Experimental Research On Chloride Threshold Level For Reinforcement Corrosion

R.C. structure is one form of structures which are applied most extensively worldwide nowadays. Lots of R.C. structures in use lose their functions ahead because of their inadequate durability, thus results in great loss in economy. On the other hand, in the R.C. structure design process, more attentions are paid to bearing-capacity design, and durability design is often ignored. Therefore, research on durability is very important in the R.C. structure research.Corrosion of reinforcements in concrete is a general event among the factors inducing durability degeneration. The predispositions inducing corrosion of reinforcements consist of carbonation of concrete and chloride encroachment in chief. Owing to the faster corrosion rate and larger nuisance resulting from chloride encroachment, chloride-inducing corrosion becomes the emphasis of research. The main contents of research on chloride-inducing corrosion include penetration of chloride, corrosion- initiation conditions and reinforcement corrosion rate, and present paper mainly points to the corrosion-initiation condition– chloride threshold level (CTL). The reasonable assertion of CTL is of great significance to durability design and prediction of service life of structures.Predecessors have developed rather deep researches on CTL. However, owing to so many factors influencing CTL, CTL values from literatures varies largely. To ensure the reliability of R.C. structures, standards of many countries adopt a quite conservative value. This method may be an expedience before the CTL laws are revealed absolutely.Corrosion potential exceeding pitting corrosion is the sufficient and essential condition for depassivation of reinforcement, and this constitutive issue is seized now. Considering the influence of pH and temperature of the concrete pore solutions, CTL for reinforcement corrosion is researched with potentiostatic method based on the experiments and statistical method. Mean values and variation coefficients of Cl- content at the reinforcement corrosion initiation and CTL values with 95% acceptance value under different potentials, different pH and different temperatures are suggested.The following are discovered:1. Corrosion potential is the comprehensive and dominative factors influencing the state of reinforcement corrosion. When potential drops, CTL increases. Potential is divided to two sections according to the degree of influence on CTL, and the interface between the two sections relates to pH. The linear fitting of log(mean value of Cl- content at the reinforcement corrosion initiation) and potential is suggested.2. pH of the concrete pore solutions is one of the key factors influencing CTL. The exponential fitting of mean value of Cl- content at the reinforcement corrosion initiation and pH is suggested. On the other hand, carbonation of concrete also has an influence on CTL. When carbonation exceeds a certain degree, CTL may increase with the carbonation degree.3. Temperature is one of the key factors influencing CTL too. Mean value of Cl- content at the reinforcement corrosion initiation decrease 50% when the temperature increases from 5oC to 25oC. Temperature plays a role through influencing the reaction rates of the electrochemical processes and the micro-environment of corrosion.Finally an invented monitoring sensor for reinforcement corrosion is tested in laboratory. Compared to the similar sensors abroad, the sensor tested can indicate the depth of CTL in concrete perfectly, and have some advantages such as more simple, smaller volume and more reliable data etc.