Water in the form of vapor, liquid, and solids presents below-grade construction with many unique problems. Water causes damage by vapor transmission through porous surfaces, by direct leakage in a liquid state, and by spalling of concrete floors in a frozen or solid form.
Water conditions below-grade make interior spaces uninhabitable not only by leakage but also by damage to structural components as exhibited by reinforcing steel corrosion, concrete spalling, settlement cracks, and structural cracking.
Below-grade waterproofing materials are subject to water conditions that are typically more severe than above-grade envelope areas. Structure elements below-grade are often exposed to hydrostatic pressure from ground water tables that can rise significantly during periods of heavy rainfall. At the same time, below-grade materials are not subject to the harsh environmental conditions of exposed envelope components, including wind-driven rain, ultraviolet weathering, and acid rain.
Manufacturers of below-grade waterproofing systems can then concentrate on the properties to ensure effective barriers to water penetration without having to contend with the elements encountered above-grade. For example, membranes used below-grade can have substantial elongation properties since the manufacturer does not have to supplement the product with ultraviolet resistant properties that tend to limit elongation capabilities.
Below-grade systems are all barrier systems; there are no appropriate new construction drainage systems designed for adequate protection under hydrostatic pressure. Diversion systems are frequently included in the design of below-grade waterproofing, and in fact are highly recommended for use in conjunction with any below-grade system, with the possible exception of hydrous clay materials that require the presence of adequate water supply to maintain their hydration and waterproofing properties.
Proper below-grade design begins with adequate control of water conditions. There is no reason to subject any below-grade envelope components to unnecessary amounts of water that could otherwise be diverted away form the structure for supplementary protection. Both surface and groundwater should be diverted immediately away from the structure at all times.