A detailed case study of carbonation profiles in prestressed concrete beams from an aggressive marine environment

The measurement of the depth of carbonation in reinforced and prestressed concrete structures is typically determined via freshly cut cores or broken concrete surfaces (often adjacent to superficial evidence of corrosion) and sprayed with a pH indicator phenolphthalein solution. These measurements are representative of only a small percentage of the entire concrete surface and rarely give insight into the overall carbonation profile. This may therefore lead to an over- or underestimation of the depth of carbonation, and presents difficulties in assessing the likely corrosion risk due to carbonation of a reinforced or prestressed concrete structure. The current paper presents unique information on the progression of the overall carbonation front in cross-sections taken from three 45-year-old precast, prestressed concrete T-beams located in an aggressive marine environment. Deeper carbonation was found with increasing height of the beam cross-section, particularly at junctions, and towards the more sheltered areas of the diaphragms. Maximum carbonation depths of 50mm were recorded in some locations, exceeding reinforcement depths and approaching the depth of the prestressing. Carbonation fronts were irregular, but were not always influenced by aggregate. Zones of carbonation were observed around cracks but this was not always the case. The present study highlights that carbonation can be influenced locally by geometry, environment, orientation, and properties of the concrete. Further research is required to assist in the development of improved carbonation field investigations.