Effects of hybrid fiber reinforcement and silica sand dosage on the mechanical properties of ultra-high-performance alkali-activated concrete

Abstract

Effects of hybrid reinforcement and various silica sand dosages on the mechanical properties of ultra-high-performance alkali-activated concrete Gi Woong Kim Dept. of Architectural Engineering The Graduate School Hanyang University The growing demand for high-rise and large-scale structures has spurred advancements in construction technology, leading to an increased focus on high-strength and high-performance concrete. These trends have increased the need for the development of ultra-high-performance concrete (UHPC) and ultra-high-performance fiber-reinforced concrete (UHP-FRC). However, the environmental impact of excessive cement use in UHPC and UHP-FRC is a significant concern, given the cement industry’s contribution to greenhouse gas emissions. Therefore, in response to global climate challenges, the paper discusses the potential of alkali-activated concrete (AAC) as a sustainable alternative to traditional cement-based materials. Specifically, ultra-high-performance alkali-activated concrete (UPH-AAC) is highlighted for its remarkable properties, such as high compressive strength, low permeability, and resistance to fire and sulfates. In this paper, similar to UHPC, UHP-AAC uses steel fibers as reinforcement to address its inherent brittleness. In addition to this, ductility was enhanced by incorporating a hybrid combination of different Polyethylene (PE) fibers. In addition, the appropriate combination of hybrid fiber reinforcement ratio and Sand-to-binder (S/B) ratio was confirmed by adjusting various S/B ratios according to the substitution ratio of the fiber. The hybrid fiber reinforcement ratio used was steel fiber: PE fiber, 2%: 0% to 0%: 2%, with a total of 5 substitution rates of 0.5%. A total of 4 S/B ratios were applied: 0.16, 0.32, 0.48, and 0.64. The performance of UHP-AAC was evaluated using porosity, compressive strength, tensile strength, tensile strain capacity, tensile strain energy density, cracking behavior, and PSH indices. To analyze the relevant evaluation indicators, MIP analysis, compressive strength and elastic modulus measurements, three-point bending test, direct tensile strength test, and single-crack tensile strength test were performed. Substituting steel fibers with PE fibers resulted in reduced compressive and tensile strengths, with the most favorable performance observed in hybrid specimens featuring a moderate S/B ratio. As the PE fiber replacement ratio increased and the S/B ratio decreased, the occurrence of multiple microcracks escalated. Notably, the S/B ratio had a more pronounced influence on the variation of pseudo-strain hardening indices compared to the fiber replacement ratio. The highest recorded complementary energy, amounting to 1,762.5 J/m2, was achieved in UHP-AAC reinforced with PE fibers at an S/B ratio of 0.16, while the lowest value, 309.6 J/m2, was noted in UHP-AAC reinforced with steel fibers at the same S/B ratio. Thus, optimizing the hybrid reinforcement of UHP-AAC necessitates a thoughtful consideration of both the S/B ratio and fiber replacement ratio.