Deterioration Mechanisms and Durability of Sprayed Concrete for Rock Support in Tunnels

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 of H2S within the rock mass and subsequent re-oxidation to sulfuric acid near tunnel space. This process was likely assisted by sulfate reducing bacteria and Acidithiobacilli, respectively. Thaumasite carbon was derived from calcite in the Alum Shale and atmospheric CO2, whilst the shale organic carbon was inert. A novel composite degradation mechanism was discovered in subsea tunnels, leading to partial to complete breakdown of the cement paste matrix and destructive steel fibre corrosion. Such degradation was evident after < 4 years of exposure, and the rate of disintegration varied from < 0.5-10 mm/year. This was due to: a) acidification caused by redox reactions within layered Mn-Fe biofilms (Leptothrix sp. and Gallionella sp.) and b) infiltration of saline ground waters and abiotic transformation of the cement paste due to decalcification, magnesium attack, TSA and PCD. At advanced stages gypsum deposits, with crystalline carbon derived from biofilm material, had formed on the outer concrete surface. Important consequences for engineering were derived. The concrete standard EN 206-1 has important shortcomings, because it does not involve classification relevant to biodegradation, hydrogeological environment and effects of evaporation. It was established that sprayed concretes subjected to a hydraulic gradient were more degraded than similar concrete under static conditions. Moreover, H and O isotopic evidence showed that evaporation of tunnel water due to tunnel drought can cause a 100 % to 1000 % relative increase in ionic concentrations. Consequences for service life, maintenance/repair and rules and recommendations were discussed. The challenge of future studies into durability of sprayed concrete for rock support is to fully understand the interaction of biotic and abiotic deterioration mechanisms and the complex impact of water and rock mass instability.Materials & EnvironmentCivil Engineering and Geoscience