location of rebar in 6 slab

huangyhg

location of rebar in 6" slab
where should rebar be placed in a 6" concrete bridge deck? a friend is suggesting 2 layers of rebar, one about 2" below the top surface, the other 2" above the bottom. are 2 layers really necessary?
what is the best spacing to use between the bars?
this is a vehicle bridge (cars), about 10' x 22'. many thanks for sharing your expertise
-joey

the answer depends on the spacing of the supporting beams underneath and the amount of slab overhanging on each side of the two exterior beams (assuming there are more than two to begin with).  most bridge slabs are design looking at two cases 1. the overhang forces/moments and 2. the section of slab between the beams for both positive and negative moment.  if the overhang or the interior negative moment controls the design then the rebar required by design will be placed near the top of the slab.  the remaining rebar is for load distribution, temperature and serviceability.
while this is most likely a private application, most bridge deck slabs are 8.5" with the minimum being 7.5.  thus you should give some thought to increasing the slab thickness.  the smaller slab will result in larger rebar, which, in turn, will result in serviceability problems and placement problems.
the spacing of the reinforcing would be controlled by the design.
for more information, please contact your local building enforcement official.  they can be very helpful and provide some insight so that the constructed product doesn't fall apart only a few months later.
i agree with qshake on the thicker slab.  a 6" slab is quite minimal for most bridge deck applications.  most dot's use double layers in their decks.  some, even, go so far as to recommend no reinforcing, allowing the deck to actually arch across the span to the supporting girders.  there has been some research in recent years to look into this as a bridge deck with no rebar is a bridge deck that doesn't deteriorate as fast.
if just topping slab and bars are only temperature reinf, then 2" from the top is good location.  this provides cover for chloride penetration and bar location is ok to 'trap' cracking (most people don't see the underside <g>).  try spacing bars 16"+/- if possible this makes it easier for finishers to step through bars while working on it.
if bars are structural, then slab thickness might be a tad thin.
bars could be epoxy coated.
i guess i should have made it clear that this is just a bridge that's part of my driveway, leading up to my house. the only traffic on it is me & the missus goin out for bread and beer. hope to get away with that 6" under these cirumstances.
i understand, but the local building official may have other critiera for your own safety.  not to mention saftey vehicles such as fire trucks and ambulances.