Unbonded Bars Research Articles

Development of reinforced concrete column with embedded concrete hinge and lateral restraints for Anti-Catastrophe oriented design

In Japan, since the 2011 Tohoku Earthquake, there has been a growing interest in addressing earthquakes that exceed design assumptions. Consequently, the concept of “anti-catastrophe” has been proposed as a new approach to the seismic design of infrastructures. This approach necessitates considering the conditions after significant structural damage, a concept not explicitly addressed in current seismic design codes. This study proposes two types of reinforced concrete (RC) columns: Embedded Concrete Hinge RC column (ECH-RC) and Embedded Concrete Hinge unbonded bar RC column (ECH-UBRC) to minimize the uncertainty of failure behaviors of RC columns during Beyond Design-Basis Events (BDBE) while ensuring high ductility during Design-Basis Events (DBE). ECH-RC features a Mesnager hinge embedded at its base, while ECH-UBRC incorporates unbonded high-strength bars in addition to a Mesnager hinge. ECH-RC and ECH-UBRC are designed for bridge columns subjected to low axial loads. To ensure the stability of the entire bridge structure during BDBE, providing lateral restraints at large displacements is essential. Results from cyclic loading tests revealed that embedded Mesnager hinges effectively prevented the uncontrollable failure of RC columns during BDBE by limiting axial and shear deformation at the plastic hinge, while the unbonded bars enhanced the stability of RC columns.

Finite element model of beam-end specimen with different qualities of reinforcing bar encapsulation

The proper encapsulation of reinforcing bars in shotcrete has represented for many years a concern among engineers. Thus, recent research has investigated the impact of imperfections specifically behind reinforcing bars using ASTM A944-10 “beam-end” specimens in the laboratory. Here, a 3D non-linear Finite Element model at rib-scale with different reinforcing bar encapsulation qualities was developed to validate such experimental results and obtain additional data; different concrete covers and void sizes and lengths around the bar were investigated. The results support recent findings by suggesting a 20% un-bonded bar perimeter limit beyond which an excessive bar stress reduction and slip are observed. Beyond this limit, the failure mode of the specimens is thought to change from splitting to pull-out. The limit becomes 30% when voids cover half of the bar’s length. These limits could allow engineers to determine if design actions or corrections guarantee the proper bond behavior of structural elements.

Flexural behavior of ungrouted post-tensioned concrete masonry beams with unbonded bars

Precast and post-tensioned systems increase the efficiency of masonry beams in terms of labor and time of construction. This study is aimed at evaluating the flexural behavior of ungrouted post-tensioned concrete masonry beams, with eccentric bars, and the effect of the main design parameters on their structural performance when subjected to monotonic and unidirectional cyclic loading until failure. All beams failed in flexure by crushing of the masonry in the compression region. Masonry strength was the design parameter that had the most significant effect on the load-deflection behavior of post-tensioned masonry beams. The test results showed that the measured nonlinear load-deflection behavior and the correlation between bar stress increase and midspan deflection can be estimated using a multi-linear relationship. Beams tested under monotonic and unidirectional cyclic loading exhibited comparable load-deflection envelope response.

Experimental calibration of parallel-to-grain strain penetration length for internal epoxied bars

Tensile strain penetration effects have been first investigated in the study of reinforced concrete components as experimental studies highlighted that theoretical values for the equivalent plastic hinge length did not reflect the observed experimental results. This is due to the elongation of the bars beyond the theoretical interface of the structural element with another component (e.g. beam-to-column or column/wall-to-foundation connections). The concept of an equivalent strain penetration length was first introduced for flexural analysis of concrete elements and the definition of the element’s plastic hinge length.Experimentally calibrated expressions for the equivalent strain penetration length have been developed in literature and extensively adopted in the design of traditional reinforced concrete connections.Internal partially unbonded bars have more recently been used is precast concrete dissipative post-tensioned rocking systems, developed during the PREcast Seismic Structural Systems (PRESSS) program, and more recently in timber rocking (Pres-Lam).As per bars embedded or post-grouted into concrete, internally glued bars in timber are affected by tensile strain penetration effects. An important parameter involved in the design of dissipative rocking section as well as more traditional epoxied rods connections is the additional slip and consequent loss of stiffness, strength and energy dissipation due to strain penetration.The paper shows the determination of an equivalent strain penetration length in internal epoxied bars through the experimental testing of several specimens accounting for different geometric parameters. The final part of the paper discussed the experimental results in terms of bar slip proposing simplified analytical formulas.

