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

Black Carbon from smoke coming out of vehicles, tyres and bitumen top get washed away by rains and stain the sides of the concrete bridge.

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.