International Concrete Abstracts Portal

This study investigated the serviceability behavior and strength of polypropylene-fiber (PF) reinforced self-consolidating concrete (PFSCC) beams reinforced with glass fiber-reinforced polymer (GFRP) bars. Five full-scale concrete beams measuring 3100 mm long × 200 mm wide × 300 mm deep (122.1 × 7.9 × 11.8 in.) were fabricated and tested up to failure under four-point bending cyclic loading. Test parameters included the longitudinal reinforcement ratio (0.78, 1.18, and 1.66%) and polypropylene fiber (PF) volume (0, 0.5, and 0.75% by concrete volume). The effect of these parameters on serviceability behavior and strength of the test specimens is analyzed and discussed herein. All the beams were evaluated for cracking behavior, deflection, crack width, strength, failure mode, stiffness degradation, and deformability factor. The test results revealed that increasing the reinforcement ratio and PF fiber volume enhanced the serviceability and flexural performance of the beams by effectively restraining crack widths, reducing deflections at the service and ultimate limit states, and decreasing residual deformation. The stiffness exhibited a fast-to-slow degradation trend until failure for all beams, at which point the beams with a higher reinforcement ratio and fiber volume evidenced higher residual stiffness. The cracking moment, flexural capacities, and crack width of the tested beams were predicted according to the North American codes and design guidelines and compared with the experimental ones. Lastly, the deformability for all beams was quantified with the J-factor approach according to CSA S6-19. Moreover, the tested beams demonstrated adequate deformability as per the calculated deformability factors.

Noncorrosive fiber-reinforced polymer (FRP) reinforcement presents an attractive alternative to conventional steel reinforcement, which is prone to corrosion, especially in harsh environments exposed to deicing salt or seawater. However, FRP rebars’ lower axial stiffness leads to greater crack widths when FRP reinforcing bars elongate, resulting in significantly lower flexural stiffness for FRP-reinforcing bar-reinforced concrete members. The deeper cracks and larger crack widths also reduce the depth of the compression zone. Consequently, both the aggregate interlock and the compression zone for shear resistance are significantly reduced. Additionally, due to their limited tensile ductility, FRP reinforcing bars can rupture before the concrete crushes, potentially resulting in sudden and catastrophic member failure. Therefore, ACI Committee 440 states that through a compression-controlled design, FRP-reinforcing bar-reinforced concrete members can be intentionally designed to fail by allowing the concrete to crush before the FRP reinforcing bars rupture. However, this design approach does not yield an equivalent ductile behavior when compared to steel-reinforcing bar-reinforced concrete members, resulting in a lower strength reduction, ϕ, value of 0.65. In this regard, using FRP-reinforcing bar-reinforced ultra-high-performance concrete (UHPC) members offers a novel solution, providing high strength, stiffness, ductility, and corrosion-resistant characteristics. UHPC has a very low water-to-cementitious materials ratio (0.18 to 0.25), which results in dense particle packing. This very dense microstructure and low water ratio not only improves compressive strength but also delays liquid ingress. UHPC can be tailored to achieve exceptional compressive ductility, with a maximum usable compressive strain greater than 0.015. Unlike conventional designs where ductility is provided by steel reinforcing bars, UHPC can be used to achieve the required ductility for a flexural member, allowing FRP reinforcing bars to be designed to stay elastic. The high member ductility also justifies the use of a higher strength reduction factor, ϕ, of 0.9. This research, validated through large-scale experiments, explores this design concept by leveraging UHPC’s high compressive ductility, cracking resistance, and shear strength, along with a high quantity of noncorrosive FRP reinforcing bars. The increased amount of longitudinal reinforcement helps maintain the flexural stiffness (controlling deflection under service loads), bond strength, and shear strength of the members. Furthermore, the damage-resistant capability of UHPC and the elasticity of FRP reinforcing bars provide a structural member with a restoring force, leading to reduced residual deflection and enhanced resilience.

Portland cement has played a significant role in the construction of major infrastructure and building structures. However, in light of the substantial CO₂ emissions associated with its production, there is a growing concern about environmental issues. Accordingly, the development of eco-friendly alternatives is actively underway. Geopolymer represents a class of inorganic polymers formed via a chemical interaction between solid aluminosilicate powder with alkali hydroxide and/or alkali silicate compounds. Concrete made with geopolymers, as an alternative to Portland cement, generally demonstrates comparable physical and durability characteristics to ordinary Portland cement concrete (OPC). Research on the material properties of geopolymer concrete (GPC) has made extensive progress. However, the number of large-scale tests that were conducted to assess its structural performance is still insufficient. Additionally, there is a shortage of comprehensive studies that compile and analyze all the structural experiments conducted thus far to evaluate the GPC’s potential. Therefore, this study aimed at compiling and analyzing a number of bond, flexural, shear, and axial strength tests of GPC to assess its potential as a substitute for OPC and to identify its distinctive characteristics compared to OPC. As a result, it is considered that GPC can be used as a substitute for OPC without any structural safety issues. However, caution is needed in terms of deflection and ductility, and additional experiments are deemed necessary in the aspect of compressive strength of large-scale members.

End hooks of steel fibers provide a stronger bridging force across the concrete matrix in steel fiber-reinforced concrete (SFRC). In this work, SFRC beams were prepared with steel fibers of the same length and diameter but different types of end hooks (straight, three-dimensional [3D], four-dimensional [4D], and fivedimensional [5D]) at increasing fiber volumes (0.0, 0.5, 1.0, 1.5, and 2.0%). Four-point bending tests conducted on each SFRC beam yielded load-deflection curves, from which the first cracking strength, flexural strength, and fracture toughness up to certain deflection-to-beam length ratios were obtained. The test results showed that the presence of end hooks remarkably enhanced the flexural strength and toughness of the SFRC beams, and this enhancement was amplified with an increasing number of hooks. Quantitative analysis revealed the hooking index, a factor introduced herein to delineate the efficiency of various types of hooks, was 1.00, 1.30, 1.60, and 2.10, respectively, for straight, 3D, 4D, and 5D steel fibers used in the present study. Lastly, empirical models for predicting flexural strength and toughness were established with high prediction accuracy.

This study investigates the behavior of recycled steel fibers recovered from waste tires (RSF) and industrial hooked-end steel fibers (ISF) in two single and hybrid reinforcing types with different volume content, incorporating microstructural and macrostructural analyses. Scanning electron microscopy (SEM) is used to study the microstructure and fractures, focusing on crack initiation in the fiber interface transition zone (FITZ). The macrostructural analysis involves using digital image correlation (DIC) software, Ncorr, to analyze the split tensile behavior of plain and FRC specimens, calculating strain distribution, and investigating crack initiation and propagation. The SEM study reveals that industrial fibers due to the presence of hooked ends promoted improved mechanical interlocking, anchors within the matrix, frictional resistance during crack propagation and significantly improved load transfer have better bonding, crack bridging, and crack deflection compared to recycled fibers. Recycled steel fibers significantly delay crack initiation and enhance strength in the pre-peak zone. The study suggests hybridizing recycled fibers from automobile tires with industrial fibers as an optimum strategy for improving tensile performance and utilizing environmentally friendly materials in FRC.