concreting in space

--- In ssi_list@... "Combs, Mike"
> OK, here's the cite. I was wrong about the article being in the
JBIS
> (probably was just remembering that the author was British), it was
> "Spaceflight". The author's name is D. J. Sheppard, and here's
the cite:
>
> "Concrete Space Colonies"
> Spaceflight, Jan 1979

Now that I've reflected on this matter for a moment, I recalled that
Henry Spencer did a synopsis of the article back in 1998.

I'm copying below Henry Spencer's post to the sci.space.policy group.

Would you say that Henry's treatment hits all of the points that
Sheppard made in his article, or did Sheppard go into a lot more
detail?

TangoMan

BEGIN QUOTE

Let's see here, I finally got around to digging up the Jan 1979 issue
of Spaceflight... "Concrete Space Colonies", Dr. D.J. Sheppard,
Spaceflight v21n1, Jan 1979, pp 3-8. Summarizing quickly...

Normal civil-engineering practice accepts working stresses in steel
cables of 1100-1600 N/mm^2, about six times what is acceptable in
plate or rolled sections. The mass of steel in a Stanford Torus is
reduced by about a factor of 4 by using prestressed concrete rather
than a steel shell, using the same design rules (ASME Boiler and
Pressure Vessel Code, Section III, 1977) for both. (Other design
rules give somewhat different numbers for one or both -- there is a
table in the paper -- but the advantage of reinforced concrete
remains large.) Given that you need a lot of rock for shielding
anyway, it makes sense to use it structurally to reduce your need
for steel.

A quick sketch of a design concept for a Stanford Torus shows 1kt
slabs of fused lunar rock, 17x13x1.6m, as the basic materials. The
compressive stress is limited to 40 N/mm^2, about 10% of available
compressive strength, at full prestress before imposition of spin
and pressure loads. Window sections could either concentrate the
cables in narrow concrete ribs serving as window frames, or perhaps
(if glass manufacturing is easy and techniques for casting thick
slabs of high-transparency glass can be developed) simply replace
some rock slabs with glass slabs (compressive strength of glass is
quite high). Prestress would be applied gradually as construction
proceeded, eliminating most need for scaffolding etc. There is no
separate shield, the pressure hull is the radiation shield.

Prestressed concrete is safer than a metal shell for several reasons:

1. Cracks are harmless, whereas in a metal shell they are of great
concern.

2. There is extensive structural redundancy, almost impossible in
metal.

3. It is difficult to arrange a metal shell to leak before it fails -
- a highly desirable property -- while this is trivial in
prestressed concrete (the rock goes into tension, cracks, and leaks,
at a point where the steel cables are still in their elastic region
and hence have considerable reserve strength left).

4. All portions can be strength-tested during construction. The
rock is at maximum stress -- 2-3 times operating load -- before
pressurization and spin-up, and since this stress is all from the
prestressing, it can be applied as necessary for testing during
construction. The cable can be 100% proof-tested, in the same load
environment it will see in operation (single-axis tension), as part
of its production (this being one reason why the permissible working
stresses of cables are so high). Comparably realistic tests are not
possible for metal shell sections.

5. Rock is known to be stable and inert in both the internal
environment and the external one, whereas steel structures suffer in
the former (unless they are continuously repainted) and are unknown
in the latter. The cables are in a protected environment of their
own within the rock.

6. Cables can be removed for inspection, testing, and replacement
while the structure is in service. Depending on design, this might
even be done from inside. There is not much that can go wrong with
the rock, and repairs to it (using ordinary concrete) are simple.

We have multi-thousand-year structural-life experience with rock,
and not with steel. "With a metal hull one could only hope for a
hundred-year life... [with concrete] we can think in terms of a
thousand-year colony."

The main need for Earthly materials for construction will be alloying
elements to turn lunar iron into steel. Not only does the concrete
design use less steel, but steel cable needs smaller percentages of
alloying elements than high-grade (crack-stopping) steel plate.

Cable is easier to make than large flawless steel plates, and
eliminates the need for large-scale welding and X-ray inspection.

The thickness and non-critical nature of the rock part of the
construction simplifies anchoring of structures to its inside,
something that would have to be done rather carefully with a thin
steel shell.

END QUOTE
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