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Fast set concrete - HELP!

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couch67

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I am filling in a passenger door in my garage, which first required me to build up a concrete curb before building the wall in. The curb is 8" deep, 10" tall and about 36" wide. I used Sakrete Fast Set concrete, which says it 'can be walked on' in about 3 hours. Mixed per directions, two full bags, and used exactly the amount of water specified. Hand mixed in a wheelbarrow and I'm confident it was mixed well. Temperature was about 10 C, well above the -5 rating. I left it alone most of the day but noticed it was still soft before I went to bed.

This morning I take off the forms and it is still quite soft! I easily plunged an awl down the middle of it. :shocking:

Is this normal? Should this product not have set up firmer than this after almost 24 hours? Not sure if I should rip it out (which should be easy) or should I just give it more time? I've done concrete work in the past but first time wiht the fast set.

couch
 
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matt_i

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I just patched a hole in my driveway with Quikcrete 5ksi, just 1 bag. I even mixed it with hot water. I put one area under a piece of cardboard and 2 layers of movers blankets since it was supposed to get to 30F here overnight.

That concrete that was protected and sorta insulated was harder than the area which sat unprotected, but was still not fully hardened. After 1 more day they all seem appropriately hardened.

The concrete cures slower when cold, especially with small bag amounts which are easily affected by the ambient vs the mass of a 10 cu yard load.
 
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Strouty

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When setting up, concrete creates a chemical reaction that heats things up, being cold and your really small quantity of material made that heat dissipate quickly, we would use foam or insulating blankets. Too late now, you are just going to have to wait, could be days.
 
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brownbagg

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yes, cold retards cement. at around freezing, it wont do anything, concrete needs heat to hydrate
 
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joes169

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brownbagg said:
yes, cold retards cement. at around freezing, it wont do anything, concrete needs heat to hydrate
Click to expand...

Concrete doesn't set off of air temperature, it set's off of material temperature, which is why it is often poured below freezing, successfully.

I've never used the Sakrete version, but we do use the Quikrete version in the smaller, red bag often. It actually set's too fast most times, and we mix some 5k Quikrete in with it to get a little more time.

As long as it didn't freeze, I would probably leave it alone and cover it with a blanket or something for a few days.
 
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couch67

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Just catching up, thanks for the replies. I had failed to mention in first post that I had put a small heater on it overnight but only many hours after I had placed the concrete (too late!). I suspect because of the colder temp, and I used cold water (and the bags were cold as they were in the garage overnight), combined with the small volume, the 'fast' reaction just not going to happen.

I was in a pickle as I'm 4 days away from being on vacation for 2 weeks, and don't like the idea of having a hole where the door used to be! So I decided I couldn't wait for the concrete to set and ripped it out. It wasn't much stiffer than when I placed it!

Went to a different store, bought some Quikcrete fast set, and used warm water (in the veeery remote chance I had a bad batch!) After 20 minutes it was harder than the first batch that had 24 hours set time! I covered it with plastic sheet and a blanket for about 3 hours. After a few hours I checked the temperature with an infrared, it was 24 C (and 8 C outside)!.

It cost me a few bags of cement and extra labour, but looks like it worked out. I learned a good lesson this weekend. Thanks again guys.

Dave
 
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66cj225

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Came from a batch that was suppose to go to the islands for hurricane repair? Likely would have worked good if it went on your vacation with you.
 
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couch67

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lol absolutely. Had I not procrastinated this job and finished it 2 months ago, would have not been an issue! Looks like 2nd attempt with the warm-hot water and room-temp bag mix made all the difference (thank you again for the feedback).

Concrete is now firmly set, I've removed the forms and have framed / sheathed in the door opening. Kept the plastic on both sides and leaned the forms back on the concrete, although there's no chance of freezing here til the weekend potentially.

If I can get the siding back on before I leave that is a bonus, but not critical now. Outside temporarily weathered in with plastic. !:beer:
 
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couch67

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Well, I suppose there's a remote chance that the product was defective but I'd put my money on operator error. I had read the instructions and they have general instructions for what would for most climates, then notes on 'cold' weather to use warm water, but don't specify what temperature is 'cold'.

Air temperature (and the bags) was 10C, well above freezing (they say bags need to be > 5C). I'm willing to bet if I had used warm water, and the bags were at room temp to start, the first batch would have fast set.

So if I were to say anything constructive about the mixing instructions, they should likely be more prescriptive. I think the advice the guys here gave me were bang on.
 
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SGKent

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I've poured an A/C pad of regular concrete in December here in NorCal at 33F with a low of 24F, and still had it properly set overnight so my guess also is you got a bad batch. For that pad I used some cardboard and a blanket on it once the excess moisture was gone. That was 15 years ago, and it is doing fine. Your wooden forms would have helped insulate your pour too.
 
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Daniel Dudley

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SGKent said:
I've poured an A/C pad of regular concrete in December here in NorCal at 33F with a low of 24F, and still had it properly set overnight so my guess also is you got a bad batch. For that pad I used some cardboard and a blanket on it once the excess moisture was gone. That was 15 years ago, and it is doing fine. Your wooden forms would have helped insulate your pour too.
Click to expand...

Bigger pours tend to fire off better because of the extra mass.
 
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alexb2000

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When mixing small batches at home in cold weather I do a couple of things:

Bring the sacks inside the house (or heated shop) a day before the pour.

I make sure the formed area is covered until just before the pour to let the ground heat do as much as it can to the dirt that will be touching the fresh concrete. If it is rocky or existing concrete or another cold surface will be touching the new pour, I heat everything up with a propane roofing torch just before the pour and it seems to help.

Use hot water

Try to find a mix with Type III cement as it works the best in cold weather.

Then like others I just cover the pour with something insulating. I have used a residential electric blanket with good success as a substitute for a heated concrete blanket.

Doing all the above I have poured down to about 20 without any problems.
 
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jubilee

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Add a little calcium chloride to the mix. I try to buy 95%+ pure cc ice melt, dissolve in water, then mix in cement. Work into small experimental batch to see how hot you want it.
 
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