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.

 

[Enable]

I do shoring and re-shoring designs in the Ontario market for these kinds of projects. In general, curing methods are less than ideal in all seasons due to the rapid pace of construction. Somewhat ironically then, I have found that conventional curing protocols in the winter actually produce the best results due to moderate slab temperatures that reduce the drying surface potential. If your site-cast cylinder results are satisfactory, I would have no concerns. BTW the site-cast cylinders should be cast and held in the EXACT conditions of the slab (e.g. not down below in a cooler box but above the formwork surface) if you are using those to assess validity of the on-site curing procedures.

Most commonly the slab is heated from below with tarps enclosing the entire perimeter. It is so hot that the workers usually wear t-shirts even if it is -20 or so outside. This heat causes quite the thermal gradient across the slab such that even in moderate sub-zero ambient temperatures, the slab will be entirely above 0 and most likely above 5 degrees. Thus, the induced gradient plus heat of hydration allows the slab to cure while not freezing. That is the key, you cannot let the slab freeze prior to achieving 7mPa. If the ambient temperatures are so cold that this is a concern, then you'll absolutely require heating blankets and an accelerator in the mix.



Another issue is that interior slabs are usually not air entrained, and so it becomes more critical to ensure that snow is not allowed to melt and subsequently freeze on the surface (especially repeatedly). However, in terms of the actual cure itself, if you are using certain types of concrete then all CSA A23.1 requires (type 1 or 2) is that temperatures are controlled, not that they are wet cured. This is what contractors rely on generally in the winter (that they control temperatures and that the slab has reduced drying potential so that evaporation does not occur at a harmful rate - thus providing a form of moisture loss prevention). Here's a link to the RMCAO best practice guide.