Abstract:

This paper deals with the use of fibre reinforced concrete (FTC) in underground construction activities. The correct choice of a steel fibre for structural use is a challenge to engineers and researchers. It is to be remembered that the appropriate choice of fibres strongly depends on the requirements of the application. Wide use in shotcrete and segmental linings worldwide has now and again shown the importance of steel fibres in such applications. Establishing quality assurance of FTC needs to be implemented. For this, proper batching systems and methods need to be enforced in project sites by choosing the right kind of systems based on experience and expertise of leading service providers.

1.0 INTRODUCTION

Steel fibres are more and more being used in structural concrete members since they increase the ductility of the concrete matrix. Fibres provide the brittle concrete with a post-cracking strength which can be considered in the structural design. Steel fibres are, in particular, used for providing the post crack load bearing capacity of concrete. Thus fibres are used to improve the overall behaviour at the Ultimate Limit State where they can partially or totally substitute conventional reinforcement. The fibres increase the impact strength and fatigue strength and have a crack-distributing effect so that the cracks formed are smaller. Fibres are used to improve the behaviour in the Serviceability Limit State since they can reduce crack spacing and crack width, thereby improving durability and robustness. Along with steel fibres, special micro polypropylene mono filament fibres are used in concrete for underground applications. On the one hand to improve the plastic shrinkage behaviour due to the ability to hold water on their vast specific surface; on the other hand for passive fire protection, since they markedly reduce or entirely prevent explosive concrete palling in tunnel fires. The large concrete cover that is often required for fire resistance can in this way be reduced.

2.0 STEEL FIBRES

Steel fibres have been prevalently used in various Steel Fibre Reinforced Concrete (AFC) applications across the globe. They are classified depending on the manufacturing process i.e. cold deformed, cutting, milling or melt extraction or the chemical composition of the base material i.e. low or high carbon content or stainless steel. ATM A 820–04 [1] and EN 14889–1:2006 [4] categorises steel fibres based on the manufacturing process and their base materials. Steel fibres can have various cross sections from circular to sickle shaped. The surface may be straight, corrugated, grooved or profiled. To ensure proper anchorage into the concrete matrix, the ends of the fibres are usually deformed with double or multiple hooks.

2.1 Steel fibres versus macro synthetic fibres

Apart from steel fibres, synthetic fibres are also available commercially. Applications where shrinkage cracks need to be eliminated or reduced or where fire protection is required, micro synthetic fibres show their beneficial performance in concrete. Whereas, for structural applications steel fibres are suited over synthetic fibres, which do not contribute to mechanical load bearing due to their low Young's modulus.

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