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Concrete softening (Carbonate imbalance)

On long term immersion in water (such as in storage reservoirs), cement has a tendency to soften and dissolve, the extent of which is dependent on both the pH and the hardness of the water: the softer the water, the faster the rate of dissolution.

 

Similarly, limestone’s can also suffer dissolution as they comprise primarily of calcium carbonate. Concretes made with limestone aggregates can therefore be particularly susceptible to this problem, as both aggregates and cement paste can dissolve.

 

Excessive cleaning using high pressure water jetting will remove the softened cement, opening the matrix and this should be avoided.

 

The susceptibility of stored water to dissolve calcium from concrete or limestone can be determined from the Langelier Index.  This is an approximate indicator of the degree of saturation of calcium carbonate in water.

 

If the Langelier Index is negative, then the water is under saturated with calcium carbonate and will tend to be corrosive in the distribution system.  If the Langelier Index is positive, then the water is over saturated with calcium carbonate and will tend to deposit calcium carbonate forming scale in the distribution system.  If the Langelier Index is close to zero, then the water is just saturated with calcium carbonate and will neither be strongly corrosive or scale forming.

Concrete softening in a storage reservoir
 
 
Other references:CIRIA Report 138 Underground service reservoirs – waterproofing and repair manual
CIRIA Report 69 Review of concrete behaviour in acidic soils and ground waters
 

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Loss of surface with water