Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?

Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?
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Answer :

In some cases, it may be due to the accumulation of dust and dirt. However, for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges. After rainfall, the bridge surface absorbs water and retains it for a certain period of time. Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric pollution   and   proximity   of   plants   provide   nutrients   for   their   growth.   Improvement   in drainage   details   and application   of painting  and  coating   to bridges  help  to  solve  this problem. Reference is made to Sandberg Consulting Engineers Report 18380/X/01.
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