Experimental Study On Mechanical Properties Of High Performance Fiber Reinforced Shotcrete

With the rapid development of railway and highway tunnels,urban subway and underground engineering in China,the amount of shotcrete is increasing rapidly.However,nowadays,it is difficult for shotcrete to meet the requirements of tunnel engineering construction in areas with high ground stress and large deformation,because of its low strength grade and insufficient toughness.Therefore,it is of great significance to study high performance fiber reinforced shotcrete.Based on the concrete mix proportion design method,the mix proportion of different fiber reinforced shotcretes were given in this paper through workability,spray ability and strength tests.The influences of four kinds of fibers single doping and two fibers hybridization on the strength of shotcrete were studied,and the constitutive equation of shotcrete was given.The chloride ion permeability and crack resistance of shotcrete were also studied.The strength development law and fiber strengthening mechanism of fiber reinforced shotcrete were explored.The main work and conclusions are as follows:(1)Considering the characteristics of shotcrete,the theoretical mix proportion design was carried out firstly,and then the reference mix proportion of shotcretes were given through workability tests.The influences of the kinds and dosage of accelerator,length-diameter ratio of fiber and the strength grade of cement on the performance were studied.Finally,the mix proportions of shotcretes with different kinds of single and hybrid fibers were given through workability,spray ability and strength tests.(2)The influence laws of different contents of four kinds of fibers on the strength of shotcrete were studied.Four kinds of fibers all had a significant enhancement effect on the compressive strength of shotcrete.With the increase of the content of hooked-end and corrugated steel fiber,the flexural strength of shotcrete increased linearly.With the increase of the content of imitation steel fiber,the flexural strength of shotcrete increased first and then decreased,and the improvement of polypropylene fiber on the flexural strength was not significant.The strength of hooked-end steel fiber and polypropylene fiber hybrid shotcrete was improved,while the strength of hooked-end steel fiber and imitation steel fiber hybrid shotcrete was reduced.(3)The stress-strain curves of fiber shotcretes were studied,the ductility of shotcrete was improved by adding hooked-end steel fibers,the constitutive equation of shotcrete was given.The permeability of chloride ion in shotcrete matrix was very low.With the increase of the content of hooked-end steel fiber,the shrinkage strain of concrete decreased.The crack resistance of shotcrete could be improved obviously by adding 30kg/m~3 hooked-end steel fiber or 30 kg/m~3 hooked-end steel fiber and 0.6kg/m~3polypropylene fiber hybrid.Considering the improvement of the performance of shotcrete by fiber,the shotcrete with 30kg/m~3 hooked-end steel fiber single doping,30kg/m~3 hooked-end steel fiber and 0.6 kg/m~3 polypropylene fiber hybridization can achieve the purpose of high performance.(4)The strength development law of fiber reinforced shotcrete was studied,and the development formulas for compressive strength and flexural strength were given.Based on the mechanics theory of composite materials,considering the distribution characteristics of the fibers,the equivalent bond-anchorage strength between different fibers and matrix was compared.The strengthening mechanism of different fibers was analyzed,the formula for calculating flexural strength of fiber reinforced shotcrete considering the influence of hooked-end was given.