Where building substructures enclose basements, parking garages, or other usable space, groundwater must be kept out. Concrete alone is rarely adequate for this purpose. Moisture can migrate through its microscopic pores, or through other pathways created by shrinkage cracks, form tie holes, utility penetrations, and the joints between concrete pours. To ensure a substructure’s resistance to water entry,
two approaches are used: drainage and waterproofing. Drainage draws groundwater away from a foundation, reducing the volume and pressure of water acting on the foundation’s walls and slabs.
Waterproofing acts as a barrier, stopping water that reaches the foundation from passing though to the interior.
Drainage, consisting of some combination of drainage backfill (well-sorted crushed stone or gravel), drainage mat, and perforated drain piping, is used with almost every building substructure (
Figure 2.60). Drainage mat is a manufactured component that may be made of a loose mat of stiff, inert fibers, a plastic egg-crate structure, or some other very open, porous material. It is faced on the outside with a filter fabric that prevents fine soil particles from entering and clogging the drainage passages in the mat. Any subterranean water that approaches the wall descends through the porous material of the mat to the drain pipe at the footing. Perforated drain piping is frequently laid around the outside perimeter of a building foundation. The pipes are 4 or 6 inches (100 or 150 mm) in diameter and provide an open channel in the crushed stone bed through which water can flow by gravity either “to day-light” at a lower elevation on a sloping site, to a municipal storm sewer system, or to a sump pit that can be automatically pumped dry whenever it fills. The pipes are laid at least 6 inches (150 mm) below the top of the basement floor slab tomaintain the groundwater level safely below that of the slab. Perforations in the pipes face downward so that water is drained from the lowest possible level. Where groundwater conditions are severe, rows of perforated pipe may be installed under the basement slab as well (
Figure 2.61).
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Figure 2.60 Two methods of relieving water pressure around a building substructure by drainage.
The gravel drain (left) is hard to do well because of the diffi culty of depositing the
crushed stone and backfi ll soil in neatly separated, alternating layers. The drainage
mat (right) is easier and often more economical to install. |
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Figure 2.61 For a high degree of security against substructure fl ooding, drainage both around and
under the basement is required, as seen here in a section view. Above-slab drainage is used
in buildings with mat foundations. |
On most foundations, some form of water-repelling barrier is also used to protect against the pas-
sage of groundwater. Dampproofing is a moisture-resistant cement plaster or asphalt compound commonly applied to residential basement walls and to other substructures where groundwater conditions are mild or waterproofing requirements are not critical. Cement plaster dampproofing, or parge coating, is light gray in color and troweled on. Asphalt or bituminous dampproofing is dark in color and is applied in liquid form by spray, roller, or trowel. Dampproofing is less expensive and less resistant to water passage than true waterproofing.
Waterproofing, unlike dampproofing, can prevent the passage of water even under conditions of hydrostatic pressure. It is used where groundwater conditions are severe or the need to protect subgrade space from moisture is critical. Waterproof mem-branes are most commonly formulated from plastics, asphalt compounds, or synthetic rubbers and come in a great variety of forms.
Liquid waterproofing is applied by spray gun, roller, or squeegee and then allowed to cure in place. It is easy to install and easy to form around complex shapes. When fully cured, the finished membrane is seamless and fully bonded to the underlying substrate. However, because liquid membranes are formed in the field, they are subject to uneven application, and the surfaces to which they are applied must be clean, smooth, and dry to ensure reliable adhesion of the membrane.
Preformed sheet membrane waterproofing may be adhered or mechanically fastened to substructure walls
or laid loosely over horizontal surfaces (
Figure 2.62). Fabricated under controlled factory conditions, sheet membranes are reliably uniform in material quality and thickness.
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Figure 2.62 A diagrammatic representation of the placement of sheet membrane waterproofi ng
around a basement. A mud slab of low-strength concrete was poured to serve as
a base for placement of the horizontal membrane. Notice that the vertical and
horizontal membranes join to wrap the basement completely in a waterproof enclosure. |
However, they can be more difficult to form around complex shapes, and the seams between sheets, which are sealed in the field, may be subject to lapses in quality. Sheet membranes that are loosely laid or mechanically fastened can be used over substrates that will not bond with liquid-applied or adhered sheet membranes. They are also a good choice where substrate cracking or movement may be expected, because such movement is less likely to stress or damage the membrane. An advantage of adhered membranes (both sheet and liquid) is that in the case of a defect, water cannot travel far under the membrane, limiting the extent of water damage that may occur and simplifying the tracing of leaks.
