Chloride Transport And Reliability-based Servicelife Prediction Of Concrete Under Load

Concrete structures generally play an important role in the infrastructure construction. The corrosion of reinforcing steels in concrete structures is one of the main causes of the degradation and reduction of their service life, especially when they are exposed to marine environment or de-icing salts. An understanding of the process of chloride transportation in concrete is of great importance for engineers to predict the service life of concrete structures. However, most of the durability design and service life prediction of concrete structures in chloride environment have not considered their stress state. Therefore, the transport behavior of chloride ion in concrete under load was investigated, based on which a reliability-based model for service life prediction of concrete under load was proposed and validated in a realistic concrete project. The main aspects presented in this dissertation are as following:1. A set of test setup that can provide a uniform, stable and non-eccentric axial tensile or compressive load was build up t o investigate the transport behavior of chloride ion in concrete under external load. Together with a salt circulation system with precise control of flow rate, the corresponding test method for chloride diffusion test in concrete under external load was proposed.2. Degradation process of concrete under synergetic mechanical load and chloride attack was investigated using the designed test setup and the proposed test method. Chloride diffusion coefficient of concrete under compressive load decreases first and then increases with incremental compressive stress ratio, while chloride diffusion coefficient of concrete under tensile load increases with applied tensile load ratio. In the meanwhile, the diffusion coefficient decreases with the exposure time, no matter whether an external load is applied or not.3. A probabilistic model for the chloride ingress into concrete, based on the Fick’s second law of diffusion, was developed that takes into account the uncertainties in the relevant mechanical load(axial tensile stress or axial compressive stress) and environmental factors(temperature, relative humidity and ambient chloride concentration, etc.) influencing the diffusion process of chloride. This model can be used for the service life prediction of concrete structures under chloride attack and under synergetic mechanical load and chloride ingress.4. A reliability based service life prediction model for concrete structures under mechanical load and chloride ingress was proposed, taken the chloride concentration on the steel surface reaching the critical chloride concentration as the limit state of reinforced concrete structures. From the previous laboratory and field investigation results reported by other researchers, the quantitative effect of the factors, e.g. temperature, relative humidity, test method and stress state, on the chloride diffusion coefficient was studied statistically. Furthermore, the accurate critical chloride concentration values that initiate corrosion of reinforced steel were collected and analyzed statistically. Hypothesis testing results show that the critical chloride concentrations are lognormal distributed as Ccrit, t ~ Lognormal(0.818, 0.494) in terms of total chloride and Ccrit, t ~ Lognormal(0.612, 0.299) in terms of free chloride.5. The reliability based service life prediction of concrete structures in the Hong Kong-Zhuhai-Macau sea link project for a d esigned service life of 120 years was carried out using the established service life prediction model. Results show that after 120 years, the reliability index of pier caps concrete structure with compressive stress in splash zone and tidal zone are 1.615 a nd 1.85, respectively. In addition, the reliability index of concrete structure that links the cast-in-situ bored pile and pier caps with tensile stress in splash zone and tidal zone are 1.385 and 1.614, respectively. After 120 years’ servicing, the reliability index of the tested concrete structures are all above the target value of 1.3.