Longitudinal Steel Bars Research Articles

Experimental study on strengthening of RC beams with Side Near Surface Mounted technique-CFRP bars

This paper presents an experimental program that was carried out for the purpose of studying the global flexural performance of reinforced concrete beams strengthened internally with Carbon Fiber Reinforced Polymer (CFRP) rods using the Side Near Surface Mounted (SNSM) technique. The CFRP rods were placed laterally, adjacent to the longitudinal steel bars inside precut grooves. The strengthening length and position of CFRP rods as well as the type of filling material were the main variables investigated in this study. Moreover, a detailed comparison between the Side Near Surface Mounted (SNSM) and the Near Surface Mounted (NSM) techniques for the strengthening of RC beams using CFRP rods was conducted as well; in order to validate and assess the effectiveness of the SNSM technique.The test results showed that using the SNSM-CFRP rods technique allowed improving in a significant way the load carrying capacity of RC beams but decreased their ductility along the deflection at maximum load. The results obtained indicated that the failure mode was influenced by the length of CFRP rods and the filling material characteristics, while the strengthening position did not have significant impact. The SNSM strengthening technique can be used as an alternative method to the NSM method, and in some cases, it may be used to prevent the non-conventional failure modes, due to degradation of the NSM strengthening system, such as the pull out of CFRP-rods or premature debonding failure. The conventional analytical model accurately predicted the strength capacity and mid-span deflection of the SNSM strengthened beams.

High-performance composite bridge deck with prestressed basalt fiber-reinforced polymer shell and concrete

In this study, a basalt fiber-reinforced polymer (BFRP) shell–concrete composite bridge deck with prestressed BFRP strips was investigated. The effectiveness of the prestressed shell was verified based on a preliminary study. Eight full-scale specimens were designed to examine the static and fatigue behaviors, including the failure modes, ultimate capacities, load–displacement curves of the bridge deck, and key areas of the BFRP shell and prestressed BFRP strips. The parameters of the bridge deck included the surface conditions, fiber layout of the shell, position of the reinforcement, and adoption of the prestressed BFRP strips. The theoretical formulas of the composite bridge deck were also derived. The results show that the composite decks exhibit excellent integrated behavior including high stiffness, high capacity, and reliable interfacial bonds. The specimen with adhered sand achieved an evidently higher stiffness and capacity than the control specimen. The corrugated teeth were beneficial for the stiffness but irrelevant for the ultimate capacity of the specimens. The layout of the fiber mat rather than that of the fiber filament in the BFRP shell could slightly enhance the ultimate capacity but not affect the stiffness. The arrangement of the longitudinal steel bars in the tension zone and stirrups in the deck could significantly enhance the stiffness and capacity of the deck, respectively. The fatigue test proved that this bridge deck could survive 10 million cycles under a load level of 0.439 and a load amplitude of 96.5 kN, for which the residual capacity and stiffness reduced by less than 8%. The finite element models could reflect the real failure process and yielded a reliable capacity value, whereas the theoretical formulas provided conservative results of the capacity and stiffness.

Flexural behavior of hybrid concrete-filled fiber reinforced polymer tube columns

The concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) system has been widely studied as a durable alternative design for bridge columns. Research has shown that utilizing a small number of longitudinal steel bars in the concrete core provides the energy dissipation characteristics required for resisting extreme loads, such as seismic and blast. However, the construction complexity associated with the addition of rebar discourages the implementation of this system. In a novel approach, this study proposes and investigates the structural performance of CFFTs with a hybrid glass/steel fiber reinforced tube. This hybrid CFFT (HCFFT) system integrates the required longitudinal steel reinforcement into the FRP shell in the form of 30-µm fibers. This study details the experimental results from two CFFT and four HCFFT circular specimens tested under half-cyclic four-point bending. The effect of concrete slippage inside the tube was investigated. The flexural capacity, residual deformation, damage, and energy absorption capacity were experimentally evaluated. The HCFFT specimens exhibited higher energy absorption and lower degradations in stiffness compared to the CFFTs at comparable flexural deformations. Moment-curvature analysis was used to quantify the contribution of the concrete core when it is allowed to slide inside the tube.

