Steel fibre reinforced concrete is used for many years in spray concrete for tunnel as temporary lining and even final lining. Multiple research studies and tests on the behavior of steel fibre reinforced concrete have been carried out in recent years in various countries. They have greatly contributed to a better characterisation of Steel Fibre Reinforced Concrete (SFRC), and have thus allowed to gain a better understanding of the behaviour of this material and to specify minimum performance requirements for each project. The state of the art is well known and lot of international standards provides clear guidance and performance criteria to used safely steel fibre reinforced concrete. Macro synthetic fibre is also proposed today for different application. Specific technical strength and weaknesses of the different fibres, are often less well known, and lead to confusion. This paper discusses the important characteristics of steel and polymer fibre reinforced spray concretewhen used for ground support and provide the last test result from different laboratory.

1.1 Modulus of Young of the fibres

The reinforcing ability of a fibre depends on the anchorage of the fibre into the concrete, the tensile strength and modulus of Young. The Young's modulus of concrete is typically 30.000MPa, of steel fibre typically 210.000MPa, and of polyolefin fibre typically 3.000 to 10 000MPa. For well anchored fibres, and equal solicitation of the fibre, the elongation of the polymer fibre, and the corresponding crack width in concrete, might be considerably higher compared to steel fibres. This might have an impact on the durability of the concrete, especially in combination with traditional reinforcement.

1.2 Tensile strength of the fibres

The tensile strength of steel wire is typically 1.000–2.000MPa, versus 300–600MPa for macro synthetic fibre.

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