Quantitative Descriptions of Steel Corrosion in Concrete Using Resistivity and Anodic Polarism Data

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Title: Quantitative Descriptions of Steel Corrosion in Concrete Using Resistivity and Anodic Polarism Data

Author(s): Daksh Baweja, Harold Roper and Vute Sirivivatnanon

Publication: Special Publication

Volume: 170

Issue:

Appears on pages(s): 41-64

Keywords: Chlorides; corrosion; data acquisition; steels.

Date: 7/1/1997

Abstract:
This paper presents results from a major long-term study on chloride induced steel corrosion in concrete. Specifically, data on concrete resistivity and estimated corrosion rates of steel in concrete have been measured and compared. The performance of 50 reinforced concrete slabs made with a range of portland and blended cement binders was evaluated. A high C3A cement, a low C3A cement, a blended fly ash cement and a blended blast furnace slag cement were used. All reinforced concrete slabs were exposed to high chloride conditions by partial immersion in simulated sea water conditions. Reinforcement was cleaned and weighed prior to embedment into the concrete slabs. Periodic non-destructive measurements of concrete performance included half cell potential monitoring, concrete resistivity and electrochemical measurements of rates of corrosion of steel in concrete using potentiodynamic anodic procedures. In addition, individual slabs were broken for reinforcement recovery at predetermined times during the study and measurements made of the area of corrosion and the weight loss of steel through corrosion. As opposed to the initiation-propagation model frequently cited in the literature, three distinct segments were apparent when the estimated corrosion current data were plotted against the concrete resistivity over a period of five years for reinforced concrete slabs considered in this study. The first stage was described as the Quiescent Stage, during which it was found that resistivity increased with time and estimated corrosion current values were low. Upon reaching a maximum resistivity value, a second stage of corrosion took place. This stage was described as the Active Stage, during which resistivity values decreased and estimated corrosion current values increased. After this, a third or Breakaway Stage of corrosion was reached, during which resistivity values decreased at a lower rate while estimated corrosion currents increased significantly. The findings for each of the above stages were consistent with the exposure conditions applied to the slab, in that, after chlorides had reached the steel and caused depassivation, the concrete resistivity decreased while the steel corrosion rate increased. Using the above information, it was found that the time taken to reach the maximum resistivity value could be used as a criterion for corrosion onset as, after this time, steel corrosion rates in concrete could be expected to increase.