A culvert balloon is an inflatable rubber/PVC form used to create circular culverts, drainage pipes, tunnels, and concrete pipe structures without using permanent rigid formwork.
Typical specifications:
Diameter: e.g. 900 mm
Length: e.g. 12 metres
Material: Heavy-duty reinforced rubber/PVC
Inflation: Air pump/compressor
Application: Reinforced concrete culverts and drainage structures
Reusable: Yes, when properly handled and maintained
For your 900 mm × 12 m culvert balloon, it can be used as an internal former