Why Concrete Cracks on the Central Coast (and How to Prevent It)

Reactive clay in the hinterland. Back off the water, through the Gosford hinterland and up around Kariong, the ground is reactive clay and shale. It swells when it soaks up rain and shrinks back as it dries out over summer. That constant up-and-down movement lifts and drops a slab unevenly, and a slab that is not built to span it will crack across the middle or at the corners. This is the single most common cause of serious cracking away from the beach.

Loose sand on the Peninsula. Down at Umina, Ettalong and Woy Woy the blocks are coastal sand. Sand drains beautifully, which is a genuine advantage, but it has no cohesion, so if the base is not properly confined and compacted it can spread sideways under the weight of a car. When the sand moves out from under an edge, the slab loses support there and cracks. The concrete is rarely the problem on the Peninsula, the confinement of the base is.

A high water table near the water. On the low-lying blocks around Brisbane Water and the Tuggerah Lakes the water table sits high, and after wet weather it can sit very high. A saturated base is a soft base, and water moving under a slab can wash the fine material out from beneath it, so the slab settles unevenly and cracks follow. Here the enemy is water, and the answer is drainage.

Salt near the beach. At Terrigal, Avoca and Wamberal there is a second problem sitting on top of the ground one: salt. Salt-laden air settles on the surface, and repeated wetting and drying drives spalling, where the top few millimetres flake and crumble away. It is a surface attack rather than a structural one, but left alone it eats a driveway from the top down, so beachside slabs need extra cover, the right mix and sealing.

The right base for the ground. Reactive clay wants a deeper, better-drained base that can carry movement, sand wants proper confinement and compaction, and a wet block wants drainage before anything else. Matching the base to your ground is the biggest single thing that stops cracking.

Steel mesh on chairs. Reinforcing mesh only works if it sits up in the middle of the slab on chairs, not down in the dirt. Placed right, it holds any crack that does form tight and stops it opening and stepping.

Control joints. Cut or tooled joints give the slab planned lines to shrink along, so the inevitable shrinkage crack lands on a neat line you chose rather than wandering across the surface.

Concrete cover and a coastal mix. Near salt, enough cover over the steel and a suitable mix keep the salt away from the reinforcement and slow surface breakdown.

Sealing near the beach. A sealer on beachside slabs keeps salt and water out of the surface and is the cheapest ongoing defence against spalling.