Depassivation of reinforcing steel due to accelerated-carbonation in limestone calcined clay cement concretes

The effect of carbonation on corrosion of reinforced steel was investigated in five limestone and calcined clay (LC3) concrete mixtures designed adjusting their cement SO₃ content. Accelerated carbonation tests were carried out according to BS EN 12390-12, while simultaneously the rebar corrosion activity was also monitored using electrochemical tests such as the linear polarization resistance (LPR). This paper reports on the methodology proposed to evaluate the reinforcement's response to corrosion while concrete carbonates. Results showed that LC3 concretes had a significant increase in the carbonation rate and demonstrated a Carbonation Index (C_I) between 0.6–0.9, a parameter defined as the carbonation-depth to concrete-cover ratio, indicating the presence of active corrosion in the rebar, measured in terms of corrosion potential (E_oc) and current density (i_corr). This condition represents an early stage of depassivation because the steel corrosion processes started before the carbonation front reached the rebar (C_I < 1). This finding goes against traditional durability models in which the propagation stage begins when all cover concrete is already carbonated (C_I = 1). Consequently, concrete using very high limestone and calcined clay replacement levels are much more vulnerable to accelerated carbonation.