Replies

1. 
   Guest_ | Feb 22, 2000 08:15pm | #1

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   I am planning to build a house on a concrete slab in Quebec, Canada. Does anyone have any information about the insulation method? Where should the insulation be? Right above the concrete or in the floor...? Thank you for your help.
   Alois

2. 
   Mark_T. | Feb 22, 2000 11:14pm | #2

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   Renovator:

   The purpose of the gravel, other than drainage is to provide a uniform, strong base to support your slab thereby reducing the possibility of cracking.

   When I say strong, know this: Sandy loam supports 2000 PSF (pounds per square foot) and gravel supports 5000 PSF. These are off the cuff numbers - I'm not gonna take the time to look up the actual figures. The gravel forms a less resilient substrate, which is to say that it much less likely to compress as live load (say a vehicle) or dead load (the weight of the slab) bears down on the material under the slab.

   When I say uniform, it is important to understand that besides the strength of the concrete, an important factor in preventing cracks in concrete is to support the slab evenly. If one end of the slab has softer dirt below it, that end will eventually crack off and fall. Let me ask you a question: Is the soil below the proposed slab all compacted to 100%? Or in other words is some of the soil fill and some virgin soil? What? you say you don't know? Fine. Add a layer of gravel, and you now have a more uniform support for your slab (although it is stilll important that your soil is compacted, it not
   i as
   important).

   Now let's talk about point load. Let's say you are working on a truck on the slab, and place the truck on a couple of jack stands that are sitting on the concrete. The jack stands have a 4" square base. Each jack stand is supporting 3000 pounds. The concrete is standard 3000 PSI concrete - no problem with the load. Now realize that the load is bearing down
   i through
   the slab at roughly a 45 degree angle. The slab is 4" thick. This means that the load spreads out 4" in each direction creating a "footprint" below the slab 12" x 12" or 1 SF. So, you have 3000 pounds of weight bearing down on 1 SF of 2000 PSF sandy loam substrate - you got a problem, whereas if you have the 3000# load bearing down on 5000 PSF gravel - no problem. Now, let's say you have a gravel substrate that is 4" thick. Your 4" x 4" 3000# load is bearing on 1 sf at the bottom of the 4" slab and 1 sf at the top of the gravel. Once again, the load is spread at aprox 45 degree angle, so the footprint at the bottom of the 4" gravel bread spreads an additional 4" in each direction - or an area 20" x 20" or roughly 2.5 sf. Now you have your 3000# load bearing down on 2.5 sf of 2000 psf sandy-loam supstrate - no problem! Did that make sense?

   The above anolgy was intended to give you an idea of the concept of supporting concrete. In actuality, I was mixing compressive strength with shear strength (or is it tensile strength), but the same concepts still hold true.

   Sorry for the spelling errors - I do not have access to spell check right now.

3. 
   Renovator_ | Feb 22, 2000 11:23pm | #3

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   Hey All,
   What is the need for a sub-surface of 5/8"- or pea-gravel for concrete slabs? Especially if the soil is sandy loam with good drainage!
   I'd appreciate your imput.
   Dan