Experimental Study On Bearing Performance Degradation Of HPC Components Under Chloride Salt Environment After Repeated Loading

In the process of construction and use of concrete structures,there are various factors（such as the change of external loading,service environment,etc.）that affect the service performance of concrete structures.For example,in the case of coastal engineering,reinforced concrete bridges,offshore structures,coastal dam and other structures not only bear the action of static loading or repeated loading,but also bear the action of marine environment erosion,resulting in their service life less than the designed service life.At present,the research of the performance deterioration and degradation mechanism of concrete structures under the combined action of repeated loading and chloride salt is less.Therefore,supported 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 Loading"（51878319）,high performance concrete（HPC）structures under the combined action of repeated loading and chloride salt erosion environment were selected as the research object in this paper.The internal damage,mechanical properties,durability performance of HPC and the load-bearing performance of center pull-out specimens and beams with steel bars were carried out from two aspects of materials and components.The main works and conclusions are as follows:1.MaterialsExperimental study and comparative analysis of the relative dynamic elastic modulus and compressive strength degradation of HPC materials under repeated loading,chloride salt dry-wet cycles,and the coupling of repeated loading history and the chloride salt dry-wet cycles（first apply the repeated loading and then perform the chloride salt dry-wet cycles）were carried out.The HPC blocks which were applied 10 and 30 times repeated loading respectively（the level of loading is 0.4）,and then the HPC samples were placed in the atmospheric environment and chloride salt dry-wet cycles environment for a total of 280 days,the relative dynamic elastic modulus and compressive strength were finally measured.The test results shown that: 1)With the increase of the number of repeated loading,the relative dynamic elastic modulus of the HPC test blocks first decreased rapidly,and then the descending speed slowed down and gradually stabilized;in addition,the corresponding compressive strength also had a similar change rule.2)As the time of the chloride salt dry-wet cycles increased,the relative dynamic elastic modulus and compressive strength of HPC blocks showed a trend of first increasing and then decreasing.3)Under the combined effect of repeated loading history and chloride salt dry-wet cycles,the relative dynamic elastic modulus and compressive strength also showed a trend of first increasing and then decreasing,and the rate of increase got faster when the degree of damage were larger.2.Components（1）The experiments on the bonding performance of HPC and steel bars under repeated loading,chloride salt dry-wet cycles,and the coupling of repeated loading history and the chloride salt dry-wet cycles were carried out.The central drawing members which were applied 10 and 30 times repeated loading respectively（the level of stress was 0.4）,and then the central drawing members were placed in the atmospheric environment and chloride salt dry-wet cycles environment for a total of 280 days,to explore the influence of the bond performance of steel bars and HPC by the three damage mechanisms.The test and analysis results show that: 1)As the number of repeated loading increased,the bond performance between HPC and steel bars gradually deteriorated.2)As the time of chloride salt dry-wet cycles increased,the bond performance would increase first and then deteriorate.3)Under the coupling effect of repeated loading history and chloride salt dry-wet cycles,the bond performance of HPC and steel bar were similar to that of the specimens without load.The bond performance also showed a trend of first increasing and then decreasing.However,the greater the degree of damage,the worse the bond performance.4)Compared with the relative dynamic modulus damage,the compressive strength loss was better than the evaluation of bond performance degradation.（2）The experiments on the durability and flexural performance of HPC beams under repeated loading,chloride salt dry-wet cycles,and the coupling of repeated loading history and the chloride salt dry-wet cycles were carried out.The HPC beams which were applied 10 and 30 times repeated loading respectively（the level of load is0.4）,and then the HPC beams were placed in the atmospheric environment and chloride salt dry-wet cycles environment for a total of 280 days,to explore the influence of the the durability and flexural performance of HPC beams by the three damage mechanisms.The results show that: 1)Under the same powder extraction depth,the chloride ion content in the area subjected to bending and compressive stress was higher than that of no load.2)When the cracks width（the cracks width in the repeated loading process）were 0.095 mm,the chloride ion content increases sharply and a sudden change occurs.3)After the first loading-unloading was completed,the HPC beams had a large plastic deformation,and then the deflection tends to be stable.4)As the number of repeated loading increased,the flexural capacity of the HPC beams gradually decreased.Under the action of the chloride salt dry-wet cycles,the ultimate bending moment of the test beams showed a greater increase.The ultimate bending moment of the HPC beams under the coupling effect of repeated loading history and the chloride salt environment showed a trend of first increasing and then decreasing.When chloride salt dry-wet cycles 280 days,the ultimate bending moment was reduced by 3.32%～4.81% compared with the reference beam.5)The use of compressive strength loss was better than evaluating the flexural performance of HPC beams.Therefore,the HPC compression strength fitting formulas were introduced to establish the calculation model of HPC beams flexural performance under three damage mechanisms.The calculation results were consistent well with the test values.