Concrete shrinkage problem... 1

[youngstructural]

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

 

[dbuzz]

A 4% loss of thickness (0.5" in 13.5") due to shrinkage seems large.

What is the structural arrangement, i.e. overall plan dimensions, number of panels, etc.? How long has the structure been in service?

 

[youngstructural]

Hello Dbuzz;

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

 

[dbuzz]

One way forward may be to discuss with professors at your nearest engineering school.

 

[flamby]

Normal concrete shrinks about 0.04% (0.0004 in fraction). Worst possibles know no limits. If you pool up cement solution, basically water, and call it concrete, it will shrink 100%. 3-4% is too much even for a grout mixture and I think you may also want to examine other issues like whether the compaction/ vibration etc. during concreting were at fault.

 

[iandig]

Just a quick question to get it clear in my head, how soon was the 'movment' evident, and is it still moving?
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.

 

[VoyageofDiscovery]

Hi youngstructural,

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

 

[BigH]

SOrry to one and all: iandig - re: London; can you eM me at bohica@indiatimes.com (got rapped before on eM in thread - ).

 

[jike]

My first reaction is that perhaps the shear transfer, along the interface between the new and existing slab, has allowed some slip to occur. What does this detail look like?

I have a hard time believing that this is related to shrinkage.

 

[jheidt2543]

Just wondering: You say the 13.5" thick slab was poured on 1.5" metal deck form to fill in a hole in a floor slab. What is the deck span? Was the deck shored to support the wet concrete load on the deck during the pour [(13.5/12)*145 pcf= 163 psf]? Is it possible that the deck deflected during the pour. You do say it is level, did you actually shoot elevations from the bottom side?

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.