Re: Concrete permeability

[DaveIrons@aol.com]

In theory concrete with the optimum amount of water added to the
concrete mix would make the would make the hull impenetrable to
water.  In practice the optimum amount of water creates a mix that
is too dry to be workable.  Even the highest quality concrete
contains microscopic voids.  Concrete, immersed in water,
becomes saturated.  However, even with medium quality concrete
no water penetrates into the interior.  

Water penetrating the hull and flooding the interior is not a
problem.  In the Seacon experiment a full sized habitat was placed
at 600 feet for 314 days.  The only water found in the hull was a
tiny trickle due to a hull penetrator.

As water saturates the hull, a submersible or habitat can change
buoyancy.  High quality concrete absorbs about 1% water.  In a
large underwater structure that could amount to tons of buoyancy.  

As pressure increases concrete absorbs more water.  Therefore,
apparently the mircopores in concrete never become completely
saturated.  The microprobes are more correctly called gel pores.  
Some of the tests have been continued as long as eleven years.

Hulls that have been coated with epoxy do not absorb water.  

David Irons

Thanks for the input.  At this point I would like to examine every
possible problem before I invest too much time in this project.

In a message dated 3/14/99 4:06:00 PM EST, jonnie@chronic.lpl.arizona.edu
writes:

>About concrete hulls, I seem to recall that permeability (as the
material is somewhat porous from a geological point of view) was
also a limiting factor, and that the concrete hull could fill with
water through pores before crushing.  Because pore pressue can
equal hydrostatic pressue, this could be a rather important
consideration. Treatments are often applied to concrete to reduce
porosity and permeability, but do they continue to be as effective at
high hydrostatic pressures?

							John

>John Brownlee
Lunar and Planetary Lab
University of Arizona
jonnie @ lpl . arizona . edu


>Re: Concrete permeability

>Contrary to some popular belief concrete is one-hundred percent
waterproof.  Epoxy or acrylic sealants applied to the high pressure
side of a concrete pressure hull have excellent adhesion.

>David Irons