Concrete Warships vs Steel Warships

Question

Humanity in my world has to deal with the invasion of inter-dimensional beings (Their tech level is around WW2 and in certain departments they're more advanced than we were around that period and in others a bit less).

Now things aren't going so swell. The invaders come in waves that are seemingly random, fortunately it seems to be random for the invaders as well meaning that whatever they're using to enter the world is not something they fully control either. (small portals and the occasional big portals appear often enough that they can supply their forces, but not the kind of mass portal spam you need for a massive armada to come through in a timely matter.)

Since my world has way more islands than ours the naval theater is quite important as most of the fighting is similar to what went on in the Pacific theater.

Anyway this whole thing has been going on for a while now and in certain places humanity isn't doing too good anymore. Child soldiers, encouraging the 'burdens' on society (cripples, chronically ill) to enlist in suicide units,... Dark stuff al around. The invaders also go on the extermination route so bloody are literally to the last man on both sides, which make battles extremely bloody affairs.

However supplies are important too and on certain places it is kind of getting difficult to get enough steel to make proper full steel warships, so those places are trying to build warships made out of concrete to compensate.

They've got experience with building concrete ships as many, many auxiliary vessels are already concrete ships and some shitty second line 'warships' nobody actually expects to do much of anything should they encounter an enemy. They're mostly just extra heavily armed merchantman running on coal or something else (gasification) as oil too is getting too expensive to waste on non-combat ships.

Anyway, my question is since as far as I know no actual warships were build out of concrete IRL and concrete is quite a different material than steel. What are the things one should keep in mind when building warships (ranging from destroyers to battleships and aircraft carriers) out of them?

Will they not be able to mount the same kind of caliber guns say the Nagato, Iowa and Yamato battleships could or would the material make them unusable for aircraft carriers? Would they be more durable or less in case of an attack?

In other words how does a ferrocrete warship hold up against a conventional steel one?

Like I said people of this world are in certain areas more advanced (in certain areas) then we were during the 40's, 50's and 60's though of course due to the nature of the war it's not always easy to swap supply lines to actually mass produce the new stuff.

Answer 1

Preliminaries

Concrete is a good material under compression stress, but lousy shearing stresses and abysmal in tension (elongation stresses) and bending - try to bend a concrete fence post, it will simply crack. To compensate, one:

1. reinforces the concrete with rebar - the usual way to make the tall vertical buildings resist lateral stresses cause by high winds (bending) or earthquakes (bending and shearing). For the vertical structures, this is enough, since the gravity compresses them already
2. precompresses the beams - by running some cables inside the beam (they call them "tendons") and using some plates and screws at the end to squeeze the beam between its ends and "pack" it solid. Usually, this is used for horizontal structures (like the runway for the planes to takeoff or land).

Compressing the beams will significantly increase their ability to withstand non-compressive stresses but does run at a price: you simply can't run tension with a curved tenon, so you won't see curved structures set under tension (can be done, but the geometry of tenons placement and differential tensions between tenons running in different directions makes it an engineering nightmare)

Main

The above provide the explanation for why building ships from concrete instead of steel plates will result in huge thicknesses for the hull walls in comparison with a steel hull: one need to have enough concrete mass to squeeze in compression without the risk of the concrete losing cohesion and crumbling. I'd not be surprised if instead of having 1" steel sheet, one would need 1-2 feet of concrete for the hull only.

Huge thickness of hull walls translate in a huge mass for the ship. Which further translates in:

1. low speed - you won't have corvettes and frigates built from concrete
2. very poor maneuverability - trying to change the direction of a huge mass in motion is an exercise to remind one that mass is a measure of inertia. If you need a maneuverable capital ship, you don't build if from concrete.
3. the low speed also translates into impossibility on using mixed fleets - a convoy will always travel at the speed of the slowest ship.
4. huge mass means also huge displacements to ensure enough buoyancy so they can stay afloat. Don't use them in shallow waters. (sea depths? port depths? how do you resupply them?)

I'm sure someone can think of many other disadvantages tactical disadvantages of concrete ships.

The only point at which concrete floating structure would "excel" - floating islands. Platforms that provide support for presence where there are no natural islands. But since your world already "has way more islands than ours", artificial floating islands are a waste, serve no purpose.

What you can do in "way more islands" situation:

1. use lots of small cheap ships with "composite hulls". You know? Like the pleasure boats with hulls made of fiberglass mats and fabric impregnated with phenolic or polyester resin. Not a platform for heavy fighting, but invaluable at logistics and mine placement (fortress defense)- fast, maneuverable, working well in shallow waters, can be used for fast army displacements where needed, etc.
   Since you have "way more islands", it means they'll not likely need to be far away from a port.

2. use the "way more islands" as platforms for long range artillery and airports - if they are so many of them, you shouldn't have troubles with coverage. May I suggest missiles instead of ballistic projectiles? I mean, you can reposition them faster than large bore cannons.

Now that I think of it, with enough range for those missiles and "way more islands", I can't see the reason for running steel hulled large capital ships. Heaps of cheap "fabric and plastic boats" carrying 3-4 missiles each makes a pretty good strategy, they make a dispersed target.