Asbestos-cement composites

From the beginning of this century, the demand for asbestos fibers grew in a spectacular fashion for numerous applications, in particular for thermal insulation in steam engines and technologies (4). Moreover, the development of the Hatschek machine in 1900 for the continuous fabrication of sheets from an asbestos—cement composite opened an important field of industrial application for asbestos fibers. 

The reinforcing capacity of asbestos fibers in a cement matrix constitutes another key criteria for the evaluation of asbestos fibers. This property is assessed by preparing samples of asbestos —cement composites which, after a standard curing period, are tested for flexural resistance. The measured mpture modub are converted into a parameter referred to as the fiber strength unit (FSU) (34). 

Hosseinpourpia, R., Varshoee, A., Soltani, M., Hosseini, R, Tabari H.Z., 2012. Production of waste bio-fibre cement-based composites reinforced with nano-Si02 particles as a substitute for asbestos cement composites. Construction and Building Materials, 31, pp. 105-111. 

Table 3.1 Mechanical properties of asbestos cement composites, after Hannant (1978) and other sources...

Akers, S. A. S., Garret, G. G. (1983) The relevance of simple fibre models to the industrial behaviour of asbestos cement composites . International Journal of Cement Composites and Lightweight Concrete, 5(3) 173-9. 

Allen, H. G. (1971) Tensile properties of seven asbestos cements . Composites, 2, June 98-103. 

The absence of CH-rich zones in the vicinity of the fibres was reported by Akers and Garrett [41] for asbestos-cement composites and by Bentur and Akers [42] for cellulose FRC composites produced by the Hatscheck process. This may be the result of the affinity of these fibres for the cement particles, and the processing treatment which involves dewatering, both of which lead to a system with very little bleeding, and probably reduce the extent of formation of water-filled spaces around the fibres in the fresh mix. This is reflected in the nature of the fibre-matrix bond failure in asbestos composites, the cement matrix was sometimes seen to be sticking to the asbestos fibre bundle. This suggests that a strong interface was... 

S.A.S. Akers and G.G. Garrett, Fibre-matrix interface effects in asbestos-cement composites , J. Mater. Sci., 18,1983, 2200-2208. 

In reviewing asbestos-cement composites, one must present a treatment that combines the concepts involved in the production technology with those of the reinforcement of the hardened composite. These two aspects are a product of the special structure and properties of the asbestos fi bres. I n this chapter, the production technology will first be discussed, followed by a description of the fibre structure and properties, and of the fibre-matrix interaction in the hardened composite. 

When considering the properties of asbestos-cement composites in relation to the production processes, the orientation effect must also be taken into account. The data reported by Allen indicate that the strength in the transverse direction is only about 60-85% of that in the longitudinal direction, with similar ratios reported for the ultimate elongation. Zevin and Zevin [9,10] determined the orientation of the fibres by X-ray methods. They then calculated the strength ratio between the longitudinal and transverse directions, which was found to compare favourably with... 

Table 9.4 Effect of fibre processing on the properties of asbestos-cement composites ...

Table 9.5 Longitudinal to transverse strength ratio in asbestos-cement composites produced by different processes (after Zevin and Zevin [ 10])...

The two main approaches for dealing with the mechanics of asbestos-cement composites are based on composite materials concepts (rule of mixtures) and fracture mechanics, Microstructural characterization of the composite has usually been carried out in conjunction with these studies, to determine the numerical parameters required for the modelling, such as the aspect ratio, and to resolve the pull-out and fracture processes during failure. The results of these microstructural studies wi II be reviewed first, since they provide the background required for the modelling of the processes which control the mechanical performance of the hardened composite,... 

SEM characterization of the fractured surface of asbestos-cement composites was reported by Akers and Garrett [4,11] and Mai [12], It was shown that in the actual composite, a considerable portion of the fibres was in the form of bundles... 

Figure 9.9 The bundled nature of the fibre in an asbestos-cement composite, as seen during in situ testing of a composite (after Akers and Garrett [I I]).

The composite materials approach, based on the rule of mixtures (reviewed in Section 4.3) has been applied by various investigators to account for the modulus of elasticity, tensile strength and flexural strength of asbestos-cement composites. 

Figure 9.14 Calculated and analytical relations between tensile strength and fibre content of asbestos cement composites (after Mai [12]).

Fracture mechanics has been applied to model some of the fracture characteristics asbestos-cement composites, especially those related to crack bridging by the fibres. Mai [12] analysed the behaviour of both notched beams in flexural loading and double cant lever beams (DCB) to determine the critical stress intensity factor, /Cc and the specific work of fracture, WF, which measures the average fracture energy per apparent unit crack surface over the entire fracture process. The effect of fibre content on these parameters is shown in Figure 9.15. The trends resemble those observed for the effect of fibre content on strength (Figure 9.14). 

Lenain and Bunsell [19] characterized the fracture behaviour of the asbestos-cement composite by determining the F-curve. Fibre bridging over microcracks which formed ahead of the crack tip was shown to be a major mechanism in resisting crack propagation, and it thus played an important role in controlling the / -curve. 

S.A.S Akers and G.G. Garrett, The influence of processing parameters on the strength and toughness of asbestos cement composites , int J. Cem. Comp. Ltwt. Conor. 8, 1986, 93-100. 

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. 

Andonian etal. [27] used the composite materials approach, in the form of the rule of mixtures, to account for the strength properties of the composite. They used the same concepts as those described for asbestos-cement composites (Eqs 9.8 and 9.9, Chapter 9), namely, that strength can be calculated as the sum of the effects of the matrix and the fibres the fibre contribution is governed by pull-out and is a function of r /c/, while the matrix contribution is a function of the strength of a void-free matrix, umo, multiplied by its solid content, (1 - Vq). Therefore, For tensile strength ... 

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