Metal-reinforced concrete compared to Fiber-reinforced concrete
Metal-reinforced concrete is the type of concrete most commonly used in modern structural construction. It consists of iron or mild steel reinforcing bars ("rebar"), formed into shapes and tied together using steel or iron wire, with concrete poured over the resulting reinforcement and consolidated to drive out air voids using mechanical agitation including push rods and vibration.
A mold, often made from temporary wood or permanent steel structure, surrounds the metal mesh and concrete to form the shape while the concrete sets. The shape of the mold and reinforcement are very carefully engineered and constructed to meet structural performance requirements.
While concrete without reinforcement is very strong in compression (pushing), it's very weak in tension (pulling) and torsion (twisting). The reinforcement adds strength in tension and torsion and does so in a way that's greater than the sum of the parts. The reinforcement improves the performance of the concrete, and the concrete improves the performance of the reinforcement, in a synergistic way. (As such, reinforced concrete is a composite material, as are plastic composites like fiberglass and carbon fiber composites. The two materials in the composite improve each others' properties when used together.)
A common failure mode with metal reinforced concrete is for the metal reinforcement to corrode, for example due to rainwater or seawater intrusion. Attempts to mitigate the corrosion by placing plastic coatings over the metal have not been not fully successful as water can migrate between the plastic and metal and still corrode the metal. Structures made from metal reinforced concrete have failed due to corrosion of the metal reinforcement.
One way to solve this problem is to use non-metallic reinforcement, in particular using materials that are strong in tension and highly resistant to corrosion such as plastics or mineral fibers. For example, glass, basalt and various plastic fibers have been tried. Glass and basalt are minerals and essentially don't corrode. If the reinforcement doesn't corrode, the resulting reinforced concrete structure can be far more durable. The resulting material can be called Fiber-reinforced concrete.
Wikipedia has some additional information about Fiber-reinforced concrete.
Uses for seasteading
While reinforced concrete is generally too voluminous to be used for building ships, it has been proposed to build barge-like platforms for seasteading, such as proposed by Deltasync.
Compared to Ferrocement
A related, but different technique called Ferrocement applies a relatively thin layer of cement over a metal mesh. The mesh is somewhat similar to the reinforcement used with concrete, but the amount of cement used is much thinner, essentially a thin shell over the metal mesh.