h a l f b a k e r yA riddle wrapped in a mystery inside a rich, flaky crust
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Reinforced concrete (steel REinforcing BARs are the key) is a
fairly popular construction material. However, it has a couple of
flaws. One is that if water seeps into the concrete, the steel
rusts, and EXPANDS, breaking the concrete. Not very good for a
long-term structure!
Another problem
is weight; steel is about 3 times as dense as
concrete. This limits how tall a reinforced concrete thing can be.
Both problems can be addressed by using titanium instead of steel,
as the reinforcing material. In terms of strength, the metal is
pretty much the same as steel. But in terms of corrosion....
Shortly after being made, a bar of titanium will quickly and
naturally acquire an oxide/nitride coating that protects the metal
from deeper corrosion. So if titanium rebars are added to
concrete, even if there are cracks that let water soak the metal
for a century, it won't be affected. Thus the reinforced concrete
structure can be expected to last a long time.
Next, titanium has about half the weight of steel. Therefore a
significantly taller structure can be built, than if steel rebars are
used.
titanium rebar bridge
http://www.newscien...t.html#.VU8p659ArqA
nitinol rebar for earthquake resistant bridge [xaviergisz, May 10 2015]
[link]
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Titanium costs about 10× more than steel, which is
probably the main reason it isn't used. |
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Corrosion is mainly avoided by sealing and
sacrificial anodes. Stainless steel would be another
option. |
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At least Stainless has a similar modulus of elasticity as steel,
whereas Titanium has around half. To get the same crack
resistance (caused by stress), it seems that twice the cross
sectional area of Titanium would be required. |
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Stainless steel doesn't have anything like the mechanical strength of carbon steel. |
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Standard rebar can be quite effectively protected by hot-dipping in polymer or powder-coating with epoxy. |
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Coefficients of thermal expansion should be matched up, also. |
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//steel is about 3 times as dense as concrete. This
limits how tall a reinforced concrete thing can be.// |
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Is that really a significant factor? I'm assuming that
the rebar adds up to, maybe, 5% of the cross section
of the concrete. That means that a beam with rebars
will be about 10% heavier than one without. |
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The density of titanium is about twice that of
concrete, making a beam with titanium rebars about
5% heavier than one without. |
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So, you're saving 5% of the weight; and that's
assuming that titanium has the same modulus as
steel. If, as [Ling] said, its modulus is about half, the
weights would come out the same. |
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