Hello All;

    Here's a challenge for all you concrete guys:  I'm having a hard time determining the maximum possible amount that a concrete slab could shrink, given the worst concrete mix or whatever other contributing construction horrors.  I should probably give you all a little background as to why I'm looking for this:

    Our firm designed a system of beams and columns, covered by a 13.5 inch thick concrete slab, to fill in a hole in an industrial floor.  The floor has developped an obvious sag around the edges (looks just like a depressed membrane, or a meniscus in mercury) and the forklift drivers are nervoud driving over it.  The sag varries between .25 inch and .50 inch in drop of elevation.  You can readily get below the area, and there is no evidence at all of any beams having moved or of the slab creeping (I say this because the bottom of the slab is still perfectly flat).  It really looks like the concrete is pulling out and away from the centre and shrinking down in total volume at the same time, resulting in a perfectly flat bottom in tension and depressed top as a result of the change in volume.  Of course, I really don't think this is the case, since there is no cracking evident and I would think that if tension was keeping the bottom flat under the creep which typically goes hand in hand with high shrinkage, there would be evident cracking.

     I have heard of various amounts of shrinkage by various factors (1% overall volume change due to very high water/cement ratios, etc...)  but I cannot find any information on just how bad this can get.  And that is my question:  Given the worst possible situation (and please outline the situation you are thinking of), what is the most concrete will shrink?

     I'm really curious to see what you're all going to say.  I'm really hoping there are some people out there who have seen this kind of thing before.

Thanks in advance,

Young Structural