flaking, spalling and rust marks rather than a neat crack line.
Two Wollongong conditions to watch
Some of those causes hit harder around Wollongong. Reactive clay is widespread on the coastal plain and foothills, the escarpment has shale and sandstone, and near the beaches and Lake Illawarra you'll find sandy soil with a high water table. Clay soaks up water during a wet stretch and shrinks back as it dries, and that constant swelling and shrinking keeps working on any slab above it. Below the escarpment, cut-and-fill driveways bring an added risk: fill that was poorly compacted settles over time and leaves the slab hanging. Then there's the rain. Big storms and East Coast Lows push water into every gap, softening and scouring the gravel base, so prompt crack sealing and good drainage away from the slab count for more here than in drier parts of the country.
Cosmetic or structural? How to read your cracks
It mostly comes down to two questions: is it moving, and how far down does it go? Cosmetic cracks are fine, stable and shallow. Structural ones are wider, still changing, and the concrete either side has shifted.
- Cosmetic: hair-thin, unchanged over time, flush on both sides, sometimes a fine web across the surface.
- Structural: about 3mm or more across, lengthening or widening, stepped, cracked right through, or beside a patch that has dropped or heaved.
There's a simple check anyone can do. If a fingernail barely catches in it and it looks no different from a year ago, it's most likely cosmetic. If it has opened up, one edge has sunk, or you can see daylight through the slab, get someone to look at it. Our concrete crack repair crew assesses and fixes both kinds.
| Sign | Likely cosmetic | Possibly structural |
|---|---|---|
| Width | Hair-thin, barely visible | Roughly 3mm or wider |
| Depth | Top surface only | Right through the slab |
| Alignment | Flush on both sides | One edge sits proud |
| Over time | Unchanged | Lengthening or widening |
How control joints keep cracks in line
Shrinkage stress can't simply vanish. Control joints are lines of weakness we saw or tool into the slab on purpose so that stress releases exactly where we choose. At around a quarter of the slab depth, the joint offers the easiest path, and the crack forms tidily out of view at the bottom of the cut.
There are two ways joints let you down. Sawn too late, the slab has already cracked on its own terms. Sawn too shallow, they're a decorative line that controls nothing. Spacing counts too: set them too far apart and cracks appear between them, and long skinny bays tend to crack across their width regardless.
Stopping serious cracks before the pour
Nearly all of the prevention is done before any concrete is delivered. The concrete is seldom the weak link.
- Excavate for the ground you have: take out topsoil, roots and soft material to whatever depth your site needs rather than a stock figure.
- Compact in stages: place and compact the crushed rock in thin layers, not one thick dump that settles under traffic later.
- Position the steel correctly: mesh left on the ground achieves very little. Held mid-slab with proper cover, it keeps a crack from turning into two loose pieces. See our reinforced concrete page for more.
- Control the water: grade the slab away from the house and add drainage so storm runoff can't soak the ground underneath.
- Saw joints promptly: at the right depth and close enough together that shrinkage cracks land just where they were planned.
When to get a crack checked
Get an inspection if a crack is more than roughly 3mm wide, has grown since you spotted it, has one edge higher than the other, or appears in a retaining wall or close to the house footings. Anything less than that, seal the fine cracks against rain and keep watching them.
No concreter can promise a slab that never cracks. Quality work keeps cracks fine, predictable and tucked inside the joints, and on a driveway that's what success looks like.