|FCR: Fibre Concrete Roofing (SKAT, 1987, 185 p.)|
|3. Technical rationale|
Roofing elements of natural fibre concrete have been in use since 1977. The experiences so far include both success and failures. To secure a good quality roofing material made of concrete or mortar with natural fibres one has to keep the following in mind:
The cement matrix
- The properties of the hardened concrete are affected by the mix proportions, the mixing procedure, the compaction and the curing during the first 7 days.
- The chopped fibre plays its main role in giving the fresh mix a better cohesion and it enables the moulding of corrugated products.
- Fibre length less than 25 mm is recommended.
· All types of natural fibres are possible to use in FC as long as they are clean so that they do not have a negative influence on the setting and hardening of the concrete.
- Concrete with long natural fibres may be improved in flexural strength but this effect is lost when the fibres are decomposed.
- Concrete with chopped natural fibres is a material with properties more like burned clay than asbestos cement and it has to be looked upon as an unreinforced material. The addition of 1 percent by volume of chopped fibres does not improve the properties of the hardened concrete with exception of a slight increase in toughness.
- The natural fibre is decomposed in the alkaline environment of the concrete . This has the effect of a reduction in toughness but not of the other strength properties.
- The durability of natural fibre concrete is good if the batching, mixing, compaction and curing are done properly and if the material is not subjected to stresses exceeding its capacity. There is need for a good quality control program for the production of natural fibre concrete elements.
- The stresses within the roofing element are dependent on its dimensions and how it is fixed. In this respect the roofing tile seems to have an advantage compared with the roofing sheet.