Negative comments for approved practice design of a raft foundation

[mar2805]

Hi people.
Plase look at the sketch attached.
Short description:
Virgin ground under the future raft slab is removed (aboth 20cm).
Beam trenches are dig out.
Beams are poured first and serve later as the "side formwork" for good hardcore compaction.
Virgin ground (slab part) is compacted and then filled with harcore compacted filling (fractions raging from 1-32mm).
An "mud slab" is poured (above the harcore fill) wich then serves as the leveling plane for the proper reinforecement placement.
Beams are poured first and later serev as the "side formwork" for good hardcore compaction.
Hidroisolation is placed on the raft surface as the first layer of the residential floor.

note: virgin ground level (left in green) and the down side of the RC raft slab are at the same elevation.

Im interested in your negative comments for this design.
Thank you.

 

[mar2805]

Hi!
Ive been reading more and more on USA recommendations for floors on ground insulating solutions.
Ive found one article here that goes in-depth on explaining how the capilary action + water vapor under the slab are two main concerns.
The articel is located here:

http://www.jdtechnical.com/VaporBarriersNus_Nec_PAC_Article.htm

I agree on most part mentioned on capilary action that can occur under the slab and its effect on the concrete slab, BUT, I cannot agree on the part thats written on the water vapor coming from ground into residental space.
Water vapor always travels from higher pressure to lower pressure!
If we assume that, in winter,
- inside heated residential spaces have temperature of 70F and relative humidity of 50-60%.
I cannot agree on values below 30% of relaitve humidity couse youll probably have serious sympthoms of "dry throat"
- temperature of soil below the slab dependes on many factors, air temperture, type of soil, depth at wich its measured...but lets say that outside temperature is around 40F and for simplicity let assume that thats also the temperature of the soil.
If we assume the worst case scenario that the soil under the slab has 100% relative humidity the water vapor pressure would be around 150psi
So internal water vapor pressure 180psi > soil water vapor pressure 150psi
Vapor would travel from the inside towards the ouside (soil).
Your thoughts...

 

[mar2805]

Guys I frogot to ask, how do you thermo-insulate your residential slabs on grade?
Where do you place thermo-inslulation?
Whats usual practice in USA?

 

[mar2805]

Can nonwoves geotextiles be used as capilary break under the slab?
Ive found an article that explains this very good but Ive always thought geotextiles where used when you wanted to direct waterflow?
Anyone has any experience?

 

[dik]

I'd have put the PEVB underneath the concrete (encapsulating the edge beams). This stops the water from entering the system... as Hokie noted, put the PEVB on the wet side.

It's my experience that raft foundations are generally used with poorer soils and that the consolidation of the granular fill may be an improvement (if required). As long as the soil is essentially undisturbed, I can take no exceptions to your detail. I'd have used hooked dowels in the top all around and then spliced straight bars for the top reinforcing.

Dik

 

[mar2805]

Can someone explain a bit the difference in geotextile weight and they performance.
I mean, when to choose wich model.
For non-woven there are 4oz 6oz 8oz 10oz...
Can any of these model be whitstand the pressure that will a slab on grade produce?