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In the recent 6.4 earthquake that hit Taiwan, it was seen in a newswire photograph of rubble that metal cans were embedded in the concrete.
The quote from an official regarding this practice:
I suspect that the reason to use it is that the cans and the associated empty space is cheaper than actually having solid concrete (and may be partly necessary to reduce the weight of the concrete and thus associated stresses with it).
However, the 'Styrofoam and formwork boards' being the acceptable standard now makes me wonder how different that actually is from empty cans.
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You're probably (mostly) correct. For beams, reducing the weight of the beam itself will reduce stresses in the beam and the members supporting it, as well as reducing the material cost of construction.
However, for columns, reducing weight is not necessarily a good thing. Columns are influenced by what's called P-M behavior, which is the combination of axial load and bending moments on a column. In general, if one increases the axial load on a concrete column, one also increases the bending capacity of that column (until one of these parameters hits a critical value). The image below shows a simplified P-M curve, and you can see that increasing axial load increases moment capacity up to about midway up the curve.
If you remove part of a column's cross-section, you remove it's axial capacity, which is not a good thing.
Also, you'll notice that there are little layers of concrete between the cans. Ideally, one would want these larger so that the "webs" of concrete can transfer force between the layers. Having essentially two separate layers of concrete ~3" thick isn't strong at all.
In practice, there's probably not a huge difference since both introduce a void in the concrete. Styrofoam might be a bit better since it won't deform as much as a tin can and thus hold up to the hydrostatic pressures of concrete (during a pour) better. Also, it's probably easier to get styrofoam with manufactured to a controlled process than it is to get consistent tin cans.
What you end up with is something like a waffle slab, just that the holes in the waffle are interior to the slab.
As an aside, the engineer in me really recoils at the thought of doing this to a concrete slab. If I saw a contractor doing that to one of my designs, we would have problems.
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The inclusion of cans was almost certainly to reduce the volume of (expensive) concrete and had nothing to do with any weight savings by including voids. This type of practice has been seen throughout the world when building standards are lax.
The quote about this type of construction not being illegal before 1999 may be due to confusion on the part of the journalist. It could be a case where any building standards were not enforced before 1999. If the new (1999) standard allowed for Styrofoam voids, it doesn't mean that the concrete was designed to use them. It would just be allowable if engineered that way from the beginning.
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As one can see, an organised, designed way of construction does work! Styrofoam blocks have been used in the past. Controlling uplift is the big challenge. This has been addressed by the various manufacturers of the void formers.
The most notable building is the Square at the Frankfurt airport which is above the railway station and houses a Hilton.
Creating voids in slabs using voidformers (cans, or in this case plastic balls) is an accepted way of construction and fullfills Eurocode requirements.
The benefits:
More detail can be found at www.cobiax.com.
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