Chloride threshold for rebar corrosion in concrete with addition of silica fume

The effect of silica fume on the chloride threshold for the initiation of pitting corrosion of steel in concrete was investigated. Laboratory tests were carried out in concrete specimens made with ordinary Portland cement and with 10% of silica fume. Chloride contents up to 2% by mass of cement were added to the mixes, in order to investigate the corrosion rate of embedded bars made of both strengthened and mild steel. A lower chloride threshold was observed in the bars which were embedded in concrete with silica fume compared to those embedded in concrete made of Portland cement

Monitoring the chloride concentration in the concrete pore solution by means of direct potentiometry

Determination of the chloride concentration at the depth of the rebar is important when studying the mechanisms of chloride-induced corrosion. It is generally assumed that only the chloride ions dissolved in the concrete pore solution can initiate corrosion, while those bound by the constituents of the cement paste are considered harmless. Attempts to use ion selective electrodes in concrete, to non-destructively monitor the free chloride concentration, have thus been made by several researchers. In the present work, results from laboratory experiments with such chloride sensors are presented. It was found that the presence of concentration gradients such as pH differences markedly influences the measurement of the sensor potential and thereby the accuracy of this chloride measurement technique. This has to be taken into account carefully when designing an experimental setup involving the use of ion selective electrodes

Potentiometric determination of chloride ion activity in cement based materials

The chloride content at the reinforcement is one of the decisive factors for the initiation and propagation of localised corrosion in concrete structures. A monitoring technique for the chloride concentration which is accurate, non-destructive and continuous would thus be highly desirable. For this reason, the performance of ion selective electrodes (ISEs) was investigated both in alkaline solutions and embedded in mortar. The Ag/AgCl electrodes used in this work showed Nernstian behaviour with a slope of -59 ± 1 mV per decade and a detection limit for chloride ions below 10-2 mol dm -3 even at pH close to 14; the selectivity coefficient for hydroxide interference was estimated at kpotCl-, OH- ≈ 4 · 10-3. The Ag/AgCl membranes show good long-term stability over more than 6 months even in highly alkaline solutions as long as chloride ions are present; in the complete absence of chloride the measured potentials were affected by the pH of the solution. The sensors are, however, able to recover fast as soon as they come into contact with chloride. When using ISEs embedded in concrete, diffusion potentials between the reference electrode and the ISE, as arising e.g. from gradients in pH, significantly affect the potential measurement and present a most important error source for the application of direct potentiometry to concrete. To minimise such errors, the reference electrode has to be positioned as close to the ISE as possible

Chloride induced reinforcement corrosion: electrochemical monitoring of initiation stage and chloride threshold values

Reinforcement steel embedded in six different concrete mixes was exposed to chloride by wetting/drying cycles. Various parameters were continuously monitored during more than 1 year. Cement replacement with fly ash had beneficial long-term effects regarding chloride penetration resistance. Concerning corrosion performance, the by far most dominant influencing parameter was the steel/concrete interface since corrosion initiated on the lower side of the rebar (with respect to casting direction) regardless of binder type and w/b ratio. In many cases, after the first signs of depassivation, a marked increase in chloride content was required to prevent repassivation and to enable stable pit growth

Chloride induced reinforcement corrosion: rate limiting step of early pitting corrosion

Transition from passive state to stable localised corrosion of reinforcement steel in concrete owing to chloride ingress takes place over a period of time rather than being a one-step-occurrence. The depassivation process was characterised by frequent measurements of corrosion potential, polarisation resistance, and macro-cell currents when short-circuiting the working electrode with additional cathode surface. In addition, the concrete resistivity was continuously monitored and cathodic and anodic polarisation curves were measured. The results are consistent and imply that the localised corrosion process is in the initial phase of pit growth under mixed anodic/ohmic control. With time, the anodic reaction kinetics become more limited and the corrosion rate gets almost entirely determined by anodic control. The observed relationship between achieved maximum corrosion current and concrete resistivity indicates that the extent to which the anodic reaction kinetics are restricted is determined by the concrete microstructure and its ability to retain ionic movement

Critical Chloride Content in Reinforced Concrete - A Review

Chloride induced corrosion as the major cause for degradation of reinforced concrete has been the subject of great research efforts over the last fifty years. The present literature review summarises the state of the art by presenting the concept of the critical chloride content, discussing influencing factors, and assessing available measurement techniques. A large number of published chloride threshold values together with the respective experimental details are collected. While today's experience is mostly based on Portland cement, more modern studies with non-traditional binders often reported contradictory results. The present literature evaluation highlights the strong need for a practice-related test method, and, in this regard, focuses especially on experimental procedures by discussing advantages and drawbacks of methods and setups. It clearly emerges that many of the setups used to determine critical chloride contents are not suited to give realistic results