Freezer Floor

[Crikkky]

Hi everyone

I am in the process of designing a slab on ground for a new freezer. The construction is of the usual type, concrete slab on 4" rigid insulation, etc...

I have calculated the thickness of the slab, using the charts found in the Ringo and Anderson book for post loads. It gives me a thickness of 9 inches.

My question is about reinforcement. The calculation that I have done assume there is no reinforcement at all. But I want to prevent curling and cracking as much as possible.

Since our client doesn't want to use fibers, my first idea was to put reinforcement 2" from the top of the slab (#4 @ 12"" c/c both ways) but I am afraid of "alligator cracks". Also, I plan to put saw cuts at 15 feet apart maximum.

Am I in the right direction?


Chritian Laroche, P. Eng.

 

[Ron]

Jointing is the key to crack control, not reinforcement. The reinforcement will keep the cracks from widening, but they will still occur. Don't waste your money on it.

Your joint spacing looks good, but make sure that three things occur:

1. Sawcut the joints as soon as the saw blade can be run through the concrete without causing ravelling at the cut. In general, this will be 4 to 6 hours after placement. Don't wait any longer to do this. It is important that it be done quickly.

2. Make sure the sawcut extends at least 2-1/2 inches into the slab. Another very important point if you want to cause true crack control.

3. Make sure your mix design uses the largest size aggregate practicable for your placement method. Don't use a small aggregate pump mix as this will cause your cement and water demand to be higher (yes, you'll get higher compressive strength, but so what), which will cause more shrinkage, thus more cracking.

In addition, use wet curing for at least 14 days. Yes, I know it is a freezer slab, but it will be a long time likely before you do the freezer drawdown, so go with wet curing if you can.

 

[Crikkky]

Thanks for your answer, Ron


I forgot to mention that I wanted to prevent the reinforcement from crossing the joints. So my rebar would be 14'-8" long (the sawcuts are at 15' c/c.

What do you think of that?


Christian

 

[Ron]

I don't see the need for the rebar.

 

[jheidt2543]

Another importand consideration for a freezer floor is the subgrade. Even though you have insulation under the slab (which is really to reduce the heat transfer from ground to freezer), the sub-grade can freeze and heave the concrete slab. So, be sure that you have a good depth of course gravel or crushed stone base, 8"-12" to prevent capillary action.

 

[Crikkky]

Thanks for your input jheidt2543.

Under the insulation, a glucol temperature maintaining system will be installed. This system is designed by a specialised engineer so I think this part of the problem is well covered.

My main concern is really cracking and curling of the concrete slab.

Thanks again!

 

[structuresguy]

I have done many freezers, but they have all been walk-in restaurant/industrial kitchen sized. Just curious, how big is this freezer?

 

[Crikkky]

It is about 80' x 100' with a free height of 18 feet.

 

[Ron]

Crikkky...one thing I forgot to mention is that when you are ready for the temperature "pulldown", be sure that it is done with some hold and stabilization times. I have investigated failures of freezer floors caused by improper pulldown sequencing (they usually try to do it too fast, resulting in concrete problems).

Further, make sure the concrete has the lowest practicable water-cement ratio to get the concrete properly consolidated and finished. Most ready mix suppliers are going to throw you a mix design with a W/C ratio of 0.55 or higher. This is too high for the application. Make sure it is below 0.50 and preferably in the 0.45 range. Use admixtures to achieve workability if necessary.

 

[bjb]

for saw cutting crack control joints we specify the use of the "soff cut" saw, and have had very good results. With this saw you can get on the slab to cut very early- about 2 hours after finishing.

http://www.soffcut.com.

We specify a w/cm ratio of 0.45, and require 1.5" max coarse aggregate. I like to see a small percentage retained on the 1" sieve, which doesn't always happen so I recommend having the mix supplier submit a sieve analysis of the coarse aggregate.

Regarding rebar in the slab, the Corps of Engineers does have a design method for adding rebar to a slab-on-grade to increase the strength of the slab, it is more that just an attempt at temperature control. I do agree that there is no substitute for proper jointing of the slab. Whenever I have reinforcement in a slab-on-grade, wether it is structural or not, I always specify it to be discontinuous at the control joints.

 

[Crikkky]

Thanks Ron and bjb,

A few more questions:


what would be a typical temperature pulldown. How many days would you say are needed to do it correctly?

Also, what kind of influence does entrained air can have in this situation? Do you think it would be a good idea to use concrete without air?