Cold Weather Concreting 1

[SteelPE]

I have heard that a contractor on one of my jobs would like to pour a 3-1/2” suspend slab early next week. Weather forecast put the temperature at max = 33 degrees and min = 17 degrees. I have already expressed some of my concerns about the planned pour but before I confront the contractor I need to make sure I am doing everything correctly.

ACI 306 contains the recommendations for pouring concrete during cold weather. However, they don’t really discuss the insulating requirements for a slab that is this thin. I can extrapolate from the temperature charts but I’m not sure this would be accurate. Also, if the contractor were to provide insulation over the slab (by using blankets), isn’t the same insulation required under the suspended slab? This is not specifically discussed in ACI 360… but I would think common sense would prevail.

The floor construction is a 3-1/2” slab on 9/16” form deck with steel joist spaced a 2’-6” o.c.

 

[StructuralEd]

If you venture into telling a contractor how to perform his task, then you are assuming the liability associated with any potential issues or failures that may result.
I'd hazard a guess that your insurance company might frown on that.

To play devils advocate, what if the weather gets more extreme, colder or hotter, windy etc. Then perhaps whatever your directive was may not be appropriate and you share the blame.

I once saw a job where the forecast called for cold temperatures so quite a bit of accelerator was added to the concrete for the slab.....the sun came out and the temperature went to 60 degrees during the finishing. The slab turned out looking like a jigsaw puzzle.

 

[jike]

A sudden drop in temperature can ruin unprotected slabs even at warmer temperatures! We once had a 25 degree temperature drop in a short period after the pour and got a jigsaw puzzle looking slab.

Cold weather pouring is done all the time with enclosure and heat below and insulated blankets on top. We have poured slabs like your description down to about 20 degrees ambient with great sucess. The key is to take precautions and be properly prepared.

 

[BAretired]

Placing concrete goes on all year round in northern Canada. It is expensive but cheaper than waiting until spring. In very cold temperatures, hoarding and heating is required so that the concrete is kept well above freezing until cured.

The heaters must be properly ventilated to remove carbon monoxide and carbon dioxide fumes which react negatively with workers and with freshly placed concrete.

It is the marginal conditions such as described in this post which are the most difficult to deal with. If the weatherman predicts only a few degrees of frost, the contractor is tempted to save money by skimping on cold weather protection.

BA

 

[concretemasonry]

I understand the concept of "degrees of frost", but the real engineering concern is the temperature of the concrete for curing purposes. Without surface moisture there is no "frost".

I have seen concrete placed at -42F, but there was planning, preparation and maintenance of heat for setting and initial curing before any freezing.

There must be proper, controlled conditions before the concrete is placed and then protection for curing to occur. A few days ago I saw a basement built properly (excavation, footings and walls) with the morning lows of about -10F to -14F because of planning, scheduling and protection over the 3 days it took. If you can build a basement, why is it so difficult to build a controlled commercial project?

It just takes scheduling, controls, preparation, quick construction and proper protection.

 

[hokie66]

StructuralEd,
What accelerator? Surely not calcium chloride! Are there accelerators which are not deleterious to durability?

 

[conceng]

hokie66,

In most of our market, if you ask for an accelerator, you get a non-chloride accelerator, which is not deleterious to steel. Most of the ready mix companies don't even stock calcium chloride anymore.

Greg

 

[hokie66]

Good. There were major problems with chlorides when I practiced in the US in the 70's. We have no need for accelerators in Australia.