Numerical parametric study on ultimate load and ductility of concrete encased equal-leg angle steel composite columns

Steel-concrete composite high bridge pier has been applied increasingly in China and around the world. Most applied steel type in the composite piers are H-shaped steel and steel pipe, while seldom research or practice is associated with angle steel. This paper conducted parametric study on the composite column with equal-leg angle steel and aimed to investigate the ultimate load and displacement ductility of the composite columns with different parameters. The parameters include the type of shear connector (stud and perfobond connectors), the type of structural steel (H-shaped steel and angle steel), steel-plate hooping ratio, shear-span ratio, and axial compression ratio. Finite element analysis was conducted for each specimen, which incorporated the concrete confinement effect, as well as the inelastic behavior of concrete, structural steel, and longitudinal and transverse steel bars. The equal-leg angle steel composite column was found to have slightly higher strength and displacement ductility than H-shaped steel composite column. The increase of steel-plate hooping result in larger strength and displacement ductility for the composite column, and the increase of shear-span ratio and axial compression ratio decrease the displacement ductility. Research results suggest stud and perfobond shear connectors should be applied as axial compression ratio being larger than 0.2 and 0.3, respectively. This paper provides reference for research and engineering practice of the concrete encased angle steel composite columns and bridge piers.

Behavior of circular stainless steel stub columns internally strengthened by longitudinal carbon steel bars

In this paper, experimental and numerical investigations into the performance of circular unfilled and concrete filled stainless steel tubular stub columns strengthened by carbon steel bars welded to the inner surface were presented. The carbon steel bars were fabricated to be a structural component of the stub column and directly participate in supporting the axial load. Ten stub columns of 141.3 mm outer diameter and 3.4 mm thickness were tested under axial compression load for different numbers and sizes of carbon steel bars. 3D finite element models (FEMs) were carried out for the strengthened stub columns by using finite element program ABAQUS and validated with the experimental results. The validated numerical work was utilized to carry out a parametric study to assess (i) the practical configuration of the carbon steel bars, (ii) the possible reduction of stainless steel thickness, which can be compensated for by the addition of carbon steel bars, (iii) the effect of the diameter to thickness ratio (D/t) on the stub column performance. The numerical work was further utilized to generate extensive data to validate the load carrying capacity predicted by the ACI and Eurocode4 codes and the continuous strength method (CSM). The experimental and numerical results demonstrated that this strengthening technique enhanced the axial load carrying capacity for unfilled and filled stainless steel stub columns. The results from the parametric study confirmed that (i) strengthening by two carbon steel bars is the most practical configuration to minimize the welding process, (ii) a substantial reduction in stainless steel thickness can be achieved by using carbon steel bars, (iii) the ultimate load carrying capacity is inversely proportional to the D/t ratio. CSM was the most accurate method for predicting the failure load of the strengthened stainless steel tube.

Effect of reinforcement grade and concrete strength on seismic performance of reinforced concrete bridge piers

This paper experimentally investigates the effect of the yield strength of longitudinal steel bars and concrete compressive strength on the seismic performance of reinforced concrete (RC) bridge piers. Cyclic loading tests of ten large scale RC bridge piers (including five circular section and five rectangular section) with different longitudinal steel bars grades (varying among HRB335, HRB500E and HRB600) and concrete compressive strengths (i.e., 32.6 MPa and 48.2 MPa) were conducted. Two different steel replacement methods, i.e. equivalent volume and equivalent strength replacements, were adopted for comparison. The seismic performance indexes lateral bearing capacity, deformation capacity, hysteretic energy dissipation and residual displacement were compared and analyzed. The test results indicate that all RC bridge piers show typical flexural failure mode while circular piers show better deformability and hysteretic energy dissipation than rectangular piers. Replacing conventional steel bars HRB335 with same amount of high-strength longitudinal steel bars (HRB500E and HRB600) (i.e. equivalent volume replacement) will increase the lateral bearing capacity (as expected), the yield displacement and the total deformation. Due to the increase in yield displacement, the displacement ductility, the viscous damping ratio and the residual displacement will reduce to some extent. Still, the RC piers reinforced with high-strength steel bars (HSSB) show larger plastic deformation and hysteretic energy dissipation because of greater total deformation and bearing capacity. The transverse reinforcement configurations of circular section provide better restraint to the buckling of longitudinal steel bars than rectangular section, thereby RC bridge piers with circular section show better deformability and hysteretic energy dissipation than rectangular section. The increase in concrete compressive strength reduces the yield displacement, thus enhances the hysteretic energy dissipation and viscous damping ratio at same total deformation. It is worth noting that the effect of concrete compressive strength on the seismic performance of rectangular section is more remarkable than circular section. Replacing conventional longitudinal steel bars with reduced amounts of high-strength steel bars (i.e. equivalent strength replacement) provided comparable flexural strength and deformation capacity, but reducing the residual displacement and hysteretic energy dissipation.

