Ferrocrete (Ferrocement) is mentioned regarding certain Infinity Patrol constructions and in a lot of Star War material. According to wikipedia "is a system of construction using reinforced mortar or plaster (lime or cement, sand, and water) applied over an "armature" of metal mesh, woven, expanded metal, or metal-fibers, and closely spaced thin steel rods such as rebar" with the term dating to the 1840s putting it late TL5 or early TL6.
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