Study On The Long-term Performance Of Marine Concrete Beams Under The Combined Action Of Chloride Salt And Repeated Loads

In the marine environment,concrete structures are susceptible to corrosion by chloride ions,causing corrosion of steel bars,which affects the long-term performance of the structure.Especially for the marine concrete structural members in the alternate dry and wet cycle area,under the long-term repeated action of the service load,the members are in a loaded or cracked state,which will inevitably affect their resistance to chloride ion corrosion,and then affect the long-term performance of the structure.This paper is funded by the National Natural Science Foundation of China "Research on the Long-term Performance Degradation Mechanism and Improvement Technology of Coastal Concrete Structures Under the Combined Action of Chloride Salt and Repeated Loads"(51878319),combined with domestic and foreign research results and the research results of this research group,using experimental research and other methods have carried out research on the long-term performance of marine concrete beams under the combined action of chloride salts and repeated loads.The main tasks are:1.Experimental study on the mechanical properties of corroded steel bars in marine concrete beams under the action of chloride salt drying and wetting cycles.The marine concrete test beams that the research team has undergone 420 d chlor-salt dry-wet cycles were broken,and the corroded steel bars in the beams were removed and the static tensile test was carried out.The test results show that the corrosion of the steel bars reduces the effective area of the cross-section of the steel bars,resulting in different degrees of decline in the mechanical properties of the steel bars.The specific manifestation is that as the average mass loss rate of corroded steel bars increases,the yield platform gradually shortens,and the yield strength and ultimate strength also decrease.2.Experimental study on chloride ion erosion in marine concrete beams under the combined action of chloride salt and repeated loads.Design 3 sets of concrete mix ratios,and make a total of 10 marine concrete test beams.Repetitive loads(load levels of 0,0.3,and 0.6)were applied to the test beams by self-anchoring in pairs,and the chloride salt dry-wet cycle test(30T,180 d in total)was carried out at the same time.The RCT method is used to detect the free chloride ion content in the concrete beam under repeated loads,and the regression analysis is carried out.The test results show that the existence of cracks accelerates the intrusion of chloride ions,and the acceleration effect is related to the width of the crack.The specific performance is as follows,the content of free chloride ions at each depth increases with the increase of the crack width.The presence of tensile stress in the component or the appearance of cracks will increase the apparent chloride ion diffusion coefficient of the marine concrete beam,but the appearance of cracks will resist it.The degradation of chloride ion erosion performance is more influential.According to the degradation effect functions F(?)and F(w),a calculation model for the equivalent apparent chloride ion diffusion coefficient of SFA concrete under the loaded state and the cracked state is proposed.3.Conduct experimental research on the flexural performance of marine concrete beams under the combined action of chloride salt and repeated loads,compare and analyze the effects of admixture types and load levels on the flexural performance of the beams.The test results show that the mid-span section strain of each group of marine concrete beams before and after repeated loading basically conforms to the assumption of a flat section,and the maximum crack width is smaller than the calculated value calculated by the maximum crack width calculation formula of the current code.The test beam is subjected to the chloride salt dry-wet cycle and repeated loads work together,the yield displacement,ultimate displacement,and flexural bearing capacity will all decrease to a certain extent.Under repeated loads of27 k N and 54 k N(load levels of about 0.3,0.6),the ultimate bearing capacity of SF marine concrete beams decreased by 5.91% and 28.67%,SFA marine concrete beams decreased by5.86%,27.41%,and SFNS marine concrete beams suffered with repeated loads of 27 k N(load level of about 0.3),the ultimate bearing capacity is reduced by 10.12%.Considering the repeated loads causing strength damage to the concrete,a calculation formula for the degradation of the flexural bearing capacity of marine concrete beams under repeated loads is proposed.4.Taking the beginning of corrosion of steel bars as the durability limit state,the Monte Carlo method is used to predict and analyze the durability life of marine concrete beams against chloride ion corrosion under repeated loads.The related results show that: repeated loading causes the durability life of marine concrete beams to decrease,and the increase in load level has more obvious impact on the durability.However,the effect of the load level on the durability life decreases with the increase of the thickness of the protective layer.Although increasing the thickness of the protective layer can prolong the durability life of the concrete structure,the appearance of cracks will greatly advance the initial rust time of the steel bar,and with the increase of the crack width,the increase of the thickness of the protective layer will gradually reduce the effect of extending the durability life.Take a crack width of 0.15 mm as an example.After the thickness of the protective layer is increased from40 mm to 60 mm,the life expectancy will increase from 10 years to 36 years.When the crack width is 0.20 mm,the corresponding life expectancy is only increased from 4 years to 17 years.At this time,the increase in the thickness of the protective layer is difficult to meet the needs of the marine concrete structure to meet the established service cycle.It can be seen that,for the durability of marine concrete structures,the control of the crack width is more important than the increase in the thickness of the protective layer.