Corrosion of steel in carbonated concrete: mechanisms, practical experience, and research priorities – a critical review by RILEM TC 281-CCC

Abstract

Carbonation of concrete is generally assumed to lead to reinforcing steel corrosion. This mindset has long dictated the research priorities surrounding the developments towards new, low-emission binders. Here, by reviewing documented practical experience and scientific literature, we show that this widely held view is too simplistic. In fact, there are many cases from engineering practice where carbonation of the cementitious matrix surrounding the steel did not lead to noticeable corrosion or to corrosion-related damage at the level of a structure. The influencing factors that can, however, lead to considerable corrosion damage are identified as the moisture state, the microstructure of the carbonated concrete, various species that may be present – even in minor amounts – in the concrete pore solution, and the cover depth.
 The circumstance that a reduced pH alone is not sufficient to lead to significant steel corrosion in concrete seriously challenges the established approach of assessing the durability performance based on carbonation testing and modeling. At the same time, this circumstance offers great opportunities for reducing the environmental impact of concrete structures with low-emission binders. To realize these opportunities, the focus in research and engineering should shift from studying carbonation to studying corrosion of steel in carbonated concrete.

Durability-based design: the European perspective

“…On the other hand, if the rate of corrosion is high and the consequences of corrosion are great, the probability of depassivation should be low because corrosion onset will rapidly lead to serious damage. The need for a differentiated probability of failure and a more discriminating use of limit states has recently been debated for carbonation-induced corrosion, where the present use of limit state depassivation works against the use of low clinker blends (Angst et al, 2020).…”

“…However, it is generally accepted that cracks facilitate ingress of aggressive substances and thus may cause early corrosion onset, so the impact of cracks on corrosion onset and propagation is dealt with in Sections 4.1.1 and 4.2 below. For discussion of carbonation-induced corrosion onset, the reader is referred to a very recent review paper prepared by a task group of RILEM TC 281 CCC (Angst et al, 2020).…”

Solubility and speciation of iron in cementitious systems

“…Chlorides, if present at sufficiently high quantities at the steelconcrete interface, can lead to the breakdown of the passive film that typically forms on steel surfaces in the alkaline environment of concrete [10][11][12]. Another possible cause for steel corrosion in concrete is carbonation of the cementitious phases and the associated loss in pH buffering capacity of the matrix surrounding the steel [13,14]. Motivated by these causes for corrosion, the modelling of chloride ingress and concrete carbonation in concrete has been a subject of research for a long time as evidenced by a range of ever-evolving (reactive) transport models for quantifying the space-time evolution of species that initiate corrosion of reinforcement [15][16][17][18].…”

Effects of model boundary conditions on simulated drying kinetics and inversely determined liquid water permeability for cement-based materials

“…Liquid water movement may carry aggressive agents, such as chloride ions, into concrete and thus leads to corrosion of steel reinforcing bars (rebars). [2][3][4] Carbon dioxide can diffuse into concrete through the empty pores and cause concrete carbonation if concrete is unsaturated with water, [5] and later causes steel corrosion once water content in concrete reaches a high level. For these reasons, the accurate simulation of moisture transport is crucial for studying the durability of reinforced concrete structures.…”