You cannot stop concrete from shrinking as it dries, but you can stop that shrinkage from turning into unwanted cracks. The job is to control where and how the concrete relieves its stress, and five things do almost all of the work. Cure the slab by keeping it damp for the first seven days, protect it from drying too fast in heat and wind, cut control joints at the right spacing and depth, use a correct mix without adding water on site, and pour onto a well-prepared, compacted base. Get those right and most cracking is avoided or hidden neatly along the joints. The table below is the short version.
| Step | What to do | Why it works |
| Cure it | Keep the slab damp for 7 days | Lets concrete gain strength before it is stressed |
| Slow the drying | Cover, shade, pour in the cool | Stops the surface shrinking faster than the body |
| Cut control joints | Right spacing and depth, cut in time | Concentrates cracking on planned lines |
| Get the mix right | Correct strength, no added water | Stronger concrete that shrinks less |
| Prepare the base | Compact and level the sub-base | Prevents uneven settlement cracks |
Cure It Properly, the Single Biggest Lever
If you do only one thing to prevent cracking, make it curing, because it does more than any other single step. Curing simply means keeping the concrete moist while it gains strength.
Concrete develops its strength through a reaction between cement and water that keeps running for weeks, and the first seven days are when most of that strength forms. If the slab is allowed to dry out in that window, it loses moisture before it is strong enough to resist the shrinkage stress, and it cracks. Keeping it damp lets it reach strength first. You can cure by lightly spraying or hosing the surface to keep it wet, covering it with plastic sheeting or wet hessian, or spraying on a curing compound that seals moisture in. Any of these works, as long as the surface stays damp rather than drying and rewetting over and over. A properly cured slab is far less likely to crack than one left to dry on its own, whatever else you do.
Slow the Drying in Heat and Wind
Closely tied to curing is protecting the fresh slab from drying too fast, because rapid surface drying is what causes the fine cracks that appear within hours. Heat, wind, and direct sun are the triggers.
When the top of the slab loses water faster than the concrete below can keep up, the surface shrinks while the body is still soft, and it tears into plastic shrinkage cracks. Hot, dry, windy days make this far worse, which is why summer pours across Central Victoria need extra care. Pour in the cooler part of the day where you can, keep the slab shaded or covered after finishing, and start curing as soon as the surface is firm enough. On a windy site, a temporary windbreak helps. If you are ordering pre-mix concrete for a summer pour, booking an early delivery and having covers ready before the truck arrives makes a real difference.
Cut Control Joints at the Right Spacing
Since the slab will shrink and want to crack somewhere, control joints let you choose where, and they are the main tool for keeping cracks tidy. A slab of any size needs them.
Control joints are grooves cut or tooled into the slab that create a deliberate line of weakness, so the concrete cracks along the joint instead of wandering across the surface. The spacing matters. As a rough guide, joints are placed in metres at about two to three times the slab thickness in millimetres divided by ten, so a 100mm slab wants joints every 2 to 3 metres. They need to be about a quarter of the slab depth, and cut at the right time, either tooled in while the concrete is still plastic or saw cut within the first day before random cracking starts. Leave joints out, space them too far apart, or cut them too late, and the slab cracks on its own terms. A clean crack running along a control joint is the system working, not a fault.
Start With the Right Mix
The concrete itself sets how well the slab resists cracking, and two mix mistakes cause a lot of avoidable cracks. Both weaken the slab from within.
The first is adding water, whether at the plant or with a hose on site, to make the concrete easier to place. Extra water raises the water-to-cement ratio, which makes weaker, more porous concrete that shrinks more and cracks more easily, so it is one of the worst things you can do to a pour. The second is using a strength grade that is too low for the job. The fix is to order the correct strength grade in MPa for what the slab will carry, keep the mix as delivered, and reach for a plasticiser rather than a hose if it needs to flow better. Matching the right concrete to your project from the start removes a whole category of cracking before the pour even begins.
Prepare and Reinforce the Base
What sits under the slab is just as important as what goes into it, because uneven ground cracks even good concrete. The slab can only be as stable as its base.
Before pouring, the sub-base should be levelled and compacted, with any soft spots, loose fill, or tree roots removed, and a uniform layer of compacted crushed rock laid down. A base that settles unevenly once the slab takes weight will crack it, no matter how well the concrete was cured. Steel reinforcement, usually mesh and sometimes bar, ties the slab together and helps it resist and distribute the stresses that cause cracking. On reactive clay soils, which move with the seasons across parts of the region, heavier reinforcement or an engineered design may be needed. Our owner-builder concrete guide covers getting the base and reinforcement right before delivery day.
Finish It Without Overworking It
A few things during placing and finishing also help keep the slab crack-free, and they cost nothing but attention. The aim is to let the concrete do its thing.
Pour the whole slab in one continuous go where you can, since stopping and starting creates cold joints that become weak lines, so order enough for the job in a single load, which our ready mix buyers guide helps you plan. Do not overwork the surface with the trowel, as too much finishing brings excess water and fine material to the top, which forms a weak, crack-prone skin. And never finish over bleed water sitting on the surface, since sealing that water in weakens the top layer. Wait for the sheen to go, then finish, then cure.
Frequently Asked Questions
Can you completely stop concrete from cracking?
No, because all concrete shrinks as it cures. What you can do is control the cracking so it is hidden along planned control joints and kept to harmless hairline levels, by curing well, cutting joints, using the right mix, and preparing the base.
How long should I keep new concrete damp?
Aim to keep it damp for at least the first seven days, since that is when most of the strength forms. In hot or windy weather, hose or cover it more often, as the surface dries out quickly and is most at risk of cracking early.
Does wetting concrete while it cures stop cracks?
Yes, keeping the surface damp during the first week is one of the most effective ways to prevent cracking. It lets the concrete reach strength before the drying shrinkage can stress it, as long as the surface stays consistently damp rather than drying and rewetting.
