Weathering concrete is something more than concrete that is structurally durable. It is also concrete that continues to be visually acceptable as it ages. If it undergoes changes in color or surface texture, the change should be both predictable and acceptable. Its appearance should ordinarily continue to be uniform, not blotchy or streaked; or if the appearance becomes non-uniform, it should do so because controlled variations have intentionally been permitted to develop as weathering proceeds in order to highlight specific features. Several properties of concrete are of special significance in their effect on weathering. Permeability affects the amount of penetration of water, dirt, or chemical agents such as acidic rainwater, carbon dioxide, and sulfur dioxide. Absorption should be uniform as well as low. Non-uniform absorption results in variable wetting that is manifested in blotchiness of color and texture; water courses accentuate these differences. Roughness determines how much dirt can be held and also determines how uniform will be the flow of water across the surface. Surface integrity is a surface quality in which the strength, density, porosity, and general freedom from cracking of the surface are equivalent to those of the main body of concrete of good quality. Concrete with good surface integrity will resist dusting, erosion, abrasion, and mechanical damage. Adequate air content will help protect against damage from freezing and thawing or from salt scaling during freezing. It will usually be necessary to choose concrete materials and mix proportions with weathering specifically in mind rather than on the basis of just strength requirements. It may also be necessary to give more than the usual amount of attention to providing adequate cover for reinforcing steel. Yet before the architect proceeds this far in his design he must give careful attention to the effects of his exterior details on how the building will weather. Openings for windows and doors, their shapes, and how they relate to adjacent elements affect the degree of staining from water flow. Surface textures, wall slopes, location of drips and ledges, and joint location must be chosen with more in mind than the visual appearance of the newly erected wall.