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close this bookAppropriate Building Materials: a Catalogue of Potential Solutions (SKAT; 1988; 430 pages)
View the documentPreface
Open this folder and view contentsIntroduction
Open this folder and view contentsFundamental information on building materials
Open this folder and view contentsFundamental information on building elements
Open this folder and view contentsFundamental information on protective measures
Open this folder and view contentsExamples of foundation materials
Open this folder and view contentsExamples of floor materials
Open this folder and view contentsExamples of wall materials
close this folderExamples of roof materials
View the documentEarth reel roofs
View the documentSoil brick roof
View the documentClay tile roofs
View the documentGypsum-sisal conoid
View the documentPrecast concrete channel roof
View the documentFerrocement roofs
View the documentCorrugated fibre concrete roofing sheets
View the documentFibre and micro concrete tiles
View the documentDurable thatch with stiff-stem grasses
View the documentBamboo roof structure
View the documentPole timber roof structures
View the documentBamboo and wood shingles
View the documentCorrugated metal sheet roofiing
Open this folder and view contentsExamples of building systems
Open this folder and view contentsAnnexes
 

Precast concrete channel roof

KEYWORDS:

Special properties

High production rate, minimum formwork and space

Economical aspects

Medium to high costs

Stability

Very good

Skills required

Average construction skills

Equipment required

Special steel moulds

Resistance to earthquake

Good

Resistance to hurricane

Good

Resistance to rain

Good

Resistance to insects

Very good

Climatic suitability

All climates

Stage of experience

Experimental

SHORT DESCRIPTION:

• 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.


FIGURE

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