Concrete cracks because it shrinks as it cures, because the ground beneath it moves, because it gets loaded harder than it was built for, or because nobody gave the shrinkage a tidy line to follow. Most of the cracks people worry about are harmless. A few are genuinely telling you something. The useful skill is knowing which is which, and in Hobart there are two local factors that tilt the odds.
What follows covers what actually causes cracking, how to read a crack, and what makes the difference between a slab that develops a few hairlines and one that comes apart.
What actually makes concrete crack
Concrete is very strong in compression and weak in tension. It holds up enormous downward load and splits readily when something pulls it apart or takes away its support. Nearly every crack traces back to one of these:
- Shrinkage as it cures: fresh concrete loses moisture and contracts slightly. This is the most common cause of fine cracking and it cannot be designed away, only directed.
- Ground movement underneath: if the base settles, washes out or was never compacted properly, the slab loses support and cracks under its own weight. On reactive clay this is the big one.
- Loads it was not built for: a slab poured for cars will crack under a delivery truck or a loaded trailer, particularly at the edges where support is weakest.
- Joints missing or badly placed: with nowhere sensible to relieve shrinkage, the slab picks its own line, usually somewhere visible and untidy.
- Poor mixing or curing: too much water in the mix, a surface allowed to dry too fast in sun and wind, or a frost before the slab has gained strength gives you a weaker slab that crazes and cracks early.
- Freeze-thaw and corroding reinforcement: water in cracks freezes and expands on frosty nights, slowly widening them, and moisture that reaches the steel makes it rust and expand. This shows as scaling, flaking and rust staining rather than a clean line.
Telling a harmless crack from a serious one
Two of those deserve particular attention here. Reactive clay underlies much of Greater Hobart, and it swells when it takes up water through the wet winter and shrinks as it dries out over summer — a seasonal cycle that works at any slab sitting on it. And frost: Hobart's cold winter nights drive freeze-thaw cycles that open up any crack water can get into, which is why sealing cracks early matters more here than in milder climates.
The difference between cosmetic and structural is mostly about movement and depth. Cosmetic cracks are narrow, stable, and confined to the surface. Structural cracks are wider, they change over time, and the slab either side is doing something.
- Cosmetic: hairline width, stable over time, no level difference between the sides, often in a fine map-like pattern across the surface.
- Structural: roughly three millimetres or wider, growing, stepped or offset in height, running full depth, or accompanied by a section that has sunk or lifted.
A rough field test: if you can barely get a fingernail into it and it looks the same as it did last year, it is almost certainly cosmetic. If it has opened, if one side has dropped, or if it runs through the full depth, get it looked at.
| Sign | Likely cosmetic | Possibly structural |
|---|---|---|
| Width | Fine / hairline | Wide (about 3mm+) |
| Depth | Surface only | Through the slab |
| Level | Both sides even | One side raised or sunken |
| Change over time | Stable | Growing or spreading |
What control joints are really doing
Concrete will relieve its shrinkage stress somewhere. A control joint is simply a weakened line cut into the slab to make that happen where you chose. Cut to about a quarter of the slab depth, it creates a plane of least resistance, and the crack forms down inside the joint where nobody sees it.
Two things go wrong with joints. They get cut too late, by which point the slab has already cracked on its own terms, or they get cut too shallow, in which case they are a decorative groove that controls nothing. Spacing matters as well — joints too far apart let cracks form between them, and long thin panels tend to crack across the middle regardless.
What actually prevents the serious kind
Almost all of it happens before the concrete arrives. The slab itself is rarely the problem.
- Dig out properly: remove topsoil, vegetation and soft material to a depth that suits the ground you actually find rather than a standard figure.
- Compact in layers: crushed rock laid and compacted in stages, not one deep lift that will consolidate later under load.
- Reinforce, and chair it up: mesh lying on the base does nothing. Held at the correct height, with proper cover against moisture, it holds the slab together across any crack that forms.
- Manage water: fall away from the house and drainage that stops winter rain soaking the clay under the slab.
- Cut joints on time: to the right depth and spacing, so shrinkage follows the lines you chose.
When to get someone to look at it
If a crack is wider than about three millimetres, has grown since you first noticed it, has one side higher than the other, or sits in a retaining wall or near a house's footings, have it assessed rather than waiting to see. Otherwise, seal fine cracks to keep water out and keep an eye on them.
None of this guarantees an uncracked slab, because nothing does. What it does is keep cracking fine, predictable and in the places you chose, which for a driveway is the realistic definition of success.