Precast concrete channel roof
• This roofing system, developed at the National Building Research Institute, Pretoria, South Africa, is based on a precast concrete trough-shaped element, which is cast with great speed and ease, requiring very little working space.
• The cross-sectional dimensions are shown in the diagram overleaf and the length used in the project was 4.27 m, resulting in a total weight of about 107 kg (or 25 kg/m). Seven 4 mm steel bars provide reinforcement along its length, and stirrups of 3.3 mm steel are placed every 30 cm. The elements are self-supporting, and can span 3.50 m with a cantilever on either side of the walls.
• The assembly of the roof is done manually. After placing the boughs side by side, the gaps between them and the top of the walls are closed by inserting precast filler blocks and sealed around the edges. A polythene sheet is laid over the troughs, which are covered with a 20 mm layer of loose gravel, for improved thermal performance and to protect the sheet. The gravel is kept in place by precast, shaped, no-fines concrete blocks placed dry at the ends of the troughs. Rainwater that collects in the troughs percolates through the no-fines concrete and can be collected. Hence, a 5 % slope is suitable.
Further information: Jorge L. Arrigone, Senior Chief Research Officer, National Building Research Institute, P.O. Box 395, Pretoria 0001, South Africa; Bibl. 23.02.
Precasting the Trough-Shaped Units
The steel mould consists of a trough-shaped base with supporting ribs, fixed to the concrete floor, a swell as moveable parts, ie side risers and end closer plates. The inner surface of the mould is covered with a polythene sheet and pushed in place with a steel trough-shaped form. The side and end risers are bolted into position, and a fairly dry mortar mix 1: 3 (cement: coarse sand) poured end distributed evenly, 33 mm thick on the horizontal parts and 22 mm thick on the sloping sides. The reinforcing grid of 4 mm steel bars is placed on the mortar, pushed down, and the surface evened out by tapping the sides of the mould.
About an hour later, a new polythene sheet is placed over the element, pushed in place with the steel form, the side and end risers bolted down and the procedure repeated as before. Up to 10 units are cast one on top of the other, each one taking about 20 minutes to complete. On average, six roofing units are made per mould per 8-hour working day. The units are cured wet for two weeks and dry for another two weeks.
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