Tngolfer, I acknowledge that your view is the majority one. But the problem with it is that shrinkage and temperature change (both volume changes) do not cause concrete to crack, restraint does. The restraint is at the bottom, so that is where the crack initiates.
If you are interested in thinking about this more, there is an excellent paper in the July 1997 issue of Concrete International, "A Structural Looks at Slabs on Grade" by Cesar Kiamco, which explains it in detail much better than I can.
But, as tngolfer says, if the bottom of the slab (with its mesh) restrains the top, couldn't cracks then form in the top without extending to the bottom?
As the slab tries to shrink, you would get a "smiley face" deflection if the ground wasn't present and you would have tension in the bottom. Since the ground is present and won't allow the slab to deflect, the top still tries to shrink but the bottom will not allow it. This causes a crack to form in the top. One large crack would form at the weakest section unless you have steel to limit the crack width and create many small cracks. If the steel is in the bottom you would still get one large crack.
I'm a Canadian Structural and did my intership in Ontario. While there my experiences were pretty much the same as JAE's expression of "harping", however now that I work in New Zealand I never have problems with chairs anymore. The finite attention paid to concrete detailing here (due to the very serious seismic considerations) means that contractors do not expect to get away with much, and don't really try. Chairs are almost always in place, the chairs are securely tied and you can walk on the mesh without much deflection thanks to the normally quite generous number of chairs supplied. And it's always chairs too, not small pieces of brick or other junk from around site! *sigh* And in case you're curious, WWR is simply called "reinforcing Mesh" and abbreviated "Mesh" in a soft metric system... ie 6x6 6/6 becomes 665 MESH (only approx equal).
While I find that with concrete work the quality is in general much better here than in Canada, that's not to say that everything is better....
Most steel quality if comparable between the two countries, and falls into the classic (pretty much always good out of the shop, usually good on site), timber is again the same with good quality work being the norm, and the occasional horrifying nightmare job being the exception to the rule.
That said, Site Safety is no where near as good here in New Zealand as it was at home. Also awareness is very different, and the guys here on site often chuckle and call me a "blouse"; Sort of a polite/teasing "You're a girl" when I wear safety gear. Except for the once in a while someone gets hurt while I'm still there. Always happy to help with the first aid when that happend... *evil smile*
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
The twists and turns of this thread. From designation of reinforcing, which is apparently different in every country, to where it is best placed in a slab on ground, and to quality in different places.
We use a lot of reinforcement mesh in Australia, both in slabs on ground and in suspended slabs. Probably because it is used so much in suspended slabs, our workers seem to give it as much respect as bars in terms of placing accuracy. Our standard bars are now deformed, 500 MPa, and the standard sizes are 5, 6, 6.75, 7.6, 8.6, 9.5, 10.7, and 11.9. Square meshes are usually at 200 centres each way. Rectangular meshes with the main bars at 100 centres are readily available for one way slabs, and special orders are often done for large projects.
Hokie66, I appreciate your reference to the article by Kiamco. I have noticed over several years and millions of dollars of construction that it doesn't seem to matter where the wwf is in the slab - it still seems to be effective. At least, better than no wire or plastic fibers. I understand the frustrations of those trying for an exact placement - still, it's a pretty forgiving product.