Limit Equilibrium Method-based Shear Strength Prediction for Corroded Reinforced Concrete Beam with Inclined Bars.

Shear strength is a widely investigated parameter for reinforced concrete structures. The corrosion of reinforcement results in shear strength reduction. Corrosion has become one of the main deterioration factors in reinforced concrete beam. This paper proposes a shear strength model for beams with inclined bars based on a limit equilibrium method. The proposed model can be applied to both corroded and uncorroded reinforced concrete beams. Besides the tensile strength of longitudinal steel bars, the shear capacity provided by the concrete on the top of the diagonal crack, the tensile force of the shear steel at the diagonal crack, the degradation of the cross-sectional area and strength of the reinforcements induced by corrosion are all considered. An experimental work on two groups accelerated corroded beams was performed. Good agreements were found between the proposed theoretical predictions and experimental observations.

Experimental Investigation on Columns of Steel Fiber Reinforced Concrete with Recycled Aggregates under Large Eccentric Compression Load.

To improve the structural application of steel fiber-reinforced concrete with recycled aggregates (SFRC-RA) composited in gradation by large-particle natural coarse aggregate and small-particle recycled coarse aggregate, the large eccentric compression behavior of eight SFRC-RA columns was experimentally investigated in this paper. The main parameters considered were the strength of the SFRC-RA and the volume fraction of the steel fiber. Details about the sectional concrete strain, the longitudinal steel bar strain, the lateral displacement, the cracking load, the crack distribution and crack width, and the ultimate load of the SFRC-RA columns were measured. The beneficial effects of steel fiber on these attributes were discussed, and the ductility corresponding to the lateral displacement of the SFRC-RA columns was also analyzed. Based on the test results and design principles, formulas were proposed for predicting the cracking resistance, crack width, and lateral displacement of SFRC-RA columns in a normal service state. The ultimate loads of the SFRC-RA columns under a large eccentric compression load were calculated, considering the second-order effects.

Experimental Investigation on Shear Fatigue Behavior of Reinforced Concrete Beams with Corroded Stirrups

Stirrups in RC beams are more vulnerable to corrosion than longitudinal steel bars due to the thinner concrete cover, and the shear behavior may deteriorate faster than the flexural behavior under fatigue loading. An experimental study was conducted to investigate the deterioration mechanism of the shear fatigue behavior of RC beams with corroded stirrups. RC beams were corroded by an impressed current before fatigue loading was applied. The fatigue test results showed that all beams failed with fatigue fracturing of the stirrup, and the corrosion of stirrups had a significant influence on the shear fatigue behavior and fatigue life of RC beams. It was observed that fatigue cracks initiated and propagated rapidly around corrosion pits, and the stirrup fractured at the minimum cross section where the corrosion pits had formed. The strains in the stirrups, longitudinal steel bars, and concrete increased with an increase in the number of loading cycles, and a significant increase of the midspan deflection was observed in all beams. With the increasing corrosion degree of stirrups and load amplitude, the stress amplitude of the stirrups increased obviously, especially at the minimum cross section, and the stress concentration at the corrosion pits put the stirrups in a more adverse situation, resulting in faster deterioration of the shear fatigue behavior and a decrease in the fatigue life of RC beams with corroded stirrups.

