| The phenomenon of dry-wet cycle is very common in nature,such as the ebb and flow of river during flood season,the ebb and flow of seawater,the rise and fall of reservoir water level,the fall and evaporation of rainwater,etc.are all inducements of the dry-wet cycle.The soil and water in the eastern coastal areas and northwest saline areas of China contain a large amount of chloride salts,which can cause damage to concrete structures and reduce the durability of concrete structures.In summer,the surface temperature of river or sea water is relatively high.The temperature of the water surface can reach about 30-45℃ during the day and about 20℃ at night.The cyclical day and night change is similar to putting concrete in an environment where the water temperature changes.Considering the effects of dry-wet cycles at different water temperatures,the study of chloride ion erosion and diffusion is more in line with actual working conditions.Floating objects in river water or seawater or passing ships may impact the bridge pier concrete,causing damage and destruction to the bridge pier concrete.The concrete specimens were analyzed for physical properties such as water absorption,density,longitudinal wave velocity,and apparent characteristics.The static uniaxial compression test and the dynamic SHPB test were carried out to analyze the static and dynamic mechanical characteristics of the pier concrete,combined with the SEM test and the XRD test analyzes the damage and deterioration mechanism of concrete under the combined action of temperature-chloride salt-dry-wet cycle.The main research contents and conclusions are as follows:(1)With the increase in the number of dry-wet cycles,the water absorption and density of the specimens show an increasing trend and tend to be stable.The higher the temperature,the lower the final density of the specimen,the more chloride salt crystals formed on the surface of the specimen,and the more severe the damage.(2)In the uniaxial compression test,with the increase in the number of dry-wet cycles,the strength of the specimens first increased and then decreased.The higher the temperature of the chloride salt solution,the more the strength of the specimen decreased compared with the specimen in the dry state.When the number of dry-wet cycles is the same,the higher the temperature of the chloride salt solution during the immersion process,the greater the length of the compacting stage of the micro-cracks in the stressstrain curve of the specimen,and the smaller the length of the elastic stage.When the number of dry-wet cycles is 30,the broken shape and degree of the specimens become more severe with the increase of the temperature of the chloride salt solution.(3)In the uniaxial impact test,three impact air pressures of 0.25 MPa,0.35 MPa and 0.45 MPa were used.After 5 and 10 dry-wet cycles under the same impact air pressure,the dynamic strength of the specimen increased slightly,and the stress-strain curve The length of the medium compaction stage gradually decreases,and the length of the elastic stage gradually increases;after 20 and 30 dry-wet cycles under the same impact air pressure,the strength of the specimen is significantly reduced,and the higher the chloride salt solution temperature,the specimen The greater the decrease in the strength of the stress-strain curve,the length of the compaction stage in the stress-strain curve gradually increases,and the length of the elastic stage gradually decreases.(4)When the temperature of the chloride salt solution and the number of dry-wet cycles are the same,the strength of the test piece increases with the increase of the impact air pressure;under the same impact air pressure for 30 dry and wet cycles,the broken shape of the test piece increases with the temperature of the chloride salt solution.The increase gradually intensifies,and the size of the broken particle size gradually decreases with the increase of the temperature of the chloride salt solution;when the number of drywet cycles is the same as the temperature of the chloride salt solution,as the impact pressure increases,the specimen is destroyed The post-crushing form gradually intensified,the large-particle fragments gradually decreased,and the proportion of smallparticle fragments became larger and larger.(5)The XRD test results show that a large amount of Friedel salt formed by the reaction of calcium hydroxide and calcium aluminate hydrate and chloride salt in the concrete in the later stage of the dry-wet cycle produces expansion force inside the specimen,and the higher the temperature,the faster the reaction rate,and the resulting Friedel The more salt,the more cracks generated inside the specimen.The SEM image shows that after 30 dry and wet cycles,with the increase of the temperature of the chloride salt solution,the number of cracks and pores inside the concrete specimen gradually increases,and gradually develops into interpenetrating cracks.(6)It can be seen from the SEM image that when the temperature of the chloride salt solution is 60℃,with the increase of the number of dry-wet cycles,the inside of the specimen is more dense in the early stage of the dry-wet cycle,and the interior of the specimen gradually becomes loose in the later stage of the dry-wet cycle;In the early stage of the wet cycle,the appropriate amount of Friedel salt formed by the reaction of calcium hydroxide and calcium aluminate hydrate in the concrete with the chloride salt fills the internal pores and cracks to a certain extent,and a large amount of Friedel salt produced in the later stage of the dry-wet cycle is inside the specimen.Produce expansion force,causing internal cracks.Figure 42 table 3 reference 80... |
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