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Abstracts: Recently, Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) has offered notable advantages over other types of concrete. Therefore, a comprehensive investigation of the latest developments in Ultra-High-Performance Concrete (UHPC) is necessary to provide essential information for materials testing requirements and procedures and to expand its practical applications. The present work is a comprehensive four-part review of the UHPFRC. The current first part of the review focuses attention on the developments, principles, and raw materials of the UHPFRC. Part II covers the hydration and microstructure of the UHPFRC. Part III reviewed the fresh and hardened properties of the UHPFRC. Part IV covers the durability properties, cost assessment, applications, and challenges of the UHPFRC. This review is expected to advance the fundamental knowledge of UHPC and promote further research and applications of UHPC.

The direct tensile behavior of ultra-high-performance surface-modified steel fiber-reinforced concrete was evaluated in this study. Various chemical modifications were applied to steel fibers, including acetone washing, hydrochloric acid washing, zinc phosphating, silica coating, and chelation using an ethylenediaminetetraacetic acid (EDTA) electrolyte solution, which improved the tensile strength, tensile strain, and g-value to a maximum of 17.76 MPa, 1.22%, and 144.51 kJ/m3, respectively. Acetone washing and EDTA chelating were the most effective methods for improving the tensile strength, whereas silica coating was the best for improving the strain capacity and energy absorption capacity. The optimal treatment time for EDTA chelation treatment was approximately 6 h, and the tensile performance decreased considerably after 12 h of treatment. Comparing these results with those of pullout experiments revealed that a high shear stress should be maintained after the fiber has fully debonded to effectively enhance the post-cracking tensile performance.

This paper presents a study on the flexural behavior of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) beams, which included conventional reinforcing bars. The study focuses on critical design aspects, such as concrete shrinkage and cracking implications on the tension-stiffening phenomenon. An experimental program with two different sized flexural reinforced UHPFRC beams was run. Beams were cast and tested in a four-point bending test (4PBT) using UHPFRC with different amounts of fibers: 130 and 160 kg/m3 (1.66% and 2.00% in vol.) to cover a wide range of strain-softening and strain-hardening constitutive UHPFRC behaviors. A non-linear finite element model (NLFEM) was developed to validate the mechanical tensile characterization of UHPFRC when applied to reinforced elements. Both shrinkage and tension-stiffening effects were considered to improve the model. After the NLFEM simulation, very reliable results were obtained at both the service and ultimate load levels compared to the experimental ones. Finally, some aspects about the design of reinforced UHPFRC cross-sections under bending forces are addressed and satisfactorily compared to the experimental results.

In this study, we investigated the effects of steel fiber type and specimen thickness on the uniaxial and biaxial flexural behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC). For this purpose, three types of steel fibers (straight, three-times twisted, and six-times twisted) and three thicknesses of specimen (24, 48, and 72 mm) were used. The test results indicated that, owing to the larger perimeter of the triangular shape and mechanical anchorage effect, the twisted steel fibers exhibited better pullout resistance than the straight steel fiber with a circular shape, and its effectiveness increased with the number of ribs. In contrast, the best flexural behavior of UHPFRC was observed when the straight steel fiber was used under both uniaxial and biaxial stress states, and the six-times twisted steel fiber exhibited the worst flexural performance owing to the excessive bond strength of the composites. The uniaxial and biaxial flexural strengths of UHPFRC were insignificantly influenced by the sample thickness; however, the normalized toughness decreased with an increase in the thickness. A higher flexural strength, normalized toughness up to the peak, and deformability were observed under the biaxial flexural stress state than those under the uniaxial flexural stress state. The use of twisted steel fibers was more effective for slabs subjected to biaxial flexural stress than that for uniaxial beams.