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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.

cans in concrete

The quote from an official regarding this practice:

For such purposes in construction, it was not illegal prior to September 1999, but since then styrofoam and formwork boards have been used instead

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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    $\begingroup$ Do you have a link to this article? I'd like to read some more into this. The picture isn't really clear entirely what's going on here. $\endgroup$
    – grfrazee
    Feb 9, 2016 at 21:25
  • $\begingroup$ @grfrazee unfortunately, the articles are a bit scarce on engineering details and more focused on the rescue efforts. That photo was from ibtimes. There is similar information over on cnn. And a google news search with the same quote and many times, the same image. $\endgroup$
    – user348
    Feb 9, 2016 at 21:32
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    $\begingroup$ The usual method is to use a dead body. That gets rid of some scum about to testify to a grand jury, and saves concrete at the same time. If the building inspector requires too much of a payment, then he becomes the next fill. More savings all around. $\endgroup$
    – Olin Lathrop
    Apr 29, 2016 at 21:51
  • $\begingroup$ Should have used Weetabix. That stuff dries hard as rock. Who needs concrete $\endgroup$
    – Jodes
    May 3, 2016 at 10:07

3 Answers 3

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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).

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.

P-M
(source)

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.

However, the 'Styrofoam and foamwork boards' being the acceptable standard now makes me wonder how different that actually is from empty cans.

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 that is 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.

waffle slab
(source)

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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  • $\begingroup$ A tangent question that might be interesting to explore in another post is the glass bottle filler material sometimes seen in self made green houses (the glass is reused bottles): inspirationgreen.com/index.php?q=glassbottlewalls.html and the plethora of iamges out there. $\endgroup$
    – user348
    Feb 9, 2016 at 23:20
  • $\begingroup$ @MichaelT, I've seen those before, and they're quite an interesting upcycling of materials. For most cases, since they're only for a single-story wall, the loads are pretty low. Build too high with construction like that and you'd probably have issues with walls cracking. $\endgroup$
    – grfrazee
    Feb 10, 2016 at 14:33
  • $\begingroup$ As I understand it, glass is actually really neat for strength. There is an infomercial from Corning on "The Glass Age" - Part 1: Flexible, Bendable Glass and Part 2: Strong, Durable Glass - the second one being the more applicable one in this case (though again, it would be interesting to see how far one could take that construction approach). $\endgroup$
    – user348
    Feb 10, 2016 at 14:37
  • $\begingroup$ @MichaelT, glass used for bottles isn't produced in a very controlled manner (in terms of structural performance, I mean). There are structural glasses, like what they use in the Sears Tower Skydeck in Chicago. $\endgroup$
    – grfrazee
    Feb 10, 2016 at 16:03
  • $\begingroup$ Agreed that structurally there shouldn't be any difference between cans and styrofoam, but could there be a durability difference? I'm wondering whether the cans might cause a long term corrosion issue, e.g. due to bi-metallic corrosion. $\endgroup$
    – AndyT
    Feb 11, 2016 at 17:12
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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:

  • reduction in self weight
  • reduction in member sizes due to less load being carried
  • floor-to-floor height of buildings is reduced because beams are smaller
  • foundations can be smaller
  • cost savings due to the above
  • CO2 footprint of the building is less due to less material being used and the CO2 emissions during production of cement are reduced.

More detail can be found at www.cobiax.com.

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