Heavy gauge steel tubes (or fabricated sections) can be used as the permanent formwork for columns. The steel and concrete act compositely. In this application the steel section tends to be the main load carrying member with the concrete adding stability and providing some additional load carrying capacity. The fact that the tubing acts as permanent formwork is a secondary consideration.
Light gauge tubes and hollow sections are used as void formers in thick sections, such as bridge decks. In this situation the formwork is not assumed to act compositely with the concrete and hence does not contribute to the strength of the structure. A specialised form of tubing is that used to form the ducts for post-tensioning tendons (see also Prestressing/Accessories/Duct liners).