Concrete cracks: when to worry, when to ignore.

Nearly every crack we get called to in this region is one of four things, and they are easy to tell apart once you know what you are looking at:

• Crazing. A fine web of shallow surface lines, like crocodile skin, usually on a slab poured on a hot windy day. Ugly, entirely cosmetic, no structural meaning. Common on summer pours where the surface dried too fast.
• Plastic shrinkage cracks. Short, roughly parallel cracks that appeared within hours of the pour. Caused by the surface losing water faster than the concrete could bleed it. Shallow. Annoying, rarely serious.
• Drying shrinkage cracks. Straight, often running from a corner or between joints, appearing in the first months. Normal concrete behaviour finding a release. Should be in the joints. When they are not, the joints were cut too shallow or too late.
• Structural cracks. Wider, often stepped or tapered, frequently diagonal from a corner, and they keep changing. This is the ground moving. This is the one that matters.

The tell for the fourth type in Rockhampton is the season. Reactive black soil across Gracemere and the Fitzroy flats swells through the wet and shrinks through the dry, so a structural crack here breathes on an annual cycle. Cosmetic cracks do not care what month it is.

The slab usually tells you through the house before it tells you through the floor. Watch for doors that bind in the wet season and swing free in the dry, stair-step cracking in the brickwork above windows and doors, cornice separating from the ceiling in a straight line, a gap opening between the skirting and the floor, and windows that suddenly stop latching. The seasonal pattern is the giveaway. Damage that comes and goes with the wet and the dry is reactive black soil moving the slab, and it is the classic Rockhampton signature.

People miss this for years because the symptoms are so ordinary. A sticking door in February gets planed. It stops sticking in August and everyone forgets. Three summers later the cornice opens up and it becomes a real problem. If you have planed a door and it came good on its own, write the date down. That is data, and it is exactly what an engineer wants.

And if the movement is real, understand the cause before spending: it is nearly always the footing design, the drainage, or a leaking pipe. Slab prep on reactive soil covers what a slab needs to survive this ground in the first place, and does insurance cover a cracked slab covers who, if anyone, pays.

It depends entirely on whether the cause has stopped. If the crack is stable, meaning it is not getting wider month by month and there is no vertical step, it can usually be routed out, filled with a flexible polyurethane sealant and left. If the ground is still moving, any repair is temporary and the money is better spent fixing the drainage or the footing first. The honest test is to mark each end of the crack with a pencil and a date, measure the width, and check it again after a full wet season.

2026 numbers so you can plan. Routing and sealing a stable crack runs about $45 to $90 per lineal metre with a minimum call-out. An engineer’s inspection and report is $900 to $2,200. Removing and replacing concrete is $40 to $80 per m² for demolition plus tip fees, then $90 to $140 per m² for a new engineered slab, with reactive-soil footings adding 15 to 30 per cent.

A worked example from Parkhurst. A 45m² driveway, one 3mm crack, flat, no step, not moved in two years since it was first marked. Route and seal about 6 lineal metres: roughly $500. The owner was quoted $6,200 to replace the lot. The replacement would have been genuinely wasted money, because the slab is fine, it just has a crack in it. That is a very different situation from the same crack with a 5mm step and a door that sticks every February. Full rates are on the cost guide.

Is it normal for a concrete slab to crack?

Yes. All concrete cracks, and any concreter who promises otherwise is either new or lying. Concrete shrinks roughly half a millimetre per metre as it dries, so a 10 metre slab wants to get about 5mm shorter and something has to give. The job of a good slab is not to avoid cracking, it is to control where the cracks go: into the saw cuts and control joints, where nobody sees them and nothing structural depends on them. A hairline crack in a joint is the slab working as designed. A 5mm crack wandering diagonally across the middle is not.

How wide does a crack have to be before I should worry?

Use width and step together, not width alone. Under 0.3mm, about the thickness of a business card edge, is cosmetic and needs nothing. From 0.3mm to 1mm, watch it and seal it if it is outdoors. From 1mm to 5mm, get it looked at, especially if it appeared quickly. Over 5mm, or any crack where one side sits higher than the other, or any crack you can see daylight or moisture through, get an engineer. The step matters more than the width: a 2mm crack with a 4mm vertical step is a worse sign than a 6mm crack that is dead flat.

What are the early warning signs of slab movement in a Rockhampton house?

The slab usually tells you through the house before it tells you through the floor. Watch for doors that bind in the wet season and swing free in the dry, stair-step cracking in the brickwork above windows and doors, cornice separating from the ceiling in a straight line, a gap opening between the skirting and the floor, and windows that suddenly stop latching. The seasonal pattern is the giveaway. Damage that comes and goes with the wet and the dry is reactive black soil moving the slab, and it is the classic Rockhampton signature.

Why does my driveway crack in the same place every year?

Because something in the ground or the design keeps concentrating stress at that spot, and patching the surface never touches the cause. The usual culprits are a missing or too-shallow control joint, a re-entrant corner where the slab changes width, a service trench backfilled with loose spoil that keeps settling, a tree root, or a section poured over the join between cut and fill. Re-cracking through a repair is not the repair failing. It is the slab telling you the diagnosis was wrong.

Can a cracked slab be repaired or does it have to be replaced?

It depends entirely on whether the cause has stopped. If the crack is stable, meaning it is not getting wider month by month and there is no vertical step, it can usually be routed out, filled with a flexible polyurethane sealant and left. If the ground is still moving, any repair is temporary and the money is better spent fixing the drainage or the footing first. The honest test is to mark each end of the crack with a pencil and a date, measure the width, and check it again after a full wet season.