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Steel fibre reinforced sprayed concretes used for rock support in tunnels are subjected to variable and complex exposure conditions. Structurally weakened concretes (5 to 35 years old) were investigated with respect to deterioration mechanisms, sources of aggressive agents and related engineering aspects. The evidence was based on field characterisation/sampling, concrete petrography, chemical microanalysis, X-ray diffraction, water chemistry and stable isotope systematics. The Alum shale and subsea tunnel environments represent the most severe ground water conditions in Norway, whilst ion poor waters had much lesser effects on concrete durability. Alkali-Aggregate Reaction was unimportant. Historically, Alum Shale has caused severe and rapid cement paste degradation, previously interpreted to be due to ettringite and sulfuric acid attack. However, examination of historical test samples within this study proved this was due to Thaumasite Sulfate Attack (TSA). TSA was closely associated with decalcification of the cement paste matrix and internal detrimental carbonation, such as Popcorn Calcite Deposition (PCD). Two novel reaction mechanisms were found: a) development of high crystallisation pressures during thaumasite deposition and b) TSA was most efficient after partial or complete decalcification of the cement paste matrix: Thaumasite had formed at the expense of remaining amorphous silica and secondary calcite (PCD). Sulfuric acid caused outer leaching, making way for deep effects of aggressive ions. Modern sprayed concrete made with Sulfate Resisting Portland Cement and silica fume had suffered similar but less rapid degradation, leading to local spalling with destructive fibre corrosion after < 13 years. Anhydrite in Alum Shale was the main source of sulfate, whilst pyrite and pyrrhotite oxidation was less important than previously claimed. A multiproxy study suggested that thaumasite sulfate originated from partial reduction of anhydrite sulfate and formation, Materials & Environment, Civil Engineering and Geosciences

Steel fibre reinforced sprayed concretes used for rock support in tunnels are subjected to variable and complex exposure conditions. Structurally weakened concretes (5 to 35 years old) were investigated with respect to deterioration mechanisms, sources of aggressive agents and related engineering aspects. The evidence was based on field characterisation/sampling, concrete petrography, chemical microanalysis, X-ray diffraction, water chemistry and stable isotope systematics. The Alum shale and subsea tunnel environments represent the most severe ground water conditions in Norway, whilst ion poor waters had much lesser effects on concrete durability. Alkali-Aggregate Reaction was unimportant. Historically, Alum Shale has caused severe and rapid cement paste degradation, previously interpreted to be due to ettringite and sulfuric acid attack. However, examination of historical test samples within this study proved this was due to Thaumasite Sulfate Attack (TSA). TSA was closely associated with decalcification of the cement paste matrix and internal detrimental carbonation, such as Popcorn Calcite Deposition (PCD). Two novel reaction mechanisms were found: a) development of high crystallisation pressures during thaumasite deposition and b) TSA was most efficient after partial or complete decalcification of the cement paste matrix: Thaumasite had formed at the expense of remaining amorphous silica and secondary calcite (PCD). Sulfuric acid caused outer leaching, making way for deep effects of aggressive ions. Modern sprayed concrete made with Sulfate Resisting Portland Cement and silica fume had suffered similar but less rapid degradation, leading to local spalling with destructive fibre corrosion after < 13 years. Anhydrite in Alum Shale was the main source of sulfate, whilst pyrite and pyrrhotite oxidation was less important than previously claimed. A multiproxy study suggested that thaumasite sulfate originated from partial reduction of anhydrite sulfate and formation, Materials & Environment, Civil Engineering and Geosciences