Chloride Transport in Concrete - Measurement and Prediction
Doctoral thesis, 1996
This doctoral work has been carried out to try to classify the basic theories involved in chloride transport in concrete, and at the same time to develop a measurement method and a prediction model for concrete structures submerged in sea water.
Chloride binding accompanies almost all the transport processes and has a significant influence on chloride transport. It has been found that the chloride binding capacity non-linearly decreases as free chloride concentration increases. The relationship between bound and free chlorides may be simply expressed by the Freundlich equation.
Diffusion is one of the main transport mechanisms, especially when the pores in concrete are filled with water. A concept of "intrinsic diffusion coefficient" with the dimension m 2_x per second has been proposed to evaluate chloride diffusion through the pore solution in concrete. This intrinsic diffusion coefficient facilitates the comparison between different test results and the modelling for predicting chloride penetration.
Under the action of an external electrical field, the chloride transport process can be greatly accelerated. The rate of migration is theoretically related to the diffusion coefficient. Therefore, the chloride diffusion coefficient can be quickly determined through a migration test. The CTH rapid method for determining the chloride diffusion coefficient has been developed based on the theory of diffusion-migration. Among different rapid methods the CTH method is theoretically clearest and experimentally simplest.
Numerical approach is a better way for modelling of such a complicated transport process. A windows based program ClinConc has been developed for this numerical approach. In this numerical model most of the factors affecting chloride penetration have been considered. The main input data in the program include concrete mix design, workmanship and exposure conditions. The only parameter which must be measured is the chloride diffusion coefficient by the CTH rapid method. The chloride penetration data obtained from both laboratory and field studies were used to test the model. The preliminary results show that the predicted chloride profiles correspond fairly well with the measured data, in spite of different exposure conditions and durations.