Why is my concrete sleeper discolouring??

Have you ever built or seen a concrete sleeper retaining wall using unsealed charcoal sleepers, then had a look at the wall 6 -12 months later? I’m guessing you noticed the wall started to get uneven white blotches all over it. Perhaps even half of the wall had defined white blotches and the other had noticeably less defined white blotches. Well the reason for this is a little scientific and universally applicable to any and all coloured concrete sleepers.

The process of concrete sleepers turning white is known as efflorescence. This is essential when water within the concrete begins to rise to the surface of the sleeper than evaporates, leaving behind dissolved salts that appear as white blotches. 

Additionally, concrete undergoes two processes when drying; endothermic and exothermic reactions. Exothermic is the reaction of the cement, water, sand & gravel mixing together, this actually produces heat and cures the concrete. The endothermic reaction is the effect the outside elements may have in increasing or decreasing this reaction. Regarding our unsealed charcoal concrete sleepers, this may affect them in two major ways. 

Firstly, if there is lots of rain, dew, fog or high humidity, the process of efflorescence increases due to increased amounts of water available. 

Secondly, if the concrete sleepers are in the sun or high heat areas, it will also increase the process of efflorescence due to increased heat in the sleeper. 

So how does this affect a concrete sleeper retaining wall?

Well, let’s say that half of your sleeper wall is covered by trees, and the other half is fully exposed to the sun. with unsealed charcoal sleepers, you are going to see the two halves of the walls discolour at different rates. This will become more and more noticeable over time. However, It is important to note that efflorescence will not affect the structural integrity of your concrete sleeper or wall. 

So in conclusion, efflorescence happens to all concrete. It is inevitable. No matter who you buy your sleepers through (hopefully me), your unsealed concrete sleepers will always discolour. For some this doesn’t cause any alarm as they plan for the wall to discolour, just like a good pair of boots. But if you’re looking for a wall that looks the same 10 years later as the day it was built, it might be worth looking at a sealed concrete sleeper. Check out our Sealed vs Unsealed textured concrete sleepers post for more information.