Feasibility of Reducing the Fiber Content in Ultra-High-Performance Fiber-Reinforced Concrete under Flexure
In this study, the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) is examined as a function of fiber length and volume fraction. Straight steel fiber with three different lengths (lf) of 13, 19.5, and 30 mm and four different volume fractions (vf) of 0.5%, 1.0%, 1.5%, and 2.0% are considered. Test results show that post-cracking flexural properties of UHPFRC, such as flexural strength, deflection capacity, toughness, and cracking behavior, improve with increasing fiber length and volume fraction, while first-cracking properties are not significantly influenced by fiber length and volume fraction. A 0.5 vol % reduction of steel fiber content relative to commercial UHPFRC can be achieved without deterioration of flexural performance by replacing short fibers (lf of 13 mm) with longer fibers (lf of 19.5 mm and 30 mm).
Biaxial flexural behavior of ultra-high-performance fiber-reinforced concrete with different fiber lengths and placement methods
Abstract This study investigates the effects of fiber length and placement method on the biaxial flexural behavior and fiber distribution characteristics of ultra-high-performance fiber-reinforced concrete (UHPFRC). A number of UHPFRC panels including three different fiber lengths were fabricated using two different placement methods, and an image analysis was performed to quantitatively evaluate the fiber distribution characteristics such as fiber orientation, fiber dispersion, and number of fibers per unit area. The biaxial flexural performances including load carrying capacity, energy absorption capacity, and cracking behavior were found to be improved with the increase in fiber length up to 19.5 mm. The biaxial flexural performances were also influenced by the placement method; the specimens with concrete placed at the center (maximum moment region) showed better flexural performances than those with concrete placed at the corner. These observations were confirmed by the image analysis results, which showed poorer fiber orientation and fewer fibers across the crack surfaces at the maximum moment region for the specimens with concrete placed at the corner, compared with their counterparts.
Fiber distribution and orientation in UHP-FRC beams and their effect on backward analysis
AbstractIn this study 23 ultra-high performance fiber reinforced concrete (UHP-FRC) notched beams were tested in order to investigate the effect of fiber orientation and distribution on the bending behavior. Due to the small fiber diameter and high fiber volume fraction used in UHP-FRC, the number of fibers crossing a 150 × 150 mm (6 × 6 in.) beam section can be as high as 26,000. An opto-analytical method was used to investigate the number and orientation of the fibers near the critical crack in each test beam. For the casting method used, which was in accordance with the recommendation of Association Francaise de Genie Civil (AFGC) and Service d’etudes techniques des routes et autoroutes (SETRA), a high degree of uniformity in fiber distribution and orientation was found over the beam cross section. Based on the analytically determined fiber orientation and distribution, shape functions were derived that take into account variations over the beam height. The proposed shape functions were incorporated in a backward analysis of the results from bending tests to estimate the uniaxial tensile behavior of the UHP-FRC used. The sensitivity of the backwardly calculated tensile behavior of UHP-FRC to variations in fiber density and orientation was evaluated. Based on the bending test results of the notched beam specimens, and for the casting method used in this research, the influence of these variations on the backwardly calculated tensile strength was found to be small, with a maximum stress variation of ~5 % for any given crack width.
