腐食鉄筋によるひび割れ発生前後の腐食挙動およびひび割れ進展

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  • 篠原 保二
    東京工業大学未来産業技術研究所　准教授・博士（工学）
  • 丸山 裕生
    清水建設　修士（工学）
  • 山下 海斗
    東京工業大学環境理工学創造専攻　大学院生
  • 南 正 樹
    東京工業大学フロンティア材料研究所　研究員・博士（工学）

書誌事項

タイトル別名
  • CORROSION BEHAVIORS BEFORE AND AFTER CRACKING INDUCED BY CORRODING BARS AND CRACK PROPAGATIONS
  • フショク テッキン ニ ヨル ヒビワレ ハッセイ ゼンゴ ノ フショク キョドウ オヨビ ヒビワレ シンテン

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抄録

 Number of experimental studies particularly aimed to relate the corrosion cracking appearance on the concrete surface with the corrosion ratio of steel bars have been conducted using an accelerated corrosion test to save time. The corrosion ratios during the test were generally evaluated by a linear interpolation from the relations of the accumulative current density and the measured corrosion loss after the test. However, the corrosion rate may be changed before and after cracking because an electric resistance reduces after cracking.<br> To improve the evaluation of corrosion ratios during an accelerated corrosion test, a number of small specimens were prepared to measure the corrosion loss several times and to make clear the corrosion rate before and after cracking.<br> The specimens have a small cross section of 150×150 mm and 300 mm in length, and contain a single reinforcement of D16. All sides except for exposed one were coated by epoxy resin to avoid penetration of NaCl solution. The variable factors of specimens are cover depth (40 and 60 mm) and accumulative current density. The accumulative current density when cracking was determined based on the previous test results.<br> The specimens were cured for 28 days before accelerated corrosion test was applied. During the accelerated corrosion test, the specimens were placed in the tank containing 3 % of NaCl solution. A direct-constant 10 voltage was given and the current was monitored and recorded using a data logger in 60 minutes increments. The cracks on the concrete surface were visually observed and the crack width at certain locations was frequently measured using a digital microscope having a resolution of 0.01 mm. After accelerated corrosion tests, the corroded steel bars were removed from the surrounding concrete and soaked into 10% diammonium hydrogen citrate solution. After chemically and mechanically cleaning, the weight loss of corroded rebars was measured.<br> A corrosion rate before cracking was slower with increasing a cover depth, and the corrosion ratios at cracking for 60 mm and 40 mm cover were half and 60 % of the theoretical value respectively. A current flow increased after cracking and a corrosion level of reinforcement approached the theoretical value. The evaluated corrosion ratios when cracking through the linear interpolations were overestimated by 30 % and 20 % for 60 mm and 40 mm cover respectively. Test results indicated that a bi-linear interpolation having a break point at cracking improved an evaluated corrosion ratio during the accelerated corrosion test.<br> In order to connect the width of corrosion cracks on the concrete surface with the corrosion level of reinforcing bars inside the concrete, a corrosion cracking behavior has been divided into three phases; (1) until the corrosion product appears on the concrete surface, a corrosion ratio is denoted by β, (2) a little corrosion product flow out through a crack until 0.2 mm of crack width in this model, the first slope of corrosion ratio to crack width is denoted by α1, and (3) an outflow of corrosion product from a crack over 0.2 mm in width, the second slope of corrosion ratio to crack width is denoted by α2. Since corrosion crack behaviors are greatly affected by concrete strength and cover depth, the effect of concrete strength and cover depth on β, α1, and α2 are investigated by considering the movements of corrosion products and the pore structure of concrete around corroding bars. Finally a prediction model to evaluate a corrosion level of steel bars inside concrete from a width of corrosion cracking on the concrete surface was proposed together with previous data. It's an easy and simple method.

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