• Ferrocement components are extremely thin (15 to 25 mm), but have a higher percentage of reinforcement than reinforced concrete, thus achieving a higher tensile-strength-to-weight ratio. Further strength and rigidity is achieved by curvature or folds.
• Ferrocement roofs can be made in situ or with precast components, the former being useful for free forms, the latter being appropriate for modular and repetitive constructions.
• Depending on the design, ferrocement roofs can be made to span large areas without supporting structures, thus saving costs and providing unobstructed covered areas. If the ferrocement surface is properly executed (complete cover of wire mesh, dense and smooth finish, cracks sealed) no surface protection is needed, thus saving further costs. However, it is advantageous to apply a reflective coat on the outer surface to reduce solar heat absorption.
Further information: Bibl. 10.02, 10.03, 10.04, 23.01, 23.13, 23.22.
Framed Ferrocement Roof (Bibl. 23.01)
• Once the walls are erected, no reinforced concrete ring beam is required, as the roof is designed to clamp the walls together.
• Around the top, outer edge of the walls, a timber frame (6 x 6 cm) is fixed, as well as two tripod frames above the floor area. The surfaces described by these frames are hyperbolic paraboloids (hypars), which are made up of straight lines. This simplifies the fixing of the wire mesh.
• The mesh (2 or 3 layers) is stretched over the frame and nailed or stapled onto it. The frame is only needed to hold the mesh during construction, as the structure will be self-supporting once plastered.
• Reinforcing bars are fixed around the wall and along the folds of the roof.
• The roof is plastered by a team on top forcing the mortar through the mesh, while another team below recovers the falling mortar to plaster the inside.
• This curved roof system, developed by P. Ambacher, France, permits the wind to blow around smoothly, making it very suitable for hurricane prone areas.
Precast Trough Element (Bibl. 23.22)
• These elements function on the principle that folded plates have much higher strength than plates of the same thickness but without folds.
• The roofing element shown here, developed at the Structural Engineering Research Centre, Roorkee, is made either on a stationary brick-and-concrete mould or on a portable wooden mould, and can be in the form of a trough or inverted.
• A reinforcement cage is prepared on the mould.
• Before placing the mortar, a thin coat of rich cement slurry is applied to the reinforcement cage with a brush. The mortar is then applied and pressed into the reinforcement. This is done in 2 or 3 layers. A specially designed vibrator, operated by two men, compacts the mortar.
• The finished element is moist cured for one week, before it is removed from the mould. The lower side is finished with a coat of cement slurry and cured for at least another week, before handling and installation.
Precast Segmental Element (Bibl. 23.13)
• The alternative to trough elements, shown on the previous page, is a segmental element, made principally in the same way.
• The segmental element shown here was developed at the Regional Research Laboratory, Jorhat, India.
• The element is 60 cm wide, 250 cm long and 2 cm thick. The reinforcement in each element consists of 5 bars of 6 mm 0 in the longitudinal direction and 10 bars of the same diameter in the transverse direction, with two layers of hexagonal chicken wire mesh. The mortar comprised 1 part cement: 2 parts sand by weight.
• Long-term performance tests have shown very satisfactory results.
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