Concrete reinforcement

[DaveIrons@aol.com]

I don't have any hard data on the effects of various aggregates on concrete
strength.  That information can be obtained from the American Concrete
Institute, Detroit Michigan.  However, in my experience crushed granite
or hard limestone increase the strength of concrete.  Although some of the
studies on concrete pressure hulls have been done with washed river rock. 
By river rock I mean the rounded gravel or stone often used in concrete.  It
may come from a beach or a gravel pit, however, at one time it was rolled
around in water until the surface was smooth.  River rock seems like a
poor choice.  River rock is a mixture of rock types, some of which may
weaken the concrete mix.  The irregular shape of crushed stone seems to
enhance the bonding between the concrete and aggregate.  I would
recommend that test samples should be made and tested to destruction.  

Adding polymer (PIC) in place of water produces a 20,000 psi pressure
hull.  The depth rating increases to 4,000 feet.  The concrete used in one
study had a uniaxial compressive strength of 6,000 to 11,000 psi.  With
good quality control 9,000 psi concrete can be consistently maintained.  

Polymer-Impregnated Concrete (PIC) has a tensile strength of 1,600 psi. 
The durability of PIC is better than concrete.  It is more resistant to
chemical attack and it is impermeable to water.  
My primary interest in PIC is to beef up areas which may be subject to
increased stress or wear.  For example: around hull penetrations.  Another
application is to strengthen a cold joint.  That is a portion of the hull that
set-up before the entire hull was formed.  The joint would be left rough to
produce a better bond between the old and new concrete.  The old concrete
would be primed with polymer to increase the bond strength.  The cold
joint treated with polymer would be about twice the strength of the rest of
the hull.  Another use of PIC would be to produce a new hull penetration. 
For example to install a viewport.  Break a hole in hull.  Prime the
concrete with polymer.  Place the viewport seat in the opening and tuck-point
around the viewport with PIC.  Tuck-pointing is a technique in
which voids in concrete or masonry are filled using a thin trowel in the
above example a margin trowel would be used.  This is not a job for an
amateur.  A skilled workman can perform a repair which is stronger that
the original concrete.  

Although concrete is waterproof prolonged immersion in water saturates
the concrete and reduces the strength by about 10%.  However, concrete
has the unique property of increasing in strength for the first 100 years. 
Concrete spheres that have been immersed in seawater for years increased
significantly in compressive strength.

All of the concrete hemispheres were bonded together with epoxy.

How to increase the tensile strength of concrete
submersibles is a complex question.  The ideal hull would be of steel
sandwich construction.  Steel sandwich construction is a method in which
interior and exterior steel liners are used, with concrete in-between.  Steel
sandwich construction sounds like a good design for very deep
submersibles or habitats.  Exploring the mid-Atlantic ridge for example. 
However, I'm not planing to go any where near that deep.  It is difficult to
justify the cost of building two steel hulls for depths under 3,000 feet.

I would like to include a few design ideas for further discussion.

Placing steel reinforcement on the outside of the hull makes sense.  The
bending stress of the hull would be greatest at the outside surface of the
hull.  For example: The hull is cylindrical with hemisphere end caps. 
Steel cables are stretched from bow to stern.  The cables are pretensioned
using a 100,000 pound hydraulic bottle jack.  The cables are welded to
steel bands at the bow and stern.  Steel cables are stretched around the
circumference of the hull.  The cables are pretensioned and welded into
place.  A seacrete shell (Sea Water Electrolytic Mineral Accretion) would
be grown around the cables to bond them permanently to the hull and to
protect them from corrosion.  The method is similar to standard
pretensioned concrete construction except that the steel cables would
ordinarily be placed inside a concrete slab.

David Irons

>As you pointed out, you discovered problems that you hadn't thought
>about.  I was surprised to discover that re-bar re-enforcement actually
>WEAKENS a concrete pressure hull.  I would have assumed the
>opposite.

>Rick


>Rick Lucertini
>empiricus@sprint.ca
>(Vancouver, Canada)