The aim of this study is to identify a specific degradation of concrete that has been observed in bridge decks made of reinforced concrete (RC). To control the phenomenon, fundamental studies were conducted. Compressive loads and external water pressure were cyclically applied to submerged concrete cylinder specimens with different pre-loading and restraint conditions. The turbidity of the water in the tank was generally observed during the loading, and the pH of the turbid water steadily increased as the number of cycles increased. Thereafter, fine aggregates without a cement matrix were found on the inner surfaces of split specimens. These phenomena were quantitatively analyzed, and the analyses suggested that cyclic water pressure acted on the inside of pre-cracked specimens and washed out their cement matrix. The degradation of a rough cracked surface was also examined using the cyclic shear test, with/without a water supply to the crack. The shear slip and the orthogonal displacement were clearly amplified with an increase in the number of cycles when the water supply was present. The mechanical properties of cracked concrete with water in shear was discussed in accordance with that of liquefaction. These fundamental studies could help to determine the acceleration factors of the degradation and provide certain thresholds for practical use.
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