Experimental Research Of Chloride Threshold In Concrete

Corrosion of reinforcement induced by chloride penetration is an important influence on the service life of concrete structures.Steel does not corrode when embedded in concrete. Portland cement concrete pore solution is mainly consisted of saturated Ca(OH)2 and presence of NaOH and KOH increases the pH to more than 13. Under normal field conditions, the high alkalinity allows the steel to develop a stable passive film. When a critical chloride content reaches the steel surface, the passive film is broken and the corrosion of steel initiates. The critical chloride content is called chloride threshold level (CTL).Many people have used different experimental methods to confirm chloride threshold level. Glass summarized the chloride threshold levels inferred from published data and also the type of the sample and its exposure condition, chloride threshold level changes between 0.17% and 2.5% when it is presented as a total chloride content expressed relative to the weight of cement. The distribution and varying rule of chloride threshold level is not clear now.The reason for not finding a unique chloride threshold level is attributed to the number of parameters influencing the process. The influence factors can be divided into two parts, direct factors and indirect factors. The direct factors contain pitting potential and corrosion potential of steel, while the indirect factors contain cement type, concrete properties, addition of blended materials, the temperature and humidity of environment and pH of the pore solution and so on. When corrosion potential exceeds pitting potential of the steel, the steel initiates to corrode. In the present paper, two experiments are conducted to study the relationship between chloride content and pitting potential of steel. In the first experiment chloride content is added into motor specimens when they are cast. In the second experiment chloride is inducted by penetration. The region of potentials tested includes from +100mV to -300mV (SCE). The study is carried out by means of potentiostatic tests in each sets of identical mortar specimens.Based on the experiments and the results from experiment in simulated pore solutions, a transformation for free chloride from expressed as a percentage by weight of cement to mol content in pore solution is set up. The chloride threshold in mortar specimens is higher and varies more widely than that in simulated pore solutions.