Every concrete slab cracks. That is not a defect – it is physics. What separates good concrete from bad is where it cracks, how wide the crack gets, and whether the slab stays level. In Calgary, the deciding factor is almost always freeze-thaw.
What freeze-thaw actually does
Concrete is porous. Water gets into those pores from rain, snowmelt and washing the car. When it freezes it expands by about 9%. That expansion has to go somewhere, and if there is nowhere for it to go, it pushes the surface apart from the inside.
Calgary is unusually hard on concrete because of chinooks. We do not freeze in November and thaw in March – we cycle above and below zero dozens of times each winter. A slab in Vancouver might see five or six freeze-thaw cycles a year. A Calgary driveway can see forty.
Why air entrainment is the whole answer
Air-entrained concrete has microscopic air bubbles deliberately distributed through the mix – roughly 6% by volume for exterior work. When water in the pores freezes and expands, it pushes into those bubbles instead of against the surrounding concrete.
It is a pressure-relief valve built into the material. Non-air-entrained concrete has nowhere for that pressure to go, so the surface pops off in flakes. That is spalling, and it is the most common concrete failure in this city.
This is not optional in Alberta. Any exterior residential concrete poured here without air entrainment will spall. It is not a question of if, only how many winters it takes. Ask for it in writing.
De-icing salt makes it dramatically worse
Salt lowers water’s freezing point, which means more freeze-thaw cycles per winter, not fewer. It also draws moisture into the slab. Salt does not chemically attack concrete the way it attacks steel – it accelerates the physical freeze-thaw mechanism.
- Never salt concrete in its first winter. Use sand for traction.
- Prefer sand or non-chloride de-icers on driveways and walkways generally.
- Avoid magnesium and calcium chloride. More aggressive than plain rock salt.
- Shovel before it melts and refreezes. Removing snow beats melting it.
- Watch what drips off the car. Road brine lands on the garage apron – usually the first area to spall.
The three cracks worth knowing
Shrinkage cracks – normal
Hairline, usually within the first few weeks as the slab cures. Cosmetic. This is what control joints are for: the tooled or saw-cut lines create a deliberate weak point so the slab cracks along a straight line instead of randomly. Joints should be cut at roughly 24 to 30 times the slab thickness – every 3 to 3.5 metres for a typical driveway.
Settlement cracks – a base problem
Wide, uneven, and one side sits lower than the other. The ground underneath moved. Almost always inadequate base preparation. Cannot be fixed by patching the top, because the cause is underneath.
Surface spalling – a mix problem
Flaking, pitting, or the top layer popping off in sheets. Freeze-thaw damage on concrete that lacked adequate air entrainment. Repairable with a bonded topping if the slab is structurally sound.
How to prevent it – the checklist
- 32 MPa mix with 6% air entrainment. Code minimum is 25 MPa.
- 150mm of compacted granular base, placed in lifts and compacted properly.
- Rebar on chairs, not mesh dropped on the ground.
- Control joints cut at correct spacing, within 24 hours of the pour.
- Slope for drainage – minimum 2%. Standing water is freeze-thaw fuel.
- Cure it properly. Curing compound or wet cure for seven days.
- Seal at one year, then every 3–4 years.
- No salt in year one. Worth repeating.
Already cracked? What it means
Hairline cracks along control joints are doing their job – leave them, or fill with a flexible sealant to keep water out. Surface spalling on a flat, sound slab is a candidate for a bonded topping at roughly half the cost of replacement. But if sections have heaved, settled unevenly or cracked through and shifted, the base has failed and any repair will crack again within a couple of seasons.
If you are unsure which you have, send a photo or have us look. We quote the repair when the repair is the right answer.