sand layer under surface beds? recommended?

[BauTomTom]

Hallo Engineers

I just wanted to find out if it necessary to provide a sand layer under the concrete surface bed.
Very often you see this detail in books. I guess it is there to level the natural ground

Section would be then (from top to bottom):

1. tiles
2. surface bed 100 mm
3. plastic sheet
4. sand layer 40 mm
5. natural ground


BauTomTom

 

[Ron]

BTT...it is there for several reasons. First it compacts well. Next, if sufficiently coarse, it provides a capillary barrier to bulk water rising to the slab, while conversely, it provides adequate drainage beneath the slab. It also provides a less restrictive friction surface when used with plastic as compared to a gravel layer (less restraint for the slab, thus reduced cracking potential)

 

[BauTomTom]

--to Ron--

thanks for the comment

compact well: this makes also sense, right

capillary barrier: by normal sand? gravel I would understand but by sand?

less restrictive friction surface: this didn't come in my mind but you right.

so in the end you would definately say it should be provided, right? and how thick should the layer be?

BauTomTom

 

[Ron]

For the sand to work as a capillary barrier, the fineness modulus should be more than about 2.75.

The sand layer should be 6 to 8 inches thick.

 

[BauTomTom]

200mm sandlayer ??? this is to much for my case. This would mean further excavations and costs

higher then 2.75mm sand this makes sensse

BauTomTom