Every layer is chosen to bond to the next so the finished sphere behaves as one body: the balloon that shapes it, the shotcrete that is it, the fibre that reinforces it, and the aircrete that lines it. For a head-to-head on the hull material itself - concrete vs steel, aluminium, composites and titanium - see 06 Hull materials.
| Option | Why | Watch-outs | Fit |
|---|---|---|---|
| PVC-coated polyester | Proven Monolithic-Dome air-form fabric; weldable seams, holds low pressure for weeks | UV degrades if left bare; single-digit-kPa working pressure | primary |
| TPU-coated nylon | Tougher, more abrasion + tear resistant, reusable more times | Costlier; seam welding needs control | reusable |
| EPDM / reinforced rubber | Cheap, very airtight | Heavy, hard to seam into a true sphere | backup |
| Component | Role | Target |
|---|---|---|
| Sulfate-resisting / marine Portland (or GGBS blend) | Binder, seawater durability | 380-420 kg/m³ |
| GGBS or fly ash replacement | Lower heat of hydration, denser matrix, chloride resistance | 30-50% of binder |
| Water / binder ratio | Low w/b = low permeability (keeps chloride + water out) | 0.36-0.40 |
| Silica fume | Densifies, cuts permeability, boosts shotcrete cohesion | 5-8% |
| Superplasticiser (PCE) | Workability at low water | to spec |
| Set retarder | Keeps successive lifts "live" so the shell cures monolithic | to spec |
| Corrosion inhibitor + hydrophobic admix | Protects any steel; reduces wicking | to spec |
| Fibre | Strength / stiffness | In seawater concrete | Where to use it |
|---|---|---|---|
| AR-glass (alkali-resistant) | Good tensile, low cost | Must be AR grade - plain E-glass is eaten by cement alkali | Primary shell mesh |
| Basalt | Higher than AR-glass, excellent | Superb chloride + alkali resistance, non-corroding | Primary mesh (marine first choice) |
| Carbon | Highest strength + stiffness; electrically + thermally conductive | Inert, non-corroding | High-stress zones + the thermal patch (see 07) |
| Steel rebar (epoxy/GFRP-clad) | Cheap, ductile | Corrodes unless well-covered or clad; avoid bare in saltwater | Only if clad; prefer non-metallic |
A poured aircrete (foamed concrete, ~400 kg/m³) liner bonded to the still-green structural shell gives a low-conductivity insulating skin and a fair interior surface, curing into the same body. It is buoyant, non-combustible, and easy to shape into built-in furniture. Keep it out of the keel thermal patch, where you want conductivity, not insulation.