Concrete cracks because it shrinks as it cures, and outdoors in Ontario freeze-thaw and road salt make it worse. You cannot stop cracking entirely, but you can control it with the right mix, a compacted base, and control joints. Done right, exterior concrete flatwork stays sound for decades.
Concrete shrinks as it cures, and shrinking means cracking
Start with the part nobody can change. Concrete goes down wet and cures hard. As it cures it gives up water and shrinks, and that shrinkage pulls on the slab from every side at once. Concrete is strong when you push on it and weak when you pull on it, so the tension wins and the slab cracks. Every slab does this. There is no mix, no additive, and no crew good enough to pour a large exterior slab that never cracks. Anyone who promises crack-free concrete is selling you something.
So the goal is not to prevent concrete cracking altogether, because you cannot. The goal is to control it: to decide where the crack goes, keep it tight and straight instead of wide and wandering, and build the slab so frost and salt cannot tear it apart from there. The rest of this comes down to four things we control: the mix, the joints, the base, and the reinforcing.
What Ontario winters do to exterior concrete
An indoor slab lives an easy life. Exterior flatwork here does not. Two things go to work on it every single winter.
- Freeze-thaw: water soaks into the surface, freezes, and expands. Then it thaws and the cycle repeats, over and over, all winter long. Each cycle pries at the concrete from the inside.
- De-icing salt: the salt that melts ice on your driveway and steps drives even more freeze-thaw cycles right at the surface. That is what makes concrete flake, pit, and peel, what the trade calls scaling and spalling.
This is why the concrete in your driveway or front walk needs more than the concrete in a basement floor. It has to be mixed and built to take a freeze-thaw and salt beating that an indoor slab never sees.
The mix that stands up to salt and frost
The single biggest upgrade for exterior concrete in Ontario is air. Air-entrained concrete has millions of tiny air bubbles built into it that give freezing water somewhere to expand into, instead of cracking the surface. For flatwork that faces freeze-thaw and de-icing salt, the right spec under CSA is clear:
- Air-entrained, 5 to 7% air: the bubbles that absorb freeze-thaw pressure and stop the surface from scaling off.
- CSA Class C-2, about 32 MPa: the exposure class and strength built for flatwork exposed to freezing and salt.
- A low water-cement ratio: less water in the mix means a denser, stronger, less permeable slab that does not soak up water and salt.
- Freeze-thaw-durable aggregate: stone that holds up to the cycle instead of breaking down inside the slab.
None of that shows on the surface, and that is exactly why it gets cut on a cheap pour. We pour air-entrained 32 MPa (C-2) concrete on exterior flatwork because that is what stands up to Ontario winters and road salt.
Concrete control joints decide where the crack goes
Since the slab is going to crack, we tell it where. That is the entire job of concrete control joints, also called contraction joints. They are tooled or saw-cut grooves that run partway down into the slab, creating a deliberate weak line. When the slab shrinks and the tension builds, it cracks at the bottom of that groove, down where you never see it, instead of randomly across the top.
Two numbers make joints work:
- Spacing no more than 2.5 m apart: joints have to be close enough to relieve the shrinkage before a random crack beats them to it. On a wider slab that means a grid, not one line.
- Depth about one quarter of the slab: the groove has to go deep enough to force the crack to start there. Too shallow, and the slab cracks somewhere else anyway.
Get the spacing and depth right and the cracks land in the joints, tight and straight, where they read as part of the pattern instead of a flaw. Skip the joints or space them too far apart, and the slab picks its own line across the middle.
The base does half the work
A perfect mix poured on soft ground still fails. What the slab sits on matters as much as what it is made of. Concrete needs a solid, compacted granular base under it: crushed granular brought in, laid in lifts, and compacted so it does not settle or move. When a slab is poured on loose fill or soft topsoil, it settles unevenly, and uneven support is what turns a hairline into a wide, stepped crack you can catch your toe on.
The base matters most on heavy clay, and around here that is a lot of lots. Clay is common on the Mountain and through Ancaster, and clay holds water and heaves hard with the frost. A compacted granular base spreads the load and gives water somewhere to drain instead of pooling under the slab and lifting it. That is why concrete cracks in a driveway so often trace back to the base: too thin, never compacted, or poured straight onto clay. The concrete gets blamed, but the ground underneath is what moved.
Reinforcing and curing: strength you cannot see
Two more things separate a slab that lasts from one that does not, and like the rest, neither one shows once the job is done.
- Reinforcing for anything driveable: a slab that carries a vehicle, like a driveway or a garage apron, gets rebar or wire mesh run through it. The steel does not stop concrete from cracking, it holds the two sides of any crack tight together so it cannot spread, step, or open up under the weight of a car.
- Proper curing: fresh concrete has to be kept damp and protected as it cures so it gains its full strength slowly. Concrete left to dry out too fast on a hot or windy day comes up weaker and more prone to surface cracking. Curing is slow and boring, and it is a big part of why one slab outlasts another.
Steps and porches have their own rules
Flat, ground-level concrete is one thing. Once concrete goes up, the Building Code steps in. A raised step or porch is not just a slab, it carries load and it lifts people off the ground, so it gets built to a higher standard.
- Footings down to frost: steps and porches need footings that bear on undisturbed soil below the frost line, roughly 1.2 m (4 ft) down in this area. Without that, frost gets under them and heaves them away from the house over a winter or two.
- Guards where it is high enough: once the walking surface sits more than 0.6 m (about 24 inches) above grade, the Building Code requires a guard, the same threshold that triggers a deck permit.
That is the difference between a porch that stays tight to the house and level, and one that pulls away and tips within a couple of seasons.
Frequently Asked Questions
Are cracks in my concrete driveway a problem?
It depends on the crack. Fine, tight hairlines from normal shrinkage are cosmetic and expected on any slab. The ones to watch are wide cracks, cracks where one side has dropped lower than the other, and cracking that keeps spreading. Those usually point to a weak base or missing reinforcing under the slab, not just surface shrinkage.
How do you prevent concrete cracking?
You cannot prevent concrete cracking completely, because every slab shrinks and cracks as it cures. What you can do is control it: pour an air-entrained mix built for freeze-thaw and salt, on a compacted granular base, with control joints cut at the right spacing and depth, reinforced if it gets driven on, and cured properly. That keeps cracks tight and in the joints.
How far apart should concrete control joints be?
For exterior flatwork, control joints should be no more than 2.5 m apart, and cut to about one quarter of the slab's depth. Closer spacing relieves shrinkage before a random crack forms, and the depth forces the crack to start at the bottom of the groove instead of wandering across the surface. Wider slabs get a grid of joints, not a single line.
Pour it once, pour it right
Exterior concrete only lasts when the mix, the base, the joints, and the reinforcing are all done right for Ontario winters, and that is how The Hammer Construction pours driveways, walkways, patios, and steps. Licensed and insured. Get a free quote at our online estimator, call or text (647) 424-1148, or email hello@thehammer.construction