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There is a fairly new type of lightweight concrete that contains some styrofoam beads to make it lighter and can be sprayed on over the rebars to the required thickness that may be better suited to what you are doing. This would reduce the amount of bracing required.
There are also other materials that can by sprayed on to create a room that do not contain concrete but are instead made of an expanding foam. The foam can have a hard shell sprayed over it after it is cured.