Chloride Penetration and Reinforcement Corrosion in Fly Ash Concrete Exposed to a Marine Environment

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Title: Chloride Penetration and Reinforcement Corrosion in Fly Ash Concrete Exposed to a Marine Environment

Author(s): M. D. A. Thomas and J. D. Matthews

Publication: Special Publication

Volume: 163

Issue:

Appears on pages(s): 317-338

Keywords: chlorides; concretes; corrosion; diffusion; fly ash; reinforcing steels; underwater structures; Materials Research

Date: 8/1/1996

Abstract:
Steel reinforced concrete specimens, with nominal strength grades of 25, 35, and 45 MPa and different fly ash levels (0 to 50 percent), were exposed to various curing treatments during the first 28 days prior to exposure in the tidal zone of a seawater exposure site. Chloride concentration profiles and reinforcing bar weight losses were measured for specimens after one, two, and four years exposure. Chloride profiles were also measured for specimens after 28 days immersion in seawater under laboratory conditions. Fly ash concretes showed substantially increased resistance to the penetration of chlorides compared with control specimens. The improved resistance of fly ash concrete to the penetration of chlorides resulted in reduced corrosion of steel bars embedded in the concrete. Threshold chloride levels for corrosion were estimated from relationships between steel weight loss and chloride content at the location of the steel. The threshold concentration decreased with increasing fly ash content. Chloride concentration profiles after 28 days of immersion in seawater showed that considerable chloride penetration occurred during this period due to sorption (capillary suction) of the seawater into the unsaturated specimens. This results in a significant error in diffusion coefficients calculated from the concentration profile using the standard solution to Fick's second law. The error may be substantial for fly ash concretes in which the penetration of chlorides due to sorption in the first few days far outweighs subsequent diffusion during continued seawater exposure. Alternative methods of solution that take account of initial sorption effects are presented.