Is it OK to lay a new slab foundation now?


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Old 12-15-05, 05:11 PM
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Is it OK to lay a new slab foundation now?

We're looking into having a 2 car garage built. We've got the money and would love to rock and roll, but it's basically the middle of winter. We live in Iowa, and average highs now are in the mid-20s. The contractor we've been talking with says there are a number of ways to overcome the cold with laying new cement like heat blankets and heaters and what not. Is he full of hot air, or is it OK to lay a new slab foundation? Is there much/any risk to laying a new foundation now, or am I just be over cautious? Thanks!
 
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Old 12-15-05, 06:59 PM
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Is it OK to lay a new slab foundation now?

You southerners in Iowa worry too much about the cold.

You can put in a slab now if you just take the PROPER precautions.

1. Make sure the base is not frozen. This may take thawing, but the latent heat of the soil will help some since we are not too late in the season.

2. Use high early cement (touchy to finish properly). Here the minimum recommended for a driveway is 4000 psi and 5% air. Mosy contractor like to use 500 psi and 5% air. DO NOT USE FLY ASH FOR A WINTER SLAB POUR!!! A garage is almost as critical for freeze-thaw durability because of the concentrated salt droppings, so many contractors use the same for a garage slab since the cost is not that much more and provides good insurance.

3. Cover with insulated blankets (can rent) to keep the concrete above freezing for a few days. The heat of hydration of the Type III cement is greater and will decrease the length of protection required.

Despite the extra cost for precautions, some builders feel winter construction is not that bad because good contractors like to keep the good employees busy.

Dick
 
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Old 12-16-05, 03:32 AM
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In addition to Dick's method, you may want to ask your contractor about freeze-proof concrete. Around here they call it FreezeGuard. It is not actually freeze PROOF, but they do guarantee it not to freeze down to about 15 degrees or so. In many instances you don't even need to cover it. Ask the local ready mix concrete supplier for full details. If your contractor has not used it, he should be warned that it sets up FAST! He should be sure to have enough guys on hand to handle it.

Pecos
 
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Old 12-16-05, 11:17 AM
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Is it OK to lay a new slab foundation now?

There "freezeproof" concrete is probably high early (Type II) cement and an acceperator. It can have long term problems if BOTH the contractor and supplier are not experienced with it.

FAST setting is not the word for it. If the aggregate is warmer than expected - you can turn around, and it is set when you turn back.

Dick
 
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Old 12-17-05, 02:50 PM
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Well Dick, what you suppose may or may not be so. That's why I told them to ask their ready mix supplier and contractor for details. The internet searches I've done don't mention type II cement though. The only one that gave any detail at all mentioned type I and their proprietary additive.
In addition to Freeze Guard, some other names for these type products are Polar Set and Pozzitech. I've used Pozzitech and it does indeed set up quickly. That's why I mentioned it in my first post.
Anyway, if your contractor is competent and has placed a lot of cold weather concrete, I would trust his judgement as to whether you can pour now or not. Internet forums are good in general, but things are done differently in different parts of the country. No one person's advice should ever be taken as gospel.

Pecos
 
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Old 12-17-05, 06:02 PM
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Is it OK to lay a new slab foundation now?

sx460 -

I made a mistake - I typed in Type II when I meant Type III cement. Type III (High early strength) is manufactured to give higher early strengths of concrete with ultimate strengths about the same as normal Type I cement. Alternately, special propietary admixtures are used to create a quicker set and hopefully higher strengths wher permitted. Combining two different materials for the same purpose must be done carefully.

Pecos was right saying that the quick setting mixes can set up very fast and sometimes to fast if you are not prepared with enough manpower to finish a slab. Foundations and walls are usually not a problem.

There are a number of totally different additives used to acheive quick setting. The earliest was calcium chloride, but it is not used commonly because of the corrosive effects on reinforcing steel. Sodium chloride (normal salt, rock salt or Halite) is very rarely used. The "Freeze Gard" mentioned by Pecos is liquid magnesium chloride and is also used as a deicer similar to calcium chloride and potassium chloride. A different type, "Polar Set" is a proprietary material advertised as a non-chloride additive. The "Pozzitech" is a pozzolanic by product promoted by carbon-based combustion group.

The goal of all of these is get get a fast initial set so the concrete can be finished quickly and protected. Damage to the concrete can be prevented if the moisture in the concrete is tied up with the cement reaction to the point where the concrete is not saturated when it ultimately freezes (1 or 2 days depending on the cover and temperatures). At low temperatures, the cement does not react and the gain of strength is stopped until the temperature rises to above 40 or so. Then it begins to cure again.

That is the technical part -

From a practical standpoint, you must have a good contractor and a good concrete supplier when you do cold weather concrete (isn't that what you want with any concrete?). The experience, equipment and manpower is just more critical when it comes down to winter concrete. Make sure your contractor is experienced and has a track record with cold weather concrete and you should have no problem. In our area, cold weather concrete is just a fact of life and the protection costs are justified by permitting construction to go ahead.

Good luck.

Dick

p.s. - My Iowa concrete friends and I constantly discuss "what is the best thing to come out of Minnesta/Iowa?" We all agree on Interstate 35, but when can't agree on the direction
 
 

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