Reinforced concrete jacketing and steel plate jacketing are widely used to increase the flexural strength of existing reinforced concrete bridge piers. Efforts are being made, therefore, to develop a more efficient and economical seismic retrofit method using a thinner jacketing material. This study proposes an economical seismic retrofit method for increasing the flexural strength of an existing reinforced concrete pier by placing small-diameter steel bars around the pier for flexural and shear reinforcement and jacketing the pier with a thin and light layer of sprayed mortar. In the proposed method, small-diameter deformed prestressing steel bars or high-strength steel reinforcing bars (USD685) are used for axial reinforcement. To ensure ease of installation and minimize effects on footing structure, the embedment end of each axial reinforcement bar is pressed into a flatter and wider shape so that the embedment length of the steel bar in the footing can be made shorter. Reversed cyclic loading tests on a 1/4-scale specimen mimicking an existing reinforced concrete railway bridge pier showed that the proposed method is a useful means of increasing the flexural strength of an existing reinforced concrete pier and that the load-carrying capacity and deformability of piers retrofitted by the proposed method can be evaluated by using conventional evaluation formulas.
More than 50 years have elapsed since the fist introduction of PC sleepers, and some damage sleepers have also been confirmed. Although it is necessary to detect and replace damaged PC sleepers properly, it is difficult to find the damages in the ballasted track because inspectors can only see the upper surface of the sleepers. In this study, various vibration and sound measurements were conducted to develop a damaged PC sleeper detection method using modal characteristics. The bending tests, vibration and sound measurements, and numerical analyses revealed that the natural frequencies are clearly dropped due to open cracks at the cross section of rail seat, and the vibration measurement of the actual damaged PC sleepers clarified that the natural frequency of third mode have high correlation with the damage degree. In addition, a simple detection method using a sound level meter was proposed and applied to the test line and actual rail lines. As a result, it is possible to detect damaged PC sleepers efficiently with the similar accuracy of multi-point detailed measurement method.
The cyclic loading experiments and FE analyses were conducted to make clear the effect of stirrups on shear fatigue load carrying mechanism of RC beams with rebar corrosion cracks. To simulate the most severe state of rebar corrosion cracks, artificial cracks were installed in the beam specimens. As a result of the loading experiments, tied arch mechanism appeared in the beams which had artificial crack tips inside of vertical stress non-contributed region (B-region). No remarkable difference was observed on the load carrying mechanism between sound beams and the beams which had artificial crack tips inside of vertical stress contributed region (D-region). In addition, the dowel action of stirrups was identified from the comparison with the slip displacements on the artificial crack surfaces of the cracked beams with and without stirrups. After the loading experiments, the validity of reproducing method of load carrying mechanism by using artificial cracks was confirmed based on the result of beams which had real corrosion cracks. As a result of decomposition of the load carrying mechanism based on FE analysis, in the beams which had artificial crack tips inside of B-region, the absorbed energy of arch mechanism was decreased by the slip resistance on the artificial crack due to the dowel action of stirrups. It was confirmed that when the absorbed energy of arch mechanism was high, the fatigue life became short regardless of the existence of stirrups and position of the artificial crack tips.
Cathodic protection is a highly reliable corrosion protection method for re-bar in concrete. Generally, “100-mV Polarization Shift or Depolarization Shift” is used as a criterion of cathodic protection for rein-forcing steel in concrete. In this study, we examined the relationship between potential shift values and effect of cathodic protection, using concrete specimens in which small, electrically-connected steel splits were embedded. As the result, we found that the corrosion rate can be reduced by the cathodic protection applied in the range of less than 100-mV polarization shift. However, from the point of view to stop the corrosion reaction electrochemically, we concluded that 100-mV polarization shift is appropriate as the criterion. Furthermore, we proposed a method of supplying protection current, the installation position of a reference electrodes, and the design method of criterion of cathodic protection for corrosion rate.
In order to improve construction productibity of bored cast-in-place concrete piles for structures such as railway viaducts, which have narrow construction space or small overhead clearance, the authors have developed deployable reinforcement pile cage system. In the cage, PC steel strands are used as longitudinal reinforcement. Special devices, which are able to make the PC strands rotate 90 degrees to form the cage, are arranged at the intersections of the longitudinal reinforcement and the spiral reinforcement. With those devices, the cylindrical cage can be deformed into helical shape by twisting and easily deployed by twisting in the opposite direction. As a result, the construction time of the cage can be reduced, especially when the overhead clearance is small. The characteristics of the cage with the consideration of the deformed shape and stiffness of PC strands are presented in this paper.