International Concrete Abstracts Portal

This paper presents the behavior of unreinforced cylindrical concrete elements confined with a hybrid system, consisting of an ultra-high-performance concrete (UHPC) jacket and basalt fiber-reinforced polymer (BFRP) grids. For exploring the feasibility of the proposed strengthening scheme, a series of tests are conducted to evaluate material properties and to obtain results related to interfacial bond, load-bearing capacity, axial responses, and failure modes. To understand the function of the individual components, a total of 57 cylinders are loaded under augmented confining conditions, including plain cores with ordinary concrete (CONT), plain cores with UHPC jackets (Type A), and plain cores with UHPC jackets plus BFRP grids (Type B). By preloading the cores at up to 60% of the control capacity (60%fc′) before applying the confinement system, the repercussions of inherent damage that can take place in vertical members on site are simulated. The compressive strength of UHPC rapidly develops within 7 days, whereas its splitting strength noticeably ascends after 14 days. The adhesion between the ordinary concrete and UHPC increases over time. While the Type B specimens outperform their Type A counterparts in terms of axial capacity by more than 18%, reliance on the BFRP grids is reduced with the growth of UHPC’s strength and adhesion because of the interaction between the hardened UHPC and the core concrete. The adverse effects of the preloading are noteworthy for both types, especially when exceeding a level of 30%fc′. The BFRP grid-wrapping alleviates the occurrence of a catastrophic collapse in the jacketed cylinders, resulting from a combination of the axial distress and lateral expansion of the core. Analytical models explain the load-carrying mechanism of the strengthened concrete, including confinement pressure and BFRP stress. Through parametric investigations, the significance of the constituents is clarified, and design recommendations are suggested.

The present study demonstrates the feasibility of using longitudinal hybrid reinforcement in concrete columns in seismic zones. In this research, four concrete columns were constructed and subjected to quasi-static cyclic loading, featuring a combination of steel and glass fiber-reinforced polymer (GFRP) longitudinal reinforcement. Two reference columns were fabricated and reinforced in the longitudinal direction with steel bars. These columns had a 400 × 400 mm (15.8 × 15.8 in.) cross-section and 1850 mm (72.8 in.) overall height. All the columns were reinforced with GFRP crossties and spirals in the horizontal direction. The variable parameters were the transverse reinforcement spacing, axial load ratio, and column configuration. The outcomes of this research clearly showed that reinforced concrete (RC) columns that are properly designed and detailed longitudinally with hybrid reinforcement (GFRP/steel) could achieve the drift limitation in building codes with no strength degradation. Further, these hybrid-RC columns displayed enhanced energy dissipation capacity, superior ductility, and improved post-earthquake recoverability compared to columns reinforced longitudinally with steel. The promising results of this study represent a step towards the use of longitudinal hybrid reinforcement in lateral-resisting systems.

This paper investigates the mechanical characteristics (encompassing compressive strength, flexural strength, toughness, and impact resistance) of ultra-high-performance fiber-reinforced concrete (UHPFRC) incorporating polypropylene (PP) and polyvinyl alcohol (PVA) fibers. An experimental program was conducted, based on which the polymer and metallic fibers were used at the same fiber content, and different sets of single and hybrid fiber reinforced composites were fabricated and tested. Despite the fact that it has been exhibited through previous research that the hybridized PVA-PP fibers do not result in the development of the mechanical characteristics of engineered cementitious composites (ECCs), the UHPC composites incorporating such hybrid fibers show augmented levels of toughness, flexural strength, and resistance to impact loads. A comparison was also made to assess the potentiality of the used fibers in terms of environmental impact and cost. Based on the results, hybridization with PVA and PP fibers leads to remarkable improvement in technical performance and mitigation of the economic and environmental impact of UHPFRC composites.

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.

Flexural strength and ductility of exclusively bonded or unbonded steel prestressed concrete (PC) members are well covered and documented in the literature and codes of practice. However, current design methods are limiting the use of hybrid (i.e., a combination of unbonded and bonded steel and Fiber Reinforced Polymer (FRP)) tendons, particularly when using brittle material such as FRP tendons. In this paper, a general procedure for evaluating the nominal moment capacity and ductility of hybrid PC members was developed using the strain compatibility approach. The procedure is applicable for members with any combination of bonded or unbonded steel and FRP tendons. Using a capacity design approach based on strain compatibility, the ductility performance of several hybrid systems with different parameters was compared. The parameters included, among others, the level of “net tensile strain” in the tension reinforcement at nominal strength adopted in ACI 318-19 as a measure of ductility; concrete compressive strength; and the newly defined hybrid prestressing ratio (HPR). HPR represents the ratio of the moment contribution of the unbonded tendons to the total moment capacity of the member with hybrid tendons. Non-linear analysis was carried out to generate the entire load-deflection and moment-curvature responses of the different systems. The accuracy of the nonlinear analysis was verified by comparing with available experimental data and the analysis results were used to compare traditional curvature ductility measures of the various systems against the ductility measure specified in the ACI Building code. A design example is provided in Appendix A to illustrate the use of the strain compatibility approach.