Estimating Critical Corrosion for Initiation of Longitudinal Cracks in RC Structures Considering Phases and Composition of Corrosion Products

Abstract

Premature deterioration and reduction in the life of reinforced concrete (RC) structures due to corrosion of the embedded reinforcing steel bars is a major concern across the world, and the time for formation of longitudinal cracks has been largely accepted as the “service life” of the structure. It is also widely known that the formation and deposition of expansive corrosion products (of iron) on the bars exert radial pressure on the cover concrete, which finally cracks. In this case, it is reasonable to assume that the composition of the corrosion products, in terms of the different phases of the oxides and hydroxides, and their relative percentages, play a vital role in determining the loss of weight (of iron) that causes sufficient pressure for the onset of such longitudinal cracks. It is interesting that virtually no quantitative information is available in the literature on this subject. The main thrust of the present work is to understand the effect of composition and relative amount of different rust phases with longitudinal crack initiation in RC structures. A laboratory study was carried out where corrosion in the reinforcement embedded in concrete specimens was accelerated by impressing a direct current. The amount and composition of the corrosion products were determined using sophisticated spectroscopic and microscopic methods. This composition was compared with that of the corrosion products taken from the embedded reinforcement in a 50-year-old concrete building; it was found to be very similar. Carrying the work further, this composition was used to estimate the critical amount of corrosion (wcr) required for the onset of longitudinal cracks using an axisymmetric plane strain formulation of corrosion pressure in RC structures. An effort was made to study the effect of concrete compressive strength and the cover-to-diameter ratio on wcr. The typical values found were in the range of 38.5 to 53 mg/cm2.