open access

https://doi.org/10.14943/doctoral.k12907

https://hdl.handle.net/2115/70838

局所的に腐食したせん断補強鉄筋を有する鉄筋コンクリートはりの力学特性

英語

2017-09-25

Hokkaido University

vi, 82p

Corrosion of steel reinforcement is one of the major causes of damage of reinforced concrete structures. Corrosion products formed have a volume six times more than the steel, which exerts pressures on the surrounding concrete. The pressure leads to cracking and spalling of cover concrete, deteriorates the bond between the steel reinforcement and concrete, and finally reduces the ultimate strength which sometimes result in brittle failure. A lot of research have been carried out to understand the detrimental effects of corrosion of flexural reinforcement in reinforced concrete beams. A large number of experimental studies have shown that the corrosion of flexural reinforcement considerably reduces load-carrying capacity and ductility. However, shear reinforcement (stirrup) has not been given much consideration and there is not much literature available on the effects of corrosion of stirrup. Therefore, in this study, a detailed experimental research has been conducted to observe the behavior of reinforced concrete beams with locally corroded stirrup. Thirty-nine beams of 1800 mm long, 100 mm wide and 150 mm high were casted, and corrosion of stirrup was electro-chemically accelerated. The beams before suffering from stirrup corrosion were designed to show the flexural failure mode or the shear failure mode. For this purpose, three kinds of flexural reinforcement were used; two D10, two D13 and two D16, all of which were epoxy coated to avoid corrosion. The stirrup was a deformed bar of 6 mm in diameter with the spacing of 80 mm, 120 mm and 160 mm. The location of stirrup corrosion is also a significant factor in this research; accordingly, stirrup was locally corroded in the shear span, in the middle span, or the full span while using 120 mm stirrup spacing. Mild and severe corrosion levels were prepared, mass loss of which were approximately 10% and 20%, respectively. After the corrosion accelerating treatment, corrosion cracks were marked and their widths were measured to observe their distributions and influences on flexural cracking in the bending test. Four-points bending test was applied to observe the ultimate strength of the corroded beams. Finally, the stirrup was taken out to check the degree of corrosion. It was found that the most of the corrosion cracks lie in the crack width range of 0.03-0.05 mm, which is the narrowest crack width range. The corrosion cracks in the severely corroded beams were more, and wider cracks were observed as compared with the mildly corroded beams. At the critical locations, the corrosion cracks acted as the pre-defined failure paths and the flexural or shear cracks followed the corrosion cracks during the bending test. The flexural or shear cracks were wider in the corroded beams than those generated in the control beams. Although stirrup is not responsible for flexural capacity but the results showed reduction in the flexural capacity after corrosion of stirrup. In all cases, the beams with stirrup corrosion had less flexural capacity than the control beams. The reduction in the capacity was more in the beams where the stirrup was corroded in the full span and the shear span for mild and severe corrosion and shear failure occurred in the severely corroded beams. The stirrup in the middle span did not contribute to the flexural capacity of the beam, as there is zero shear force and maximum bending moment in the middle span. However, the stirrup corrosion induced the corrosion cracks in the middle span. The flexural cracks followed the corrosion cracks during the bending test, and were mainly responsible for reducing the flexural capacity of the corroded beams. The corrosion cracks occurring due to stirrup corrosion were vertical cracks along the length of the stirrup with some horizontal or connecting cracks which passed through the vertical cracks. These horizontal cracks and the vertical cracks tended to widen during the bending test. At higher values of applied load, the horizontal and vertical cracks presented in the middle span at the top of the beam in the compression zone, also tended to widen, separating the concrete cover in the compression zone resulting in spalling of the concrete cover. This reduced the cross-sectional area of the beam as the top concrete cover was spalled out, reducing the width of the compression zone and hence lowering the flexural capacity of the corroded beam. This phenomenon was more obvious and clear for full span or middle span corrosion as all stirrups in the middle span were corroded and had corrosion cracks. In case of shear span corrosion, the corrosion cracks were critical at the junction of shear and middle span, just under the applied point load. Because of these reasons, the failure mode of a few beams with full span or middle span corrosion were changed from flexural tension failure to flexural compression failure, after stirrup corrosion. The severely corroded beams had higher strength loss and all D13 and D16 severely corroded beams with shear span or full span stirrup corrosion failed in shear. D10 beams had the flexural reinforcement ratio well below the ratio at balanced failure and the shear reinforcement ratio is high enough and the probability of shear failure is minimal after stirrup corrosion. This is the reason, only one D10 severely corroded beam failed in shear and all other failed in flexure. The ultimate capacity loss of corroded beams was more for higher transverse (shear) reinforcement ratio, which was the case of closely spaced stirrup. When the stirrups were closely spaced, the strength contribution of the stirrup was more, and once the stirrup was corroded, the strength loss observed was also more. The failure modes of the corroded beams were also changed particularly when the stirrup was severely corroded. Depending on the location of corrosion, most of the severely corroded beams failed in shear despite having yielding of flexural reinforcement for the control beams. The beams failed in shear had higher capacity loss than those failed in flexure. Hence, the stirrup corrosion strongly influenced the capacity of the beam, which further depended on the location and the amount of corrosion. The stirrup corrosion had a strong tendency to change the failure mode and even to reduce the flexural capacity of reinforced concrete beams. The deflection ductility was also significantly reduced after the stirrup corrosion. For lower flexural reinforcement ratio, the deflection ductility was reduced considerably as the deflection ductility of the control beams was much higher for the lower flexural reinforcement ratio which was governed by the control beams was much higher for the lower flexural reinforcement ratio which was governed by the flexural design of the beams. The residual shear capacity of the stirrup corroded beams was predicted using some empirical models including the width of corrosion crack. The predicted values did not give enough accurate results as these empirical models were developed considering one or two straight corroded rebars which were larger in diameter and mostly used as longitudinal reinforcement. However, the stirrups are rectangular in shape and the diameter of stirrup is usually smaller than the longitudinal reinforcement. Moreover, there is an interaction of the tensile stress induced due to the corrosion of the adjacent stirrups which results in more cracking which are vertical and horizontal cracks. When the stirrup is closely spaced and the shear reinforcement ratio is higher, more horizontal corrosion cracks are observed which pass through the vertical cracks. These corrosion cracks are the indication of confinement loss provided by stirrup which also results in the loss of compressive strength of concrete, and are absent in case of longitudinal reinforcement corrosion. This makes the behavior of stirrup corrosion a bit different from the straight rebars which are widely used as longitudinal reinforcement. Therefore, more research should be carried out to study the effect of stirrup corrosion by varying the design parameters and new empirical models should be established considering the stirrup shape, interaction of the tensile stresses due to the stirrup corrosion while using smaller diameters.

2017-09-25

甲第12907号

博士(工学)

(主査) 教授 横田 弘, 教授 上田 多門, 准教授 西村 康志郎

工学院（北方圏環境政策工学専攻）

博士論文

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