A mason who did my slab was adamant that I lower the rebar so that it is at least 2 inches away from the surface.   He said if rebar is too close to the surface cracks can occur.  OK fine, but looking at my 4" core reward blocks I notice that they have plastic horizontal rebar position holders 1 inch away from the styrofoam wall.   That would mean that the rebar is 1 inch away from the concrete surface.    Why can rebar be 1 inch away from the surface in ICF's and should be 2 inches away from the surface in a slab?  

The requirements for cover (depth to surface) can vary depending on exposure to corrosion and bar size. In a slab there is exposure to the ground and, possibly, standing water on the surface.

For walls with stay in place forms, the minimum concrete cover requirement for the rebar is 3/4". As mentioned above, the cover requirement is greater for footers and slabs because of the greater chance that water can stand or other wise get to the rebar and corrode it.

One way to avoid corrosion in rebar is to use RockRebar instead of iron rebar. RockRebar can be placed safely closer to the surface.

Rebar cover is most important where they could be salt. On an interior slab, I doubt is makes much if any difference, corrosion wise.

Does this minimum 3/4" cover requirement applies to vertical rebar as well? 

Posted By jacktca on 15 Aug 2011 12:40 PM
Does this minimum 3/4" cover requirement applies to vertical rebar as well? 

Yes.

From your post, I wasn't sure whether you were talking about the slab, wall, or both. Arkie is right, 3/4" min clear from any rebar, including ties, etc, for walls inside ICF. The surfaces of the wall and door openings will be slightly more.
In slabs, keeping the rebar within the middle 1/3 is a good rule.

Your rebar should always be closer to the tension side...

for walls it is hard to say which face is the tension face - usually the inside due to wind pressure or backfill - but sometimes it can be the outside due to negative wind pressure...

Basement walls - position rebar closer to the inside face
Above Grade walls - dead center because you just don't know...
swimming pool walls - dead center (because water and backfill neutralize each other & you should need no rebar but if the pool is empty then you will likely need a double mat of rebar (because the wall is unrestrained at top)... So place rear everywhere

slabs are trickier.. a good rule of thumb is to place it in the dead center and forget about it... however you might want to have it closer to the bottom in the middle of the room and maybe closer to the top around the perimeter..
but for a free floating slab - just leave it in the center. .