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 thermally-oxidized iridium/iridium oxide (IrOx) 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 IrOx 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, give insight in kinetic processes such as transport and phases transformations. Additionally, 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.
Highlights
Reinforced concrete is the most common building material used in civil engineering
We propose an iterative calculation algorithm taking into account diffusion potentials arising from pH changes, permitting the reliable, non‐destructive determination of the pore solution pH over time
The pH of the concrete pore solution is in the range pH 13‐ 14 due to the alkalinity provided by the dissolved sodium and potassium oxides present in Portland cement [2, 3]
Summary
Reinforced concrete is the most common building material used in civil engineering. Durability of this material can, be compromised due to the corrosion of reinforcement steel. Other methods to determine the pH of concrete are pore solution expression or crushing concrete samples and exposing them to a leaching agent [6, 7] While these approaches may provide more accurate results than indicator tests, they are time consuming and present difficulties at low moisture levels as well as in the presence of sharp pH gradients, where limitations regarding the minimum sample volume needed render the application of these methods impossible. We report the pH response of IrOx electrodes embedded at different depths in mortar samples that were exposed to accelerated carbonation These sensors allowed monitoring the pH of the pore solution continuously while the cement paste was carbonated.
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