huangyhg

support of wwf in slab on grade
i have a project where the contractor wants to pour the concrete for the slab on grade then pull the wwf up through the wet concrete into position. is this explicitly prohibited by the code (ibc or aci)?
thanks
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what your contractor is proposing is accepted practice. usually "real-time" construction management is needed to make it happen.
it might be standard practice, but if you reference aci as a designer, consider at the following
aci 318, 7.5.1 "reinforcement...shall be accurately placed and adequately supported before concrete is placed..." (emphasis mine)
i would argue that the 'pull it up through the wet concrete method' is disallowed by inference.
that is normally what i see happen even if the mesh is supported. if it is a critical application, consider using tied bars spaced far enough apart so that the concrete crew can step around them during the pour.
thanks for responding. i checked the wire reinforcement institute web site and it states: "it is not recommended that reinforcement be placed on the grade surface then pulled up (so called "hooking"),...."
i've always asked the contractor..."what do you stand on when you pull it up through the concrete?" usually, its the wwf itself so all they are doing is bending up a small portion of the wire (if they can even do that) and there is absolutely zero control over how high they pull it, how evenly it is set in the vertical thickness of the slab, or even if they do it in time to not damage setting concrete.
almost every slab i've ever seen demolished, which had wwf in it, the wire was on the very bottom of the slab, or at least within 1/2" of the bottom.
and since slab reinforcing should be within the top 1/3 of the slab to do any good in controlling cracks...well, we've stopped specifying it for slabs on grade...unless the contractor chairs it with bars at 4' o.c.
we specify to support the mesh on concrete bricks. it probably gets trampled down to the bottom anyway.
i have found that if you pour 2/3 of the slab thickness and then walk the mesh in place and then pour the remaining 1/3 of the slab thickness, most of the mesh is pretty close to where you want it to be. obviously, where you step will push it down, but most of it stays up.
we have had some luck getting contractors to do it this way, however, other contractors are much more resistant to trying anything new. this method obviously requires more labor.
wwf in slab on grade is just a waste of money. either use rebar or consider closer joint spacing.
if as an eor you specify wwf and you are not there to personally observe that it is placed correctly or you don鈥檛 have very persistent inspector, forget it. it's not where you want it. in my line of work i see a slab-on-grade pour on average twice a week. that hooking business is just crap. once the labor gets tired, forget it, it's on the bottom. if i stand right on top of the guy with the hook then it's perfect but the second i leave the hooking stops. the one guy was right about them standing on the wwf as they hook it and bend it.
specifying the placement of concrete bricks for support is a waste of time because 1) far too many are required to do it right, and 2) the wwf is walked down until you just have a bunch of "u" shaped wwf (over the bricks and down to the bottom and the over the bricks again). the best chairs for wwf are slab bolsters with sand feet placed 4' oc but the "u" effect occurs here too.
you guys would be a lot better off specifying #3 or #4 bars at 18鈥?ew, bricks or slab bolsters with sand feet, and closer joint spacing that is sawed in as soon as practicable but not causing raveling of the stone.
techmaximus
the wire reinf. instite states that wwf shall be supported. in reality, it seldom is supported. wwf requires alot more supports because it is more flexible. one of the main reason of not supporting the wwf is that you can run the concrete truck over the subgrade and place the concrete where you need it. the concrete trucks will often leave ruts which will affect your cracking(acts like a shear key). some say the wwf should be in the top 1/3 of the slab, some say the bottom 1/3, and others compromise and say the middle of the slab. it appears best in the top 1/3 for cracking, bottom 1/3 for bending strength, and the middle as a compromise. i'm not so sure what the granular material does. as far as a drainage layer, the building is covered and all the water around the outside of the building is shead away from the building. we just use it as shimming material to level the site. if you put a vapor barrier over the top of the granular, little water will get to it. if you put the granular material over the vapor barrier you take the chance of it raining, before you pour, trapping the water. then your worse off.
it seem, that in concrete everything is a trade-off and it all depends on what you want to trade.

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