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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
Get PDF鉄筋コンクリート（RC）は，建物やインフラ施設の建設に広く使用されてきました． 特にRC橋脚は，経済性，施工性，耐久性，耐震性，耐食性などの利点から，高速道路の高架橋，山岳橋梁，渡河橋等に広く活用されています．RC橋脚の設計・施工においては，鉄筋とコンクリートの接着性能が重要です．特に，信頼性の高い応力伝達を得るには，材料間の十分な接合強度を確保することが必要不可欠です． 多くのRC構造では柱境界部やフーチング内の鉄筋とコンクリートの接着強度が低下するとすべり挙動が生じ，橋脚の耐荷力および剛性の低下や，RC構造の耐震性能そのものの低下を招くおそれがあります．
これまでの研究により，軸方向鉄筋の直径と配置が接合部の接合性能に大きく影響することが示されてきました．同程度の軸方向鉄筋比の配筋であっても，細径の軸方向鉄筋が密に配置したRC柱では，定着性能の低下により軸筋とフーチングコンクリートとの結合が失われ，破壊形式が設計で想定している曲げ破壊からロッキングモードへと移行し，想定外の被害が発生する可能性があります．
このような背景から，ＲＣ橋脚の限界状態や耐震性評価に関する研究分野では，軸方向鉄筋の抜け出しを防ぐために，軸方向鉄筋の付着強度や定着長の要求性能に関する研究が行われてきました．しかし，建築分野のように，軸方向鉄筋の直径や配置等の違いが接合部の定着性能に及ぼす影響だけでなく，同部材の限界状態の耐力や変形性能に及ぼす影響に関する統合的な研究は多くありません．
本研究では，同程度の軸方向鉄筋比と耐力を有する，異なる鉄筋径と鉄筋本数の軸方向鉄筋により構成されるRC縮小模型柱を対象とした正負交番繰り返し載荷実験による変形性能の評価と，有限要素解析に基づくRC柱の変形挙動の再現に関する研究を行いました．また，これらを通じRC橋脚の付着－すべり現象が，高密度配筋された細径軸方向鉄筋を有するRC柱の耐震補強性能に及ぼす影響について研究を行っており，本論文の構成を以下に記述します．
・２章では，既往の研究で多用される標準的なRC縮小模型橋脚と同程度の軸方向鉄筋比を有する細径鉄筋を密に配筋したRC柱供試体を用いた繰り返し載荷実験を行い，細径鉄筋による配筋がRC柱の変形耐荷性能や破壊メカニズムに及ぼす影響について評価を行った．特に，軸方向鉄筋のひずみ履歴や荷重～ひずみ関係の履歴と供試体内部における鉄筋の損傷状況，また，柱基部載荷両側における鉛直方向変位に算出された回転変形挙動に着目して考察を行った．
・３章では，非線形有限要素法に基づいて繰り返し載荷実験の再現解析を行っているが．軸方向鉄筋とコンクリート間の接合を考慮する必要があることを明らかにし，RC柱の軸方向鉄筋とコンクリート間の付着－すべり現象を再現するための新たなモデル化手法を提案した．接合部における鉄筋の付着－すべり現象や，異なる配筋状況によって生じる付着破壊性状の違いに焦点を当て，それらがRC柱の全体的な変形耐荷性能にどのように影響するのかを詳細に分析した．これらの分析・検討の結果から，細径鉄筋を密に配置したRC柱の性能や破壊メカニズムについて取りまとめた．
・４章では，細径鉄筋を有するRC柱について耐震補強の可能性について検証を行った．既存の交通インフラ施設の各種補強工事が数多く行われているが，旧耐震基準に基づいて設計・建設された既存RC橋脚はその多くが現行の基準に比べて細径の軸方向鉄筋が用いられており，こうしたＲＣ部材を補強しても十分補強効果が期待できない可能性があることから，細径の軸方向鉄筋を補強した供試体を作成し，これを用いた繰り返し載荷実験に基づき耐荷変形性能の評価を行った．補強により軸方向鉄筋の付着破壊や塑性ヒンジ区間での回転変形などの抑制効果に焦点を当てて詳細に検証した．これにより，高強度PCM材の打ち込み補強工法が既存部鉄筋の付着破壊を抑制し，既存部のロッキング変形を抑制できることを明らかにした．
・5章では，補強された供試体を対象として，前章の繰り返し載荷実験で対象としたPCM材による巻き立て補強が既存部における軸方向鉄筋の定着不良や，ロッキング変形への抑制効果について非線形有限要素法に基づく検証を行った．PCM補強部と補強部鉄筋を適切にモデル化することで，繰り返し載荷実験に観測された除荷再載荷履歴のピンチング現象を再現でき，供試体基部に付着破壊による生じた塑性ヒンジ部のロッキング変形も抑制できたことを明らかにした．
最後に，各章の結論をとりまとめ，付着－すべり現象に着目した高密度に配置された細径軸方向鉄筋を有するRC橋脚の耐震補強性能に関する研究成果の総括を行いました．また，本研究では解決できなかった課題を挙げることで，今後の研究課題について記述しています．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.博士(学術)山口大学Yamaguchi Universit
