Concrete shrinkage problem...
Concrete shrinkage problem...
(OP)
Can't believe I actually posted this to the steel group by accident... anyways, please read on:
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
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





RE: Concrete shrinkage problem...
What is the structural arrangement, i.e. overall plan dimensions, number of panels, etc.? How long has the structure been in service?
RE: Concrete shrinkage problem...
Thanks for your reply...
The area is 17' by 25'4.5" and, of course, filled with a 13.5" thick slab. The slab is supported by a 1.5" steel deck, and below that a very stiff framing system. I have checked the design (which was not mine, and reviewed before going out as per standard), and there is no way this trouble could be caused by deflection of the framing system. Remember that there was absolutely no evidence of any movement or additional deflection, nor was there any cracking below the slab. In my opinion, young as that may be, this is a concrete shrinkage problem. I just need to be able to show it, because as we all know, 4% is huge.
Besides the fact that I am inexperienced, we do have our fair share of very experienced P.Eng staffers, including our most senior partner who has been licenced since 1954. In everyone's opinion, this is shrinkage, nothing more. Unfortunately, it is very difficult to show and I cannot find any guidelines on the maximum possible shrinkage. That is really what I need.
Thank again,
Young Structural
RE: Concrete shrinkage problem...
RE: Concrete shrinkage problem...
RE: Concrete shrinkage problem...
I know it may sound simple, but there can be many reasons why the surface profile changes, and shrinkage can be caused by lots of factors.
RE: Concrete shrinkage problem...
That's a thick slab! It appears you have an inadequate longitudinal shear transfer mechanism along your top of beams.
Research in shrinkage is an area recently being revived especially in Canada and the Canadian steel code CAN/CSA-S16.1-94 has guidance in Appendix L.
HTH
Regards
VOD
RE: Concrete shrinkage problem...
RE: Concrete shrinkage problem...
I have a hard time believing that this is related to shrinkage.
RE: Concrete shrinkage problem...
I have seen floor slabs shrink away from construction joints, but it was a SOG poured on a PVC vapor barrier. It is hard to believe that much shrinkage would take place on a fluted metal deck without some cracking around the flutes highs and lows.