Simplified analytical model to assess key factors influenced by fiber alignment and their effect on tensile performance of UHPC

Abstract

The increase in single-fiber pullout strength usually improves tensile strength and toughness of UHPC. An increase in fiber orientation angle (θ) up to 45° to the direction of pullout load was found to enhance the single-fiber pullout strength. However, such increase in predominant θ value along tensile strength specimens led to a reduction in tensile strength of UHPC. In order to elucidate this controversial observation, analytical models combined with experiments were carried out to assess the key factors influencing the tensile properties of UHPC due to variations of fiber orientation. The test parameters involved the evaluation of the effect of θ (0°–60°) and fiber embedment length (le) of 5 and 10 mm on the single-fiber pullout performance. The influence of fiber orientation on UHPC tensile performance was also investigated by casting tensile strength samples having predominant θ values of 25°, 30°, 40°, and 50° along the tensile loading direction. Test results indicate that the key factors influencing the tensile properties of UHPC included the fiber-matrix bond strength (τθ), le, and fiber number in a given cross section (Nf). The highest τθ was obtained at a θ value of 45°. The increase in θ from 0° to 90° led to a consistent reduction in le and Nf. A simplified model that considers these factors was proposed to assess the effect of each factor on UHPC tensile strength. The model indicates that le had the greatest influence on tensile strength, followed by Nf and τθ. The decrease of predominant θ in tensile strength specimen from 45° to 25° enhanced le by 65% and Nf by 60%, compared to a 7% reduction in τθ, thus leading to a net increase in tensile strength of UHPC by 60%.