Author Topic: Concrete floor ..how thick???  (Read 17566 times)

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Offline 2zwudz

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Concrete floor ..how thick???
« on: July 07, 2009, 02:16:41 AM »
  How thick does a concrete floor need to be that is being poured over a conventional floor (floor joist and plywood)? I know the floor needs be strong enough to hold the concrete but I was wondering about the concrete cracking.  Has anyone ever had experience with this?

Mark

Offline Redoverfarm

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Re: Concrete floor ..how thick???
« Reply #1 on: July 07, 2009, 04:13:45 AM »
I am not sure this is what you are looking for but a "rule of thumb" is that at the least it should be 2" on concrete with some reinforcement.  You might look at your joist and determine the load that the concrete will create.  Some polymer fortified overlays are much thinner but this is usually over a previous concrete substraight. 

Offline glenn kangiser

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Re: Concrete floor ..how thick???
« Reply #2 on: July 07, 2009, 05:51:53 AM »
The apts I'm working are using lightweight concrete with fibermesh, 1 1/2 inches thick, however, they still crack even with reinforcement in some places.  They are going to be the finished floor and that is acceptable to them apparently.

These are cut into no larger than 10'x10' sections by scoring with a diamond blade - but still occasionally get the random cracks through the sections.
"Always work from the general to the specific." J. Raabe

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Offline 2zwudz

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Re: Concrete floor ..how thick???
« Reply #3 on: July 07, 2009, 01:54:47 PM »
  Thanks for the replies.  I guess I should have told more of my story.  I am starting my hunting cabin and I was thinking of putting in a hydraunic heated floor over the sub floor before I begin framing. I figure I could get this far before the hunting season starts. I am in the process of laying my block for the foundation and I want to build the floor strong enough to support the heated concrete floor but was wanting to get an idea of how thick the concrete should be so I can ask YOU GUYS  :) what is the best way to support the weight?.  The span will be 16' wide X 32" long and I am guessing I will need to support it at 8' centers and use 12"X 16' TGI's??  What suggestions or ideas do you have for this application?

Thanks again
Mark

Offline rwanders

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Re: Concrete floor ..how thick???
« Reply #4 on: July 07, 2009, 03:01:18 PM »
The TGI distributors can provide engineering support/info for your application---I have usually seen TGIs placed at 16 OC and 1 1/8 plywood subflooring under 1 1/2 inches of lightweight concrete.
Rwanders lived in Southcentral Alaska since 1967
Now lives in St Augustine, Florida

Offline glenn kangiser

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Re: Concrete floor ..how thick???
« Reply #5 on: July 09, 2009, 06:04:31 AM »
You are adding about 15 lbs per square foot dead load to your normal loading so using a span calculator you could  calc your loads at around 55 lbs per square foot or tell the TJI /TGI distributor what you are doing and their engineers will calc it as RW said.

The span calculator is for wood joists.

http://countryplans.com/smf/index.php?topic=989.0

Looks like #2 DF -north will span 9'4" with a 2x6 on 16" centers at 40 lb live load and 15 lbs dead load with deflection L/360
"Always work from the general to the specific." J. Raabe

Glenn's Underground Cabin  http://countryplans.com/smf/index.php?topic=151.0

Please put your area in your sig line so we can assist with location specific answers.

Offline archangel

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Re: Concrete floor ..how thick???
« Reply #6 on: July 12, 2009, 10:13:36 AM »
Also, have you ever notice some old/ancient concrete driveways, or other cement pourings that did not crack, and held up as if they would last forever?
Don't believe a contractor when they say "it just happens" when talking about concrete slabs cracking!

The reason concrete slabs crack is the individuals pouring it ask the mixer driver to add water so it will flow easier.
That makes it much easier to work with, but the quality suffers badly.
The more water you add the more it will shrink and crack and the lower the quality of the finished product

Also, pay a little more and get a stronger PSI mix with more bags of Portland cement in it.

Quality is also determined by how many bags of Portland cement are in the concrete mix per yard.
More bags of Portland cement per yard will also make the slump stand up taller.
More Portland cement in the mix and less water are the main (but not the only) key to strong concrete.
You can also have them add some "fibemesh" to increase strength. (http://www.westrocinc.com/Concrete/Additives/additives.html)

There is a measurement called "slump"
They use an inverted cone (like a volcano) stepping on it to hold it still and after filling it with the cement to be used (big end down) and tamping it (poking into the small end on top to mix it) with a smooth rod, they then slide the cone off and it simply slumps down.
By turning the cone upside down so that the small end is on the ground next to the cement that was inside of it, you can measure the "slump".
If it drops 3 inches, it's called a 3 inch slump.

You should ask for a slump meter/guage be delivered with the mixer driver (or supplied by the contractor) so it can be verified before adding any more water and ruining the mix.

And make sure it is no more than a 3” slump (I would specify a 2” slump), no matter how much the workers complain!
And have them work the surface repeatedly as it (compacts?) makes the surface stronger and less likely to chip if hit.

Oh, and preventing the concrete from drying too fast will help strength and quality.
If the concrete freezes before it's completely cures, it will destroy it!

They will only have to deal with it being thick for a short while as they work it, but you will have to live with the end result for a long time!

 

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