Experimental Study On The Effect Of Freeze-thaw Cycles On Chloride Ion Transport In Concrete

For the reinforced concrete structure in the marine environment of the frozen area,the freeze-thaw cycle causes micro-cracks inside the concrete,increases the chloride ion transport channel,and accelerates the chloride ion erosion process.At present,the research on the influence of freeze-thaw cycles on chloride ion transport in concrete mainly exists the following problems:In the existing chloride ion transport model,the chloride ion diffusion coefficient is mainly corrected according to the number of rapid freeze-thaw cycles.On the one hand,the influence of freeze-thaw cycle on the surface chloride ion concentration can not be neglected.On the other hand,the difference of indoor and outdoor freeze-thaw cycles leads to the poor practicability of the model.Therefore,it is necessary to further establish a chloride ion transport model considering the influence of freeze-thaw damage on the surface chloride ion concentration and the instantaneous chloride ion diffusion coefficient.The value of freezing minimum temperature is relatively simple?-18°C?.It is worthwhile to further study the influence of different freezing minimum temperature on chloride ion transport characteristics in concrete.The freeze-thaw cycle of indoor rapid salt freezing test is relatively short?3h?,thus it is impossible to realistically simulate the influence of the coupling process of chloride ion transport and freeze-thaw cycles on chloride ion corrosion in concrete in natural environment.Therefore,corresponding outdoor tests should be carried out to study this problem in depth.In order to solve the above problems,the combination of theoretical analysis and physical test was used,and the index of freeze-thaw damage was proposed to reflect the effect of freeze-thaw cycles on the transport characteristics of chloride ions in concrete.Besides,the influence of the number of freeze-thaw cycles?the minimum freezing temperature and outdoor coupling process of chloride ion transport and freeze-thaw cycles on the degree of concrete freeze-thaw damage was explored.And based on that,a practical chloride ion transport model considering the effects of concrete freeze-thaw damage was established.The specific research contests are as following:?1?The instantaneous chloride ion diffusion coefficient(D_ins)of concrete can reflect the change characteristics of the internal pore structure of concrete caused by freeze-thaw cycles in real time,and can be used to study the influence of freeze-thaw cycle on the transport characteristics of chloride ion in concrete.Based on Fick's second law,the mathematical relationship between D_ins and the apparent chloride ion diffusion coefficient?D_a?was derived by theoretical analysis,and the calculation method of D_ins was proposed.The quantitative relationship between chloride diffusion coefficient D_RCM and D_ins and D_a was established by conducting RCM test and indoor diffusion test.?2?The antifreeze performance and chloride ion penetration resistance of concrete specimens were investigated by conducting indoor rapid freeze-thaw cycle test and RCM test.The quantitative relationship between the degree of freeze-thaw damage of concrete and the number of freeze-thaw cycles and the quantitative relationship between chloride diffusion coefficient D_RCM and the degree of freeze-thaw damage were established.The degree of freeze-thaw damage of concrete was proposed to reflect the effect of the freeze-thaw cycle on chloride ion transport characteristics in concrete.?3?The chloride ion transport law in the concrete specimens after different freeze-thaw cycles was explored by conducting indoor rapid freeze-thaw cycle test and the natural diffusion test in the artificial ocean tide environment.The surface concentration freeze-thaw influence coefficient?and diffusion coefficient freeze-thaw influence coefficient?were proposed respectively to reflect the influence of freeze-thaw cycles on the surface chloride ion concentration and instantaneous chloride ion diffusion coefficient.Finally the chloride ion transport model considering the influence of concrete freeze-thaw damage was established,and the correctness of the model was verified by compared with the test results.?4?The effects of freezing minimum temperature on freeze-thaw damage of concrete and chloride ion transport law were studied by conducting indoor rapid freezing-thawing cycle test under different freezing minimum temperature conditions and natural diffusion test in artificial marine tidal environment.Combined with the chloride ion transport model considering the influence of concrete freeze-thaw damage and the variation of the degree of freeze-thaw damage of concrete with the freezing minimum temperature,a chloride ion transport model considering the effects of freeze-thaw damage on concrete with different freezing minimum temperatures was established,and the correctness of the model was verified by compared with the test results.?5?The chloride ion transport law in concrete under the coupling process of chloride ion transport and freeze-thaw cycles was explored by conducting the experiment of winter outdoor natural freezing and thawing cycles in Tianjin area.According to the comparison of the degree of freeze-thaw damage of concrete specimens between the indoor and outdoor freeze-thaw cycles,the indoor-outer relationship of concrete frost resistance was preliminarily determined.By introducing the time-varying law of concrete freeze-thaw damage in the chloride ion transport model considering the effects of concrete freeze-thaw damage,a chloride ion transport model under the outdoor coupling process of chloride ion transport and freeze-thaw cycles was established.Finally,the model was temperature corrected,and the correctness of the modified model was verified by the test results...