Cyclic Flexural and Shear Performances of Beam Elements with Longitudinal Glass Fiber Reinforced Polymer (GFRP) Bars in Exterior Beam-Column Connections

The objective of this study is to examine the effect of the poor anchorage length of glass fiber reinforced polymer (GFRP) bars used for longitudinal reinforcement on the flexural and shear performances of beam elements in exterior beam–column connections made using high-strength materials. Six exterior beam–column connection specimens were tested under reversal cyclic loads applied at the free-end of the beam. The selected strength categories of materials in the beam element were as follows: 35 MPa and 70 MPa for the design compressive strength of concrete, 400 MPa and 600 MPa for the yield strength of conventional longitudinal steel bars, and 800 MPa for the tensile strength of the GFRP bar. All the longitudinal steel bars of the beams satisfied the minimum requirements of the provisions of ACI 318–14, whereas all the longitudinal GFRP bars of the beam were linearly anchored into the column section, resulting in poor anchorage length, especially for the beam with the concrete compressive strength of 35 MPa. The flexure-governed beams with GFRP bars exhibited a greater increasing rate in displacement at the pre-peak state and did not display the plastic flow characteristic after the peak load when compared with companion beams with steel bars. The beams with GFRP bars possessed lower diagonal cracking strengths and shear capacities than the companion beams with steel bars although the shear capacities of the beams with GFRP bars could be conservatively predicted using the design equation of ACI 440.1R–15 provision. The low elastic modulus and elongation capacity of GFRP bars resulted in large displacements and brittle post-peak beam performances. Furthermore, the lack of anchorage length of GFRP bars in exterior beam–column connection significantly reduced the flexural strength and ductility of the beam element.

Flexural performance of steel fibre reinforced concrete beams designed for moment redistribution

This paper presents an experimental study to investigate the flexural and moment redistribution performance of six full-scale two-span continuous reinforced concrete beams with and without steel fibres. The beams were reinforced with conventional bar reinforcement with longitudinal steel reinforcement ratios of between 0.0069 and 0.0138; four of the six beams also contained either 30 kg/m3 or 60 kg/m3 of steel fibres. The specimens were tested under monotonically increasing displacement-controlled loading. The primary variables within different test specimens were the arrangement of longitudinal steel bar and the dosage of steel fibres. The beams were designed for 30% of positive and negative moment redistribution with respect to the linear-elastic condition, the maximum allowed by different codes and standards. Test results showed that the inclusion of steel fibres increases the load carrying capacity of the reinforced concrete beams. Furthermore, the addition of steel fibres increased the number of cracks and decreased the crack spacing. For the steel reinforcement ratios tested, the RC-SFRC beams were found to have good ductility. It was shown that the RC-SFRC beams achieved full moment redistribution demand to form the collapse mechanism, and maintained their load carrying capacity for large plastic-hinge rotations. The guidelines of AS 5100.5-2017 were found to give a good prediction of the load carrying capacity of the tested RC-SFRC beams.

Multiscale analysis of non-contact splices at drilled shaft to bridge column interface

The contact and non-contact splices of the longitudinal steel bar and dowel bar have been widely adopted as the interface connection between pier columns and drilled shafts in bridge engineering. With the popularization and application of bridge substructures composed of the rectangular column and circular drilled shaft, the suitability and accuracy of the design provision specified in the current codes, which is derived from the investigation of the uniform circular components, are becoming the research hotspots in engineering and academia. In order to further study the mechanical behavior of the pier column-drilled shafts interface with inconsistent geometrical shapes, the experimental and numerical analysis of the interfaces with the overlap spacing of 0, 4, 6 and 8 in. are carried out systematically. The experimental results indicate that with the increment of lap spacing, the load-bearing capacity of the components will be consistently reduced, and the material strength also plays an important role in the structural performance. Compared with the specimen with the contact splice, the non-contact lap splice between the longitudinal bar and dowel bar will lead to additional shearing cracks and splitting cracks, and the inclination angle of the shearing cracks gradually increases with the enlargement of lap spacing as well. The relationship of the lateral displacement at the top of the column and the vertical reaction agrees well with that of the numerical analysis. Moreover, the fracture distribution and the interface separation can be explicitly depicted by 3D/2D numerical models based on homogeneous concrete assumption, which fully validates the rationality of the experimental observations. In order to exclude the influence caused by the variation of the concrete strength in tested specimens, the material properties in 3D and 2D simulation are uniformly specified according to the measured material strength of Specimen 1 and the numerical findings show that the structural stiffness of Specimens 1–4 decrease with the increment of lap spacing. However, the ultimate bearing capacity of the component is not apparently affected while using non-contact splices, accompanied by satisfied ductility. The detailed failure pattern and the crack evolution between overlapped steel bars are explicitly revealed by the 2D multiscale models. The maximum angle of diagonal cracks can be detected at the end of the overlap area, which is about 45°. The inclination of shearing cracks within the overlap area is slightly smaller and deteriorates with the enlargement of lap spacing. Meanwhile, the microcracks located in the non-overlap area are typically horizontal bending fractures. The research conclusion of this study based on large-scale experimental and numerical investigation is highly referential for the practical application of the interface connection of bridge column to drilled shaft with inconsistent geometry using non-contact splices.

