Why Put gravel under concrete?

[Mark Hudson]

replying to clint wood, Mark Hudson wrote: Concrete is permeable. If it were not permeable you could sweep water off of concrete and it would not appear wet. Especially most slab concrete because it is air entrained to protect against freeze/thaw cycles and most slab concrete is 3000 psi. Concrete can be non-permeable with the correct mix design, but that is not usually the case for slabs. If concrete is non-permeable why does the code require vapor barriers????

[Rockhead]

Appreciably, Alan

[Kevinzep]

Good question. 6-6-10 Welded Wire Mesh - It is used often to keep the slab in compression. Concrete has poor tensile strength. Many times the mesh will sink to the bottom of the slab and will be pratically useless unless it is physically pulled up into position after the pour. I prefer #3 rebar at 18" o.c. placed on concrete dobies or chairs especially for driveways and large slabs. Kevin Szczepankowski, HouseArte.com