Cold weather curing challenges for high-rise concrete slabs

[Sibroe]

Hi everyone,

I'm currently working on a high-rise concrete residential tower in Canada, and I'd like to hear your insights on flat slab curing in cold weather.

The structure is being built through the winter at higher floors (20+), and we've noticed that the slabs aren’t receiving any curing. Flying forms are being stripped 24–48 hours after the pour, and heating is applied from the floor below. That means there’s no curing on either face of the slab, and no insulating blankets on top, since the workers are already on that floor the next day.

The contractor claims that all high-rise projects in the area follow the same approach, as there’s no curing compound that works effectively in cold temperatures, meaning proper curing would require pausing work for a few days at each level.

We’re concerned about the potential impact of this on durability and deflections (2-way slabs, 215mm thick, 6m spans). While I know this lack of curing isn’t compliant with the code, nor ideal, is it really a significant issue? Is skipping curing really common and accepted practice in cold weather high-rise construction?

Side note : This is my first post here, but I’ve read hundreds of discussions on this forum over the years and have learned so much. Huge thanks to everyone who shares their knowledge! And a special shoutout to KootK! You’re a legend in my office!

 

[ANE91]

What are your cylinder breaks like?

Historically, the biggest issue I’m aware of is shoring/reshoring that was too expedient for the strength gain of the slabs. Towers have collapsed.

Is ACI 306.1 incorporated into your construction documents? If so, then they have to cure “properly.”

You could have finish issues if, say, you’ve spec’d for polish.

 

[Sibroe]

What are your cylinder breaks like?

Historically, the biggest issue I’m aware of is shoring/reshoring that was too expedient for the strength gain of the slabs. Towers have collapsed.

Is ACI 306.1 incorporated into your construction documents? If so, then they have to cure “properly.”

You could have finish issues if, say, you’ve spec’d for polish.

They typically use Lok-Tests for formwork removal and cylinder breaks later. Concrete strength has been over 75% of f’c at removal, around 95% at 7 days, and exceeding 100% at 28 days, so strength gain is not an issue.

As for shoring and reshoring, the general contractor is checking this and reports that the approved procedure is being followed (removing a single fly and reshoring immediately before stripping adjacent fly).

Surface finish is fine in this case.

Regarding ACI 306.1, we don’t reference it since this is a Canadian project, but I took a look at the guide. It doesn’t align with typical high-rise construction practices.

 

[ANE91]

Regarding ACI 306.1, we don’t reference it since this is a Canadian project, but I took a look at the guide. It doesn’t align with typical high-rise construction practices.

306 is the guide; 306.1 is the spec. Now you got me curious — which parts don't align? Hot mix water is common in the northeast US.

 

[Sibroe]

306 is the guide; 306.1 is the spec. Now you got me curious — which parts don't align? Hot mix water is common in the northeast US.

Both ACI 306 and 306.1 don’t address methods for ensuring proper curing while allowing work to continue on top of the slab during this period. I'm all for good concrete curing, but I also want to go beyond theory and understand the actual industry practices that balance durability with construction efficiency and tight scheduling.