The Influence of Micro Steel Fiber Orientation on the Strength of Ultra-High Performance Concrete

In recent years, UHPC has been widely applied in infrastructure construction due to its excellent properties such as ultra-high strength, high durability, and high density. In UHPC, micro steel wire fibers are one of the commonly used reinforcing materials. They can effectively improve the tensile strength, impact resistance, and crack resistance of UHPC. The UHPC micro steel wire fibers are usually randomly dispersed. The fibers not aligned with the tensile stress direction cannot fully play the role of reinforcement. Only the micro steel wire fibers distributed along the direction of tensile stress in UHPC can fully exert the reinforcing effect. Studies have shown that directionally arranged micro steel wire fibers concrete has excellent mechanical properties, with a flexural strength increase of 50%~130% compared to randomly distributed micro steel wire fibers concrete. Over the past few decades, the orientation efficiency factor and volume fraction of micro steel wire fibers in UHPC have been explored in terms of their impact mechanism on mechanical properties. Many theoretical models have been proposed to describe the relationship between the mechanical properties of micro steel wire fiber concrete and the orientation efficiency factor and volume fraction of the fibers. However, these models are all based on the linear elasticity of composite materials before cracking and do not consider the discreteness of the cement-based composites.

Research Content on the Orientation of Micro Steel Wire Fibers

This paper studies the effect of directionally micro steel wire fibers on the mechanical properties of UHPC. First, the magnetic induction intensity required to prepare ASF-UHPC specimens is determined. Then, ASF-UHPC specimens are prepared and tested. Two parameters are introduced to account for the discreteness of UHPC and the dispersity of micro steel wire fibers , thereby establishing theoretical models of tensile, flexural, and compressive strength. Finally, the reinforcing mechanism of directionally arranged micro steel wire fibers on UHPC is analyzed. Linear regression analysis is performed on the uniaxial tensile strength results of micro steel wire fibers with volume ratios of 1.5% and 2.5% , obtaining the undetermined coefficients k1 and k2 in the tensile strength model of ASF-UHPC specimens. The calculated values of k1 and k2 are 0.90 and 0.26, respectively, incorporated into the equation. The tensile strength of RSF-UHPC and ASF-UHPC specimens is considered when the volume ratio of micro steel wire fibers is 2.0%. The theoretical values are in good agreement with the experimental values of the ASF-UHPC and RSF-UHPC specimens, indicating that this model can accurately predict the tensile strength of micro steel wire fiber-reinforced UHPC .

Conclusions on the Effect of Micro Steel Wire Fibers on UHPC Mechanical Properties

Using composite material theory to analyze the stress of ASF-UHPC specimens, this study considers the orientation and aspect ratio of the micro steel wire fibers and the fiber-matrix interface adhesion. By determining model parameters based on experimental results, the uniaxial tensile, flexural, and compressive strengths of micro steel wire fiber-reinforced UHPC can be effectively predicted.

The orientation of micro steel wire fibers in UHPC can be adjusted using an electromagnetic field, to prepare ASF-UHPC specimens. When the micro steel fiber volume content is the same, the tensile, flexural, and compressive properties of ASF-UHPC specimens are significantly improved compared to RSF-UHPC specimens. When the micro steel wire fiber volume content is 1.5%, the tensile work and fracture energy of ASF-UHPC specimens are increased by 21.1% and 47.0%, respectively, compared to RSF-UHPC specimens.

When the fiber direction in UHPC reinforced with directionally arranged micro steel wire fibers is parallel to the main tensile stress direction, the fiber direction is perpendicular to the crack direction, allowing more micro steel wire fibers to effectively bridge the cracks and bear the load. Moreover, in ASF-UHPC specimens, a single micro steel wire fiber has less impact on the surrounding matrix when pulled out. Therefore, the enhancement effect of directionally arranged micro steel wire  fibers is significantly greater than that of randomly distributed fibers.