Concentrated and Distributed Plasticity Models for Seismic Repair of Damaged RC Bridge Columns

Two models, Model Fiber and Model Rotational Spring (RS), simulating the seismic performance of repaired column-to-cap beam/footing connections using a carbon fiber-reinforced polymer (CFRP) donut are presented in this paper. In Model Fiber, distributed plasticity was assumed over a plastic hinge length of the nonlinear beam-column element. In Model RS, concentrated plasticity was considered using a nonlinear moment rotational spring located at the repaired cross section. Previous concrete damage and low-cycle fatigue of longitudinal steel bars as well as bond-slip between the damaged steel bars and surrounding concrete were included in the proposed numerical models. Numerical simulations show that the results are in good agreement with the experiments in terms of structural response, cumulative hysteretic energy, and moment-rotation capacity. Model Fiber can also predict the local response and low-cycle fatigue of the longitudinal steel bars. Using appropriate pinching parameters, Model RS can model accurate pinching behavior for both the repaired cast-in-place (CIP) and precast concrete specimens.

Experimental study on the durability of FRP bars reinforced concrete beams in simulated ocean environment

Abstract In this paper, the flexural performance of fiber-reinforced polymer (FRP) bars reinforced concrete (RC) beams after conditioning in a simulated seawater wet-dry cycling environment for 6, 9 and 12 months is experimentally investigated. Two types of FRP bars, i.e. basalt FRP (BFRP) bars and steel-FRP composite bars (SFCBs), are adopted. Steel bars are employed for comparison. During the conditioning, a constant load is coupled to the beams. A total of 24 simply supported beams are tested. In addition, microscopic damage to the conditioned BFRP bars is detected by scanning electron microscopy (SEM). The test results indicated that total corrosion was observed at the surface of the bottom longitudinal steel bars after a maximum exposure of 12 months. However, the degradation of the macro-mechanical properties of the steel bars RC beam was not distinct. The ultimate load of the BFRP bars RC beams after 6 months, 9 months and 12 months was reduced by 22%, 33% and 42%, respectively. The yield load and ultimate load of the SFCBs RC beams were reduced by a maximum of 18% and 38%, respectively. The SEM observations revealed that there were distinct damages at the outer layer of the BFRP bars after 12-month conditioning.

Modeling of Shear Behavior of Reinforced Concrete Beams with Corroded Stirrups Strengthened with FRP Sheets

AbstractBecause stirrups corrode earlier and faster than longitudinal steel bars, reinforced concrete (RC) structures may fail in shear instead of bending. Since shear failure is brittle and catast...

Contribution of longitudinal glass fiber-reinforced polymer bars in concrete cylinders under axial compression

Contribution of longitudinal glass fiber-reinforced polymer (GFRP) bars in concrete columns under compression has been ignored by current design guidelines. This paper challenges this convention by testing 21 concrete cylinders (150 mm × 300 mm) reinforced with longitudinal GFRP and steel bars in compression. It was observed that GFRP bars could sustain high level of compressive strains long after the peak load of the specimens without any premature crushing. The results of a new coupon test method showed that the elastic modulus of GFRP bars in compression is slightly higher than that of in tension, however the compressive strength was obtained 67% of tensile strength. An analytical model was successfully implemented to predict the axial capacity of the tests specimens and it was found that the contribution of the bars in the load capacity of the specimens was within 4.5–18.4% proportional to the bars reinforcement ratio normalized to the elastic modulus of steel bars.

