Concrete surfaces can become discoloured for a variety of reasons. As the concrete ages, the surface alkalinity is reduced by carbonation and the action of rainfall, thus providing a more suitable environment for biological growth.
Organic surface growth starts with primary colonisation by micro-organisms, algae, fungi and various types of bacteria. This is followed by visible growths of algae and lichens. Dirt then collects on the surface and, together with the decaying remains of dead organisms, provides an environment suitable for more advanced biological activity.
In advanced cases, dead lichen can provide a footing for mosses and larger plants, which can affect drainage and lead to more serious damage. The roots of plants can grow into cracks and weak spots in the concrete, resulting in bursting stresses that can increase the size of cracks and may lead to spalling.