Bentonite waterproofing is made from sodium bentonite, a naturally occurring, highly expansive clay. It is most often applied as preformed sheets consisting of dry clay sand-wiched within corrugated card-board, geotextile fabric, or plastic sheets (
Figure 2.63). When bentonite comes in contact with moisture, it swells to several times its dry volume and forms an impervious barrier to the further passage of water. Bentonite sheets can be placed directly on the soil under a concrete slab on grade or mechanically attached to uncured, damp concrete walls. In slurry form, bentonite can be sprayed even onto highly irregular, rough stone walls. The swelling behavior of bentonite clay also allows it to adjust to cracking and movement in the substrate.
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Figure 2.63 Waterproofi ng in progress on a concrete foundation. Leftmost: the
bare foundation wall remains exposed. Middle: bentonite waterproofi ng panels
are fastened in place. These panels are lined on the outer face with a black-
colored, high-density plastic that adds to the waterproofi ng qualities of the panel.
Right: drainage mat has been installed over the waterproofi ng. The mat’s outer
face of fi lter fabric is lightly dimpled, telegraphing the egg crate structure of
the underlying molded plastic panel. The top edge of the mat is secured in
place with an aluminum termination bar that holds the panel in place and keeps
dirt and debris from falling behind the panel. Lower right: white perforated
drain piping can be seen, temporarily supported on wood blocking and
running alongside the footing. (Photo by Joseph Iano) |
Integral waterproofing includes cementitious plaster or crystalline admixtures for concrete or mortar
that react chemically to stop up the pores of these materials and render them watertight. It may be applied
to the surface of existing concrete or masonry or used as an admixture in new concrete. Unlike most other waterproofing materials, many integral waterproofing materials can be applied as negative side waterproof-
ing, that is, applied to the inner side of a concrete wall acting to resist water passage from the opposite side.
Blind-side waterproofing is installed prior to the pouring of concrete walls. This occurs most commonly when a substructure wall is built close to a property’s edge, and excavation cannot be enlarged beyond the property line to permit workers access to the outer face of the wall after its construction.
Drainage matting is first applied directly to the excavation sheeting, and then any of a number of possible waterproofing membranes are applied over the drainage mat. Later, the concrete wall is poured against the membrane. The sheeting remains permanently in place (
Figure 2.64).
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Figure 2.64
Blind-side waterproofi ng is used where there is no working space between a sheeted excavation and the outside of the foundation
wall. The drainage mat and waterproof membrane are applied to the sheeting; then the basement wall is poured against them. |
Joints in construction require special attention to ensure water-tightness. Preformed waterstops made of plastic, synthetic rubber, or metal can be cast into the mating concrete edges of both moving and nonmoving joints to block the passage of water (
Figures 2.65 and 2.66). Waterstops for nonmoving joints such as between concrete pours of a wall or slab can also be made of strips of bentonite or mastic that are temporarily adhered to the edge of one pour. After the adjacent pour is complete, these stops remain embedded in the joint, where they form a watertight barrier (
Figure 2.67).
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Figure 2.65 A synthetic rubber waterstop is used
to seal against water penetration at
movement joints and at joints between
pours of concrete in a foundation. The
type shown here is split on one side so
that its halves can be placed fl at against
the formwork where another wall will join
the one being poured. After the concrete
has been poured and cured and the
formwork has been removed from the
first wall, the split halves are folded back
together before the next wall is poured. |
Most waterproofing systems are inaccessible once building construction is complete; they are ex-
pected to perform for the life of the building, and even small defects in installation can allow the passage of large volumes of water.
For these reasons, waterproofing membranes are inspected carefully during construction and horizontal membranes are often flood tested (submerged for an extended period time while leak-checking is performed) to detect the presence of defects while repairs can still be easily made. Once inspection and testing are complete, membranes are covered with a protection board, insulation board, or drainage matting to shield the membrane from prolonged exposure to sunlight and to prevent physical damage during soil backfilling or subsequent construction operations.
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Figure 2.66
A rubber waterstop ready for the next pour of a concrete wall, as diagrammed in
Figure 2.65. (Courtesy of Vulcan Metal Products, Inc., Birmingham, Alabama) |
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Figure 2.67
A bentonite waterstop is adhered to
a concrete footing prior to casting
of the concrete wall above. Later,
if groundwater seepage occurs, the
bentonite will swell to fully seal the joint
between the two pours. The waterstop
is positioned to the side of the steel
reinforcing bars closer to the wall’s
exterior, also protecting the reinforcing
from moisture and corrosion. However,
because of bentonite’s expansive force,
the waterstop must not be positioned
too close to the surface of the wall, or
when it swells, it could cause portions of
concrete to split away or spall. |
