|Journal of the Network of African Countries on Local Building Materials and Technologies - Volume 1, Number 3 (HABITAT, 1991, 46 p.)|
* Concrete roof tiles and roof-tile machines, by ECO System Ltd., Blantyre, Malawi.
Since 1986, the company has been producing concrete roof tiles and roof-tile machines. The original tiles and machines were made according to designs made by the Malawi Government's Rural Housing Project staff. As a result of one year of intensive research work, a new, reliable and durable machine, and tiles with a considerably improved profile and accuracy are being offered.
A. Roof tiles
Tiles are designed in such a way that they can be produced out of mortar or concrete without using fibres. A typical composition for a cheap tile is a mortar mix of 1 part cement to 2.5 parts river sand. For a high-quality tile, a mix of 1 part cement to 1 part quarry dust (or fine sharp sand) and 2.5 parts quarry stone of 3 to 4 mm (or similar small pebble stones) is recommended.
The new profile provides a double-touching large overlap. Top and bottom radii are not the same; therefore, the tiles close perfectly (see profile sketches). Two different tile profiles are available - a standard profile and an improved profile. The standard tiles have a constant thickness: the improved ones are 6.5 mm thick at the ridge and 9mm at the troughs. The improved tiles have high strength and fit tightly. The covering width is reduced to 240mm, and the covering length is 530mm. The real tile size is 285 × 600mm. One square metre requires 7.86 tiles. Standard tiles weigh 2.3 kg or 19 kg/m2, and the improved tiles weigh 3 kg or 24 kg/m2.
3. Roof pitch
The recommended roof pitch still stands at 26.5° (1:2), but this is subject to further research. Rain tests carried out indicate that pitches might be lowered. It is expected that sound recommendations can be provided in this regard in the near future.
4. Further research
The company is working on the design of a new ridge tile, which should fit the tiles accurately, as well as the design of special tiles for the eaves. Further research will be mainly concentrated on quality improvement, with specific focus on extensive rain tests with simulated storm conditions.
Figure 1. Old and new tile profile (Dimensions exaggerated to show differences more clearly)
Figure 2. Typical roof construction for ECO beton tiles
Figure 3. Concrete rooftile machine for improved tile
Figure 4. Concrete rooftile combination machine
B. Moulding machines
1. Profile improvements
The new profiles described above made it necessary to redesign the moulding frame and moulding top of the existing machines. The machines for standard tiles have a flat top; those for improved tiles have a concave top (see figures). Because of this concave top, the machines for improved tiles can only be used for moulding the tiles, and a separate machine is required for manufacturing the moulds and ridge tiles. It is, therefore, advisable for a new unit to start production with one single machine - a so-called "combination machine" which is capable of producing standard dies, moulds and ridge tiles, and, later, when the business expands, a second machine with concave top, which produces only improved tiles, can be installed, leaving the combination machine to produce ridge tiles and moulds.
2. Quality improvements
The machines have been improved technically in many ways. One improvement is the introduction of special machines for production of improved tiles. The technical advantage is that the moulding frame only touches the table surface over a narrow strip of 2 mm, so that no dirt accumulates under the frame, which results in the improvement of tile quality and working speed. This does not apply to the combination machines where the frames have to be kept clean. Another important improvement is the fixing of the machines on a brick-masonry table, instead of a light steel frame. The machine stays firmly in place when the mortar is applied with the trowel during strong vibration. Other improvements made are a steel top instead of formica, strong supporting springs, accurate rattlewheel and improved bearings, bearing fixings, hinges and locking system.
Figure 5. Concrete rooftile machine for standard tile
Figure 6. Concrete rooftile mould and ridge machine
Figure 7. Brick pillar support of tile machine
C. Moulding system
Mothermoulds are now of preformed plywood and are accurate. To increase accuracy, the traditional grandmothermould has been omitted. Mothermoulds are supplied with the machines, so that moulds can be made directly.
Moulds are made of concrete (same mix as for tiles) on the mothermoulds. They can be made in two ways, with or without stacking ridge. Moulds with a stacking ridge can be piled up in stacks of five tiles (see figure). Plain moulds are stacked in simple wooden frames.
Tiles can be stacked five in a frame and seven frames high (35 tiles). This stacking system results in low shrinkage cracks during the hardening period.
3. Two-stage concrete-tile moulding
The idea of two-stage tile moulding was developed in the late 1970s. The difference between two-stage tile moulding and normal is that, instead of casting concrete in one stage in one mould, the tile is moulded in two stages, viz:
(a) Mortar is spread on a moulding table into a flat sheet;
(b) The flat sheet is then transferred to a curved mould and shaped into a tile.
This principle was first applied to the production of large fibre-cement roof sheets. Later, it was adapted to make roof tiles. It works as follows:
· A frame, which is fitted on a flat moulding table, determines the size and thickness of the tile.
· This frame is lifted, and a polythene sheet is spread under the frame on the table. The frame is replaced clamping the sheet to the table.
· A mortar bed 6.5 mm thick is then spread on the table with a trowel. Most moulding machines are equipped with a vibrating mechanism which is operated at this stage, to assist spreading and increase compaction.
· After spreading of the mortar, the moulding frame is taken off.
· A mould is placed on special supports beside the table, and the polythene sheet, with the compacted mortar, is pulled over this mould. The mortar, which is still pliable, bends itself into the form of a tile.
· The moulds with the freshly moulded tiles are stacked on top of one another, and the tile is allowed to harden for 24 hours.
· After 24 hours, the tiles can be taken from the moulds and are ready for further curing, at least for seven days under water.
Figure 8. Plain moulds and stacking frames