A slab cracks when it shrinks during curing, when the soil under it shifts, when it carries more weight than it was designed for, or when no joints were cut to guide the shrinkage. Most cracks owners worry about turn out to be harmless, while a handful point to a real problem. The trick is telling them apart, and Cairns has two local conditions that make some cracks more likely.
This guide looks at the underlying causes, how to read the cracks you can see, and what separates a slab with a few fine lines from one that breaks up.
The main causes of cracking
Concrete copes brilliantly with being squeezed and poorly with being stretched. It will carry huge loads pressing down but splits easily when pulled apart or left unsupported. Almost every crack comes back to one of the following:
- Drying shrinkage: new concrete gives up water as it cures and contracts a little. It is behind most fine cracks, and while it cannot be eliminated, it can be guided.
- Movement in the soil below: when the base sinks, is washed away or was left loose, the slab ends up bridging a hollow and snaps under its own load. Reactive clay makes this the most common serious cause.
- Overloading: a drive designed for cars may crack under a concrete truck, a removalist or a heavy trailer, usually first along the edges, which have the least support.
- Missing or poorly placed joints: without a planned weak line, the slab cracks wherever the stress peaks, often right across the middle of a panel.
- Mix and curing faults: a watery mix, or a surface that dries out too fast in tropical sun and breeze, produces weaker concrete prone to crazing and early plastic shrinkage cracks.
- Moisture and rusting steel: rain entering cracks scours the base and keeps the slab damp, and once moisture reaches the steel it rusts and swells, sooner still in salty air by the coast. The result is flaking, spalling and orange staining rather than a neat line.
Two local factors to keep in mind
Two of those causes deserve extra attention in the Far North. Reactive clay occurs in pockets around Cairns: it absorbs moisture and expands over the wet season, then dries and contracts through the dry, and that yearly cycle keeps working at any slab resting above it. Then there is the rain itself. Heavy tropical storms push water into every gap, softening and scouring the gravel below, so filling cracks quickly and directing runoff away from the slab count for more here than in drier parts of the country.
Cosmetic or structural: how to read a crack
Telling them apart is mainly about whether the crack moves and how deep it goes. A cosmetic crack is narrow, steady and shallow. A structural one is wider, keep changing, and the concrete on either side is shifting.
- Cosmetic: hairline, unchanged from year to year, both sides level, often a fine web of lines across the surface.
- Structural: about three millimetres or more, getting longer or wider, with a step between the sides, passing through the full depth, or beside an area that has dropped or heaved.
A simple home check: if a fingernail barely fits and the crack looks just as it did a year ago, it is very likely cosmetic. If it has widened, one edge has sunk, or you can see it passes right through, ask someone to inspect it. Our concrete crack repair service can assess and fix both kinds.
| Sign | Likely cosmetic | Possibly structural |
|---|---|---|
| How wide | Hairline, barely visible | Roughly 3mm or wider |
| How deep | Just the top layer | Right through the concrete |
| Alignment | Edges flush | A step, with one edge higher |
| Over the months | No change | Lengthening or widening |
How control joints guide cracking
Shrinkage stress has to be released somewhere. Control joints are lines of weakness sawn or tooled into the slab on purpose so the release happens in a spot you picked. Sawn to roughly one quarter of the slab thickness, it gives the stress an easy path, and the crack forms out of sight at the bottom of the groove.
Joints let you down in two ways. If they are sawn too late, the concrete has already cracked wherever it chose. If they are too shallow, they are just a line on the surface that guides nothing. Spacing counts too: joints placed too far apart allow cracks to open up in the gaps, and long skinny panels usually break across their width regardless.
Preventing serious cracks before the pour
Nearly all of the prevention happens before the truck turns up. The concrete itself is seldom to blame.
- Excavate to suit the site: strip topsoil, roots and soft soil to whatever depth the ground calls for, rather than a stock number.
- Compact in stages: place and compact crushed rock in thin layers, rather than one thick layer that settles later under traffic.
- Lift the steel: mesh lying on the gravel adds little. Positioned mid-slab with ample cover, it stops a crack from separating into two loose pieces. See our reinforced concrete page for more.
- Plan for water: slope the slab away from the house and provide drainage so storm runoff does not soak the soil beneath.
- Saw joints promptly: deep enough and close enough together that shrinkage cracks stay where you intended.
When a crack needs a professional look
Have a crack inspected if it measures more than roughly 3mm across, has spread since you first saw it, shows a step between its edges, or appears in a retaining wall or close to a house's footings. For everything else, fill fine cracks to keep the rain out and check them now and then.
No method can promise a slab that never cracks. Good practice keeps cracks narrow, predictable and inside the joints, and for a driveway that is what success really looks like.