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Ultrahigh-performance concrete (UHPC) has been considered for a variety of structural engineering applications, owing to its superior strength and durability. Among the main properties of UHPC, flexural performance characteristics largely depend on the type and dosage of fibers incorporated into the mixture. Noting this critical aspect and the fact that the availability and cost of fibers directly contribute to choosing UHPC over other cementitious composites, this study explored the possibility of a partial replacement of steel fibers conventionally used in UHPC mixtures with synthetic fibers. With a focus on workability and strength properties, a total of 15 mixture designs were developed, investigating five different synthetic fibers, including nylon, polypropylene, polyvinyl alcohol, alkali-resistant glass, and carbon fibers. Three mixtures were prepared for each fiber type by varying the dosages of synthetic and steel fibers. From the conducted tests, load-deflection curves were extracted, further to flexural strength and toughness characteristics. To properly understand the progress of crack formation and propagation, as well as the effectiveness of various fibers in the postcracking response of UHPC, the obtained results were paired with digital image correlation data. Through holistic comparisons, the outcome of the presented study provided original information on the extent of contribution of some of the most common synthetic fibers to the strain distribution, crack resistance, and flexural performance of UHPC. [ABSTRACT FROM AUTHOR]

Ultra‐high performance concrete (UHPC) is increasingly used for various engineering applications, owing to its superior strength properties. Such properties, however, can be accompanied by workability issues, especially if the dosage of steel fibers is significantly increased. As a potential solution, this study explored the use of a combination of steel microfibers and macrofibers through a holistic experimental testing program. This was to understand the effects of steel fiber combinations on the workability and flexural strength of various UHPC mixtures. The microfibers were investigated for five different dosages from 1.0% to 3.0% (with an increment of 0.5%). In parallel, two mixtures were prepared with a 2.0% dosage of macrofibers in the form of twisted wire and hooked fibers. At the next stage, six additional mixtures were prepared with a combination of microfibers and macrofibers of various dosages. Further to the measurements extracted from the workability and flexural strength tests, digital image correlation (DIC) analyses were employed to obtain firsthand information about the widths and depths of cracks observed in the flexural specimens tested under an increasing load. The outcome of this study showed how a combination of microfibers and macrofibers can contribute to improving the flexural response characteristics of UHPC, especially without having to increase the dosage of steel fibers to the ranges that can adversely affect the mixture's workability. [ABSTRACT FROM AUTHOR]

This paper aims to comparatively evaluate the compressive and flexural response of high strength and ultra-high-performance polyvinyl Alcohol (PVA)- and polyethylene (PE)-reinforced ECCs at fiber contents of 1.75 vol% and 2.25 vol%, utilizing the same base mixture design. The base mixture was formulated with the objective of reducing the reliance on Portland cement by replacing 55% of it with supplementary cementitious materials (fly ash and silica fume). The flexural loading protocol consisted of monotonic loading with a displacement rate of 0.3 mm/min and 2.0 mm/min, and a repeated/cyclic loading with a displacement rate of 2.0 mm/min. The developed PVA- and PE-reinforced ECCs exhibited significant enhancements in average compressive strength, with PVA-reinforced ECC showing a 19% improvement and PE-reinforced ECC demonstrating a 39% improvement compared to the control specimen. In terms of flexural strength, PVA-reinforced ECC exhibited a 51% enhancement, while PE-reinforced ECC achieved an impressive 199% improvement. Furthermore, a digital image correlation system (DIC) was employed to characterize the cracking behavior and strain response of the specimens. The developed ECCs exhibited multiple cracking and strain-hardening behavior, exceeding a flexural strain of 3%. PE-reinforced ECCs displayed narrower cracks under low loading rates, whereas PVA-reinforced ECCs demonstrated reduced crack widths subjected to high loading rates. [ABSTRACT FROM AUTHOR]

• A new steel –UHPFRC composite beam structure is proposed, aiming at the full and proper utilization of both materials. • The unique use of ① UHPFRC in both tension and compression and ② a half rolled section with continuous in-built steel dowels as shear connectors. • An experimental study is conducted to characterize the flexural and shear responses of the proposed structure. • The design principles for both flexural and shear resistance of the proposed structure are introduced based on the failure mechanism. Aiming at the full and proper exploitation of both steel and Ultra High Performance cementitious Fiber Reinforced Composites (UHPFRC) materials, this paper proposes a new steel-UHPFRC composite beam structure. The unique use of ① UHPFRC in both tension and compression and ② a half rolled section with continuous in-built steel dowels in combination with UHPFRC dowels (forming composite dowels as shear connectors) is highlighted. An experimental study consisting of two composite beams is then conducted to investigate the flexural and shear responses. In particular, the failure mode, cracking pattern, and monitoring of critical crack kinematics are discussed using digital image correlation (DIC) technology. Subsequently, the design principles for both flexural and shear resistance are introduced based on the failure mechanism: ① the determination of flexural resistance is based on the sectional analysis considering the tensile properties of UHPFRC, and ② the determination of shear resistance is based on a specific approach to the lever arm of internal forces and horizontal shear resistance of the composite dowel, considering the tensile contribution of UHPFRC. According to the experimental results, the effective interlocking between UHPFRC and the steel dowel allows efficient interaction between steel and UHPFRC components, benefiting from its higher shear resistance and ductility compared with traditional welded head studs. Both the flexural and shear response of the composite beam can be characterized into five distinguished domains, in which the quasi-elastic domain is introduced especially due to its large contribution to the resistance and high structural stiffness. Finally, the design principles are validated by the tested values. [ABSTRACT FROM AUTHOR]

The nature of cracking in flexural specimens made using ultra-high-performance concrete (UHPC) is studied in the context of mechanisms of enhanced strength and ductility. The effect of fibers on the suppression of cracking and the attendant toughening mechanisms are addressed at two different fiber contents to elucidate the strain-softening and strain-hardening responses. Results indicate that the first crack strength is directly related to the fiber content, with fibers extending the stable crack growth region at high fiber contents such that flexural strengths as high as 20 MPa are obtained. Fiber bridging results in resistance to crack growth, stabilization, as well as multiple crack formation, all of which contribute to toughening. Using a combination of experimental results, digital image correlation (DIC) measurements of crack extension, and an analytical model for the flexural response, the stress and strain distributions across the depth of the specimens are determined, leading to stress-crack width relationships, and consequently, the relationship between tensile and flexural stresses in fiber-reinforced UHPCs. The results are also used to determine the tensile and compressive strain and stress distributions. Using back-calculated tensile properties from the flexural response and the 3-D DIC results, crack width profile and the stress-crack width relationship in tension are obtained for UHPCs, leading to a comprehensive understanding required for the structural design using such high-ductility materials. [ABSTRACT FROM AUTHOR]