Concrete Under 20 Degrees

[ECGTX]

I have a project in the food industry where I have been requested to remove and install new concrete without shutting down the freezer that is 20 degrees Fahrenheit. I only need a minimum psi of 4000 with a fast cure time so operations may continue. I read through this forum and saw if you raise the PSI of the concrete the cure time is quicker. What products or what technique does anyone recommend?

 

[TLHS]

This is not an easy thing necessarily. You're going to be trying to do two conflicting things, because the concrete will need to be kept warm.

What are you building? Can you precast as much as possible? Is it in an area where you can separate it off with blankets, hoarding, or other similar things? What are the serviceability requirements? How bad would it be if you ended up with thermal cracking (i.e. how much risk can you tolerate)? Do you actually need 4000 psi? What happens if you don't get it?

 

[TLHS]

Also, if operability is important, you might have to really carefully think about how much heat the concrete (and any of your heating systems) are dumping back into the freezer and if the freezer can keep up with that additional heat.

It is probably easier to rent a refrigerated truck for a few days unless this is a very small pour.

 

[dik]

If long time freezing, you might insulate the slab and pump waste heat beneath it to keep it from freezing the soil in the long term.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[ECGTX]

I will be blocking off the area where the concrete will be poured with a wall that will be insulated and place heating blankets on the concrete so it will prevent it from freezing the new concrete. I'm just not sure what I can add to the concrete to help it cure faster since this freezer must be operational at least 5 days fully cured having forklifts driving over it when the work is done.. This pour is less than a yard. Temperature after blocking everything off should be around 50 degrees.

 

[le99]

You can explore both ways - increase compressive strength and add high early strength additive into the concrete mix. I doubt it will permit forklifts after five days, but with proper support below and rigid plank above, it may be doable. Also, a curing compound that can block the rapid heat loss can be important.

 

[TLHS]

Oh, great, 50F is workable.

Five days to service is definitely achievable. Honestly, a 28 day 40MPa mix would likely get you to 20MPa around then without much help. Might as well use at least some admixtures or a fast early strength mix though.

How are your concrete suppliers? Here, I would call up a supplier (or work through the contractor) and see what mixes they have that they would be comfortable standing behind. Something with some test history behind it in production.

Five days isn't even that fast. You can get crazy mixes now from some of the major suppliers that'll hit high early strength in four or six hours. They dump heat like crazy, need to be carefully cured and seem terrifying to work with because you have no time, but you can do it.

From a durability standpoint, if you could wet cure for that full five days it'd probably be beneficial. Are these inflated tire forklifts or hard wheel?

The big thing on jobs where you're going into service or advancing work before 28 day strength is to pick the strength you feel comfortable with ahead of time. Set a limiting strength and field cure some cylinders in the same conditions as your pour. Then test them before you release the slab for operation. Stay firm on the number you decided ahead of time. It doesn't necessarily need to be the 30MPa full strength you're mentioning. If it seems to the contractor or owner like you're just going by feel, you're going to feel pressured on that hold point. If it's a hard number, then it is what it is.

 

[epoxybot]

You could talk to Sika or MB about Magnesium Phosphate Concrete (site mixed). You will get your compressive strength in a day.

 

[dik]

Thanks epoxy... I hadn't heard of the use of this before... Maybe Ron has some comments.

From another site...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[BridgeSmith]

On a bridge project where I was onsite, they achieved 3250 psi in 3 days, using the standard 4500 psi mix from the local ready-mix supplier.

You might check the design to see what kind of concrete strength you actually need to meet the loading parameters. The flexural strength of reinforced concrete is typically fairly insensitive to changes in concrete strength (i.e. you don't lose hardly any flexural strength with lower strength concrete).

Rod Smith, P.E., The artist formerly known as HotRod10

 

[mfrad]

Thaw the ground- Use heaters or heated blankets to thaw surfaces where concrete will be poured. Pouring concrete on frozen ground will quickly cool concrete well below ideal temperatures. Using a heater to prep surfaces will prevent too-quick cooling or freezing and help keep the necessary reactions going.
Use hot water in your cement mix- Typically, you should aim to have your concrete mix at 65°F or higher during winter months. Hot water can help you reach this temperature.
Use Extra Cement- Adding extra cement to your mix ( (typically an extra 100 pounds per cubic yard) will cause your concrete to hydrate more rapidly.
Use Type III Cement- Type II cement mixes produce a high early strength and hydrate more rapidly.
Remove Bleed Water- Utilize squeegees or a vacuum to remove bleed water that has a difficult time evaporating during colder weather.
Use concrete blankets to keep curing cement warm- insulated and heated concrete blanket will keep cement at ideal temperatures as it cures. This will prevent any delays due to extended cure time and strength issues with cured concrete.
This is copied from powerblanket.com

 

[dik]

I was surprised with MPC concrete. They are talking strength in hours and not in days. I'd not heard of it before.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[epoxybot]

Oh Dik, I'm sure you've heard of MBs Set45.

 

[TLHS]

I've used the Lafarge Chronolia fast mix. I think it was a ten hour mix. It's pretty crazy. Instead of breaking cylinders every day after 4 or 5 days to hit some mark to pull forms, you're breaking cylinders every hour after six hours.

Curing is more sensitive though.

You can get a four hour mix, but that feels like asking for trouble if you have any sort of problems with placement.

 

[dik]

epoxy... I've used the material... just never thought of it as MPC... thanks...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik