Effect of the bond-slip behavior of thin rebars on the seismic performance and retrofitting of RC column specimens
Title
付着－すべり現象に着目した細径軸方向鉄筋を有するRC柱供試体の耐震性能の評価および補強効果の検証に関する研究
Effect of the bond-slip behavior of thin rebars on the seismic performance and retrofitting of RC column specimens
Degree
博士(学術)
Dissertation Number
創科博甲第142号
(2024-03-18)
Degree Grantors
Yamaguchi University
[kakenhi]15501
grid.268397.1
Abstract
Reinforced Concrete (RC) has been extensively used in the construction of buildings and infrastructure facilities. Particularly, RC bridge piers have been widely utilized in the construction of highways, mountainous, and river elevated bridges due to their cost-effectiveness, ease of construction, durability, seismic resistance, and corrosion resistance. In the design and construction of bridge piers, the bond performance between reinforcement and concrete is crucial. Ensuring sufficient bond strength between the materials is essential for reliable stress transmission. In most RC structures, deterioration of bond strength between reinforcement and concrete in column boundaries and within footings leads to slippage phenomena, reducing the column’s load-bearing capacity and rigidity, resulting in a decrease in the seismic performance of RC structures.
Previous studies have shown that the diameter and arrangement of axial bars significantly affect the bond performance at the joint. Therefore, in bridge piers with densely arranged small-diameter axial bars, the bond between axial bars and footing concrete may be lost due to decreased anchorage performance, possibly changing the failure mode from flexural failure, as assumed in current designs, to a failure mode caused by rocking deformation.
In this study, considering the above background, cyclic loading tests and finite element analysis based on reduced-scale RC column models, consisting of different diameters and numbers of axial bars with similar reinforcement ratios and strengths, were conducted. Through these, the influence of bond-slip phenomena in RC bridge piers with densely arranged small-diameter axial bars on the seismic reinforcement performance of RC columns was investigated. The structure of this paper is described below.
In Chapter 2, cyclic loading tests using RC column specimens with densely arranged small-diameter axial bars, having similar reinforcement ratios and strengths compared to the standard reduced-scale RC bridge pier models commonly used in previous studies, were conducted. The influence of small-diameter axial bars on the deformation and load-bearing performance and failure mechanisms of RC columns was compared with standard specimens. Specifically, analyses and considerations were made regarding the strain history of axial bars at loading stages, load-strain relationship history, damage conditions of reinforcements inside the specimens, and rotational deformation behaviors calculated from vertical displacements on both sides of the column base.
In Chapter 3, reproduction analysis of cyclic loading tests based on nonlinear finite element methods was conducted. It was clarified that it is necessary to consider the bond between axial bars and concrete. A new modeling method to reproduce the bond-slip phenomena between axial bars and concrete in RC columns was proposed. In these numerical analysis methods, focusing on the bond-slip behavior of reinforcements at the joint and differences in bond failure characteristics caused by different reinforcement arrangements, detailed analyses were conducted on how they affect the overall deformation and load-bearing performance of RC columns. From these analyses and considerations, the performance and failure mechanisms of RC columns with densely arranged small-diameter axial bars were summarized.
In Chapter 4, the possibility of seismic reinforcement for RC columns with small-diameter axial bars was verified. Even now, various reinforcement works are being conducted for existing transportation infrastructure facilities for reasons such as improving the seismic performance of RC bridge piers, extending the life of aging structures, and taking measures against imminent heavy rain disasters. In the case of existing RC bridge piers designed and constructed based on old seismic standards, many of them use smaller diameter axial bars compared to current standards and do not have sufficient flexural strength. Also, in reinforcement, it is necessary to select a construction method that comprehensively considers seismic resistance, durability, workability, and economy. Especially when applying to river piers, it is necessary to smoothly construct within a limited construction period, and in some cases, a reinforcement method with a thin wrapping thickness is chosen to reduce the riverbed occupancy rate and maintain its performance for a long time. Since it is unclear whether the reinforcement effect can be sufficiently expected even if reinforcement is performed, cyclic loading tests were conducted on specimens reinforced with PCM materials for RC columns with insufficient deformation performance due to such reinforcements and anchorage conditions, and the load-bearing deformation performance was evaluated. Detailed verification was conducted focusing on the suppression effect of anchorage failure of axial bars and rotational deformation in the plastic hinge section caused by bond failure. It was clarified that the high-strength PCM material pouring reinforcement method can suppress the anchorage failure of existing part reinforcements and the rocking deformation of the existing part.
In Chapter 5, verification based on nonlinear finite element methods was conducted on the specimens reinforced in the previous chapter, focusing on the suppression effect of anchorage failure of axial bars in the existing part and rocking deformation due to the wrapping reinforcement of PCM materials targeted in cyclic loading tests. By appropriately modeling the PCM reinforced part and the reinforced part reinforcements, it was possible to reproduce the pinching phenomena observed in the unloading and reloading history of cyclic loading tests, and it was clarified that the rocking deformation of the plastic hinge part caused by bond failure at the base of the specimen could also be suppressed.
