|
SHELTER
NEWS: FERRO-CEMENT The ferro-cement concept was developed by Italian engineer Pier Nervi; he combined the compressional strength of concrete with the tensional capabilities of steel. The result of this union was an extremely strong and durable monolithic structural system. Its most popular use to date has been in the boating world. Two New Zealanders, G.W. Jackson and W.M. Sutherland have been particularly active in research and experimentation in this field; their book "Concrete Boatbuilding" is an excellent reference for anyone seriously thinking of building in ferro-cement. In the boating scene there are two distinct opinions regarding ferro-cement boats. One, that boats of this construction are extremely strong, very seaworthy and a joy to own; the other, the direct opposite. From discussion with both groups it appears that quality of construction is the key. Investigations into reasons for ferro-cement failure generally indicate breakdown of the structural system, caused by the reinforcement part of the system rusting away, this rusting is usually caused by wire ends left protruding from the finished skin; in short, bad workmanship. This problem of rusting is, of course, a far greater threat to ferro-cement when it is partly immersed in salt water, and thus fighting for its survival in a dynamic environment, than when it is away from the powerful rusting action of salt water, on land in a relatively static environment. If we build a ferro-cement structure on land to the same standards as a boat it will be greatly over designed thereby increasing costs and construction time. To arrive at safe minimum design standards for ferro-cement houses further experimentation and testing is badly needed. Once these standards are established, conventional thinking and tedious hard work are the only barriers stopping us from building low cost, free-form shelters. It is not my purpose to give detailed construction methods. Simply stated, ferro-cement is a system of steel reinforcing bars at 2'-0" to 3'-0" centres, crossed by smaller rods at 3'-0" centres, and covered with several layers of chicken wire or expanded metal. This is then tied together with wire, and the monolithic ghost-like shell plastered with a cement/sand mixture to an overall skin thickness of 1" maximum, any thickness over 1" is not only wasteful but burdens the structure with tons of extra mortar. This is particularly bad when the mortar is green (wet), for at this stage the structure is still two separate systems, it takes approximately 28 days (recommended curing time for concrete) before the structure achieves unification. In calculating materials needed, allow approximately 15lbs of mix per square foot, the shell needs 10lbs per square foot the other 5lbs ends up on your face or the ground. The type of reinforcing grid used depends on the form the structure will take, if you wish to build a 28'-0" diameter dome (hemisphere) you could have the main bars on 30" centres (at the base) forming great circle meridians (same as longitudinal lines around the earth) jointed into a hub at the pole of the dome, the secondary rods (on approximately 36" centres) starting at the base rising like lines of latitude until they reach the pole. This frame then needs covering with 1/2" chicken wire, in boats they use 8 layers (4 each side of the frame), for a dome of this size I am going to try 4 layers (2 each side). The structure can be supported on a conventional reinforced concrete footing around the base perimeter, 1/2" starter bars are left protruding from the footing, these are tied to the great circle bars hence providing a tie between footing and shell. A reinforced concrete floor could be poured with footing or timber floor built later, depending on personal preference. The design of your shelter is limited only by your imagination.. Ferro-cement is a pure free-form structural material and as such, can be shaped into complex double curved surfaces, large open spans, single hemispheres or groups of domes on different levels. One solution to the problem of using conventional windows and doors in a curved surface is to form dormers as an integral part of the structure. The top of the dormer can extend past the window line, giving protection from the elements. Internally dormer windows create an interesting interplay of light and shade. Internal insulation, if you think it necessary (dependent on your climate and so on) could be provided by spraying or trowelling the internal face with vermiculite or liquid insulation. The method I prefer is to weld bolts to the main bars and leave them protruding on the inside of the skin, 2" x 1" timber battens are secured to these and then natural timber finish plywood attached. This system provides; (a) the aesthetics of a timber finish and (b) a cavity which could be filled with insulation and/or used for electrical and plumbing services. Externally ferro-cement may be left natural or coated with an epoxy type paint. Boats generally require two coats, although this is not necessary for water-proofing. I think the most satisfactory finish would be obtained by adding an oxide colorant to the mortar of a similar hue to the surrounding landscape, then growing vines or creepers over the entire dome. This not only results in an almost invisible structure complementing nature, but the foliage keeps the concrete cool thus preventing hair-line cracks which may occur in extreme climates. In comparison
with conventional structures, a ferro-cement dome has many advantages: The comparison of ferro-cement with other alternative building methods is really a matter for personal judgement. There is a good supply of literature for other alternatives such as mud bricks, geodesic domes, log cabins, stone (rock) buildings etc. It goes without saying that all materials used in alternative structures should be "ecologically desirable". The
following points bear consideration when planning to build that illusive
dream home:
|
|
| essays | |