Concrete curing - Cold weather 3

[bobby83]

Hello all,
A contractor on a pretty large project is arguing that we should not apply wet curing for suspended slabs during the winter (cold weather, below freezing point). I don’t agree with them, slabs are heated from underneath and insulating blankets cover the slab... do you have any thoughts on this?

 

[1503-44]

Should not be a problem, if you can stay within the proper temperature range. That can be problematic, hence his reluctance. Many contractors prefer not to work when artificial heating is required.

 

[kostast88]

The definition of freezing is very important. The hydration of concrete stops completely under a specific temperature, which I believe could be slightly below zero (1/2 degrees Celsius) = freezing temperatures. Concrete could loose a lot of strength.

On the other hand, concreting in cold weather means probably less water and low slump in the mix. Low slump could block the contractor's pump, so I could understand his concern.

In the end, I guess he doesn't have/want frost blankets.

EDIT: Just to be complete, Early cracking should not be an issue if shrinkage was assessed properly during design and reinforcement accounts for it.

 

[Enable]

As both a contractor and an engineer I can assure you that the risks are not worth it. If you apply water and the heat dissipates, for whatever reason - propane runs out before it's refilled because the supply company has a delay, heating blankets trip the breaker, someone does something stupid and takes a thermal blanket off the top, etc, etc - the top of your slab might just turn to paste.

Have you ever seen a fresh slab frozen due to winter conditions / heating going off? I have. It's scary especially because it's not always noticed until the next floor is underway.

Keep the temperature at a reasonable range 5-10 degrees (for the first couple days) and the slab will cure slowly enough that a lack of water at the surface will matter little to your overall strength profile / shrinkage crack issues.

If you want assurance tell the contractor it is their responsibility to field core to confirm in-situ strengths. I bet he'll take that all day long over wet curing. I would.

On the other hand, concreting in cold weather means probably less water and low slump in the mix. Low slump could block the contractor's pump, so I could understand his concern

I've never heard of ready-mix suppliers changing their w/c ratios for winter concreting though I suppose that might be done. At least on my projects the w/c ratio is set by the performance based mix design criteria and that does not change. Since this is a suspended slab I would imagine the w/c ratio is already at 0.40, which is pretty low as it is. What typically happens is they superheat the water at the plant so the initial temp is quite high. Slump is addressed via superplasticizers per usual.

We also tend to order accelerators in the mix to help with initial strength gain depending on ambient conditions.

 

[dik]

Yup... and froze before hydration started... when insulated and heated... came out as good as any concrete... also a project where the masonry walls were not heated and hoarded... came by the site and stuck my pocketknife into a mortar joint and it was soft... told the contractor that he should enclose and heat and was informed the wall was constructed yesterday...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[kostast88]

@Enable
You are right to point out that this is the case with ready-mixed concrete. My reference is for designed concrete where the designer would specify it (100/120 years design life structures).

That's some interesting input. Do you think that adding a superplasticizer (say Glenium 51) would have some beneficial effect on the whole curing process? I was thinking it may help the concrete and increase its early strength requiring less wet curing - I would imagine.

 

[Enable]

Yup... and froze before hydration started... when insulated and heated... came out as good as any concrete... also a project where the masonry walls were not heated and hoarded... came by the site and stuck my pocketknife into a mortar joint and it was soft... told the contractor that he should enclose and heat and was informed the wall was constructed yesterday...

How fortunate for the contractor. I've seen a slab frozen to the point where all it took to crumble was to hit it with the claw end of a hammer. It had to be demo'd / replaced while active construction above was underway. Seen similar to the top surface (top 12mm or so) when wet cured and heat got mistakenly shut off on a Friday night. Research seems to suggest that a 50% compressive strength reduction is quite common (along with durability characteristics for subsequent freeze thaw).

Winter concreting is a necessary part of life in Canada, as you no doubt know being in Jets city, but that doesn't mean we should add additional risk factors that don't have to be there!

That's some interesting input. Do you think that adding a superplasticizer (say Glenium 51) would have some beneficial effect on the whole curing process? I was thinking it may help the concrete and increase its early strength requiring less wet curing - I would imagine.

I'm not a materials expert so I won't pretend to know how super-p effects the curing process. But even just from a practical perspective you would expect less water at the surface (increasing strength) due solely to the fact that if the mix is flowable - say a typical 150mm+/- for flatwork - the finisher will need to add less water to close the surface.

 

[dik]

It's part of a multi-storey building (like 20 storeys) in Winnipeg... and is still standing. This happened about 45 years back. I've also seen failed concrete too... as well as frost patterns on the formed surface... Also seen 'gaps' at the top of columns where the ice melted.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[bobby83]

Interesting article Enable! In my project, which is also in the great white north, wet curing is done with Ultracure blankets that are wetted once before applying thermal blankets (no continuous water flow).
As for the hydration, the way I see it, if there is a problem with the heat in the first few days, we will have a problem with the concrete properties even without wet curing. No? Also, I have a problem with the fact that curing would be less important during the winter, especially because the air is dryer and because we are heating the material, which would increase evaporation.
For the bridges project in Quebec, wet curing is mandatory during the winter. For buildings, it is not that clear...

 

[dik]

Have to be concerned about proper ventilation to make sure no CO2 buildup... messes with concrete causing 'dusting'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enable]

Ultracure blankets that are wetted once before applying thermal blankets (no continuous water flow).

I've used these before. Expensive but awesome. The knock-off variety really stain the concrete though so watch out for contractors trying to switch without you knowing (only a problem with slabs with surfaces intended to be naturally exposed)....I may or may not have been a contractor who wanted to switch thinking nothing of it

As for the hydration, the way I see it, if there is a problem with the heat in the first few days, we will have a problem with the concrete properties even without wet curing. No?

Not quite. Wet curing the top surface introduces a saturation layer that is extremely susceptible to freeze thaw. This is not true of just the concrete generally, which may maintain its properties even in the event of a heating failure (dik's experience serves as a useful example).

This is exactly why ACI306 - Guide to Cold Weather Concreting instructs us to remove water from the surface 12-24 hours prior to removal of heat. So we don't get introduce the possibility of excess water blowing up our slab.

Also, I have a problem with the fact that curing would be less important during the winter, especially because the air is dryer and because we are heating the material, which would increase evaporation.
For the bridges project in Quebec, wet curing is mandatory during the winter. For buildings, it is not that clear.

Per the above ACI document, this is true if you heat outside the range I indicated. That is, if you attempt to maintain summer temps it will be exactly like concreting in the summer where water curing is necessary. However, maintaining ambient temperature just above the minimum to kick-off the hydration process is unlikely to have such an effect. Moreover, if you cover the topside with insulated blankets and maintain the forms, water will be unlikely to escape. Forms help retain the water and the insulated blankets form a surface for the minimal amount of evaporated water to condense on; the condensed water is then redeposited into the slab via contact or droplets.

EDIT - What you could also do is use a curing compound instead of water curing. If you talk with your local Euclid rep I am sure they'll have something that will work for such circumstances. Just means you have to abrade the slab prior to floor finishes. But I assume you would do that anyways via shot blasting.

Bridges make sense to water cure because of the budget. If I can afford to have a crew standing by 24/7 I would have no trouble water curing. It's when I am in a building and budget doesn't permit where I am unaccepting of the risk.