Repair guide · Repairs, care & coastal conditions
Why does concrete crack?
By Nathan Carter, owner, Gold Coast Concrete Services. Reviewed October 2026.
Quick answer
Concrete cracks mainly because it shrinks slightly as it dries, and because the ground under it moves. Fine hairline shrinkage cracks are normal and usually cosmetic. Cracks that are widening, have one side higher than the other, or let water through are worth checking, as they can point to sinking ground, tree roots or drainage problems.
| Type of crack | Likely cause | Usually |
|---|---|---|
| Fine, random surface cracks | Surface dried too fast after the pour | Cosmetic |
| Straight crack between joints | Drying shrinkage | Cosmetic; can be sealed |
| Crack with a lip or step | Ground settlement or tree roots | Needs checking |
| Crack that keeps widening | Ongoing ground movement or water | Needs checking |
| Cracks at corners and edges | Stress points, thin edges | Repairable |
Shrinkage is normal
Concrete is mixed with more water than it needs to set, and as that water leaves, the concrete shrinks slightly. Because it is held by the ground and its own weight, it cracks somewhere to relieve the stress.
Control joints give it a planned place to crack. Most shrinkage cracks form inside the saw cuts where you never notice them. Some can still appear between joints, especially on hot, windy pour days.
Ground movement
Reactive clay soils swell when wet and shrink when dry. Over years of wet summers and dry winters, that movement can crack slabs and paths. Poorly compacted fill can settle under the weight of a slab, leaving a crack with one side lower than the other.
Tree roots and water
Roots growing under a path or driveway can lift it, while large trees can dry out the soil and make it shrink. Leaking pipes, blocked downpipes or water running under the slab can wash out the base. Fixing the water problem is part of fixing the crack.
Overloading
A driveway built for cars may crack if trucks, skip bins or heavy machinery regularly use it, especially at the edges. That is why we ask about vehicles before we design a driveway.
How cracks are repaired
The fix depends on the cause:
- Hairline cracks — clean and seal with a flexible sealant or leave alone
- Stable wider cracks — rout out and fill with a suitable repair product
- Sunken or lifted sections — saw-cut out, fix the base, and repour
- Widespread cosmetic cracking — resurface once the slab is stable
When to get it checked
Call us if a crack is getting wider, one side is stepping up, water is coming through, or it is creating a trip hazard. For house slabs and structural walls, an engineer may need to assess it first.
Common questions
Is it normal for a new driveway to crack?
Fine shrinkage cracks can occur, even on well-built driveways. Control joints keep most of them hidden. A crack with a step or one that keeps growing is not normal.
Can a crack repair be invisible?
Repairs are usually visible, because new material won’t match weathered concrete exactly. Resurfacing gives a uniform finish over the whole area.
Will sealing a crack stop it growing?
Sealing keeps water out, which helps. It won’t stop cracks caused by ongoing ground movement, so the cause needs fixing too.
Should I worry about cracks in my house slab?
Most fine cracks are cosmetic. If cracks are wide, growing, or doors and windows are sticking, ask a structural engineer to check.
More on repairs, care & coastal conditions
- Should you resurface or replace your concrete?
- Concrete near the beach: what's different on the Gold Coast
- Concrete Repairs
