Guides › Why concrete cracks
Concrete cracks on the Sunshine Coast mainly because of the ground it sits on: reactive Blackall Range clay and basalt that swell and shrink with the wet and dry seasons, acid sulfate soils on the low river flats and canal estates that corrode the reinforcing steel, and salt air near the coast. A slab that is built for its block copes with all of that. One that is not tells you about it within a couple of summers.
Some cracking is normal and harmless. The rest is almost always a base, steel or drainage problem that was baked in before the concrete was even poured. Here is what causes it locally, and what a proper build does to prevent it.
Concrete shrinks a little as it cures, and it will crack somewhere. That is not a fault, it is physics, and it is exactly why control joints exist. So before you worry, it helps to know which cracks are cosmetic and which are a warning.
If you are seeing the last two, the cause is under the slab, not in it, and that is where the Sunshine Coast ground comes in.
The Coast is not one type of ground. Where your block sits decides which problem you are up against, and each one cracks a slab in its own way.
Reactive hinterland clay and basalt. Behind the coast, up through Nambour, Palmwoods, Maleny, Montville and the Blackall Range, the soils are reactive clay and basalt. They swell when the wet season soaks them and shrink back as they dry out through winter. That constant up and down movement is hard on a rigid slab. A driveway that was poured on a shallow, poorly compacted base with the steel sitting on the dirt will lift, drop and crack across the face as the ground works underneath it season after season.
Acid sulfate soils on the river flats and canal estates. Down on the low ground near the Maroochy and Mooloolah rivers, and around the canal estates at Pelican Waters, the soils are often acid sulfate. Left undisturbed they are stable, but once they are dug up and exposed to air they produce acidic groundwater. That acid attacks the reinforcing steel in a slab. As the steel rusts it swells, and swelling steel spalls the concrete off from the inside, which is why you see rust-stained cracks and flaking on these blocks. This is a corrosion problem, and it is the reason a slab on acid sulfate ground has to be built differently.
The subtropical wet and dry swing, plus coastal salt. On top of the soil, the Coast climate itself works the ground. A heavy summer wet season followed by a dry winter drives the swell-and-shrink cycle in reactive soils and moves the water table under low blocks. Closer to the beach at Coolum, Marcoola, Mooloolaba, Golden Beach and Pelican Waters, salt-laden air adds a second corrosion source, driving rust in the steel and surface spalling much like the acid ground does. Same result, different cause: the steel is the weak point unless it is protected.
None of this is exotic. Preventing cracks on the Coast comes down to a handful of things done properly before and during the pour, matched to the ground the slab sits on.
Every one of these is invisible once the job is finished, which is exactly why it is worth asking whoever quotes you how they are handling the ground on your block.
Most cracking is a build problem, so the fix is choosing someone who reads Coast ground before they pour. Our concrete driveways page walks through how a slab is built base to finish, and the driveway cost guide explains why ground conditions are the biggest single reason two quotes differ. If an existing slab is only surface-worn rather than structurally cracked, resurfacing can be an option instead of a full replacement. And if you are weighing up a slab against a flexible surface for reactive ground, our guide on concrete versus pavers covers how the block should steer that choice.
Worried about a cracking slab, or want a new one built so it does not? Tell us your suburb and what the ground is like and we will get a local concreter out to read the block and price a build that suits it. There is no cost and no obligation.
Call 0482 392 415 or send the details through the contact page.
The Coast throws hard ground at a slab. Reactive Blackall Range clay and basalt in the hinterland swell and shrink with the wet and dry seasons, the low river flats and canal estates carry acid sulfate soils that corrode steel, and coastal blocks sit in salt air. A slab that is not built for the ground it sits on is the usual reason a driveway cracks within a couple of summers.
Fine hairline shrinkage cracks are normal and mostly cosmetic. Concrete shrinks slightly as it cures and will always crack somewhere, which is what control joints are for. The cracks that matter are wide, stepped or lifting ones, or cracks that leak rust stains, because those point to ground movement or corroding steel underneath rather than simple shrinkage.
You build for the movement. That means a properly excavated and compacted base matched to the ground, reinforcing steel lifted onto bar chairs so it sits inside the slab, adequate slab thickness, and control joints cut in a grid so the slab cracks along the joints instead of across the face. On very reactive hinterland blocks the base often needs to be deeper and better drained.
Acid sulfate soils are found on the low river flats and canal estates near the Maroochy and Mooloolah rivers and around Pelican Waters. When disturbed they produce acidic groundwater that attacks the reinforcing steel in a slab. As the steel rusts it expands and spalls the concrete off from the inside. The fix is proper cover over the steel and a corrosion-aware mix suited to acidic ground.
Local concreters who read the ground before they pour.
📞 Call 0482 392 415