New concrete most often cracks because it shrinks as it dries, and if that shrinkage is not managed the slab relieves the stress by cracking. The usual causes are drying too fast in heat or wind, missing or poorly placed control joints, too much water in the mix, a weak or under-reinforced slab, and a soft or uneven base underneath. Some fine cracking is normal and harmless, but wide, stepped, or structural cracks point to a real problem. The good news is that most cracking is preventable with the right mix, proper curing, and control joints cut in at the right spacing. The table below sorts the common cracks by how much they matter.
| Crack type | What it looks like | How serious |
| Hairline or crazing | Fine surface lines, map-like pattern | Cosmetic, usually harmless |
| Shrinkage cracks | Thin cracks between joints | Common, rarely structural |
| Control joint cracking | Clean cracks along cut joints | By design, not a fault |
| Wide or stepped cracks | Over 2mm, with height difference | Structural, needs assessment |
Shrinkage Is the Main Reason
To understand almost all early cracking, it helps to know that concrete shrinks slightly as it cures. This single fact explains most of the cracks people worry about.
As the water in fresh concrete is used up by the curing reaction and lost to evaporation, the slab shrinks by a small amount. Because the slab is held in place by friction with the ground and by its own edges, it cannot shrink freely, so tension builds inside it. When that tension is greater than the young concrete can bear, it cracks to relieve the stress. This is why cracks tend to appear in the first days and weeks rather than years later, and why the aim is never to stop shrinkage, which is impossible, but to control where the cracking happens. A quality pre-mix concrete batched to a consistent recipe shrinks more predictably than a hand-mixed or over-wet batch, and ordering enough for one continuous pour, which our ready mix buyers guide explains, avoids the weak cold joints that can crack later.
Drying Too Fast
One of the biggest triggers for early cracking is concrete that dries out too quickly, and it is also one of the most avoidable. Heat, wind, and sun are the culprits.
When the surface loses moisture faster than the concrete can cure, the top shrinks while the body is still plastic, which pulls the surface into fine plastic shrinkage cracks within hours of the pour. Hot days, dry winds, and direct sun all speed this up, which is why summer pours across Central Victoria need extra care. Keeping the slab damp through the first week, covering it, or using a curing compound slows the moisture loss and lets the concrete reach strength before it is stressed. Pouring in the cooler part of the day in summer helps too. How the slab is cured in that first week does more to prevent cracking than almost anything else.
Missing or Badly Placed Control Joints
Since concrete is going to shrink and crack somewhere, control joints are how you decide where, and skipping them is a common cause of random cracking. They are the planned weak lines that take the stress.
Control joints are the grooves cut or tooled into a slab, either while it is wet or soon after, that create a deliberate line of weakness so the slab cracks neatly along the joint rather than wandering across the surface. If they are left out, spaced too far apart, or cut too shallow or too late, the slab cracks on its own terms instead. As a rough guide, joints are spaced in metres at about two to three times the slab thickness in millimetres divided by ten, and cut to about a quarter of the slab depth, but the key point for a homeowner is simply that a slab of any size needs them. Clean cracking along a control joint is not a fault, it is the joint doing its job.
A Weak Mix or Too Much Water
The mix itself decides how well the slab resists cracking, and two mistakes show up again and again. Both weaken the concrete from the inside.
Adding water to make concrete easier to place, whether at the plant or with a hose on site, raises the water-to-cement ratio and produces weaker, more porous concrete that shrinks more and cracks more easily. Using a strength grade that is too low for the job has the same effect, leaving the slab unable to carry its loads. The fix is to order the correct strength grade, measured in MPa, for the slab, and never to add water on site to loosen a stiffening mix. Matching the right concrete to your project from the start is one of the simplest ways to avoid cracking later.
Poor Base Preparation
What sits under the slab matters as much as the concrete itself, and a soft or uneven base is a frequent hidden cause of cracking. The slab can only be as stable as the ground beneath it.
If the sub-base is not properly levelled and compacted, or if soft spots, fill, or tree roots are left in place, the slab settles unevenly once it carries weight, and that movement cracks it. Reactive clay soils, common in parts of the region, swell and shrink with moisture and can lift or drop sections of a slab through the seasons. A well-prepared base of compacted crushed rock, the right slab thickness, and proper steel reinforcement all spread loads and resist this movement. On reactive sites, heavier reinforcement or an engineered slab design may be needed, which is worth checking before the pour rather than after a crack appears. Our owner-builder concrete guide covers the base and reinforcement details worth sorting out before delivery day.
Which Cracks Should You Worry About
Not every crack is a problem, and knowing the difference saves a lot of worry. Most new slabs show some cracking, and the majority of it is cosmetic.
Fine hairline cracks and surface crazing, the shallow map-like pattern that can appear as the surface cures, are cosmetic and do not affect the slab’s strength. Thin shrinkage cracks between joints are common and rarely structural. Cracking along control joints is by design. The cracks worth acting on are wide ones, generally over about 2 millimetres, cracks where one side sits higher than the other, and cracks that run diagonally across a slab or keep growing, since these can signal settlement or structural movement and are worth having an engineer or concreter assess. If in doubt, measure the crack and watch whether it changes over a few weeks.
Frequently Asked Questions
Is it normal for new concrete to crack?
Yes, some cracking is normal. Fine hairline cracks, surface crazing, and thin shrinkage cracks between joints are common as concrete cures and do not usually affect strength. Wide, stepped, or diagonal cracks are the ones worth having assessed.
How soon can new concrete crack?
Cracking can start within hours if the surface dries too fast, producing plastic shrinkage cracks, and more commonly appears over the first days and weeks as the slab shrinks while curing. Cracks that appear much later can point to settlement or ground movement.
How do I stop my concrete from cracking?
You cannot stop shrinkage entirely, but you can control it. Use the right strength mix without added water, prepare and compact the base well, cut control joints at the correct spacing, and cure the slab by keeping it damp for the first week, especially in hot or windy weather.
Do hairline cracks in concrete need fixing?
Usually not for strength. Hairline and crazing cracks are cosmetic and can be left or sealed for appearance. It is the wider cracks over about 2 millimetres, or cracks with a height difference, that warrant repair and a closer look at the cause.
