Resistance of concrete against combined attack of chlorides and sulphates

A lot of damage is reported for constructions in marine environments, due to aggressiveness of sea water containing chlorides and sulphates. Chlorides affect durability by initiating corrosion of the reinforcement steel, and sulphates by deteriorating the concrete through formation of expansive reaction products. This research compares single-ion attack and combined attack. Combined attack simulates concrete deterioration environments more realistically. However, several factors influence cocnrete resistance against chlorides and sulphates : cement type and C3A-content, additions, age, temperature, etc. To examine these influences concrete mixtures containing ordinary Portland cement, high sulphate resistant cement, or blast furnace slag as 50 % and 70 % cement replacement were compared. Resistance against chlorides and chlorides combined with sulphates was tested by natural diffusion tests based on NT Build 443. Specimens were tested at varying ages since chloride penetration decreases with concrete age. The first test series was performed at three different temperatures namely 5°C, 12.5°C and 20°C to quantify to which extent lower exposure temperatures slow down diffusion rates. It is clear that diffusion coefficients and colour change boundaries decrease when temperatures lower. Next, the influence of sulphates on the chloride penetration was examined. The results show that, when 55 g/l NA2SO4 was added to a 165 g/l NaCl solution, the colour change boundaries and diffision coefficients for ordinary Portland cement concrete and 50 % slag concrete, but not for HSR concrete. However, for all concretes chloride binding decreases when sulphate content increases. Generally the C3A-content of the cement plays a major role in the attack mechanism and in the binding of chlorides and sulphates. So, when both ions penetrate together, C3A influences this multi-ion transport as well as the concrete durability