Influence Of Freeze-thaw Cycles On The Durability Of Reinforced Concrete Structure

Reinforced concrete structures are the most popular structures in infrastructureconstruction, and the durability of reinforced concrete structure has become widely concernednow. In cold regions, the freeze-thaw resistance of reinforced concrete structures is the mostimportant parameter to define durability. It is clear that the research of durability of reinforcedconcrete under freeze-thaw cycles is very important. In the present dissertation, two aspects ofdirect and indirect influence of freeze-thaw cycles on the durability of reinforced concretestructures are given as follows:1． The direct influence of freeze-thaw cycles on the durability of reinforced concretestructures means the influence of freeze-thaw cycles on concrete directly. It is well knownthat most of concrete structures in cold regions are subjected to both external loads andfreeze-thaw cycles. In the present dissertation, the frost resistance and compressivestrength degradation of concrete under the simultaneous action of compressive load andfreeze-thaw cycles were experimentally investigated by specially designed apparatus.Also, the influence of external loads, w/c ratio and air entrainment on experimental resultswas discussed respectively. Based on the experimental results and mechanism analysis, amodel was suggested to predict the compressive strength degradation of concrete underthe simultaneous action of compressive load and freeze-thaw cycles, and the model wastestified by the data obtained from both situ and laboratory.2． The indirect influence of freeze-thaw cycles on the durability of reinforced concretestructures means the influence of freeze-thaw cycles on the other durability problems of reinforced concrete. In the present dissertation, author focuses on chloride inducedcorrosion, in which chloride ingress into concrete is one of the most important problems.Firstly, the mechanism of ions transport operating in concrete and the parameters thatdefine them were discussed. And then, the experiment of chloride ingress into concreteexposed to freeze-thaw cycles was carried out by the natural diffusion method. Also, theinfluence of freeze-thaw cycles, w/c ratio, fly ash and air entrainment on experimentalresults was discussed respectively. With the experimental results, a time dependent modelof effective diffusion coefficient was established, and based on which, a model wassuggested to predict chloride ingress into concrete under freeze-thaw cycles. Also thepredicting model was testified by the data obtained from both situ and laboratory.3． Another important problem in chloride induced corrosion research is the research ofchloride threshold level (CTL) for corrosion initiation. In the present dissertation, theinfluence of freeze-thaw cycles on CTL was discussed. Firstly, the electrochemicalmechanism of corrosion of steel in concrete was reviewed. And the conclusion wasobtained that the essential condition for depassivation of steel in concrete was corrosionpotential exceeding pitting potential. Based on the conclusion, experiment of CTL of steelin concrete specimens under freeze-thaw cycles was carried out. And in the experiment,the potentiostatic and macro current testing methods were used to control and check thecorrosion state of steel. With the experimental results, the influence of freeze-thaw cycles,pitting potential, w/c ratio and air entrainment on CTL was discussed respectively. At last,with the research of CTL under normal temperature and high temperature, the relationshipbetween CTL and full range of temperature (low, normal and high temperature) wasobtained, and based on which, the influence of freeze-thaw cycles on steel depassivationwas discussed.In the previously mentioned research works, several difficult problems were solvedsuccessfully through experiments. All models established in the present dissertation were testified by data obtained from situ and laboratory, which enhanced the reliability andapplicability of models in practice.The author wishes to express his appreciation for the support of the National NaturalScience Foundation of China (No.50678101) and Natural Science Foundation of Shanghai(No.11ZR1417400).