Corrosion damage and bearing characteristics of bridge pile foundations under dry-wet-freeze-thaw cycles in alpine salt marsh areas

To explore the corrosion damage characteristics of bridge pile foundation in Qinghai area under dry-wet-freeze-thaw cycles, relying on the Dexiang Expressway Project, the reinforcement and concrete specimens were embedded in the field to subjected to freeze-thaw cycles for one year. The laboratory test was used to conduct the dry-wet-freeze-thaw cycles on concrete specimens for 225 times. The variation rules of concrete mass, anti-erosion coefficient, relative dynamic elastic modulus, compressive strength, micro-mechanism and reinforcement corrosion rate at different positions and different cycle times were compared and analyzed. The numerical simulation was conducted to analyze the bearing capacity change rule of unprotected pile foundation over 20 years, and protection measurements for bridge pile foundations in alpine salt marsh areas were proposed. Research result shows that as the embedment depth increases, the correlation degree of anti-erosion coefficient of pile foundation concrete specimens in the field increases, and the maximum value is 0.93. As time increases, the maximum compressive strength loss rate of pile foundation concrete specimens is 38.20%. The areal corrosion rate of reinforcement at the depth of 0.25 m is the largest, and the value is 91%. Coating epoxy resin on the surface can effectively reduce the corrosion rate of reinforcement, The mass changes of pile foundation concrete specimen and reinforcement are not obvious. In the 225 th dry-wet-freeze-thaw cycles, the corner of pile foundation concrete specimen falls off and cracks appear around, but the mass change is small. The relative dynamic elastic modulus reduces by 39.10%, the anti-erosion coefficient reduces to 0.51, the compressive strength loss rate of concrete is 65.88%. Failare nearly occurs in the interior because of the presence of Friedel salt and other expansive substances. As the spalling thickness and corrosion depth increase, the bearing capacity of pile foundation in the first eight years remains essentially unchanged. After eight years, its bearing capacity gradually decreases. Without proper maintenance, the bearing capacity of pile foundation will reduce by 34.45% by the 20 th year. It is suggested that the key protection measures should be taken for pile foundations after 8 years of service.