Swiss tunnel structures: concrete damage by formation of thaumasite

Abstract The severity of concrete deterioration is related to the risk of failure of the construction and to the costs of repair. Severe conditions may also be related to interactions with water regarded as weakly aggressive, although these processes are slow and long periods of time are needed for heavy deterioration. We often encounter thaumasite in concrete exposed to ground water with high as well as with low sulfate contents. Thaumasite may form relatively thin layers near the concrete surface in contact with the water as well as larger continuous volumes of completely altered concrete. Based on the description of macro- and microscopic textures and the chemical alterations, a succession of corrosive reactions is postulated. It seems that natural ground water conditions in the Swiss Alps preferentially lead to a sulfate attack with the formation of thaumasite rather than the formation of ettringite. The consequences of these findings are discussed for underground constructions, properties of concrete used as well as for performance testing regarding sulfate attack.