Development of a Novel Methodology to Assess the Corrosion Threshold in Concrete Based on Simultaneous Monitoring of pH and Free Chloride Concentration.

Yurena Femenias,Bernhard Elsener,Ueli Angst,Fabrizio Moro

doi:10.3390/s18093101

Yurena Femenias, Bernhard Elsener + Show 2 more

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https://doi.org/10.3390/s18093101

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Abstract

Both the free chloride concentration and the pH of the concrete pore solution are highly relevant parameters that control corrosion of the reinforcing steel. In this paper, we present a method to continuously monitor these two parameters in-situ. The approach is based on a recently developed electrode system that consists of several different potentiometric sensors as well as a data interpretation procedure. Instrumented mortar specimens containing different amounts of admixed chlorides were exposed to accelerated carbonation, and changes in free chloride concentration and pH were monitored simultaneously over time. The results revealed the stepwise decrease in pH as well as corresponding increases in free chlorides, resulting from the release of bound chlorides. For a pH drop of about 1 unit (from pH 13.5 down to pH 12.5), the free chloride concentration increased up to 1.5-fold. We continuously quantified the ratio Cl−/OH− that increased steeply with time, and was found to exceed a critical corrosion threshold long before carbonation can be detected with traditional indicator spray testing, even at admixed chloride contents in the order of allowable limits. These results can strongly influence the decision-making in engineering practice and it is expected to significantly improve condition assessments of reinforced concrete structures.

Highlights

Reinforced Concrete (RC) is the world’s most used building material [1], it is widely used in infrastructures, e.g., tunnels, garages, bridges, and in many public and private buildings
In the concrete alkaline environment, steel is in the passive state, i.e., with a negligible corrosion rate [4,5], but this passivity can be lost due to the presence of chlorides or to a decrease in the pore solution pH at the reinforcement level [4,5]
We present the application of this novel method, by monitoring pH and free chloride concentration in mortar samples containing admixed chlorides and exposed to accelerated carbonation

Summary

Introduction

Reinforced Concrete (RC) is the world’s most used building material [1], it is widely used in infrastructures, e.g., tunnels, garages, bridges, and in many public and private buildings. The service life of RC structures may be limited due to some deterioration mechanisms, amongst them, corrosion of the steel reinforcement is the most widespread in many parts of the world [2,3]. In the concrete alkaline environment, steel is in the passive state, i.e., with a negligible corrosion rate [4,5], but this passivity can be lost due to the presence of chlorides or to a decrease in the pore solution pH (for example, due to CO2 ingress) at the reinforcement level [4,5]. Corrosion initiates, leading to the loss of rebar cross section and to the decrease in structural safety. In 2013 the global cost of corrosion was reported

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