Is there any treatment that is effective for a long period in preventing the growth of algae?
A copper wire or strip can be incorporated in the detailing above the area to be protected. Particular care must be taken to prevent contact with any other metal such as reinforcing steel which would cause galvanic corrosion. The action of the copper is to produce corrosion products in quantity large enough to prevent growth of algae but not large enough to stain the concrete.