Re: Concrete permeability

[DaveIrons@aol.com]

Rick Lucertini said:

>     A few thoughts on concrete....   It may be the perfect material
>for a habitat, but would it's lower impact resistance make it
>suitable for a submarine?  

The use of concrete for habitats has been well established.  The use
of concrete in submarines is purely theoretical.  However, it is
based on well established engineering principles.  It is premature to
recommend concrete as a submarine pressure hull material. 
However, the advantages of concrete as a pressure hull material are
compelling.  The subject warrants more discussion and research.

>If I walk up to a steel hull and whack it with a hammer, I can be
>pretty confident it's not going to crack--how well would these
>concrete hulls stand up to a blow?

I have broken up, drilled, hammered, broken up and pounded on
many concrete structures.  If concrete simply cracked when it is hit
with a hammer my job would have been much easier.
Not all concrete is created equal.  There is the concrete slab that
cracked the day after it was poured and there are concrete bridges
built by the Romans two thousand years ago.  Those bridges are
not only intact today, they handle ten ton trucks on a regular basis.

During WW II  concrete boats were built as supply ships.  Since
that time concrete has been used for offshore oil storage facilities,
barges and some yachts.  

I found a book recently "Tanker Spills Prevention by design, 
National Research Council"  Concrete reinforced hulls were
considered as a method of improving impact resistance.  The
positive argument was that: 
A concrete reinforced hull "Raises the severity of bottom impact
that can occur without incurring oil outflow".
Barges could be built economically with less skilled labor than for
steel construction.
 
The negative argument was that "Unique torsion and tension
properties of concrete do not match those required to manage ship
structural dynamics".  
The authors were also concerned that the pubic would not accept
concrete as a hull material.  The reaction of the general public is
probably the reason that concrete has not been widely used as a
material for shipbuilding or submarine building.  The reaction of
most people is, go out to sea in a cement boat, are you crazy!

Has anyone heard of Simon Lake?  Simon Lake competed with
John Holland for the Navy submarine contract.  Simon Lake made
one fatal mistake.  He put wheels on his submarine.  I can imagine
the Navy brass saying, the *#^** thing has wheels on it!  We'd
look like fools. Lakes design was safer and more reliable than
Hollands.  Years later people are putting wheels on submarines and
they work well.  (SUBMARINE The Autobiography of Simon
Lake).  I guess the moral is you may have a better idea, but if
people don't buy it you'll never get anywhere with it.  

Concrete submarine hulls would seem to have better resistance to
impact and the ability to avoid storms by diving under the wind
and waves.  The boat would be more resistant to salt water,
seamless, and it would cost less to build.  There is a design for a
submarine tanker at:

http://www.toyota.co.jp/University/Symposium/Lobby/index-
e.html  The title of the link is: Superconductive Submarine Tanker

>Hull penetrations seem like a real nuisance to design (since it's
>going to be harder to get a seal against concrete than steel.)   The
>idea of concrete hulls is intriguing, but I'm a little paranoid about
>how unproved the technology seems to be.  Is the cost savings
>really that compelling?

Hull penetrations are straight forward.  The taper on the
penetration flange is identical to the taper in the hull.  The angle of
the taper would ideally be the angle directly in line with the center
of the pressure hull.  However, as long as there is some taper the
angle is not considered critical.  An O ring is installed in a lip
around the penetration flange.  All penetrations are designed in
standard sizes.  This method has been tested and can be considered
a standard construction practice for habitats.  

For a large habitat the cost savings are very compelling.  For a
personal mini sub built with salvaged parts the savings may not be
as compelling.


David Irons