Weathering of concrete
Weathering is the effect of pollution and natural forces, such as frost, rain and sunlight, on a structure. With concrete, the main weathering problem is unexpected variations in the visual appearance of the structure. Dust in the atmosphere will be deposited on the facade. The flow of rainwater will tend to wash some areas preferentially, resulting in significant differences in colour between clean and dirty areas. This phenomenon is controlled by many factors, including the surface texture of the concrete, the architectural detailing and the orientation of the structure. In the short term the visual appearance of the structure can be improved by washing but the process will have to be repeated regularly. Painting or coating the surface may help to increase the time before the surface needs to be re-washed. In some cases, it may be possible to find a long-term solution to the problem, such as the provision of alternative drainage paths.
Rainfall is usually slightly acidic due to the dissolution of carbon dioxide in the water in the atmosphere. Air pollution in the form of sulphur dioxide increases the acidity of the rain due to the formation of sulphuric acid. This will tend to etch the surface of concrete or mortar, but it is largely a cosmetic problem, unless it is allowed to concentrate.