Experimental investigation on mechanical properties of Hybrid Fibre Reinforced Concrete

In order to improve the work behaviour of Plain Concrete (PC) of the shear keys in the immersed tunnel, a series of test programs consisting of direct shear, four points flexure, uniaxial tensile, uniaxial compression and splitting tensile tests were carried out to find the optimized mixture proportion of the fibres in Hybrid Fibre Reinforced Concretes (HFRC). The optimized unit weight of the hybrid fibres was obtained through the parametric studies on the improvements of different fibre contents on the mechanical properties of the HFRC, especially the shear strength and toughness. Additionally, the influences of incorporating hybrid fibres on flexure, direct shear toughness and residual load were also studied. The direct shear strength, shear toughness and residual shear load significantly increased due to the addition of steel fibres and basalt fibres. Through extensive experimental studies, the comparisons of the mechanical properties of the HFRC with different fibre content revealed that group C2 containing 180 kg/m3 steel fibres and 4.5 kg/m3 basalt fibres performed the best in terms of shear strength and toughness. The contributions of different types of fibres to the mechanical properties of HFRC were also investigated. The observations from the tests offer a practical guidance to concrete composites designers on the shear strength and shear deformation ability of different structures, especially for use in constructing the shear keys of an immersed tunnel.