International Concrete Abstracts Portal

The bond property between deformed bars and concrete plays a significant role in the safety of construction. Numerous database-dependent empirical models are proposed to evaluate the bond behavior without considering the effect of additional confinement, whose application range is quite limited as a result of unstable accuracy. In this paper, a new model was established based on the thick-walled cylinder model and fictitious crack theory, which can predict bond strength and bond-slip response with fiber-reinforced polymer (FRP)-steel confinement. The effects of various factors on the bond behavior, such as concrete strength, concrete cover, rebar diameter, bar surface geometry, and FRP/steel confinement, were comprehensively discussed. According to the radial crack radius, the radial stress and displacement induced on the bond interface can be calculated, and thus the analytical formulae of bond strength and slip were respectively developed in conjunction with deformed bar surface geometry. Finally, a new analytical model was proposed, which can simulate the bond-slip curves of the specimens with different confinement levels, covering unstrengthened, FRP-strengthened, stirrup-strengthened, and FRP-stirrup dually strengthened specimens. Compared with existing models, the proposed model can provide better agreement with existing test results.

During past earthquakes, failures of beam–column joints have commonly been observed on the exteriors of buildings. However, only one side of these joints can be retrofitted because of the presence of beams on the other three sides. Therefore, this study aims to test four exterior beam–column joints with transverse beams, leaving the rear side as the only viable location for placing fiber-reinforced polymer (FRP) laminate. All four test specimens are designed with insufficient joint shear strength, as determined by ACI 318 equations, while satisfying the criteria for a strong-column–weak-beam mechanism and sufficient development length for bar anchorage. A total of two un-retrofitted specimens, with and without joint hoops, are constructed as controls. Subsequently, two similar specimens are retrofitted by applying an FRP laminate on the rear side. The results show that sufficient FRP laminate can enhance the seismic performance of joints in terms of deformability, energy dissipation, and failure delay.

Corrosion—one of the major threats to the integrity of concrete structures—can consequently affect structure serviceability and ultimate limit state, possibly resulting in failure. Glass fiber-reinforced polymer (GFRP) can be used as an innovative alternative for conventional steel reinforcement in concrete structures, effectively addressing corrosion issues. In addition to its corrosion resistance and high strength-to-weight ratio, GFRP is commonly selected for non-prestressed bars and stirrups due to its cost advantage over other FRP materials. The study endeavored to provide a comprehensive overview of the shear resistance in GFRP-RC beams with short shear spans. The manuscript aims to synthesize and analyze shear test data based on published studies on GFRP-RC beams with a short shear span (a/d = 1.5 to 2.5). A comprehensive literature review was conducted to compile a database comprising 64 short GFRP-RC beams to evaluate the efficiency of using the strut-and-tie model (STM) for predicting the shear resistance of GFRP-RC beams. The findings reveal that the ACI 318 (2019) STM yielded the most accurate predictions of the shear resistance of GFRP-RC beams with shear span-to-depth ratios of 1.5 to 2.5, since the current ACI 440.11 and ACI 440.1R design codes and guidelines do not include shear equations using the strut-and-tie model for predicting the shear resistance of GFRP-RC beams. Based on the findings of this study, the results could contribute to establishing shear equations in the upcoming revision of the ACI 440.11 and ACI 440.1R design codes and guidelines, specifically tailored for designing short GFRP-RC beams using the strut-and-tie model. The study also provides sufficient data to apply the strut-and-tie model in the design of GFRP-RC beams.

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 reinforcing bars’ lower axial stiffness leads to greater crack widths when FRP reinforcing bars elongate, resulting in significantly lower flexural stiffness for FRP 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 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-reinforced concrete members, resulting in a lower strength reduction, ϕ, value of 0.65. In this regard, using FRP-reinforced ultra-high-performance concrete (UHPC) members offer a novel solution, providing high strength, stiffness, ductility, and corrosion-resistant characteristics. UHPC has a very low water-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 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.

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 x 400 mm (15.8 x 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 toward the use of longitudinal hybrid reinforcement in lateral-resisting systems.