Penetrating sealers, including silanes, siloxanes, and blended products, are a cost-effective means of protecting new and existing surfaces. When applied properly, penetrating sealers can serve as chloride screens and dampproofing to improve durability against destructive corrosion of embedded reinforcement and damage from freezing and thawing. Exposed concrete building exteriors, parking structures, and bridge decks are typical applications. Sealers can be applied at the time of original construction, as part of a maintenance program, or as protection for repairs. Sealers penetrate the concrete surface and react chemically to form a water-repellent layer. Water usually will bead up on a treated surface. However, water trapped in the concrete or which soaks in can escape from the treated surface as vapor. These sealers have little or no effect on the concrete surface coloring. In addition to helping repel water and chlorides, sealers also help minimize penetration of dirt, grease, and oil, making surfaces easier to clean. For successful application and performance of sealers, surfaces should be "clean, sound, and dry." These criteria are subject to interpretation, and their implementation is project-specific. The extent of effort required to achieve these criteria depends on the nature, age, and exposure of the concrete surface.