Deterioration of shear behavior of corroded reinforced concrete beams

Steel bars in reinforced concrete (RC) structures may corrode in an aggressive environment, resulting in performance deterioration and premature failure. Stirrups begin to corrode earlier and faster than longitudinal steel bars due to their thinner concrete cover, and the shear strength decreases faster than the flexural strength. This paper presents an experimental investigation into the deterioration mechanism of shear behavior for corroded RC beams, and the prominent influencing parameters on shear behavior. The impressed current method was used to obtain corroded RC beams with different corrosion degrees in stirrups and longitudinal steel bars. Test results indicated that the beam may fail in shear instead of bending as the degrees of stirrup corrosion increases. The more severe the corrosion of stirrups was, the lower the shear strength. However, as the degree of corrosion increased in the longitudinal steel bars, the failure mode could change back to bending failure. The loss of bond strength affected the shear behavior of corroded RC beams, and the shear strength for bonded specimens was larger than unbonded ones. The shear strength decreased in beams with stirrups lacking end anchorages. Stirrups without end anchorages could still contribute to the shear strength if there was a good bond between stirrups and concrete. Otherwise, the stirrups could be ignored. With neither bond strength nor end anchorage, the beam could be treated as a beam without stirrups.

Passive dry fiber confinement for reinforced concrete stub columns

An experimental program was developed in which dry carbon and glass fiber wires bundles were spirally wrapped around five longitudinal steel bars with 8 mm diameter to act as a passive confinement inside the concrete stub columns. Stub columns were manufactured using 150 × 300 mm cylinders with a different number of fiber wires as transverse reinforcement. It was observed that bundling 20 strands of CFRP fiber yarns and bundling 30 strands of GFRP fiber yarns gave ductile response. However, the use of 20 bundled wires of CFRP out-performed the 30 bundled wires of GFRP in terms of ductility but not strength.

Experimental investigation of CFRP confined hollow core Reactive Powder Concrete columns

This paper presents an experimental investigation on the behaviour of hollow core Reactive Powder Concrete (HCRPC) columns confined with a circular Carbon-Fibre-Reinforced polymer (CFRP) tube. Sixteen circular hollow core specimens (206 mm in diameter, 800 mm in height and a 90 mm circular hole) were made with Reactive Powder Concrete (RPC) of 105 MPa compressive strength. These specimens were divided into four groups. The first group was the control group that consisted of four unconfined HCRPC specimens reinforced with conventional longitudinal steel bars and steel helices. The specimens of the second group had the same configuration as the first group except that these specimens were externally confined with CFRP tube. The specimens of the third group were externally confined with a CFRP tube and internally confined with a Polyvinyl chloride (PVC) tube. Finally, the specimens of the fourth group had no steel reinforcement and were only made with an external CFRP tube and an internal steel tube. These specimens were subjected to different loading conditions: concentric, eccentric (25 mm and 50 mm) and four-point bending. It was found that the CFRP tube confinement slightly increased the strength of the HCRPC columns, whereas the ductility was significantly enhanced. In addition, by introducing the PVC tube as internal confinement to the hollow columns the ductility was also improved.

Glass fiber-reinforced polymer-reinforced rectangular concrete columns under simulated seismic loads

This paper presents the results of nine concrete columns reinforced with longitudinal and transverse glass fiber-reinforced polymer (GFRP) bars and two concrete columns reinforced with longitudinal steel bars and transverse GFRP bars. These specimens were tested under lateral cyclic quasi-static loading while simultaneously subjected to constant axial load. The influence of the volumetric ratio of stirrups, the shear span ratio and the axial load level on the behavior of GFRP-reinforced concrete columns are discussed. The results showed that GFRP-reinforced concrete columns can have sufficient bearing capacity and achieve high levels of deformability. The lateral drift ratios of all the columns reached 4.08–5.06%, which meet seismic drift limitations in most building codes. The evaluation methodology of ductility coefficient may not assess the seismic deformability and resistance of GFRP-reinforced concrete columns objectively, while the integrated performance index can reflect the influence of the volumetric ratio of stirrups, the shear span ratio and the axial load on seismic performance comprehensively. The results further indicate that concrete columns reinforced with GFRP bars can be successfully designed and used in seismic engineering if confined well.