Finally, the conclusions of each chapter were summarized, and a comprehensive summary of the research results on the seismic reinforcement performance of RC bridge piers with densely arranged small-diameter axial bars focusing on bond-slip behavior was conducted. Also, unresolved issues in this study were raised, and descriptions were made regarding future research issues.
Previous studies have shown that the diameter and arrangement of axial bars significantly affect the bond performance at the joint. Therefore, in bridge piers with densely arranged small-diameter axial bars, the bond between axial bars and footing concrete may be lost due to decreased anchorage performance, possibly changing the failure mode from flexural failure, as assumed in current designs, to a failure mode caused by rocking deformation.
In this study, considering the above background, cyclic loading tests and finite element analysis based on reduced-scale RC column models, consisting of different diameters and numbers of axial bars with similar reinforcement ratios and strengths, were conducted. Through these, the influence of bond-slip phenomena in RC bridge piers with densely arranged small-diameter axial bars on the seismic reinforcement performance of RC columns was investigated. The structure of this paper is described below.
In Chapter 2, cyclic loading tests using RC column specimens with densely arranged small-diameter axial bars, having similar reinforcement ratios and strengths compared to the standard reduced-scale RC bridge pier models commonly used in previous studies, were conducted. The influence of small-diameter axial bars on the deformation and load-bearing performance and failure mechanisms of RC columns was compared with standard specimens. Specifically, analyses and considerations were made regarding the strain history of axial bars at loading stages, load-strain relationship history, damage conditions of reinforcements inside the specimens, and rotational deformation behaviors calculated from vertical displacements on both sides of the column base.
In Chapter 3, reproduction analysis of cyclic loading tests based on nonlinear finite element methods was conducted. It was clarified that it is necessary to consider the bond between axial bars and concrete. A new modeling method to reproduce the bond-slip phenomena between axial bars and concrete in RC columns was proposed. In these numerical analysis methods, focusing on the bond-slip behavior of reinforcements at the joint and differences in bond failure characteristics caused by different reinforcement arrangements, detailed analyses were conducted on how they affect the overall deformation and load-bearing performance of RC columns. From these analyses and considerations, the performance and failure mechanisms of RC columns with densely arranged small-diameter axial bars were summarized.
In Chapter 4, the possibility of seismic reinforcement for RC columns with small-diameter axial bars was verified. Even now, various reinforcement works are being conducted for existing transportation infrastructure facilities for reasons such as improving the seismic performance of RC bridge piers, extending the life of aging structures, and taking measures against imminent heavy rain disasters. In the case of existing RC bridge piers designed and constructed based on old seismic standards, many of them use smaller diameter axial bars compared to current standards and do not have sufficient flexural strength. Also, in reinforcement, it is necessary to select a construction method that comprehensively considers seismic resistance, durability, workability, and economy. Especially when applying to river piers, it is necessary to smoothly construct within a limited construction period, and in some cases, a reinforcement method with a thin wrapping thickness is chosen to reduce the riverbed occupancy rate and maintain its performance for a long time. Since it is unclear whether the reinforcement effect can be sufficiently expected even if reinforcement is performed, cyclic loading tests were conducted on specimens reinforced with PCM materials for RC columns with insufficient deformation performance due to such reinforcements and anchorage conditions, and the load-bearing deformation performance was evaluated. Detailed verification was conducted focusing on the suppression effect of anchorage failure of axial bars and rotational deformation in the plastic hinge section caused by bond failure. It was clarified that the high-strength PCM material pouring reinforcement method can suppress the anchorage failure of existing part reinforcements and the rocking deformation of the existing part.
In Chapter 5, verification based on nonlinear finite element methods was conducted on the specimens reinforced in the previous chapter, focusing on the suppression effect of anchorage failure of axial bars in the existing part and rocking deformation due to the wrapping reinforcement of PCM materials targeted in cyclic loading tests. By appropriately modeling the PCM reinforced part and the reinforced part reinforcements, it was possible to reproduce the pinching phenomena observed in the unloading and reloading history of cyclic loading tests, and it was clarified that the rocking deformation of the plastic hinge part caused by bond failure at the base of the specimen could also be suppressed.
Finally, the conclusions of each chapter were summarized, and a comprehensive summary of the research results on the seismic reinforcement performance of RC bridge piers with densely arranged small-diameter axial bars focusing on bond-slip behavior was conducted. Also, unresolved issues in this study were raised, and descriptions were made regarding future research issues.
Creators
SHAO PEILUN
Languages
jpn
Resource Type
doctoral thesis
File Version
Version of Record
Access Rights
open access