S. K. Singh, Principal Scientist, Ajay Chourasia, Principal Scientist
M. M. Dalbehera Scientist, and S. K. Bhattacharyya, Director, CSIR, Central Building Research Institute, Roorkee.

Introduction

Concrete is a relatively brittle material and develops micro cracks even during curing and initial stages of strength development whereas reinforcement of concrete with randomly distributed short fibres improves the initiation and propagation of cracks by improving overall properties of the conventional concrete. Fiber reinforcement is commonly used to provide enhanced toughness and ductility to brittle cementitious matrix. The reinforcement of concrete with a single type of fiber may improve the desired properties to a limited level, whereas combination of two or more types of fibres if used in optimal way in concrete achieves better engineering properties due to positive synergetic effect. This includes combination of different kinds of fibres with different shapes, dimensions, strengths and modulus to concrete matrices. The hybrid fibre utilizes the individual qualities of its constituent fibres to undermine the inherent limitations of their counterpart. To control micro cracks in concrete, microfibers are used having diameters (10-40m compared to 500m), while to control macro cracks, macro fibres are used to resists propagation and crack opening. The Fig.1 presents comparison of typical load- deflection curves for conventional FRC and hybrid FRC.

Comparison of Conventional and Hybrid FRC
Fig1: Comparison of Conventional and Hybrid FRC

The hybrid fibre reinforced concrete, is effective in arresting cracks both at macro and micro levels. This is due to positive interaction between the fibres and resulting hybrid performance exceeds the sum of individual fibre performances. This phenomenon is termed as "Positive Synergy Effect". The various hybrids are based on fibre constitutive response, fibre dimensions, fiber function as described below:
  • Hybrids Based on Fibre Constitutive Response: Where one type of fibre is stronger and stiffer and provides reasonable first crack strength and ultimate strength, while the other type of fibre is more ductile or can readily undergo considerable slippage to provide better toughness and strain capacity in the post crack zone.
  • Hybrids Based on Fibre Dimensions: Where one type of fibre is smaller (micro) and provides micro-crack control at early stage and delays coalescence. This leads to a higher tensile strength of the composite. Other fibre is larger and is intended to arrest the propagation of macro-cracks and therefore, resulting a substantial improvement in the fracture toughness of the composite.
  • "Hybrids Based on Fiber Function: Where one type of fibre is intended to improve the fresh and early age properties such as ease of production and controlling plastic shrinkage, the other fibre leads to enhanced mechanical properties."
The most commonly used fibers are steel, carbon, asbestos etc. (high modulus fibers) and polyester, polypropylene (low modulus fibers). Properties of some fibers are listed in Table 1.

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