Carbonation of concrete bridge structures in three South African localities

Rates of carbonation for reinforced concrete bridges were investigated for three localities in South Africa: Cape Peninsula, Durban and Johannesburg. Carbonation data from approximately 90 in-service bridges aged between 11 and 76 years were interpreted in terms of influence of materials and environment on carbonation rates. The data were grouped with respect to concrete strength grade and exposure condition prior to statistical analysis. Bridges in Johannesburg had the highest rates of carbonation owing to the relatively dry environment. Durban bridges had lower carbonation rates than Johannesburg bridges, but higher than bridges in the Cape Peninsula, ascribed to differences in ambient temperature and the nature of precipitation. Overall, average carbonation rates for Grade 30 concretes over a 30-year period varied from approximately 0.3 mm/annum in Cape Peninsula structures to approximately 0.7 mm/annum for Johannesburg structures. Exposure condition, characterised by degree of shelter, had little influence on carbonation rate in Durban and Johannesburg bridges, ascribed to the average relative humidity and duration of precipitation at these localities. Carbonation rates for older bridges were lower than for newer structures, attributed possibly to changes in cement characteristics with time related to the need for fast track construction in modern structures.