Effects of exposure sequence and GGBS cement replacement on performance of concrete subjected to carbonation and chloride ingress

In a variety of applications, such as in tidal zones, abutments of bridges and concrete tunnel linings, reinforced concrete is exposed to both carbonation and chloride ingress. The exposure can be either simultaneous or sequential. However, durability design rarely considers synergistic effects due to carbonation and chloride ingress, even though this may have detrimental consequences for performance. Comparative implications of exposure sequence across different concrete compositions are also unknown. In this study, an experimental investigation on the effects of the sequence of carbonation and chloride ingress was conducted, using two concretes which differ by 50% cement replacement with ground granulated blast furnace slag (GGBS). Specimens were exposed to a combination of 10% CO₂ accelerated carbonation and immersion in 3% sodium chloride solution, in either sequence, and compared with companion samples subjected to only one of these aggressive environments. The extent of carbonation was measured using phenolphthalein indicator solution, while silver nitrate and Rapid Chloride Testing provided indicators of the chloride ingress. For both concrete mixes, specimens with prior chloride ingress exhibited a decreased rate of carbonation when compared to specimens with no prior exposure. Conversely, specimens with prior carbonation displayed an increased rate of chloride ingress compared to non-carbonated counterparts and a step in the acid soluble chloride content in the region of the carbonation front. The concrete composition appeared to play a role since a greater increase in chloride diffusion coefficient due to prior carbonation was observed in the mix with 50% GGBS replacement than the mix without. These findings suggest that in concrete structures exposed to air and saline environments, the effects of sequential exposure should be characterised.