PH-monitoring in mortar with thermally-oxidized iridium electrodes

Abstract

The pH of the concrete pore solution plays a vital role in protecting the reinforcing steel from corrosion. Here, we present results from embeddable pH sensors that permit the continuous, in-situ monitoring of the pH in the concrete pore solution. These are potentiometric sensors, based on thermallyoxidized iridium/iridium oxide (IrO x ) electrodes. We propose an iterative calculation algorithm taking into account diffusion potentials arising from pH changes, thus permitting the reliable, non-destructive determination of the pore solution pH over time. This calculation algorithm forms an essential part of the method using IrO x electrodes. Mortar samples were exposed to accelerated carbonation and the pH was monitored at different depths over time. Comparative tests were also performed using thymolphthalein pH-indicator. The results from the pH sensors give insight in the carbonation process, and can, in contrast to thermodynamic modelling and titration experiments, provide insight in kinetic processes such as transport and phases transformations. It was found that the front at which the pH is decreased from initially 13-14 down to 12.5 can be significantly ahead of the common carbonation front corresponding to pH 9-10. This has major implications for laboratory testing and engineering practice.

Challenges and opportunities in corrosion of steel in concrete

“…A number of sensors have been developed over the past years to measure durability-relevant parameters on engineering structures. This includes embeddable reference electrodes [115], sensors for chloride concentrations [116][117][118][119] and pH values [120][121][122] of the concrete pore solution, and sensors for the measurement of concrete resistivity or galvanic currents [123].…”

Monitoring pH in corrosion engineering by means of thermally produced iridium oxide electrodes

“…It may be particularly interesting to study non‐Portlandite binders that likely exhibit very different shapes in evolution of pH versus time . Additionally, the IrO x electrodes permit measuring the pH in the concrete at the exposure conditions favorable for carbonation, which are below saturation state …”

“…The experiments are based on embedding the electrodes at different depths in mortar and exposing them to one‐dimensional carbonation by placing the sample in a carbonation chamber (65% RH, 21 °C and 4% CO 2 ) while the potential response of the pH sensor is recorded. Preliminary results show that the electrodes can be used to monitor changes pH in situ in the concrete pore solution of reinforced concrete . Table gives the potential response of an IrO x electrode embedded in mortar at a cover depth = 15 mm, before and after the sample was carbonated, together with the days of exposure to carbonation .…”

“…Preliminary results show that the electrodes can be used to monitor changes pH in situ in the concrete pore solution of reinforced concrete . Table gives the potential response of an IrO x electrode embedded in mortar at a cover depth = 15 mm, before and after the sample was carbonated, together with the days of exposure to carbonation . The pH was calculated from averaged E ‐ pH response curves obtained for the corresponding IrO x electrode (Table S2).…”

“…Carbonation after 5 months at this depth was validated with the phenolphthalein spraying method. It is also interesting to note that even after more than 5 months of exposure to CO 2 ingress and under relatively dry environment (65% RH), the electrode functionality is still given …”