Different types and amounts of steel fibers have varying effects on the improvement of concrete’s mechanical properties. In order to identify the most suitable steel fiber types for the practical production of prefabricated pavements and derive a formula to predict and evaluate the mechanical properties based on steel fiber volume fraction, this study conducted experimental research on the physical mechanical properties of concrete using the method of equal volume substitution of coarse aggregate. The influence of steel fiber type and volume fraction on the microstructure and failure mechanism of steel fiber reinforced concrete (SFRC) was analyzed through electron microscopy scanning. The mechanical properties of plain concrete were used as benchmark. The results showed that when the steel fiber volume fractions were 0.6% and 1.5%, the bending and split tensile strengths of milled steel fibers were 3.1% higher than those of hooked-end steel fibers and the compressive strength of SFRC was significantly increased by 13.5%. The comprehensive mechanical properties of wave-shaped steel fibers were inferior to the first two types. Considering the requirements of SFRC in engineering, milled steel fibers are more suitable to be concrete components. This is because the tangling property of the hooked-end steel fibers makes them difficult to be shaped. In contrast, the smooth surface of milled steel fibers exerts a smaller impact on the forming of specimen and they are more economical. The mechanical properties of SFRC improve as the volume fraction of steel fibers increases from 0% to 2%, but start to decline when the volume fraction exceeds 2%. A volume fraction of 0.6% for steel fibers is sufficient to meet the standard for bending and tensile strength in heavy-duty concrete pavements. Finally, the relationship expressions between the compressive strength, flexural strength, and split tensile strength of SFRC and the steel fiber volume fraction were obtained through fitting the experimental data using Origin software.

Grouting technology is an important method that has been widely adopted for the crack repair of tunnel lining, but the diffusion law of grouting slurry is still uncertain in tunnel lining cracks. For the sake of exploring the grouting diffusion law of tunnel lining cracks, this paper investigated the grouting diffusion shape and the relation between the diffusion radius and time with different crack widths in tunnel lining which by means of laboratory experiments and statistical analysis. Meanwhile, the diffusion equation of grouting in lining cracks has been established on basis of the grouting diffusion shape and experimental data. Furthermore, it is analyzed the correlation between the experimental values and the calculated values. The results revealed that the diffusion shape can be divided into a circular zone, excessive zone, and elliptical zone in the tunnel lining cracks, and the calculated values of the equation of grouting diffusion are highly correlated with the experimental values. Additionally, it is shown that the diffusion equation can accurately describe the relationship between diffusion radius and time of grouting in tunnel lining cracks. The study results would provide a theoretical basis for the prediction of diffusion radius on grouting repair for tunnel lining cracks.

The aim of this study was to investigate the effect of different mix ratios on the mechanical properties of steel fiber-reinforced concrete (LSFRC) and to determine an optimum mix ratio. The effects of four factors, namely, fly ash content, volume fraction of steel fibers, water–cement ratio, and sand rate, on the mechanical properties of LSFRC were investigated through orthogonal experiments. The microstructure of LSFRC at different mix ratios was analyzed using scanning electron microscopy (SEM), and an optimal mix ratio was derived. The results showed that the water–cement ratio and the volume fraction of steel fibers were the main factors affecting the mechanical properties of LSFRC. When the water–cement ratio was 0.38 and 0.42, the combined mechanical properties of concrete were superior. Steel fiber content between 0.6% and 1% had a significant effect on the splitting tensile strength of concrete. The effect of sand rate on compressive and splitting tensile strengths was consistent, with a significant effect on both at a sand rate of 40%. In terms of microstructure, 20% fly ash content promotes the hydration of concrete. The optimum LSFRC mix ratio determined was 0.42 water–cement ratio, 0.6% steel fiber content, 40% sand rate, and 20% fly ash content. Experimental verification using this mix ratio showed that the compressive, flexural, and split tensile strengths were increased by 3%, 19%, and 33%, respectively, when compared to ordinary concrete.

When constructing tunnels in mountainous areas, the drilling and blasting method is the most commonly used because of its economy. Ordinary reinforced concrete itself has defects such as poor crack resistance and brittleness. Therefore, when using the drilling and blasting method for ordinary reinforced concrete double-line tunnels, vibration phenomena will occur and cause cracks in the first-line tunnels, which will have adverse effects on the durability and safety of the tunnel. As a response, scholars have proposed the use of steel fiber-reinforced concrete as tunnel lining. In this paper, the LS-DYNA software is used to establish three models of plain concrete, ordinary concrete, and steel fiber-reinforced concrete, and numerical analysis is conducted with different amounts of explosives. The results show that the steel fiber-reinforced concrete tunnel lining has better performance than the other two concretes in tunnel construction.

The blasting vibration velocity is a widely used controlling index for the safety assessment of tunnel structures. How to reduce the impact of blasting vibration on the operation of adjacent railway tunnels is worth studying. In this paper, the initial phase-difference of blasting using phase-difference shock absorption is derived from the propagation theory of the blasting stress wave. When the initial phase-difference is Δφ_i=±(2n+1)π (n=1, 2, 3, ), the peak particle velocity (PPV) in the tunnel structure is decreased to the minimum. On the basis of blasting theory using phase-difference shock absorption, the blasting parameters were designed for the No. 2 drainage tunnel excavated which adjacent a railway tunnel from Guiyang to Changsha, and it carried out 4 times blasting tests. It was found that the PPV of the railway tunnel structure in X, Y, and Z directions reduced by 64.6, 66.25, and 81.26, respectively, which compared with the conventional blasting with the same total quality. Meanwhile, the amount of overbreak and underbreak can be controlled within 5. The results show that the effect of blasting technology using phase-difference vibration mitigation is obvious, which is able to match with the design requirements for the tunnel excavation contour. The blasting technology using phase-difference vibration mitigation can be achieved via controlling the blasting time interval accurately. Moreover, it is beneficial for enriching and developing the blasting vibration control technology.