Titanic tourist sub goes missing sparking search

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Blasto:
A lot has changed since then. In the 1960s, state of the art concrete was 6-10 ksi, and still included coarse aggregate and steel rebars.
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As I recall, these experimental spheres were cement and a very fine aggregate, no rebar or rock. They were about 48" in diameter with wall thickness in the 2-3" range. I think they epoxy impregnated them due to water intrusion.
 
So t
Akimbo:
As I recall, these experimental spheres were cement and a very fine aggregate, no rebar or rock. They were about 48" in diameter with wall thickness in the 2-3" range. I think they epoxy impregnated them due to water intrusion.
Click to expand...
So they were working on the same things then, figures that lab experiments are always ahead of actual use.

One major change was the addition of superplasticizers, which allowed for ultra-dry mixes, impossible to flow into shape otherwise. New generations of UHPC/RPC contain partially-reacted cement, and when water gets into any microcracks, the cement reacts and reseals them. So it's best left unsealed, which helps for underwater environments.
 
Blasto:
So t

So they were working on the same things then, figures that lab experiments are always ahead of actual use.

One major change was the addition of superplasticizers, which allowed for ultra-dry mixes, impossible to flow into shape otherwise. New generations of UHPC/RPC contain partially-reacted cement, and when water gets into any microcracks, the cement reacts and reseals them. So it's best left unsealed, which helps for underwater environments.
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I'm just thinking of all sorts of new ways that things can go wrong...
Let me try and put thoughts into words. Microscopic air passage (microcrack as you refer to them). Add pressurized water. The air compresses to a dead end at the end of the passage. The water finally hits a bit of cement mix and it seals the passage. Surface, lose the outside pressure. Now there is a pocket of highly compressed air sealed inside concrete. Now the concrete around that pocket is under severe tensile stress as the air is pushing out. Concrete isn't the greatest at tensile. That's not the end of it either. Salt water now trapped in that passage. A very slow drying out as the water eventually leaves but the salt crystals stay behind.
I'm also thinking of 25 years ago when the designer concrete had specs of surface spalling, which was created by sprinkling rock salt on curing concrete. So I remember that salt is not good for making regular concrete cure well, not sure how that plays into the special mix.
 
broncobowsher:
I'm just thinking of all sorts of new ways that things can go wrong...
Let me try and put thoughts into words. Microscopic air passage (microcrack as you refer to them). Add pressurized water. The air compresses to a dead end at the end of the passage. The water finally hits a bit of cement mix and it seals the passage. Surface, lose the outside pressure. Now there is a pocket of highly compressed air sealed inside concrete. Now the concrete around that pocket is under severe tensile stress as the air is pushing out. Concrete isn't the greatest at tensile. That's not the end of it either. Salt water now trapped in that passage. A very slow drying out as the water eventually leaves but the salt crystals stay behind.
I'm also thinking of 25 years ago when the designer concrete had specs of surface spalling, which was created by sprinkling rock salt on curing concrete. So I remember that salt is not good for making regular concrete cure well, not sure how that plays into the special mix.
Click to expand...
I was more thinking that 1) concrete is not known as being a ductile material, so how it responds to repeated pressure cycles is critical, and 2) you will have penetrations for hatches, viewports, electrical connections, etc. that will require interfacing with different materials and introduce similar concerns about how those interfaces change under pressure.
 
 
broncobowsher:
I'm just thinking of all sorts of new ways that things can go wrong...
Let me try and put thoughts into words. Microscopic air passage (microcrack as you refer to them). Add pressurized water. The air compresses to a dead end at the end of the passage. The water finally hits a bit of cement mix and it seals the passage. Surface, lose the outside pressure. Now there is a pocket of highly compressed air sealed inside concrete. Now the concrete around that pocket is under severe tensile stress as the air is pushing out. Concrete isn't the greatest at tensile. That's not the end of it either. Salt water now trapped in that passage. A very slow drying out as the water eventually leaves but the salt crystals stay behind.
Click to expand...
Indeed, that is one of the scenarios that hasn't been well-tested through offshore applications.

So fibers are still needed, even if the load is all-compressive, to keep cracks from growing above the size that can be bridged by the chemical reaction. If the amount of trapped air and salt is small enough, it just stays as minor imperfections. There's also a number of special measures, such as curing in steam or under pressure, that reduce crack growth; not home build tech of course.

Not saying it's a miracle material. But as far as new underwater material research goes, it appears to be a more promising direction than epoxy composites. You don't have to get the lightest weight possible, just light enough to be neutrally buoyant with ballast and hardware. From there on, it's about reliability and long-term behavior, where concrete has a better track record than CFRP.

Penetrations are a definite problem for both, they have to be pre-planned on any composite hull. CFRP gets damaged beyond the hole area if drilled, UHPC requires embedding the metal parts before curing. But there's a lot of experience from submarines and offshore to count on. OceanGate might have saved on expertise in there...
 
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