Corrosion rate monitoring of steel in reinforced concrete structures: comparison between external and embedded sensors

Abstract

It is well known that steel passivation in concrete is due to the highly alkaline environment (pH: 12.5 to 13.6). However, steel passivity can be destroyed by local acidification, carbonation, ingress of chloride ions and/or depletion of O2, being the corrosion of reinforcements one of the major causes of the degradation of concrete structures in aggressive environments. In order to assist the development of reliable models that allow the design of new structures durable in aggressive environments and to establish rational maintenance and repair strategies of reinforced concrete structures affected by reinforcement corrosion, various systems for permanently monitoring the corrosion on site have been developed. In spite of the extended laboratory experience in the measurement of the corrosion rate of steel in concrete few results have been reported in measurements in situ. Two main electrochemical techniques have been proposed to on site measurement the corrosion rate of reinforcing steel, one using external sensors applied on concrete surface and using sensors embedded in concrete during concreting. In the present work results from corrosion rate monitoring in reinforced concrete slabs exposed to natural environmental conditions will be reported. Corrosion currents, icorr, estimated from the polarization resistance, Rp, were measured directly on reinforcement, by means of external probes with guard ring (GEOCOR 06), and in internal sensors specially designed to be embedded in different depths in the concrete cover. Two concrete compositions were studied in different conditions of corrosion initiation, one due to the introduction of chlorides during concrete mixture and other with external chloride penetration. From this study it could be concluded that the designed embedded icorr sensor allows distinguishing between the active and passive corrosion state of steel in concrete and detects the beginning of steel corrosion induced by chlorides. It is also sensitive to the natural environmental variation of the corrosion rate. Results obtained with external probes did not allow to distinguish between the passive and active state of steel in the higher resistive concrete (water-to-cement ratio of 0,5).