slabs on grade

huangyhg

slabs on grade
i am sure that this has been discussed before, but how many of you put wwr in a slab on grade, and if so, do you follow the min. steel for temp. and shrinkage, or just a number for crack control.
also, how many require a vapor barrier/retarder?
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jirsebo:
check out these threads. the subject has been discussed fairly fequently.
p.s.
the use of vapor barrier/retarder depends on the situation.
if mositure vapor penatration through the slab would be a problem for a floor finish (carpet, tile, wood flooring, etc) or otherwise be objectionable, the i would definately use a vapor barrier.
if moisture penatration would not be objectionable (machine shop, warehouse, etc) then i would consider leaving it out.
when i do use a vapor barrier. i specify a fairly hefty one. i usually spec around the 15 mil thick stego product. tends to resisting tearing and getting holes punched in it prior to the slab being placed a lot better than thinner products.
jmho
i am working on a renovation to an existing building.  part of the renovation involved removing a large section of existing interior slab-on-grade and replacing it with a new thicker slab-on-grade.  for the new slab, i specified rebar instead of wwf.  the contractor told the owner that nobody used rebar in slabs and it is cheaper to use fibermesh.  the owner had a previous bad experience with fibermesh and refuses to use it.  he and i discussed the wwf.  part of the discussion was that wwf typically ends up in the bottom of the slab where it provides little or no benefit for crack control.  the existing slab is cracked and the owner doesn't like it, but the contractor told the owner that's the way all slabs are.  it's not that the cracks exist, but the owner doesn't like that some of the cracks are wide and the floor is not level on each side of the wide cracks.  the contractor argued against using rebar or wwf and almost won until the existing slab was removed.  all of the wwf in the existing slab was sitting directly on the sub-base.  the owner looked at that and told the contractor to put in the rebar as specified on my details.
presumably the rebar won't end up on the sub-base, why can't you achieve the same with wwf?
around here sog would typically have wwf supported on chairs to ensure it remains where it should when pourung the concrete.
my 30 year experience with wwf is that is not worth 15 cents.  every time i have had to remove certain of concrete slab on grade, the mesh was at the bottom of the slab or very near there.  even with mesh supports, it sags too.  it cannot be relied on being where you want it to be.
it is quite impossible for concrete workers to place concrete on ground without walking on their work.  even with wwf pickers, they do not really know how far they are picking up the wwf and they are invariably inconsistent in their efforts. consequently, the effort is wasted.
contractor's hate rebar for slab reinforcement because it is more labor intensive and they have to support it off the ground, tie the bars, lap the bars, and walk inbetween or on top of the mat.  they trip on it, etc.
but i force them to do it because i will never specify wwf ever again.
i also prefer rebar or fibers, depending on the specific application.
the only way that i have ever seen wwf wind up approximately where you need it is to pour the lower 2/3 of the slab thickness and then have a crew "walk the wwf in", set it on the freshly poured concrete and then pour the remaining 1/3. even though the guys will step on it and push it down at various locations with their boots, the majority will remain in the upper part of the slab. (it is just the opposite of guys trying to hook it up.)contractors, obviously, do not like this method as it increases their labor costs.
minorchord,
you are obviously not specifying enough chairs. specify plastic chairs and more of them. plastic chairs arent as likely to sink down as metal chairs.
csd
wwf will stay where the reinforcing is wanted if proper chairs, (continuous), are specified and the the size of the wire is of a magnitude to provide sufficient area of steel.  very cost effective when labor and laps are estimated.