Should I worry about hairline cracks in my new driveway?

Usually not. Fine shrinkage cracks in the first months are normal concrete behaviour. Keep an eye on them: if they stay the same width and show no staining or lipping, they are cosmetic.

What is drummy concrete?

Concrete that sounds hollow when you tap it, which means it has debonded from what is underneath, often at the reinforcement layer. It is a common early sign of spalling and worth investigating before the surface lets go.

Can cracks be injected rather than cut out?

Yes, for the right cracks. Injection works well for sealing stable structural cracks and stopping water paths. It is the wrong answer for cracks caused by ongoing movement or by corroding steel.

Concrete Cracking: What's Normal and What Isn't

Short answer

Fine surface crazing, shrinkage cracks and narrow cracks that have not moved in years are usually cosmetic. Cracks that are widening, leaking, displaced, or carrying rust staining need assessment, because they point at movement or at water reaching the reinforcing steel.

In this guide

The cracks that are usually fine
The cracks that need attention
What actually causes them
What to do about each

The cracks that are usually fine

Fine surface crazing, meaning a shallow map of hairline lines across the surface, is a finishing and drying effect rather than a structural one. Shrinkage cracks appear as concrete loses moisture, and they are the whole reason control joints exist. A crack that has obediently formed along a control joint is the joint doing its job.

Narrow, stable cracks that have not moved in years and show no staining or displacement are generally cosmetic. The useful test is behaviour over time rather than appearance on the day. A crack that has been the same width for five years is telling you something very different from one that has visibly opened since last winter.

The cracks that need attention

Take these seriously: cracks that are widening, cracks where one side sits higher than the other, cracks that leak or show damp, cracks with rust staining or brown weeping, cracks next to drummy or hollow sounding concrete, and any significant crack in a suspended slab, balcony or structural element. Diagonal cracking at the corners of openings, and stepped cracking through masonry above a slab, both suggest movement underneath rather than a surface problem.

Rust staining deserves particular attention, because it means water has reached the reinforcing steel. That is the start of the process that eventually pushes the concrete off from the inside, and it does not stop on its own.

What actually causes them

Drying shrinkage as the concrete cures, which is normal and gets managed with joints and curing. Thermal movement as slabs expand and contract. Settlement where the base underneath was not compacted properly or has washed out. Reactive soil movement, which much of Sydney sits on, heaving and shrinking with moisture. Overloading, where a slab designed for cars ends up carrying trucks. And corrosion of the reinforcement, which is the one that actively gets worse.

Poor curing deserves its own mention, because it is entirely avoidable and very common. Concrete that dries out too fast in its first days develops surface cracking and reduced strength that no amount of later care recovers.

What to do about each

Cosmetic crazing and stable hairline cracks can be left alone, or sealed if keeping water out matters. Shrinkage cracks in an external slab are worth routing and sealing. Cracks with movement, displacement or rust staining need an engineer's assessment before repair, because the repair method depends entirely on the cause. Spalling with exposed steel needs proper remediation: cut back, treat the steel, reinstate. Not a cosmetic patch.

The rule that saves the most money is simple. Never repair a moving crack until you know why it is moving. A rigid repair across an active crack cracks again, usually within a season, and you have paid twice for the same information.