Freezer floor construction and temperature drawdown

[jhobbs]

Hi Everyone,

I have designed a freezer floor. My question is around the drawdown i.e. the dropping of the temperature of the concrete in the freezer room.

We are specifiying a typical drawdown procedure.

After 21 days of casting

1. Reduce temperature from ambient (approximately 22 degrees Celsius) to 2 degrees Celsius 5 degrees per day - 4 days.
2. Hold temperature at 2 degrees Celsius for 3 days.
3. 2 degrees Celsius to minus 25 degrees Celsius (5 degrees per day) 6 days

Due to timing requirements the client wants to start the draw down process earlier than 21 days. My question is that will the concrete still attain strength after it reaches 2 degrees celsius. Assuming no special heating measures to the concrete below what temperature will the concrete stop gaining strength (hydrating).

Comments appreciated.

 

[dik]

I didn't know there was a drawdown. I would have thought that after normal curing for 7 days that it would be OK as long as it wasn't loaded. In winter after 7 days, heat is often removed and it may be -30 or colder.

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

-Dik

 

[HTURKAK]

I have cold region experience.. We kept the temperature above say 10 degr C till the concrete gains 5 Mpa strength ( one or two days ) then stopped heating and left the concrete to ambient temperature say -40 degr C.

Is the floor floating screed with tiles on heavy insulation ?

You may post the details of freezer floor to get better comments.

 

[dhengr]

HTURKAK:
Gosh…, you may quit heating after a few days or at a certain strength level, but I don’t think you will find much industry literature or recommendations that say you can then immediately open the conc. up to -40°C temps and weather. You certainly shouldn’t be pouring on frozen soil, and I would think, you should continue to protect it, keeping it covered for some longer period of strength build-up and drying time, before leaving it complete exposed.

 

[HTURKAK]

Dear DHENGR,


I fully agree with you that , the frozen soil , rock or previous concrete surface which will receive fresh concrete , shall be heated above freezing temperature before concreting starts..

My respond should not be generalized for all concreting cases. If the concrete strength reaches 5 MPa , that literally means the final setting of concrete completed and concrete gained reasonable strength. If the size , thickness of the concrete elements is not abnormal, you may stop the heating .

For example , if the subject is 4.0 m thick raft foundation or concrete gravity dam, the whole concrete block ( not only freshly poured portion) shall be heated till the concrete inner temperature does not cause thermal cracks.

 

[dik]

Use thermal tarps/insulation to maintain the temperature...

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

-Dik

 

[Enable]

I have cold region experience.. We kept the temperature above say 10 degr C till the concrete gains 5 Mpa strength ( one or two days ) then stopped heating and left the concrete to ambient temperature say -40 degr C.

Is the floor floating screed with tiles on heavy insulation ?

You may post the details of freezer floor to get better comments.

In Canada, 7 mPa (close enough to your 5 mPa) is the minimum requirement for undergoing the first frost cycle (see here) but that does not mean such strength indicates that the concrete can sufficiently handle winter conditions for which it will be subject. CSA A23.1 even mentions this explicitly and says that concrete shall be heated a minimum of 7 days at 10 degrees or more if subject to typical winter conditions (see same source above).

I've never heard of a drawdown procedure so I have zero comment on what is typical. However, what dik said would be my bet. Heat the slab with electric curing blankets for 7 days (or whenever 100% strength +/-is obtained) and remove the next day. That'll very likely be less than the 21 day specified period. I wouldn't turn off the heat at 75% because this is a freezer and presumably the concrete will never get above 0 degrees again and thus will not gain strength above the initial cure. Alternatively, specify a higher compressive strength than needed such that 0.75*f_design = f_needed, and you will surely be able to get away with a 7 day cure at most!

 

[dik]

Generally leave it on for a week, fairly typical, but we have a 'dry cold'.

When they were constructing the Lindsay Police Station, I dropped out to the site and there was a masonry wall that was fully exposed. It was well below 0C. I used my pocketknife and stuck it into one of the mortar joints, and the mortar was soft (plastic). I told the contractor that he had to cover the wall and apply heat immediately. To my surprise, he informed me that the wall was constructed yesterday. Not making this up...

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

-Dik

 

[Ron]

The key is to not freeze free water until it becomes part of the hydration process. In general, this happens fairly early, usually within the first 14 days when a lot of the free water is consumed in the hydration reaction. After that it is just a matter of not dehydrating the concrete.

Temperature has a significant affect on the strength gain of concrete. Anything below about 20C slows the strength gain quite a bit. You might take some test specimens and put them in the same drawdown environment and test them every few days to make sure you're not greatly affecting the strength as the drawdown occurs.

 

[dik]

Yup... once it starts to hydrate and you break down the gel... it unpredictable and likely toast... thanks. I think it likely froze overnight and thawed in the morning. When I arrived in the afternoon it had likely just thawed. Due to the uncertainty and the poor construction I had them remove it.

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

-Dik

 

[HTURKAK]

This is copy and paste from the reference doc. in your post;

..According to CSA A23.1-14 Note 2 to Clause 7.1.2.4 “When concrete reaches a compressive strength of 7 MPa, it normally has sufficient strength to resist early frost damage.”” However, exterior concrete flatwork exposed to freeze-thaw cycling and de-icer salts must attain a strength of at least 32 MPa in order to ensure its long term durability. CSA A23.1 mandates a curing regime of 7 days at a minimum temperature of 10oC for concrete subject to these exposure conditions. Therefore in cold weather some provision must be in place to ensure that this objective is met.

Apparently , a curing regime of seven days at a minimum temperature of 10oC is required for exterior concrete flatwork exposed to freeze-thaw cycling..
And another ref. from
ACI 332 -10 Residential Code Requirements for Structural Concrete

I just want to remind , every body is free to keep a curing regime of 7 days at a minimum temperature of 10oC for cold weather concreting. But, this will not be enough in case of mass concrete ( gravity dam const. etc)
and remind also, the concrete dries and gain strength at freezing temperatures.

Good Luck..