Ferrocement

From Freepedia

Ferrocement is both a method and material used in building or sculpture with cement, sand, water and wire or mesh material - often called thin shell. Thin shell ferrocement offers strength and economy along with broad application to include home building, creating sculptures, or building boats and ships. The building system was originated by the Italian architect Dr. Dante Bini (see www.itsa.info/bini.shtml) to construct 'blow-up' shells and it has been extrapolated by others into hyperbolic catenary structures that derive their rigitity from their complex shape. In the yacht world, many designers, mostly in the 1970's, adapted their designs to the then very popular backyard building scheme of building a boat in the ferrocement medium. Its big attraction was that, for a minimum outlay and a reasonable application of skill, an amateur could construct a fair (no bumps) strong and substantial yacht hull. The basic idea required some basic form-work to define the hull shape (achieved by a set of curved steel pipes and wooden battens) on which was laid a sandwich of fine wire mesh ('chicken wire'), steel rods, and more wire mesh. The sandwich was tied together with wire ties. Once this structure was complete the next part of the process was to procure sand and cement (sometimes with an admixture added)with which to plaster the construction. The sand required to be graded so that the sand particles would fill all voids in the matrix, and then washed to remove all clay or earth particles. Mixing and plastering was normally achieved all in one day - for a 13m yacht one would need about 30 good friends (all needing refreshment at the end of the day!) The plaster mix was dry - too much water weakened the result - and was pushed by hand through the wire sandwich taking care to not leave any air voids. A mechanical vibrator was handy for persuading the mortar to move into difficult places. Long battens moved by hand smoothed off the result after a few hours to achieve a fair hull, with care taken to make sure there was sufficient depth of mortar covering the outside layer of wire. Then the hull required curing by keeping it damp for 30 days. If built upside down the next steps were to turn the hull over with a crane and then complete the deck structure in a similar way. When all was cured, paint or fibreglass finished the job. The supporting formwork was then stripped out and the fitout (and the spending of real money!) commenced.