Carbonation & Corrosion

Carbonation refers to a set of chemical reactions and mineralogical transformations that generally convert calcium hydroxide to calcium carbonate. It is a common and typically benign phenomenon that affects nearly all portland cement-based materials that are exposed to the atmosphere. However, excessive carbonation in reinforced concrete is a durability concern as it can lead to the corrosion of reinforcing steel. Great strides have been made over the years in mitigating damage from corrosion in reinforced concrete, but it remains a major durability issue across the world.

Photograph of phenolphthalein-stained surface of a core from a concrete slab on ground. The area at the top of the core not stained by phenolphthalein is carbonated.

Cross-polarized transmitted light photomicrograph of thin section from carbonated concrete. The bright color of the paste is from the high birefringence of calcium carbonate produced by the carbonation reactions.

Carbonation can be used to date cracks. This photograph is from a core where recent seismic activity was suspected as the cause of a crack. The phenolphthalein staining showed the crack formed at an early age rather than during the earthquake, which was more than 20 years after the construction of the slab.

Reflected light photomicrograph of polished surface showing detail of corrosion product on a steel wire mesh.

EDS elemental map of epoxy-coated bar undergoing corrosion that led to major cracking of a topping slab. The peach colored areas are oxidized iron, the green is the cement paste, the purple are aggregates and the dark blue area is the epoxy coating.

Reflected light photomicrograph of polished surface of core from a parking structure deck slab where an epoxy-coated reinforcing bar (note green epoxy rim on the bar) corroded, leading to cracking and microcracking (red arrows) of the concrete. The green arrow indicates a void filled with corrosion product.

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