Concrete cracks mainly from shrinkage as it cures, ground movement or settlement beneath the slab, overloading, missing or poorly placed control joints, and poor mixing or curing. The reassuring part is that some cracking is completely normal — many fine cracks are cosmetic and harmless — while others point to a problem worth investigating. Knowing the difference is the key to keeping your concrete sound.
Below we explain the main causes, how to tell a cosmetic crack from a structural one, and the practical steps that keep cracking to a minimum.
What causes concrete to crack?
Concrete is strong in compression but weak in tension, so it tends to crack whenever it is pulled or moved rather than pressed. The most common causes are:
- Shrinkage during curing: as fresh concrete cures it loses moisture and shrinks slightly. This is the single most common cause of fine cracks and is largely unavoidable.
- Ground movement and settlement: if the base beneath the slab shifts, sinks or was poorly compacted, the concrete loses support and cracks under its own weight.
- Overloading: heavier vehicles or loads than the slab was designed for can crack concrete that is too thin or under-reinforced.
- Missing or poorly placed control joints: without joints in the right places, the slab decides where to crack on its own — often in an untidy, visible spot.
- Poor mixing or curing: too much water in the mix, a rushed pour, or letting concrete dry too fast in wind or sun all weaken it and encourage cracking.
- Temperature and moisture changes: concrete expands and contracts with the weather, and repeated movement over time can open up cracks.
What is the difference between cosmetic and structural cracks?
Not all cracks are equal. The distinction that matters most is whether a crack is cosmetic or structural.
Cosmetic cracks are usually thin hairline or shrinkage cracks. They sit on the surface, stay narrow, and don't affect how the slab carries load. They are extremely common and, in most cases, nothing to worry about — sealing them mainly keeps water out and tidies the appearance.
Structural cracks are a different story. These tend to be wider, may have one side sitting higher than the other, can run right through the slab, and often keep growing. They usually point to an underlying issue such as ground movement, an inadequate base, or a slab that is too thin for the loads it carries. Structural cracks are the ones that need professional attention.
| 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 |
How do control joints prevent cracking?
Because concrete will shrink and crack somewhere, the trick is to control where. That's the job of control joints — deliberate grooves cut or formed into the slab at set spacings and depths. They create a planned weak point so that shrinkage cracking happens along a neat, straight line rather than wandering across the surface. Correctly spaced and timed control joints are one of the simplest and most effective ways to keep a slab looking good as it cures.
How can you prevent concrete cracking?
You can't stop concrete cracking entirely, but sound workmanship keeps cracks small, hidden and harmless. The main defences are:
- Base preparation: a well-excavated, compacted and even base gives the slab uniform support so it doesn't settle and crack.
- Reinforcement: steel mesh or bar holds the concrete together, so that if a crack does form it stays tight and doesn't spread or open up.
- Control joints: cutting joints at the right spacing and depth guides shrinkage cracks to where they won't be seen.
- The right mix and thickness: a correctly proportioned mix and adequate slab thickness for the expected loads reduce the chance of overload cracking.
- Proper curing: keeping concrete moist and protected while it gains strength — rather than letting it dry out too quickly — dramatically cuts shrinkage cracking. Our curing guide explains this in more detail.
When should you get a crack assessed?
A few fine hairline cracks in a driveway or path are usually nothing to lose sleep over. It's worth getting a crack looked at, though, if it is wide, uneven in height, growing over time, letting water through, or paired with any sinking or lifting of the slab. Catching a structural crack early is far cheaper than waiting until the damage spreads.
If you're unsure, we can help. Our team handles everything from minor concrete crack repair through to larger concrete repair and slab replacement, and we'll always give you an honest assessment of what's needed. Contact us or call 09 886 7171 to arrange a look.