underlays to concrete: blinding, poly sheet, sand

[bob116]

Hello all.
Can anyone advise the different uses of blinding concrete, polythene sheeting and sand as underlays for various concrete structures poured on ground? This is for Australian conditions.I have always used whatever standard drawings were to hand but now I find myself working for a firm with no standards and am having to reinvent the wheel.As examples of concrete structures, I am thinking of footings, slabs, walls, strip footings etc. its not so simple when you start to think of it. Thanks

 

[cvg]

sand or gravel when working in mucky or wet trenches
vapor barrier when moisture coming through the slab is undesirable
blinding is good to provide a level surface for placing reinforcement, such as on a rough rock surface

 

[BigH]

Adding to cvg, I would only use blinding concrete if, as he says it is rough rock surface - it is really called dental concrete where I come from. the use of vapour barrier is beneficial if the flooring of the building cannot take the moisture that could pass through - still you want to wait as long as possible before you place the flooring as setting concrete will emit water vapour. I prefer to use crushed sand and gravel if water is not a problem rather than a blinding concrete as you will be able to place it, level it and compact it - it will form a very dense level surface for protection of subgrade from rain (if the building's roof isn't yet on) and also for placing reinforcement - most likely reinforcement will be welded wire fabric (BRC) - sitting on chairs on either crushed base or blinding - you may have to literally pull it up during the placement of the slab as walking on the wire mesh during placement will in places force it to the bottom of the slab.

Check out NRMCA CIP 29 "Vapor Retarders Under Slabs on Grade"
"Vapor Barriers Under Concrete Slabs" by Suprenant

http://www.wagnermeters.com/getting_the_vapors.php

Hope these few references help.

 

[oldestguy]

The subject of a vapor barrier is very complicated. It is not just "keeping moisture from moving up". Once you encounter floor coverings that lose adherence, show bubbles, etc. and get into the testing of moisture suction in the slab, salt content, osmotic action, etc., it can be very complicated. Salt build up in the slab surface before placing coverings will have a major effect on floor performance and then trying to tie that to the permanence of the "barrier" is difficult. If you are going to use one at all, be sure it is not just "a sheet of Visqueen". BigH's link is a start.

 

[hokie66]

Another use for blinding concrete is to serve as a substrate for a waterproof membrane for subterranean structures. For instance, in high rise buildings with multiple underground carparking floors, the lowest floor is often subject to hydrostatic uplift.