In this study, the effect of rain exposure on the progress of reinforcement corrosion and state of cover concrete was investigated through a survey of actual concrete structures. Sample data on the carbonation depth, cover depth, and degree of rebar corrosion were collected from 221 locations, as well as the presence or absence of rain exposure at each sampling location, and the relationship between corrosion state, spalling or falling off of the cover concrete, and the degree of rain exposure were examined. The degree of rebar corrosion was determined by analysis of rebars obtained from actual structures. It was found that, when there is exposure to rain, the change in moisture content is larger on the surface than inside the concrete due to repetitious wetting and drying at the surface. Consequently, the corrosion rate of rebar is larger at the concrete surface than further inside the concrete.
In this study, the influences of cover concrete properties (depth, degree of saturation, chloride ion concentrations, cement type, W/B, liquid phase on steel bar) were quantitatively examined by using electrochemical measurements. The difference due to the membrane potential (MP) between the half-cell potential measured on the cover concrete and the half-cell potential measured at the surface of steel bar could be observed. And the difference became larger as the degree of saturation of cover concrete became lower or the microstructure of cover concrete became denser. And, it was found that the impedance spectroscopy was affected by cover concrete properties. The impedance spectroscopy at low frequency response was related to the steel corrosion process, considering that the response at high frequencies was attributed to the cover concrete properties. Based on these results, the equivalent electrical circuit for reinforced concrete was proposed. And the differences between the proposed equivalent electrical circuit and Randles equivalent electrical circuit were discussed.
To optimize concrete tracks in terms of reasonable loading structure, superior workability, easy maintenance and economic efficiency, a new solid-bed track with resilient sleepers has been developed by the authors. In this research, to clarify the shrinkage behavior of this track, full-scale track members were prepared. To investigate the shrinkage behavior of the track, numerical simulation was performed using a three-dimensional multi-scale coupled model where the microscopic characteristics of cementitious composites and nonlinear structural mechanics are integrated. The results of this study indicate that shrinkage cracks can be generated from just under the corner of sleepers in an inverted V shape when the thickness of the track is relatively thin, on the order of 80 mm. However, generation of shrinkage cracks can be restricted when the thickness of the track is greater than about 280 mm, as is typical in snowy cold regions. The results indicated also that the generation of shrinkage cracks is unlikely when the thickness of the track is about 580 mm. This may be due to the fact that the age at which the main stress peaks is delayed as development of shrinkage behavior slows down when the thickness of the track is large, and therefore the tensile strength of the concrete increases with age. Another reason might be that the restraining effect of the concrete bed of the track is small when the thickness of the track is large. In addition, to control the propagation of shrinkage cracks, the position and size of crack induced joints were investigated. Based on the numerical simulation results, it was found that shrinkage cracks can be vertically generated when the sectional defect range is set to about 10%.
It has been generally accepted so far that the initial strain method may be used for crack problems due to chemical reaction induced expansion in concrete because there exists no other reliable method although the initial strain method is not adaptable in some cases. The concept of the work constant theory established by Dr. Tsuji has known to be adaptable for such kinds of problems. However, generalization of his theory has not been yet achieved. In this paper, unified analytical method is presented in order to treat varieties of chemical expansion in concrete based on the mechanical energy conservation hypothesis, which improves theoretical defects of the work constant theory. Then, some numerical simulations for experimental results such as ASR and expansive concrete are carried out and the proposed method is shown to give more reasonable numerical results than the existing method. Moreover, the proposed method is shown to give admissible numerical results for damage problem containing MgO. Finally, the importance of the accumulation of chemical expansion energy data is stressed for unified treatment in the guidelines to evaluate crack and crack width for such chemical expansion problems.