Flexural test of precast high-strength reinforced concrete pile prestressed with unbonded bars arranged at the center of the cross-section

In this study, a prestressed reinforced concrete pile that uses high-strength material to increase the pile’s flexural capacity was developed. The main structural characteristics of the developed pile include (1) the neutral axis is constantly near the centroidal axis of the pile, even if the longitudinal reinforcement yields due to a flexural moment, because the pile has a high axial compressive force that is induced by prestressed steel bars, and hence, the concrete in the compression region can contribute to increasing the flexural strength of the pile; and (2) the flexural strength of the pile increases because the high-strength concrete is confined by high-strength spirals and carbon-fiber sheets in combination with concrete infilling, and, together, these modifications provide a sufficiently high lateral-confinement pressure. The results of bending tests demonstrate that the proposed prestressed reinforced concrete pile with carbon-fiber sheets and concrete infilling had a much higher flexural capacity than a conventional precast concrete pile. In addition, an analytical approach is presented that can be used to obtain the relationship between the bending moment and the curvature of the proposed pile. Even if concrete bridge systems are constructed on strata that can experience soil liquefaction, such as very soft soil, bridge foundations that use the proposed piles could remain undamaged under the design seismic action.

TWO-LEVEL SEISMIC DESIGN METHOD USING POST-YIELD STIFFNESS AND ITS APPLICATION TO UNBONDED BAR REINFORCED CONCRETE PIERS

The two-level seismic design method using post-yield stiffness is proposed. The post-yield stifiness on the load —displacement relationship of the RC pier is found to be effective for the reduction of both the demand strength and the residual displacement under extreme earthquakes. Then, the two-level seismic design method is applied to the Unbonded Bar Reinforced Concrete (UBRC) pier that has the stable post-yield stiffness. The smaller cross section is designed than the optimal cross section of the conventional RC structure. The performance of the UBRC pier is verified by the pseudodynamic tests, and as the results it is confirmed that the UBRC piers can be rationally satisfied with the required performance of the two-level seismic design.

Strengthening reinforced concrete beams with external unbonded bars: experimental investigation

In an earlier study the authors outlined a strengthening technique for reinforced concrete beams using external unbonded reinforcement. It was shown that the technique could provide useful enhancements in load capacity in all but heavily reinforced cross-sections. The technique offers advantages in speed and simplicity of installation over other, established, strengthening techniques. The earlier work used deflectors in order that supplementary external bars would follow the curvature of the original beam. If these deflectors could be dispensed with, further simplifications could be made in installation procedures. A more extensive programme of physical testing and associated modelling of behaviour has now been performed in order to further explore the capabilities of the technique and to determine the circumstances under which deflectors should be installed. The paper provides a general description of structural behaviour of beams strengthened using the technique. Results of 27 physical tests on strengthened reinforced concrete beams are reported and compared with those from a further eight reference specimens. It is shown that the technique can provide greater strength enhancements to lightly reinforced sections and that provision of deflectors enhances efficiency. The danger of sudden collapse is found to be no greater than that for equivalent conventionally reinforced beams. The various parameters which affect strength enhancements attainable with the technique are reviewed.

Strengthening reinforced concrete beams with external unbonded bars: experimental investigation

In an earlier study the authors outlined a strengthening technique for reinforced concrete beams using external unbonded reinforcement. It was shown that the technique could provide useful enhancements in load capacity in all but heavily reinforced cross-sections. The technique offers advantages in speed and simplicity of installation over other, established, strengthening techniques. The earlier work used deflectors in order that supplementary external bars would follow the curvature of the original beam. If these deflectors could be dispensed with, further simplifications could be made in installation procedures. A more extensive programme of physical testing and associated modelling of behaviour has now been performed in order to further explore the capabilities of the technique and to determine the circumstances under which deflectors should be installed. The paper provides a general description of structural behaviour of beams strengthened using the technique. Results of 27 physical tests on strengthened reinforced concrete beams are reported and compared with those from a further eight reference specimens. It is shown that the technique can provide greater strength enhancements to lightly reinforced sections and that provision of deflectors enhances efficiency. The danger of sudden collapse is found to be no greater than that for equivalent conventionally reinforced beams. The various parameters which affect strength enhancements attainable with the technique are reviewed.

