Asbestos fibres strength

Plasters have been in use in passive fire protection, as fireproofing products, for many decades. The finished plaster releases water vapour when exposed to flame, acting to slow the spread of the fire, for as much as an hour or two depending on thiekness. It also provides some insulation to retard heat flow into structural steel elements, which would otherwise lose their strength and collapse in a fire. Early versions of these plasters have used asbestos fibres, which have by now been outlawed in industrialized nations and have eaused significant removal and re-coating work. 

Asbestos cements may be considered as one of the oldest man-made fibre-reinforced materials. They were already being produced in 1900 at a time when other fibre-reinforced composites were not known. Natural inorganic asbestos fibres were used only with neat cement paste and a composite material was obtained with high tensile and flexural strength and several other excellent mechanical properties see Section 5.2. 

The important strengthening effect of asbestos fibres is due to their high strength and Young s modulus and excellent bonding to cement paste. The fibres control cracks in the brittle matrix and the rupture of the composite is accompanied by the pull-out of fibres. The linear elastic behaviour is observed nearly to the maximum load and then the shape of the descending line is determined by the efficiency of the fibres see Figure 3.5. 

The mechanical properties of asbestos fibre cements may be calculated from the law of mixtures or by using the fracture mechanics formulae from which can be seen the specific work of fracture and R-curve. Mai, et al. (1980) observed also that crack initiation was close to the bending strength, which was related to a quasi elastic and brittle behaviour. For specimens with a depth greater than 50 mm the size effect on mechanical behaviour was negligible. For smaller specimens the pull-out fibres across cracks could not be developed before quick crack propagation took place followed by the failure of the specimen. 

Vegetable fibres are also used to replace asbestos fibres, which are expensive and dangerous to health. Coconut fibres were tested for that purpose and their strength and deformability, as well as thermal and acoustic properties, and were proved comparable with those of asbestos fibres (Paramasivam et al. 1984). Similar tests on specimens reinforced with flax fibres from New Zealand and Australia also showed their ability to replace asbestos in thin cement sheets (Courts 1983). 

Table 9.1 Strength and modulus of elasticity of asbestos fibres (after Aveston [5])...

I n the asbestos-cement industry it is common to use a blend of different types of asbestos fibres, to optimize the characteristics of the mix in its green (processing) stage, as well as the strength in the hardened state. Some of the asbestos fibres are chosen for their processing characteristics, while others are present because of their efficiency in enhancing strength. 

The asbestos fibre is unique in its properties. It permits control of the properties of the fresh mix, to make it processable by the efficient dewatering techniques, and, at the same time, it provides an excellent reinforcing efficiency in the hardened composite, due to its high strength, modulus of elasticity and bond. In addition, this fibre is stable in the alkaline cement environment, thus providing a composite of excellent durability. This combination of properties is difficult to match with any one type of fibre. There are fibres which can match the mechanical quality of asbestos, but they cannot, on the other hand, provide the processing characteristics. 

Cement composites reinforced by PVA fibres can be produced with mechanical properties superior to those of asbestos cement, while using smaller contents of PVA fibres than of asbestos fibres (Figure 10.38). This is especially the case for strength and toughness, though the modulus of elasticity is somewhat lower than that of the asbestos-cement composite. 

PPT fibres are used in combination with other types of fibres and fillers in brake linings to replace asbestos. The strength and toughness and the dimensional and thermal stability of the fibres contributes to the low creep and cracking resistance of these new materials. 

---