Keita Suzumura

Experimental Study on Fatigue Strength of Corroded Bridge Wires

AbstractFatigue tests were conducted for corroded galvanized steel wires on three corrosion levels, showing that fatigue strength of corroded wires lowers as corrosion progresses. Corrosion pits were measured on the corroded specimens, showing severer corrosion produced deeper pits in more condensed areas. Fatigue tests were then conducted for wire specimens with artificial pits whose sizes were decided by the measured corrosion pit data. Three different pit shapes were assumed: round, triangle, and triangle with a notch. The wire specimens with round pits did not break until 1 million cycles in the stress range of 400 MPa. The fatigue strength of wires with the triangular pit was lower than that with a round shape. Triangular pit specimens broke at fewer cycles for shorter pit length. The fatigue strength of wires with a notched triangle further decreased, and critical cycles did not depend on pit length. As the S-N relation of the wire specimens with triangular pits and notched triangular pits has a sim...

CORROSION MECHANISM AND PROTECTION METHODS FOR SUSPENSION BRIDGE CABLES

The main cables of several Japanese suspension bridges were found to be corroded. To determine the corrosion mechanism, the environments inside the cables were investigated, and corrosion simulation tests of galvanised wires were carried out. These studies showed that the corrosion environment depends on the position within the cable, with the side parts being most susceptible to corrosion. A new anti-corrosion system was developed using S-shaped wrapping wires and improved pastes to overcome this problem. Cables protected by this system were compared in long-term exposure tests with cables protected by conventional systems, and showed improved anti-corrosion performance. Another new method, namely to pass dry air through the cables, is proposed. Preliminary tests showed this system to be promising in terms of improving the anti-corrosion properties of the cables.

MECHANICAL PROPERTIES AND REMAINING STRENGTH OF CORRODED BRIDGE WIRES

Corroded galvanized steel wires at different corrosion levels were produced, and their mechanical properties and remaining strength were investigated. It was found that the actual tensile strength of corroded wires does not decrease, whereas the elongation, torsional and fatigue strength decrease sharply. The amount of hydrogen absorbed in the corroded wires was measured, showing that it did not reach the level to cause hydrogen embrittlement. The surface of corroded wires is uneven and this s urface roughness seems to decrease ductility of corroded wires. Broken wires cut from an old suspension bridge were also investigated. The fracture surface is similar to that caused by corrosion fatigue rather than by hydrogen embrittlement. It is estimated that the wires were fractured by the mixed effects of corrosion, cyclic stresses, high residual stresses, hydrogen and fretting.

Effectiveness of Repair Methods of Corroded Bridge Cables

It is important to repair the corroded bridge cables by proper methods so that corrosion does not progress further. Six repair methods were proposed and applied to cable specimens. Then, the specimens were exposed to the severe corrosion environments and the effectiveness of the proposed repair methods was compared. Two different types of test cables were used in this study: parallel wire strands and spiral strands. The parallel wire strand cables consist of 19 non-galvanized steel wires. This aimed at the main cables of suspension bridges. Six repair methods were applied to these cable specimens: coating with zinc or epoxy resin paint or zinc powder paste, filling with epoxy resin or oil, and dehumidification method. Then the specimens were wrapped with wet gauze and kept at 40°C for 15 months to accelerate corrosion. By investigating mass loss due to corrosion and appearance during this period, effectiveness of six repair methods was compared. As for the surface wires, the dehumidification method was the most effective followed by the epoxy resin paint and filling, the zinc powder paste, and the zinc and epoxy resin paint on the surface. The oil filling was not very effective compared with other repair methods. The corrosion of the inside wires was much less than those of the surface wires. The spiral strand cables consist of seven galvanized steel wires. This test aimed at hangers of suspension bridges and stays of cable-stayed bridges. By investigating mass loss due to corrosion and appearance of both inside and surface wires during the 16 month period, the proposed six repair methods were all very effective compared with those of unrepaired strands. This study proves that, even if a cable is corroded, proper repair works are effective in preventing further corrosion.

Strength of Corroded Bridge Wires and Repair Methods

Suspension bridge cables need to perform under severe corrosive environments. To simulate this environment, galvanized steel wires were wrapped with wet gauze and kept in an enclosed box at a temperature of 40° C. This paper investigates corroded galvanized steel wires on different corrosion levels and their mechanical properties and remaining strength. Laboratory studies found that actual tensile strength of corroded wires did not decrease with corrosion levels, whereas elongation decreased sharply after the zinc layer was partly depleted and the steel started to corrode. The accumulated amount of diffusive hydrogen of corroded wires was less than 0.2 ppm, which was well below the critical concentration of 0.7 ppm to cause brittleness. However, fatigue strength significantly decreased after steel corrosion below the galvanized layer progressed. Fatigue strength further lowered when the steel wire was cyclically stressed under wet environments. It was estimated that the wires were fractured by the mixed effects of corrosion, cyclic stresses and hydrogen. Various repair methods were applied to the corroded wires: zinc rich paint, epoxy resin paint, zinc powder paste, filling with oil and dehumidification method. The effectiveness of these repair methods were evaluated by the corrosion simulation tests. The paper shows that the dehumidification and epoxy resin paint ant filling methods are most effective repair methods.