Deterioration of service reservoirs due to soft water attack

Service reservoirs in the UK designed in accordance with EN 206 have shown deterioration due to soft water attack, which is a form of mild acid attack, within one year of commissioning. The attack is represented by aggressive water transforming the surface into a soft paste fully depleted of calcium compounds. Overall, there are three distinct zones of deterioration which are characterised by full calcium depletion (Zone 1), carbonation (Zone 2) and portlandite depletion (Zone 3). The observation of this type of deterioration had initiated the requirement to investigate the type of attack and identify durable and practical concrete mixes to reduce the rate of deterioration as far as reasonably practical. The investigation comprised the production of concrete specimens based on the recommendations of EN 206 for XA3 exposure conditions and more stringent designs based on the limiting values for DC-4 in accordance with BS 8500 and BRE Special Digest 1. The specimens of 12 different mixes were investigated by immersing them in eight service reservoirs with different aggressiveness of hardness, alkalinity and Langelier Saturation Index. The paper discusses the findings of the field and laboratory investigation after one and seven years of immersion. It provides an indication of the most resistant mix designs, effects of the use of fly ash and ground granulated blastfurnace slag, and reliable water parameters that can be utilised to assess the risk and rate of attack.