Page 37 - Construction Waterproofing Handbook

CHAPTER 2

BELOW-GRADE
WATERPROOFING

INTRODUCTION

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, con-
crete 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 prop-
erties 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 possi-
ble 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.

SURFACE WATER CONTROL

Water present at below-grade surfaces is available from two sources—surface water and
groundwater. Beyond selection and installation of proper waterproofing materials, all
waterproof installations must include methods for control and drainage of both surface and
groundwater.

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