A slab that powders under a broom was usually decided by the heater, not the mix. The surface can be scratched with a nail, the sweeper leaves grey dust in the corners, and the flooring installer refuses to warranty the adhesive. Everyone looks at the ready-mix ticket. The ticket is almost always fine.
After 38 years pouring concrete in this city, we have sat through enough deficiency walks to know that the question nobody asks early enough is which heater was running inside the hoarding, and whether its exhaust went outside. This is the one winter defect that a correct mix, a correct finish and a correct cure cannot save you from, because the damage is done by the air above the slab in the first hours after the trowel comes off.
Carbonation, not poor curing, is what turned that surface to powder
Cement hydration leaves calcium hydroxide in the paste at the surface. Carbon dioxide reacts with it to form calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. At normal outdoor CO₂ levels that reaction forms a thin, harmless white film. Inside a sealed hoarding with an unvented fuel-burning heater, the CO₂ concentration at floor level is high enough that the reaction drives down into the fresh paste instead of staying on it, and the result is a porous, chalky layer with almost no strength.
The Ready Mixed Concrete Association’s bulletin on dusting lists six causes of a weak wearing surface, and inadequate ventilation is one of them: carbon dioxide from open salamanders, gasoline engines, generators, power buggies or mixer engines “greatly reduces surface strength and hardness,” with unvented gas-fired heaters the standard example. Concrete Alberta’s own cold-weather tech tip says the same thing in one line — fossil-fuel heaters in enclosed spaces should be vented, and venting also prevents carbonation of new concrete surfaces, which causes dusting.
That distinction matters on a deficiency walk. A soft skin from insufficient curing and a carbonated surface look similar and both dust, but they have different causes, different prevention and different people on the hook. A trade analysis of carbonated slabs by Bryan Birdwell and Joe Nasvik notes that the two are confused often enough that a petrographer or floor consultant is usually needed to separate them — and in one case the suspected carbonation turned out to be excess unhydrated cement at the surface instead.
The CO₂ numbers your gas monitor will never show you
Ambient air sits around 400 ppm CO₂, about 0.04%. Laboratory carbonation chambers run 3% to 4% — 30,000 to 40,000 ppm — to force the reaction. Measurements taken near floor surfaces under unvented heaters come in at roughly 1%, about 10,000 ppm, which is twenty-five times ambient and well into the range where carbonation stops being cosmetic. Owners who write the risk into their specifications typically cap jobsite CO₂ at 4,500 ppm.
Here is the trap. The gas monitor on your site measures carbon monoxide, because that is what will hurt your crew, and the alarm on it tells you nothing about carbon dioxide. A hoarding can be entirely safe to work in and still be destroying the slab. If you are going to run fuel-burning heat over fresh concrete in an enclosure, CO₂ is a separate measurement and nobody takes it by default.
The damage depth is shallow and that is exactly why it is expensive. Field cases show surface damage of roughly 1/32 to 1/8 inch, with extreme cases to 1/4 inch — too deep for a burnish and often too shallow to argue the slab is structurally unfit. Under laboratory conditions carbonation in mortar has reached as deep as one inch.
The mixes most at risk are the ones you pour indoors anyway
Risk concentrates where the paste at the surface is plentiful and the air is still. The factors that raise exposure are a water-cement ratio around 0.5 or a little higher with no supplementary cementing materials, low airflow at floor level, a porous surface, and a high volume of paste at the top of the slab. In other words: a standard interior slab-on-grade or topping, finished hard, inside a sealed building in February.
Cold weather makes that worse before the heater is even lit. Bleed water is slow to rise and slow to leave in the cold, so crews start finishing early, and working bleed water into the top 1/4 inch (6 mm) of the slab raises the water-cement ratio right where you need strength most. The same bulletin that covers carbonation puts a hard ceiling on this: moderate slump not exceeding 5 inches (125 mm), never broadcast dry cement or sprinkle water onto plastic concrete to absorb bleed water, and never start finishing while water is on the surface. Pull bleed water off by dragging a hose across it.
Alberta’s placing and protection numbers still have to be met
Temporary heat exists to satisfy the temperature requirements, so it is worth being precise about what those requirements actually are. Concrete Alberta reproduces the CSA A23.1 permissible placing temperatures by section thickness:
| Section thickness | Minimum | Maximum |
|---|---|---|
| Less than 0.3 m | 10°C | 35°C |
| 0.3 m to 1 m | 10°C | 30°C |
| 1 m to 2 m | 5°C | 25°C |
| More than 2 m | 5°C | 20°C |
Concrete should not exceed those maximums by more than 10°C. For protection, there are two thresholds and they are not the same number: ACI recommends protecting concrete from freezing until it reaches 3.5 MPa, while CSA takes the position that twice that — 7 MPa — is what provides resistance to early frost damage. If your specification cites CSA, the ACI figure is not your target. Exterior flatwork facing freeze-thaw and de-icing salts needs to reach at least 32 MPa, and CSA A23.1 asks for a minimum seven-day cure at 10°C or higher for those exposures.
