Chloride Transport And Cover Protection Of Concrete Under Drying-wetting Cycles

Chloride induced corrosion has been the top durability issue of reinforced concretestructures in marine environment, especially the drying–wetting cycles in splash andtidal zones severely deteriorates concrete durability. Aiming at the issue of chlorideingress under drying-wetting cycles, a series of lab experiments, exposing experimentsand cores drilling from concrete after long-time exposing were performed. To beginwith, a convection-diffusion model of chloride transport was established, incombination with the experimental results the mechanism of chloride transport inconcrete was systematically studied. Secondly, the chloride transport in damagedconcrete caused by either load or shrinkage was further explored based on theexperimental study. Thirdly, the chloride threshold value under drying-wetting cyclesstatistically determined after experiments. Lastly, according to the reliability theory, anew probabilistic method of the durability design or redesign was provided for silaneimpregnation concrete and Engineered Cementitious Composites (ECC) concrete underdrying-wetting cycles. The main content and conclusions are listed as follows:(1) Chloride transport in concrete under drying–wetting cycles. Chlorideingress occurs mostly by means of diffusion and capillary suction. Considering theengineering practice of concrete under the drying-wetting cycles, theconvection-diffusion model of chloride transport was established, and the finitedifference method could be applied to solve this partial differential equation. And thenseveral specimens including common concrete are used respectively to carry out a seriesof experiments on chloride ingress in concrete under the drying-wetting cycles, thenumerical computing results of chloride profile in concrete agreed well withexperiments results, which showed the efficiency of the convection-diffusion model to preferably reflect the mechanism of chloride transport in concrete under drying-wettingcycles. Both numerical results and experimental results indicated that the ratio of dryingtime to wetting time determines the peak value and penetration depth of chloride, withthe increase of the ratio of drying time to wetting time, chloride’s peak value rises, andchloride’s penetration depth presents a trend of increase.(2) The critical chloride content of concrete under drying–wetting cycles. Thecritical chloride content influences greatly on the service life of concrete structuresexposed to marine environments. In order to obtain the critical chloride content ofconcrete under drying-wetting cycles, the reinforced concrete specimens of threedifferent water-binder ratios, three different mineral admixtures contents, and threedifferent carbonation degree were prepared and were subsequently exposed to NaCl(5.0%) solution with drying–wetting cycles (H.R.=60%). During the experiments,according to the potential and the current density of reinforcement corrosion, once thecorrosions normatively appear, the chloride profiles for each sample would be measured.Then the chloride threshold values for various concretes were statistically determinedwith the probability of90%under drying–wetting cycles. Taking the carbonation intoexperiments is suitable to the engineering practice and will provide a favorablereference for further durability design.(3) Chloride transport in cover protected concrete. Chloride could easilypenetrate the reinforcement through the cracks induced by mechanical load or shrinkage,which accelerate the initiation of corrosion in concrete and reduce the durability ofconcrete structure. Chloride transport in damaged concrete was discussed throughexperiments conducted respectively under tensile load and restrained shrinkage.Experiments revealed that chloride diffusion coefficient increased along with the tensilestress, characteristic of exponential growth mode. Even simultaneously exposed to saltysolution with the same load or restriction condition, silane impregnation concrete andECC had better performance on the resistance to deterioration than common concrete.(4) Durability design of cover protected concrete structures. According to theconvection-diffusion transport model mentioned above, and the chloride threshold valuestatically determined above, a new probabilistic method of the durability design forconcrete under drying-wetting cycles was provided based on the theory of reliability.For the concrete protected with silane or ECC, in order to meet the target reliabilityindex within the design working life, both durability design of new concrete anddurability redesign of damaged concrete were provided with chloride diffusivitycoefficient and cover thickness as the main parameters.