Ferrocement

Topics: Concrete, Steel, Construction Pages: 7 (1795 words) Published: August 23, 2013
FERRO CEMENT
Introduction
The term ferrocement is most commonly applied to a mixture of Portland cement and sand applied over layers of woven or expanded steel mesh and closely spaced small-diameter steel rods rebar. It can be used to form relatively thin, compound curved sheets to make hulls for boats, shell roofs, water tanks, etc. Ferro concrete has relatively good strength and resistance to impact. When used in house construction in developing countries, it can provide better resistance to fire, earthquake, and corrosion than traditional materials, such as wood, adobe and stone masonry. It has been popular in developed countries for yacht building because the technique can be learned relatively quickly, allowing people to cut costs by supplying their own labor.

What is ferrocement?
Standard construction cement, usually mixed with plaster sand. The standard cement is reinforced with more steel or fiber at a closer spacing than traditional construction. Reduced spacing yields uniform force dispersion and increases strength. Are special tools required?

It is necessary to be able to cut steel and rubber gloves are necessary to protect skin from cement. A filter to avoid breathing cement dust is also a good idea. Otherwise, ferrocement is a labor intensive material which does not require pumps, compressors or even a mechanical mixer; cement and sand may be mixed on the ground with shovels. [See Appendix A1.1] What are the mixing proportions?

The following proportions are dry measure. Three sand to one cement, written as 3:1 (three to one). 2.5:1 is very rich, above 3:1 is less [See Appendix A 1.2] Why is it called ferrocement if it is standard cement?

Because of the use of much steel rather than much concrete. Ferrocement is sometimes referred to as thin-shell concrete. It is also called ferrocement because the rich mixture makes it hard, like iron. Fibers such as hemp, jute, flax and cotton can be substituted for steel and sand Fiber spacing is very small and the overall strength increase is significant.

How much time is required for it to dry?
Concrete does not dry; it becomes strong chemically, in 28 moist days. Four to six days seems to be a sufficient cure for the acrylic mixed with cement projects described in this website. If concrete dries before these periods it will not reach maximum strength.

Manufacture/construction
The desired shape may be built from a multi-layered construction of mesh, supported by an armature, or grid, built with rebar and tied with wire. For optimum performance, steel should be rust-treated, galvanized or stainless steel. Over this finished framework, an appropriate mixture (mortar) of Portland cement, sand and water and/or admixtures is applied to penetrate the mesh. During hardening, the assembly may be kept moist, to ensure that the concrete is able to set and harden slowly and to avoid developing cracks that can weaken the system. Steps should be taken to avoid trapped air in the internal structure during the wet stage of construction as this can also create cracks that will form as it dries. Trapped air will leave voids that allow water to collect and degrade or rust the steel. Modern practice often includes spraying the mixture at pressure or some other method of driving out trapped air. Older structures that have failed offer clues to better practices. In addition to eliminating air where it contacts steel, modern concrete additives may include acrylic liquid "admixtures" to slow moisture absorption and increase shock resistance to the hardened product or to alter curing rates. Chopped glass or poly fiber can be added to reduce crack development in the outer skin. Chopped fiber could inhibit good penetration of the grout to steel mesh constructions. This should be taken into consideration and mitigated, or limited to use on outer subsequent layers. Chopped fibers may also alter or limit some wet sculpting techniques. FERROCEMENT OVER OTHER MATERIALS

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