**6. Effect of using hybrid fibres**

It is noteworthy to examine that the concrete failure is based on a multi-scale and a gradual process even though the research mentioned above have convinced us that remarkable improvement in mechanical performance can be achieved by using single fibre type in concrete. Therefore, significant attempts are made toward fibre combinations with different functions and constitutive responses and dimensions into cementitious composite. Potential advantages can be offered through hybrid combinations of steel and non-metallic fibres to enhance concrete properties and to reduce the entire cost of concrete production (Bentur and Mindess, 1990). Fibre fractions result in a uniform and a denser fibre distribution within the concrete as it enhances post-crack strength of concrete and reduces shrinkage cracks. This combination of low- and high-modulus fibres can arrest the micro- and macro-cracks, respectively, which could be also achieved by using a combination of long and short fibres as different lengths of fibres would control different scales of cracking.

A number of studies indicated the overall benefits of using combinations of steel fibres and polypropylene fibres (Xu et al., 2011; Sivakumar, 2011; Chi, 2014; Ding et al., 2010; Sahoo et al., 2015), while limited research was carried out on the effect of using steel fibres and other types of fibres such as glass and polyethylene (Banthia et al., 2014) or using a mix of short and long steel fibres [11].

Xu et al. (2011) found that the tensile strength of steel-polypropylene hybrid fibre reinforced concrete. The results indicated that the tensile strength of conventional concrete can be dramatically improved by mixing with hybrid steel-polypropylene fibres. The enhancing effect of hybrid fibre is better than that of single fibre, and the volume fraction of steel fibre is observed to have a great impact on the tensile strength. The same results were found by Sivakumar (2011) who studied the flexural strength, toughness, and ductility of concrete specimens containing individual steel fibres and hybrid combinations of steel and non-metallic fibres such as glass, polyester and polypropylene. He found that the ability of non-metallic fibres to bridge smaller micro-cracks was suggested as the reason for the enhancement in flexural properties compared to individual steel fibre.

The effect of inclusion hybrid steel-polypropylene fibre reinforced concrete on triaxial compression was developed by Chi (2014). The results showed that the steel fibres mainly contribute to the composite's triaxial strength that was observed to improve significantly when both the volume fractions and aspect ratios of steel fibre were increased. On the other hand, the polypropylene fibres were found to have considerable effect on improving the tensile meridian rather than compressive meridian.

Ding et al. (2010) analysed the influence of various fibre types, including steel macro-fibre and hybrid fibre (macro-steel fibre and macro-plastic fibre) on the shear strength and shear toughness of reinforced concrete beams. The results indicated that hybrid fibres can evidently enhance both the shear toughness and the ultimate shear bearing capacity.

Sahoo et al. (2015) studied the influence of using both high-modulus (steel) and low-modulus (polypropylene) fibres on the shear strength of reinforced concrete beams. A better post-peak residual strength response is noticed in the case of all FRC beam specimens due to multiple cracking associated with the fibre bridging action. The main parameters investigated are shear strength, failure mechanism and displacement ductility. The FRC specimens with combined steel and polypropylene fibres showed that the shear resistance and deformability values are improved significantly; multiple cracks of smaller crack width are noticed at the failure stage of the specimens indicating the better fibre bridging action of combined metallic and nonmetallic fibres.

Banthia et al. (2014) used hybrid fibres by using two types of macro-steel fibres and a microcellulose fibre. Flexural and direct shear tests were performed, and the results were analysed to identify the degree of enhancement in the mechanical properties associated with various fibre combinations.
