Why Does Concrete Crack? (And How to Prevent It)
All concrete cracks. That sounds blunt, but it's true — concrete shrinks as it cures, and it moves with temperature, moisture and load. The job of a good concreter is to control where and how it cracks so you never see it. Here's the honest version of why concrete cracks and what prevents it.
Shrinkage Cracks (Most Common)
As concrete cures it loses moisture and shrinks slightly. Without control joints to give the slab somewhere to crack, it'll crack randomly. We cut joints at the right depth and spacing (usually within 24 hours of the pour) so any shrinkage cracking happens inside the joints — invisibly.
Poor Base Preparation
Soft spots, organic material, uncompacted fill or poor drainage under a slab cause settlement cracks. We strip topsoil, compact in layers, and use the right base material every time. Skipping base prep is the #1 reason DIY slabs fail.
Insufficient Reinforcement
Steel mesh or bar holds a slab together when it does crack. Undersized or poorly positioned mesh leads to wider, more visible cracks. We use the right grade (SL72, SL82, N12 bar etc.) and chair it correctly so it sits in the middle of the slab.
Bad Curing
Concrete needs moisture to cure properly. Pouring on a 35°C day with no curing compound or wet covers leads to surface crazing and shrinkage cracks. We plan pours around the weather and use curing compounds and water sprays to control the cure.
Tree Roots, Frost & Soil Movement
Reactive clay soils in parts of the Southern Highlands and Illawarra move with moisture. Tree roots lift slabs. These issues need design solutions — thicker slabs, deeper edge beams, root barriers and proper drainage.
When to Worry About a Crack
Hairline cracks (less than 1mm) are cosmetic and normal. Cracks wider than 3mm, cracks with vertical displacement, or cracks that grow over time signal a structural issue worth inspecting. We repair cracked concrete across the South Coast — get in touch for an honest assessment.
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Serving Wollongong, Shellharbour, Shoalhaven and the Southern Highlands. License 376227C — fully insured.
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