Most of RC bridges in Okinawa Prefecture have been constructed near the shore and deterioration of concrete due to rust of reinforcing bars becomes serious. Several investigations such as surface observations, truck loading tests and bending failure tests of main beams have been performed. These are mainly for static and fatigue behavior of a beam becomes important for the consideration of durability. Generally, fatigue strength of materials such as concrete and reinforcing bars is the major factor in determination of strength of a beam and fatigue behavior of the beam has not been seriously studied. However, it is considered as important since significant deterioration has been observed recently. Two beams tested here were cut of a deteriorated bridge and the appearance showed spalls of concrete on surface. Apparent losses in cross section of bars and bond stress were observed. This paper presents the results of fatigue test subjected to repeated loads where increasing rates of deflection, strain of bars and crack width and effectiveness of bond stress were carefully investigated. The immediate conclusions are as follows: (1) The significant reduction of bending rigidity and increase of crack width were not observed after repeated load, but residual deflection, strain and crack width were not negligible. (2) Sudden failure of test beams occured due to brittle failure of a reinforcing bar, which could be explained as the overstressing of a particular bar as result of loss in bond stress at neighboring bars.
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