Ferro-cement is the name given by Italian Professor Pier Luigi Nervi to a thin slab or mortar reinforced with superimposed layers of wire mesh and small diameter bars. The result is a product with a high degree of elasticity and resistance to cracking which can be cast without the use for formwork. Nervi successfully proved on may jobs the remarkable strength and lightness of this method of construction and its great adaptability to virtually every conceivable architectural shape. The end result of Nervi's experiments was a medium in which the thickness of a finished slab was only a very little greater than that of the assembled layers of mesh, the difference being only as much as was necessary to provide adequate cover for the steel. This ferro-cement was found on testing to have very little in common with normal reinforced concrete, however, since it possesses the mechanical characteristics of a completely homogeneous material. Shock resistance tests on 1 inch thick ferro-cement, made by dropping a 500 pound weight on to slabs about 5 feet square from heights up to 10 feet, revealed not only the high strength of the medium but also the interesting fact that when failure did occur it was not by the appearance of a hole but as a relatively widely dispersed area of shattered mortar. The material still remained in a cohesive state, however, and offered very good resistance to the passage of water. It was this discovery that led to the initial acceptance of ferro-cement for boat construction. The only real objection to ferro-cement today appears to lie in its high rates of shrinkage and creep when using rich mortars. In this respect research has shown that the cement-sand ratio has the predominating influence, while water/cement ratio has very little effect. Also, while a greater proportion of fine sand can improve workability, it increases shrinkage at the same time. Here, also, expanded metal offers advantages over conventional woven mesh due to the greater degree of uniformity with which the steel is distributed.