Rebar

* See also Concrete_IS_Your_Friend
* See also Construction and Primary_Construction
* See also UnderWater_Construction
* See also Ryanium
need more pix of embedded rebar
 "Ferro - Concrete" https://www.bing.com/search?q=what+is+ferrocement&
Ferro refers to 'IRON' addition - structural reinforcement' (steel). 
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Used to Create Ferro-Concrete :
Metal added to Concrete to provide added Tensile strength and to prevent shear fracturing. Concrete has great Compressive strength, but is very weak for other forces. Adding Rebar to be embedded within the concrete corrects that limitation.
In Salt Water, Steel Rebar ( the most common rebar material ) has corrosion issues.
Alternate materials (and coatings) which are corrosion resistant are more expensive. For Rapture, the Rebar used in the Structural Sea Containment Shell Structures would require use of these alternate materials to have a decent lifetime.
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Steel Corroded By Salt Water :
And SORRY, NO cheap steel used ( to smear Ryan's vision ), THAT in simple form wouldn't be used as it can fail within a year with the harsh Sea environment Rapture is built in.
Impressed Current Cathodic Protection
Use of a power supply to create an opposite electrical current to defeat cathodic corrosion. ( All the Rebar is systematically wired into a circuit connected to this opposing voltage/current. )
"First trials in 1959 on bridges" - so not far off technologically for a Rapture Solution (and as technology goes, it is not difficult to implement)
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One Solution : Basalt (melted/formed rock) Rebar ... ( A real Thing - fibrous mineral component being highly corrosion resistant and a high tensile strength ).
Mass production (in surface factories) manufactured with a consistent quality could be used in the quantities required.
The manufacture of Basalt Fiber requires the melting of the crushed and washed basalt rock at about 1,500 °C ( 2,730 °F ).
Basalt IS something to possibly have on hand at a 'Volcanic' site ( Gabbros ).
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Ryanium ( from the Novel ) may not be a good Rebar material ( Hinted as being very expensive and probably is best assumed used for windows ( Novel mentions its glassy form/nature ).
It might be used for special limited use (like reinforcing windows).
Corrosion resistant metals would have many othe uses in Rapture.
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Corrosion Resistance ( and Susceptibility to Corrosion ) of many Metals which might be used for Rebar. 
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