Strengthening reinforced concrete beams with external unbonded bars: theoretical investigation

The paper describes the development of a numerical model for analysis of beams retrofitted with external unbonded reinforcement. The experimental phase of the investigation is described in a companion paper, which also outlines the advantages of the method. Because external reinforcement is not bonded to the beam throughout its span, but is anchored only at the ends, normal plane section behaviour assumptions cannot be used. The numerical model is based on section analysis, and incorporates non-linear material behaviour and non-linear geometric effects. The model is first validated against results from an experimental programme comprising 35 physical tests on laboratory specimens. The model is then used to supplement the experimental study by exploring parameters which could not receive adequate experimental coverage. The influence of the shape of the bending moment diagram and of span/depth ratio, parameters which were not adequately explored in the experimental investigation, are reported. Deflectors were typically calculated to double strength enhancement at shorter span/depth ratios; the benefits were greater with higher ratios. Their use will be advantageous in the majority of situations.

Strengthening reinforced concrete beams with external unbonded bars: theoretical investigation

The paper describes the development of a numerical model for analysis of beams retrofitted with external unbonded reinforcement. The experimental phase of the investigation is described in a companion paper, which also outlines the advantages of the method. Because external reinforcement is not bonded to the beam throughout its span, but is anchored only at the ends, normal plane section behaviour assumptions cannot be used. The numerical model is based on section analysis, and incorporates non-linear material behaviour and non-linear geometric effects. The model is first validated against results from an experimental programme comprising 35 physical tests on laboratory specimens. The model is then used to supplement the experimental study by exploring parameters which could not receive adequate experimental coverage. The influence of the shape of the bending moment diagram and of span/depth ratio, parameters which were not adequately explored in the experimental investigation, are reported. Deflectors we...

Analysis of reinforced concrete beams strengthened by external unbonded bars

The article outlines a novel strengthening technique for reinforced concrete beams using external unbonded reinforcement. The advantages of the method lie in speed and simplicity of installation. The article provides a general description of structural behaviour of beams strengthened using the technique, supported by a numerical model to represent the behaviour of strengthened beams. A set of seven laboratory tests on reinforced concrete beams strengthened using the technique are reported, results compared with those of two reference specimens, and the model validated using test data. It is shown that good efficiencies can be achieved when strengthening lightly reinforced beam sections, but that with more heavily reinforced sections only limited strength enhancements are attainable.

Analysis of reinforced concrete beams strengthened by external unbonded bars

The article outlines a novel strengthening technique for reinforced concrete beams using external unbonded reinforcement. The advantages of the method lie in speed and simplicity of installation. The article provides a general description of structural behaviour of beams strengthened using the technique, supported by a numerical model to represent the behaviour of strengthened beams. A set of seven laboratory tests on reinforced concrete beams strengthened using the technique are reported, results compared with those of two reference specimens, and the model validated using test data. It is shown that good efficiencies can be achieved when strengthening lightly reinforced beam sections, but that with more heavily reinforced sections only limited strength enhancements are attainable.

Details of reinforcement of autoclaved aerated concrete lintels for shear and anchorage

This paper is a study of the shear capacity of autoclaved aerated concrete lintels reinforced with unbonded bars. A set of tests was performed to measure the anchorage capacity of the beam ends with seven different configurations. Full lintels were also tested to failure under transverse loading for two steel details. The study of the test results confirms that the lintels fail in a brittle mode when the anchorage capacity of the longitudinal bars is reached. A steel detail producing a stiff truss action at the beam ends relieving the stresses on the concrete shows to be the best arrangement, increasing the strength of the beams and producing a relatively ductile failure.

Anchorage capacity of reinforcing bars in autoclaved aerated concrete lintels

This paper is a study of the shear capacity of autoclaved aerated concrete lintels reinforced with unbonded bars. A set of tests was performed to measure the anchorage capacity of the beam ends. Full lintels were also tested to failure under transverse loading. The comparison of the two sets of tests shows that the lintels fail in a brittle mode when the anchorage capacity of the longitudinal bars is reached. The mechanism of anchorage by tie-bearing is described. Two variations of the reinforcement details are tested and discussed.