Reinforcement of cement-based composites

The reinforcement increases the composite material tensile strength if fibre reinforcement is sufficiently effective. The tensile strength of the matrix itself is low for instance, that of Portland cement mortars and concretes is approximately equal to 10-12% of their compressive strength. Much higher composite tensile strength is obtained thanks to various systems of reinforcement, including systems with two or more different fibres (hybrid reinforcement). 

Traditional reinforcement of concrete elements in the form of steel bars and prestressing cables or tendons is not studied here as it is mentioned in Section 2.3.1 and only dispersed fibre reinforcement is considered. Macrofibres are usually of 10-60 mm in length and 0.1-1.0 mm in the least dimension. Micro-fibres are of 10-30 pm in diameter and below 10 mm in length. There are several kinds of microfibres non-metallic, including asbestos, polypropylene, mica, wollastonite and xonotlite, and steel fibres. In the following chapter both these gronps of fibres are considered made with different materials (cf. ACT 544.1R-96). 

Asbestos fibres are extracted from natural deposits in several regions of the world. The best known are in Qnebec province in Canada, near Sverdlovsk in Rnssia, in Cyprus, near Tnrin in Italy, in Zimbabwe and in the Republic of South Africa. The rock containing asbestos is crushed to separate fibres by mechanical methods. 

The most frequently available chrysotile asbestos is the fibrous form of serpentine 3Mg0-2Si02 2H20 - hydrated magnesium silicate. The fibres are thin their diameter varies from 0.012 up to 0.03 pm. The fibre length is normally 5 mm, rarely reaching 40 or even 100 mm and there is an empty channel inside a fibre (Hannant 1978). The asbestos fibres are considered as microfibres. 

For more than 40 years it has been known that asbestos dust, which is produced at all stages of asbestos fibre extraction and handling, is very dangerous to human health. The risk for occupants of buildings is also considered, but it may occur only when careless repair work or demolition is carried out. In most countries, the use of asbestos fibres in building and civil engineering is forbidden in others it is strictly limited. 

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The imposed restrictions on the application of asbestos fibres in buildings as reinforcement of cement-based composites stimulated extensive research directed at other kinds of fibres. Because of the excellent mechanical properties, good durability and relatively low cost of asbestos cements, finding an appropriate replacement has been difficult. The research was mainly concerned with cellulose, polypropylene and carbon fibres and positive results have been achieved (Krenchel and Hejgaard 1975 Mai 1979 Krenchel and Shah 1985). The properties of asbestos cements are briefly described in Section 3.2.5. 

Reinforcement of cement-based composites STRESS (MPal... 

Changes in the fiber and fiber/cement interfacial region due to environmental interactions can affect the long-term performance of cement-based composites reinforced with natural fibers. A significant mechanism of changes in composite properties is pulp fiber degradation... 

Tonoli, G., Joaquim, A., Arsene, M., Bilba, K., Savastano, H., 2007. Performance and durability of cement based composites reinforced with refined sisal pulp. Materials and Manufacturing Processes, 22(1-2), pp. 149-156. 

The last group of carbon fibres is considered as a possible reinforcement for cement-based composites for general application because of their reasonable price (Ohama et al. 1985). More expensive carbon fibres are used only for particular purposes like reinforcement in a very corrosive enviromnent or in structures in which the application of any metallic elements is excluded. Because of their dimensions they are considered to be microfibres. 

The cracks in plain or reinforced elements, made of cement-based composites and subjected to tension, are unavoidable due to the difference in Young s moduli of hardened cement and steel. However, it is the task of the designer to select appropriate structural dimensions and material properties to ensure required impermeability by the material itself, or by additional coatings. 

Design and optimization of cement-based composites Table 12.4 Examples of steel-fibre-reinforced concrete mixture proportions... 

The main fields of present and fntnre applications of cement-based composites besides ordinary concrete and traditional reinforced concrete structures are ... 

De Caso y Basalo, F.B., Malta, E, Nanni, A. (2012). Fibre reinforced cement-based composite system for concrete confinement. Construction and Building Materials, 32, 55-65. doi 10.1016/j.conbuildmat.2010.12.063 De Lorenzis, L., Teng, J.G. (2007). Near-surface mounted FRP reinforcement An emerging technique for strengthening structures. Composites Part B Engineering, 38, 119-143. doi 10.1016/j.compositesb.2006.08.003 Derrien, K., Gilormini, P. (2006). In Proceedings of the DSL 2006 Conference, Defect and Diffusion Forum, 258/260, 447-452. 

Brandt, A.M., 1987. Present trends in the mechanics of cement based fibre reinforced composites. Construction Building Materials, 1(1), pp. 28-39. 

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. 

Wu, H. C. and Sun, P. (2005), Fibre reinforced cement based composite sheets for structural retrofit . Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005), 7-9 December 2005, Hong Kong, pp. 351-356. 

Concrete-like composites are the composite materials and ordinary concrete is the most representative example of the group. They are composed of a matrix and inclusions, and possibly also of a system of reinforcement. This group of materials is by definition larger than the scope of this book, which is defined as cement-based composites. 

Wei, S., Mandel, J.A., and Said, S. (1986), Study of the interface strength in steel fibre reinforced cement based composites . Journal of American Concrete Inst., 83, pp. 597-605. 

Apparently, dispersed fibres do not appreciably modify the behaviour of cement-based elements subjected to creep. There have been few experimental studies of this problem and the results obtained are inconclusive. As mentioned above for shrinkage, modification of the internal structure of composite materials causes quite complex results and comparisons are difficult. In several reports, similar behaviour of plain and fibre-reinforced elements was observed under creep conditions, both qualitatively and quantitatively. However, extensive studies executed by Swamy et al. (1977) proved that fibre-reinforcement decreased creep considerably. Similar conclusions were formulated by Brandt and Hebda (1989) who tested elements under eccentric compression in long periods of time. The creep was small for reinforced specimens. The most important factor relating to the final creep values was the level of load with respect to the material s strength and that factor was also influenced by the volume of fibre reinforcement. That was the reason why the creep recovery measurements carried out by different authors did not furnish consistent results. 

In hardened cement-based composites the transportation of liquids and gases through pore and microcrack systems plays a very important role in many processes, such as hydration of Portland cement, pozzolane effects of microfillers, carbonation, corrosion of cement paste and reinforcement due to reaction with external agents, shrinkage and creep, etc. These processes are partly described in respective Sections 4.1, 4.3, 6.5 and 11.5. Only basic information is reiterated below concerning the flow of liquids and gases through concretes and mortars. 

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