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.

 

[cntw1953]

To lessen your concern, can the space below be shield, and the internal temperature be controlled by heating devices?

 

[SteelPE]

The concerns I have expressed have been around tenting the structure. The preliminary feedback was not positive.

 

[cntw1953]

I think the tent needn't be fancy, plastic sheets/tarps might suffice.

 

[conceng]

ACI doesn't provide guidance on means and methods, such as how much insulation it will take. They do give performance requirements to meet. In your case the delivered concrete temperature from Table 3.1, line 3 would be a minimum of 65 degrees. During the protection period, the concrete temperature must me maintained at a minimum of 55 degrees for the for the duration period determined in Table 5.3. The required protection period will depend on the load and exposure conditions. How the contractor meets the 55 degree minimum protection period temperature is up to him.

Greg

 

[SteelPE]

Conceng,

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.

 

[conceng]

I guess I misspoke. What I meant is that they are not going to tell you that you must use blankets on the top and a heated enclosure on the bottom when a contractor may hang blankets from the bottom and use ground heaters and blankets above.

Greg

 

[TDAA]

SteelPE, let them decide. It is their liability.

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.

 

[concretemasonry]

your weather conditionsa are not really that difficult.

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

 

[SEwannaB]

concretemasonry et.al. are right. Not bad conditions, but the contractor HAS to be prepared, ie he needs a plan meeting his contract which likely has all the code/engineering requirements listed, right? Therefore it's up to him to provide and maintain.

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.

 

[cntw1953]

Personally I would be very uncomfortable if there are potential problems, but stop short to point out to the owner, and the contractor.

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.

 

[SteelPE]

I agree with most of the points here. Most of the recommendation I have suggested before on other projects. However, I disagree with the fact that I should let the contractor completely decide the protection method. What if the contractor decides that he doesn’t need any protection at all (I doubt they are going to do this)? Should I just remind him that the concrete temp must remain at 50 degrees for 3 days and be done with it?

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?

 

[cntw1953]

Tell him the temperture requirement suggested by ACI, and the potential that he would suffer from not following the recommendation.

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.

 

[doka1]

There are a lot of variables that come in to play in cold weather concreting, but what you are dealing with is a whole other animal in a sense in my opinion. Cold weather concreting is a lot more intense than cold weather concrete for a footing or a wall. Too often people lose sight of what they are actually doing when they read aci 306, which is very general and strictly performance based. Furthermore, you then have people telling you they have poured concrete when it was 15 degrees out, but fail to inform you that it was on a footing which was 3 feet thick and they covered 10 minutes after placement. As a concrete contractor, this time of year our schedules are always dependent on the weather, and owners/gc like to force us to pour, citing forecasts in the mid thirties to suffice. To often people think a high of 35 is good enough, but they dont realize that it doesnt rise to that temperature until 1 in the afternoon, when you have already placed all 350 cy on the deck.

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.

 

[dik]

You haven't enjoyed cold weather concreting until you can see the 'jackfrost' patterns on the slab soffit... or, the 1/2" gap between the slab and the supporting column caused by ice on top of the formed column...

Dik

 

[cntw1953]

If there a temperature gauge (can be attached to the bottom side of deck) for this situation?

 

[BAretired]

For those temperatures, provide insulation blankets above the slab and polythene cover below. The concrete will not freeze.

BA

 

[cntw1953]

it looks like, from steelPE's most recent post, the contractor is unwilling to shield the bottom. If that is the case, what he can do to ensure the quality of the slab after it is cured, provides there might not have visible defects on the top surface which will be blanket?

 

[BAretired]

With the underside of slab exposed to fifteen degrees of frost, I would think it is marginal. It might not freeze, but the risk is too great in my opinion. Furthermore, there is no guarantee that the temperature won't fall lower than predicted.

BA

 

[cntw1953]

Yes. His delimma is the concrete slab is placed on metal deck, shield from view. Also, thickness of the slab is only 3 1/2", without better protection, rapid temperature drop/variation could have considerable impact on it.