Two more numbers get missed on heated pours. When outside air is below -5°C, cool the concrete gradually rather than shutting the heat off at quitting time — 1°C per hour for sections over 1 m³, 3°C per hour for thinner sections. And thermal cracking becomes a risk once the difference between surface and interior temperature passes 20°C, which is easy to produce with a heater blowing directly across a slab.
Four decisions that keep a heated pour off the deficiency list
Name the heater type in the subcontract, not in conversation. Prohibit unvented construction heaters in writing and specify what is allowed: indirect-fired units with the exhaust ducted outside, or hydronic and electric heat. Direct-fired units put their combustion products — including the CO₂ — straight into the enclosure, which is precisely the mechanism described above.
Settle it at the pre-pour meeting. Heater selection is normally the general contractor’s or the mechanical trade’s decision, made for cost and BTU output, and the concrete crew usually finds out the morning of. A slab conference before the hoarding goes up is where this gets decided while it is still free to change.
Move air at floor level. Fans that keep air moving across the surface reduce the local CO₂ concentration where it does the damage. Venting exhaust outside and running fans both work against the minimum interior temperature you are trying to hold, so this is a balance to strike deliberately rather than discover.
Protect and cure as if the heat were not there. Heat is not curing. Apply a liquid membrane curing compound or cover with water, wet burlap or other curing material as soon as finishing allows, cure for the first three to seven days, and keep concrete temperature above 10°C with an accelerating admixture in cold weather. Our note on why concrete curing matters covers the mechanism; our winter pouring guide covers the rest of the cold-weather sequence.
What the repair costs, and who ends up paying
Repair options run from manageable to brutal. The weak layer can be removed by sandblasting, shot blasting or high-pressure washing, then hardened with a commercial chemical floor hardener — sodium silicate, or a zinc or magnesium fluosilicate — applied to thoroughly dried concrete per the manufacturer’s directions. If dusting persists, a latex, epoxy or cement-paint coating goes over it. Severe cases mean wet-grinding to sound substrate and bonding a topping course, or writing off the finish and installing a floor covering. Note that covering adhesives will not bond to a dusting floor without that preparation, and dust migrates up through carpet.
In the shallowest documented case, diamond polishing was accepted by the owner. In the worst, floors came out. Disputes over carbonated slabs generally land on fault, payment and exposed aggregate, and the concrete contractor is usually the one blamed — which is why the risk belongs in the contract. Either price the remedy in or exclude it explicitly, and tell the owner the exposure exists before the pour rather than after. The same logic we apply to warranty and guarantee language applies here: a defect nobody allocated is a defect you absorb.
Talk to us before the hoarding goes up
If you have a winter interior slab coming and temporary heat in the plan, the useful conversation happens before the enclosure is built and the heaters are rented — not when the flooring installer calls. Reach our commercial concrete contractors in Calgary to set up a pre-pour review of the heat, ventilation and protection plan, or see our concrete pads page for interior and exterior slab work specifically.
FAQ
Can a direct-fired heater be used over fresh concrete at all? Only where its combustion products are not going into the enclosed space above the slab. The hazard is the exhaust, not the heat. Unvented fuel-burning heaters should be prohibited over fresh concrete; indirect-fired units with the exhaust ducted outside, or electric and hydronic heat, avoid the mechanism entirely.
How do I tell carbonation from a poor cure? Both produce a weak, dusting surface, and visual inspection is not reliable. Carbonation damage is typically 1/32 to 1/8 inch deep. Separating the two usually takes a petrographic examination of a core, and it is worth doing before anyone agrees to pay for a repair.
Does a stronger mix prevent it? No. Carbonation attacks the paste at the surface from above, so a higher-strength mix does not stop it. Lower water content, no bleed water worked into the surface, moving air at floor level and vented heat are what prevent it.
Will my CO alarm warn me? No. Carbon monoxide monitors do not measure carbon dioxide. An enclosure can be safe for workers and still be carbonating the slab at roughly 1% CO₂ at floor level.
What is the strength threshold before fresh concrete can be allowed to freeze? ACI recommends 3.5 MPa; CSA holds that 7 MPa is what resists early frost damage. Check which standard your specification cites, because the protection period that follows differs.
How long does protection have to continue? Cure for the first three to seven days and keep the concrete above 10°C, longer for exterior flatwork exposed to freeze-thaw and de-icing salts, where CSA A23.1 asks for a minimum seven-day cure at 10°C or higher.
Sources
- Ready Mixed Concrete Association, Concrete In Practice CIP 1 — Dusting Concrete Surfaces
- Ready Mixed Concrete Association, Concrete In Practice CIP 27 — Cold Weather Concreting
- Concrete Alberta, Concrete Tech Tip #26 — Cold Weather Concreting
- Bryan Birdwell and Joe Nasvik, Carbonation in Freshly Placed Slabs, For Construction Pros