Replies

1. 
   brownbagg | Aug 18, 2004 09:53pm | #1

   its a local decision, some people use the sand so water will migrate under the concrete, some for freeze protection, here we do not use sand. Nothing wrong with it, just not needed here.

2. 
   CapnMac | Aug 18, 2004 10:17pm | #2

   What purpose other than a base for the concrete does the sand serve?

   In the dark ages (before the internet, CDs & the like), I was taught that you used sand because is has a fine grain which meant it could be shaped precisely.  This better let a person control things like concrete thickness, which controls volume, etc.  Also, sand has "engineering" qualities like known compression, etc. which vary less between jobs.

   Or, in the field:  "Ya kin pour over dirt, but dirt ain't even; an' dirt ain't the same everys place." <g>

   Occupational hazard of my occupation not being around (sorry Bubba)
1. 
   sungod | Aug 19, 2004 07:28am | #6

   Here in Southern Calif. where there is hard clay dirt, sand will give tree roots a easy time, so it can pop up the patio or sidewalk.

   Sand on top of visqueen help remove the water in concrete so it sets faster.

   Sand does provide a homogeneous surface so the concrete will cure evenly (not crack).

3. 
   BillHartmann | Aug 18, 2004 10:43pm | #3

   http://www.buildingscience.com/resources/foundations/sand_layer_under_slab.htm

1. 
   CapnMac | Aug 19, 2004 12:49am | #4

   I'm glad you posted that link--I don't believe I've ever seen anyone visqueen under the sand base before.  Now, I've got printed reasons to "not to."Occupational hazard of my occupation not being around (sorry Bubba)

4. 
   remodelerdw | Aug 19, 2004 06:11am | #5

   We commonly use pea fill here, sand is rare for concrete work.  Primarily because pea-fill is self compacting and sand has to be jetted or it will settle unevely.  Run a hammer across concrete poured on sand after a year and you'll hear the hollows.

   reodeler

5. 
   fredsmart48 | Aug 19, 2004 08:18am | #7

   My understand and the big reason for the sand between the plastic and concrete is so concrete company can get by with a 5 or 6 inch slump instead of 3 inch slump. That is so they don’t need to hire 2 or 3 or 5 extra people to move the concrete when it just sits in one spot after it is placed and won’t self level.

   And Why is that a problem? As I am told with the plastic directly under ninth the concrete all the water in a 5 inch slump has to go up and as it moves up and sits on top of the concrete to evaporate you get separation in the concrete because the water sits on top of it to long. The concrete will not get to be the strongest it could be.

1. 
   brownbagg | Aug 19, 2004 01:34pm | #8

   it will also make the slab crack

6. 
   dIrishInMe | Aug 19, 2004 02:15pm | #9

   Just out of curiosity, what location is the slab to be in... Garage? basement? driveway? comercial building? etc?  

     

   Matt
1. 
   Rojoho | Aug 19, 2004 10:24pm | #10

   We're talking about1500 sq.ft. of ground level residental with radiant heating, two story victorian near San Francisco.

   By the way, Insul-tarp specifies "optional" granular fill on the insul-tarp, which is on 6" - 8" open drain rock. I plan to mechanically compact the rock base.