Evaporative Concentration

This is a term used to describe any situation where water containing salts evaporates from the surface leaving the salts behind so that over time the salts build to a higher level than that in the concrete. Here we will talk mainly about the effects of chlorides but concentration of all salts can occur. Typical examples include:

  • Splash zones where the concrete is splashed and the absorbed water then evaporates off leaving the salts behind. Over time a high surface chloride level builds up. Seawater splash zones are a common situation and there is data on what the surface chloride may build to and this is used in design life modelling.
  • Water penetrating through a concrete element under a pressure head. Water penetration models  (permeability, sorptivity and diffusion) have been developed that have been used to predict the build up of water on the air face of a 'Hollow Leg' situation but these are not common. Simplified modelling can be used to show that in most situations with reasonable quality concrete this mechanism will not be an issue. It has also been shown in practice that for High Performance  Concrete the water saturation front is likely to penetrating after a relatively short time even under high pressures.  However there are also reports where high pressures of saline water acting on thin relatively porous concrete have led to the build up of salts on the air face of concrete.
  • Water from the ground, or water ponded on a concrete slab, rising up a vertical concrete face in contact with the ground or concrete slab (e.g. column, plinth, wall or tunnel lining) due to capillary action and then evaporating off. This mechanism is frequently a problem as the action is underestimated. The water in contact with the concrete is often considered as benign but the concentration effect leads to a high surface concentration. Salts may either crystallise causing salt etching, cause sulphate attack of the concrete or diffuse back to the reinforcement and cause corrosion. In small elements a water repellent admixture may be a cost effective
  • Water running over the concrete surface around leaking cracks or joints evaporating off around the crack or joint. Generally the leaks should be sealed before contaminants have time to build up to be an issue.
  • Water running across a slab and down the vertical face and sometimes across the soffit. The simplest approach is to stop the water source. Where not practical it may be possible to reduce the risk by making sure the water runs off rather than evaporates off. For example use drip groves to stop water running around to the soffit where it hangs onto the surface and evaporates.

In the last three situations there is insufficient data available on the level of surface chloride that may develop over time to undertake reliable modelling and the best design approach is to avoidance.

Chlorides may also increase at the concrete surface due to 'Reaction Concentration'. This is where chlorides build up at the surface as they react with the chloroaluminates to form Friedal salts and can not then move inwards. This process is observed in fully immersed situations where there is no evaporation and the surface chloride levels build to be higher than the chloride levels that would exist due to the concrete pores being full of the penetrating water. This process of Reaction Concentration may also play a part in the build up of surface chlorides fuelled by Evaporative Concentration.

Chloride profile with depth of samples taken in encrusted area around a leaking crack in a tunnel and in an uncontaminated area.

Crack in a wall below water level.

Damp area above scupper drains in a tunnel showing the surface area where water is evaporating.

Outside wall of mining plant where wash down water has been frequently allowed to run off over the wall.

Chloride profile with depth of samples taken in capillary rise area above scuppers in a tunnel and in an uncontaminated area.