Cracking is one of the most common and frustrating issues in concrete construction. While it is true that almost all concrete will develop some degree of micro-cracking over time, significant or unsightly cracks are usually preventable with proper planning, the right mix design, and good workmanship. Understanding why concrete cracks is the first step toward eliminating the problem on your next project.
Common Causes of Concrete Cracking
Plastic Shrinkage and Rapid Drying
Plastic shrinkage cracks appear while the concrete is still wet, usually within the first few hours after placement. They occur when the surface of the concrete loses moisture faster than bleed water can rise to replace it. In Brisbane's subtropical climate, this is a particularly acute risk. Hot days, low humidity, direct sunlight, and dry winds can strip moisture from the surface far more quickly than in cooler, calmer conditions. The result is a network of short, shallow cracks that appear seemingly out of nowhere before the concrete has even begun to set.
Drying Shrinkage
All concrete shrinks as it cures and continues to lose moisture over weeks and months after placement. This drying shrinkage is a natural and unavoidable property of the material. The challenge is managing where and how the shrinkage manifests. Without adequate control joints, the shrinkage stresses will relieve themselves by cracking at random locations, often in the most visible and inconvenient places. The amount of drying shrinkage is influenced by the water content of the mix — wetter mixes shrink more — and the volume-to-surface-area ratio of the slab.
Excess Water in the Mix
Adding extra water to concrete on site to improve workability is one of the most damaging practices in the industry. While it makes the concrete easier to place and finish in the short term, excess water dramatically increases shrinkage, reduces compressive strength, increases porosity, and weakens the surface layer. Every additional litre of water beyond the designed mix proportion contributes to a weaker, more crack-prone result. If the concrete arriving on site is too stiff to work, the correct response is to use a plasticiser admixture, not a hose.
Inadequate Subgrade Preparation
The ground beneath a concrete slab must provide uniform, stable support. If the subgrade is poorly compacted, contains soft spots, or includes organic material that will decompose and settle, the slab above will move unevenly and crack. In South East Queensland, reactive clay soils present an additional challenge. These clays expand when wet and contract when dry, imposing cyclical stresses on the slab that can cause cracking over time if the slab design does not account for the soil conditions.
Prevention Methods
Install Control Joints
Control joints, also called contraction joints, are deliberate lines of weakness cut or tooled into the slab surface. They guide inevitable shrinkage cracks to predetermined, straight locations where they are aesthetically acceptable and structurally manageable. The general rule is to space control joints at intervals no greater than 25 to 30 times the slab thickness. For a standard 100 mm residential slab, that means joints every 2.5 to 3.0 metres. Joints should be cut to a depth of at least one quarter of the slab thickness, and they should be placed within the first 12 to 24 hours while the concrete is still green enough to saw without ravelling.
Cure the Concrete Properly
Curing is the process of maintaining adequate moisture and temperature within the concrete during the critical early days after placement. Proper curing allows the cement to hydrate fully, developing maximum strength and density. The most common methods include spraying a liquid curing compound onto the finished surface, covering the slab with wet hessian or plastic sheeting, or applying periodic water misting. In Queensland's warm climate, curing should begin as soon as the surface can tolerate it without damage — often within an hour or two of finishing — and should continue for a minimum of seven days. Neglecting this step is one of the leading causes of surface crazing, dusting, and premature cracking.
Specify the Right Mix Design
Working with your concrete supplier to select an appropriate mix is essential. A well-designed mix balances strength, workability, and shrinkage performance. Lower water-to-cement ratios produce stronger, more durable concrete with less shrinkage, but they also reduce workability, so the use of chemical admixtures to maintain placement ease is common. For slabs-on-ground in Queensland, a 25 MPA or 32 MPA mix with controlled shrinkage characteristics is typically specified. If your project involves large unjointed areas or is in a high-exposure environment, discuss shrinkage-limited or low-heat mix options with your supplier.
Use Reinforcement
Steel reinforcement — whether in the form of mesh, bar, or fibre — does not prevent cracks from forming, but it does hold them tight and prevents them from opening up into wide, unsightly gaps. SL72 or SL82 mesh is standard in residential slabs across Australia. For more demanding applications, engineered reinforcement layouts with deformed bar provide superior crack control. Fibre-reinforced concrete, which incorporates synthetic or steel fibres into the mix, is another option that provides distributed crack control throughout the full depth of the slab.
Manage the Pour in Hot Weather
Brisbane summers regularly bring temperatures above 30 degrees with intense sunlight. Pouring concrete in these conditions requires extra precautions. Schedule pours for the early morning when temperatures are cooler. Dampen the subgrade and formwork before placing concrete to prevent them from absorbing water from the mix. Erect temporary shade or wind breaks over the pour area if possible. Have curing compounds and covers ready to apply immediately after finishing. These measures significantly reduce the risk of plastic shrinkage cracking and surface defects.
When to Seek Professional Advice
If you notice wide cracks, cracks that are growing over time, or cracks accompanied by vertical displacement (one side higher than the other), consult a structural engineer. These symptoms may indicate a subgrade failure, design deficiency, or other issue that requires investigation beyond surface repair. For standard cosmetic cracks, a flexible sealant or epoxy injection can restore appearance and prevent moisture ingress.
At Reocrete, we supply ready-mix concrete designed for Queensland conditions. Our team can advise on mix selection, shrinkage management, and curing best practices to give your project the best possible outcome. Prevention is always more cost-effective than repair.