|FCR: Fibre Concrete Roofing (SKAT, 1987, 185 p.)|
|4. Manufacturing technology|
In the past most people have believed that the making of FCR-sheets was something simple and easy to learn. Furthermore, it was believed that it depended upon the right kind of apparatus or table used. In fact, there exists a large number of publications which describe the production of FCR sheets and there are as many technical differences as there are producers.
Most technicians confronted with FCR first think of making the sheets bigger; it seems to be an automatic reaction. At the same time, one tries to make them of better quality by seeking to improve raw materials, mix-proportions, and even the form of the sheets. Most manufacturers though eventually come to the conclusion that the elements should be smaller end that the decisive factor in production is the quality of workmanship.
The availability of the materials has to be assured, but this seldom presents problems. Portland cement according to international specifications seems to be available in most countries, and complaints usually centre on the age and possible poor storage of the cement (moisture) rather than of its ex-works quality.
Sand is available in most places: it must be clean, but its granulometry does not seem to influence the quality of the streets es much as one first assumes. Tests in the Dominican Republic have shown that the difference of workmanship from one workman to another have a greater Influence on test results than the difference between sands of varied granulometry. However some more recent tests seem to alter this finding slighty.
Whether sisal or coconut con Is used does not seem to cause much difference, once the basic cleanliness and the good cutting of the fibre Is guaranteed. Other fibres like Jute are somewhat more difficult to work with and fit Is not recommended to use grasses without proper testing.
Batching and mixing of the components can be done by hand and with simple batch-box methods though preferably by weight. The fibre should be measured by weight.
There is much controversy about whether the fibre should be added to the dry or wet mixture, but it can be said that In both cases It works just about the same.
For sheet production generally a sand-cement ratio of 1:1 by
volume is used . Small deviations
(25%)did not alter test results in the Dominican Republic to a significant degree. Much of the discussions center upon the type of table which is used to hold the mould and serve as the working surface for the making of the sheet. However, it can be said that it Is not decisive what kind of table one uses since one of the more efficient workteams known prefers to fabricate their sheets on the floor.
The more complicated and perfect the table is, the easier for the workteam, but the slower the work.
The crucial moment In the sheet fabrication Is when the wet mixture Is placed on the mould. If the mixture does not have the proper degree of stiffness, this will make the Job more difficult and result In inferior quality.
The workteam has to act simultaneously and any minor error will show up. The bigger the sheet is, the more difficult is this procedure, and the better the two or three workers have to be coordinated.
A good part of the quality Is determined by the curing procedure. The task is to keep the newly manufactured sheets humid as long as possible and not to disturb them for at least 24 hours. In some places they are left for 72 hours between plastic on the moulds; however, if they can be immersed In a wafer basin they can tee carefully demoulded after 24 hours. They should stay immersed In water between 3 and 15 days, the final quality depends to a large extent on this water curing.
Where no water tank is available, the dally molsterning of the product and covering with plastic does help somewhat, but the immersion in water is better. Even if the sheets are produced perfectly, there is no guarantee that they will do well on the roof, as the roof structure and the installation of the sheets still can be imperfect. Most cracks appear within the first days after installation or after a significant change of the support structure (warping of the wood, bending) and usually cracks are lengthwise. The cracking is due to either uneven supporting structure, to incorrect fixing of the sheets to the rafters or to inner stresses, which are much greater in sheets than in tiles.
Another very Important point Is the handling of the product, where much of the later failure on the roof Is being Induced by putting too much stress on the sheets, either by rough handling or by stacking them Incorrectly.
Tiles can be manufactured safely with a much poorer mixture, and therefore economize on cement. The strain upon each tile placed on the roof is several times smaller than upon a sheet.
For tilemaking there exists a manual that instructs in the procedures, and if this is applied, using the standard tile-kit, production Is of a general uniformity. The preparation of the materials and the mixing is much the same as for sheets (only with a lower cement proportion), but mainly in the complicated moment of forming the tile presents clear advantages compared to the sheet. The unit can be worked by just one person and therefore no co-ordination of movements between workers is needed.
Compaction of the mix Is much better shanks to the vibrating table,which guarantees better formations of the matrix. Also the handling involved Is much easier than with the sheets, mainly at the demoulding stage when the fragile elements have to be stacked in water to cure.
Curing has the same Importance as in the production of sheets and should be done whenever possible through immersion in water for several, whenever possible 10 to 14 days. The tiles are less susceptible to cracking than the sheets.
The worksite can be almost anywhere, as long as there is access for raw material delivery and some shade, but It is recommended to have some roofed area for all-wheather production and a closed room for storage of cement, fibre and tools.
This is the one area where the production process varies greatly from one site to another depending on the interest management takes in it. If we list the main failures within the production process, which determine the later appearance of cracks on the finished roof, we see that all of them could be avoided by good management and most of them could still be detected on a quality-check before the element leaves the production area.
The most common production errors are:
- bad or inconsistent raw materials
- Incorrect batching
- deficient equipment
- bad compaction
- variation In the thickness of the product
- uneven elements (Incorrect positioning on moulds)
- Insufficient curing
- rough handling
If strict quality control is applied, one can not only detect deficient products, but also motivate the workteam to take more care and to improve their performance not only in quantity but also in quality. Several projects deduct wages for deficient products.
A visual check has to be done with every single element, some more thorough procedures have to be applied to a certain proportion of the output. A very successful commercial producer in Nicaragua (workshop of the Regional Government in Granada)tests a certain percentage of the sheets dally for the following aspects (random samples):
- load bearing capacity
- water Impermeability
- Impact resistance
- detail of exact overlap/underlap
If all sheets perform well, production goes on, if any one of them is deficient, the error is looked for and the guilty worker is punished financially by a discount system. One of the mayor tasks for the near future will be to establish reasonable common standards of quality for both streets end tiles, but meanwhile we strongly recommend all producers (and buyers) to establish a routine quality-check for FC products.