Cold Weather Concreting
Cold Weather Concreting
(OP)
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.
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.






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Greg
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What about the insulation recommendations in chapter 7 of ACI 306? Doesn't this table tell you the R value required for a given ambient temperature?
I don't really feel comfortable in letting the contractor dictate how he is going to provide protection. Especially if the only realistic way to provide protection by his method is to provide the blankets on the top of the suspend slab only.
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Greg
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Be sure they know they will need to remove it if they fail to maintain the requirements of the specs. I am assuming your spec references the ACI requirements. If it says they shall, then they shall or it is removed at their expense.
RE: Cold Weather Concreting
Tent above and below in advance and heat the areas for as long as possible.
Your two concerns should be the air temperature and the steel temperature. Because of the conductance of the steel it is a "thermal short circuit" that will ruin concrete when placed in contact with it if very cold and will draw heat from it and lower the concrete quality.
Use heated concrete (usually readily available in colder areas) and even high early cement (Type III), even though the contractor will have to be better at placement and finishing.
Cold weather construction is that not difficult if the contractors and suppliers (warm concrete, etc.) and are organized. I have seen basements dug and installed around me in the last few days (although the highs were up as high as 25F) and a typical basement can be excavated and built in about 4 days with proper weather protection. Been done for years since the weather cannot be changed, but the site conditions can be controlled.
Dick
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And be sure the tent can take wind... that will be the biggest killer at cooler temps if the applied heat just get washed away and the thermo from the wind reduces your enclosure temps. If that's too cool, your concrete just won't react sufficiently and on that thin a slab, blankets may not be enough at those temps. As a heavy civil contractor, I've removed wall and footing concrete under just those conditions.
Insulating blankets on top and enclosed heat below will work. I think they do this all the time in multi-story buildings. And don't forget slab edges.
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I don't demand the contractor to do things certain ways, but he shall submit his plan, and be advised/remained the consequences.
Depending on type of contract and schedule, the owner and the contractor would sometimes both want to save some money, or time, by skip some protective measures, which are usually not clearily spelled out in the contract documents, and the codes. However, as an Engineer/Inspector, we shall at least clearily point out the catches, like citing the articles in the spec, and/or provisions in the code - maintaining temp for this case, good practices - heating aggregates, covering/shielding, and the potential negative results - strength reduction, wide/thru/hidden cracks, or require demo and reconstruction if severely damaged for lack of not-so-expensive protection. Then, let them do the math and eat the result while I can have a good sleep.
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Unfortunately this project hasn't exactly gone as planned. The contractors "fairy tale" schedule fell apart during the permitting phase. This concrete pour was supposed to take place 14 weeks ago. I don't believe he has the money in his budget for this type of work and I just want to make sure everything is done correctly.
I will find out more on Monday when I talk to the contractor.
What is the determining factor for rejecting the slab, low concrete breaks and excessive cracking?
RE: Cold Weather Concreting
I am not familiar with concrete testing, could the testing on cylinder, or speciman, cured in the field be used as an indication of quality and strength of the concrete placed under identical conditions? I think someone here can answer this question, and/or provide better QA/QC guidelines on this situation.
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What I find is you have examine what you are doing. If its a slab on deck, can you heat from below, checked that winds will be below 10mph or so? 2% n/c accelerator is a must in flatwork during the winter. Do you feel you will be able to maintain the concrete temperature simply from heating below. How will spray cure be applied if at finishing times the temperatures drop below freezing. Do you realize what a mess it becomes to put blankets on if the cure does not dry properly (blankets stick to concrete).
Its a hard sell to owners, but if the projects schedule demands it I would look into zero set,or accelgard90 from euclid. Very expensive, but it can save a lot of headaches and heartaches down the road.
SteelPE, I commend you for actually caring. Nowadays too often many engineers have the mindset of TDAA which do not realize that if either of us (contractor OR ENGINEER) makes a mistake in the construction process we could face serious legal issues. I trust SteelPE realizes that if a pour doesnt work out it wont reflect well, and could cause problems for ALL parties involved in the project.
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Dik
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BA
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BA
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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.
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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.
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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
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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.
RE: Cold Weather Concreting
What accelerator? Surely not calcium chloride! Are there accelerators which are not deleterious to durability?
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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
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