Corrugated fibre concrete roofing sheets
Corrugated FC sheets
• were the first FC roofing elements to be developed, as the aim was to substitute gci and ac sheets;
• require fairly simple, locally made equipment and a very well coordinated working team of at least two workers;
• consume about the same amount of cement es asbestos cements sheets (15 kg per m2), on account of their greater thickness and production method by manual tamping, but require no electricity;
• are difficult to handle when fresh and to cure in water tanks, because of their large size;
• are difficult to transport and install without breakage, and do not tolerate inaccurately constructed and flexing supporting structures;
• withstand strong wind forces because they are heavy and have few overlaps.
In most cases FC/MC tiles are easier to produce and install than FC sheets and therefore represent the more appropriate solution.
Further information: RAS c/o SKAT, Vadianstrasse 42, CH-9000 St. Gall, Switzerland;
Production of Corrugated FC Sheets
Materials and equipment
• Cement: ordinary portland cement (Skgper 10 mm thick corrugated sheet of 60 x 60 cm) corresponding to cement: sand ratio of 1: 1; a pozzolana (eg rice husk ash) can be added to improve fibre durability and reduce cement content, but causes slow setting, which necessitates a larger number of moulds and larger workspace.
• Sand: (5 kg per sheet) preferably with angular particles and good grain size distribution between 0.06 and 2 mm, free from silt and clay.
• Fibre: (0.1 kg per sheet) mainly natural, such as sisal, jute, coir, or banana fibre, but also synthetic fibres, eg polypropelene or glass fibre, can be used. Long fibres can be used, but require a different (more difficult) manufacturing process and result in weaker products. Short fibres, chopped to lengths of 12 to 25 mm, are easy to process, provide cohesiveness to the wet mortar, permitting reshaping without cracking, and also help to prevent cracking due to drying shrinkage.
• Water: preferably drinkable water, just enough to make the mortar mix workable (water: cement ratio 0.5-0.65 by weight).
• Admixtures: such as waterproofers may be used, if the sand is not well graded, and colorants, if the grey cement colour is not desired.
• Screeding board: a flat horizontal board with outer frame, to define the FC sheet size and clamp down the polythene interface sheet.
• Corrugated setting moulds: gci or ac sheets, enough for two days production. All sheets should be obtained from a single batch made from a single master mould, as sheets from different batches or different producers are likely to have dissimilar corrugations. Accuracy in the corrugations is vital for proper installation and trouble-free performance.
• Other equipment: standard workshop tools.
Moulding and curing
• The correctly proportioned and well-mixed mortar is trowelled evenly onto the polythene sheet, which is fixed on the screeding board; the mortar is tamped, levelled to a uniform thickness of 10 mm and smoothed off with the trowel.
• The frame is removed, the edges of the mortar layer trimmed and the screeding board tilted, such that the polythene sheet with the wet fibre concrete is allowed to gradually slide onto the corrugated mould held below.
• The fresh FC sheet and mould is placed on a stack for primary curing for 24 hours, after which they are hard enough to be demoulded and placed upright for further curing (by regular watering), or completely immersed in water tanks for about 2 weeks.
• Demoulding should not be done later than 48 hours after moulding, as the sheets tend to shrink on drying, and will crack if resisted by the setting mould.
Production of FC Ridge Tiles
• Materials and equipment: same as for sheets, but different shape of frame, and screeding board made with hinges, so that it can be bent and used as the setting mould, held in a template.
• Moulding and curing: same as for sheets.
Installation of FC Roofing with Corrugated Sheets
The corrugated FC sheets are laid on timber roof structures in much the same way as gci and ac sheets. However, FC sheets are less flexible and can be damaged if the loads are not evenly distributed. Therefore, care must be taken in constructing the substructure, to ensure that the top edges of all members are properly aligned. If nails or bolts are used, holes (of slightly larger diameter) should be drilled beforehand. Alternatively, nibs with wire loops can be cast-in during moulding, avoiding the need for drilling. Mitred corners are essential for a weathertight fit.
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