Study On The Diffusion Behavior Of Chloride Ion In Marine Concrete Under Combined Action Of Load And Freeze-thaw

The paper focuses on the marine concrete structures with the severest durability problem, obtains the load types and representative load levels for typical elements according to the load level test and analysis of the typical marine structures under actual service environment in China. The chloride diffusion processes for the marine concrete structures under load+ chloride condition and load+chloride+ freezing-thawing condition are studied. During the research, equipment for constant load and flexural load conducting was developed. Beyond the typical load types and levels of the concrete structures under actual environment, factors such as compressive and flexural load types, load level range, type and dosage of the mineral admixtures, concrete mix proportion were considered systematically. Artificial accelerate simulated experiments were carried out using the indoor sea water simulation system. The microstructures of the tested concretes were investigated. Conclusions are summarized as follows.(1) The results of numerical simulation, theoretical analysis and actual test for structures in field reveal that, the load level of the researched elements under actual service environment is between 7% and 50% according the finite element analysis. The load level of permanent load+ variable load is between 30% and 40%.(2) The relationship between concrete chloride diffusion coefficient and flexural load level is an exponential function which can be characterized as y=A·eBx. For high performance marine concrete with mineral admixtures, the constant flexural load impact factor of chloride diffusion coefficient is formulated as Dη=0.9769-D0·e1363η, while the impact factor for double effect of flexural load and freezing-thawing is Ddη=1.0224·D0·e1.6383η. In the formulas, η refers to independent variable load level, Do refers to the chloride diffusion coefficient of concrete without load, and Ddη, Dη refers to the chloride diffusion coefficient of concrete with load level of η.(3) Concrete chloride diffusion coefficient increases at first and then decreases when the compressive load increases. The lowest value is observed when the compressive load level comes to 30%. The relationship between the chloride diffusion coefficient of concrete with compressive load (Dη) and without compressive load (Do) can be characterized as formula (1), where η refers to the independent variable load level.(4) The key parameters of marine concrete structures service life prediction model which is established based on reliable theory are revised considering the constant flexural load effect. The revised prediction model is given as formula (2).(5) During the freezing-thawing process, when the freezing-thawing cycle increases, the concrete relative dynamic elastic modulus decreases and the mass loss rate increases. In addition, the concrete mass loss rate also increases along with the increase of the flexural load level.(6) Both the double effect of flexural load+ freezing-thawing and sole effect of flexural load have influence on the concrete microstructures. The mean pore size and total pore volume increase according to the increase of load level, while the pore distribution is changed too. The influence of the double effect of flexural load+ freezing-thawing on pore structure is more obvious then the influence of the sole effect of flexural load. Change of pore structure is the most important reason for change of concrete chloride diffusion coefficient.