on either side and plastering is done onto them from both sides. As the name suggests, the mould remains permanently as an integral part of the finished structure. (e.g. double T-sections for flooring, roofing, etc.) Precaution should be taken to have firm connection between the mould and the layers filled in later, so that finished product as a whole integral structural unit. Advantages The advantages of a well built ferro concrete construction are the low weight, maintenance costs, and long lifetime in comparison with purely steel constructions. However, meticulous building precision is considered crucial, especially with respect to the cementitious composition and the way in which it is applied in and on the framework, and how or if the framework has been treated to resist corrosion. When a ferro concrete sheet is mechanically overloaded, it will tend to fold instead of break or crumble like stone or pottery. As a container, it may fail and leak but possibly hold together. Much depends on the techniques used in the construction. Using the example of the Mulberry Harbours, pre-fabricated units could be made for ports (such as Jamestown on St Helena) where conventional civil engineering is difficult. Disadvantages The disadvantage of ferro concrete construction is the labor-intensive nature of it, which makes it expensive for industrial application in the western world. In addition, threats to degradation (rust) of the steel components is a possibility if air voids are left in the original construction, due to too dry a mixture of the concrete being applied, or not forcing the air out of the structure while it is in its wet stage of construction, through vibration, pressurized spraying techniques, or other means. These air voids can turn to pools of water as the cured material absorbs moisture. If the voids occur where there is untreated steel, the steel will rust and expand, causing the system to fail. In modern practice, the advent of liquid acrylic additives and other advances to the grout mixture create slower moisture absorption over the older formulas, and also increase bonding strength to mitigate these failures. Restoration steps should include treatment to the steel to arrest rust, using practices for treating old steel common in auto body repair. Insurance issues During the 1960s in Australia, New Zealand and the UK, home boatbuilders realised that, for a given budget, ferrocement enabled a much larger hull than otherwise possible. However, some builders