Waterproofing and Drainage - building substructures enclose basements, parking garages, or other usable space _ Engineering and Construction.pdf

27.02.2015 Waterproofing and Drainage - building substructures enclose basements, parking garages, or other usable space | Engineering and Construction

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Waterproofing and Drainage - building substructures enclosebasements, parking garages, or other usable space

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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 substructures 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 foundations 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 dif culty of depositing the

crushed stone and back ll soil in neatly separated, alternating layers. The drainage

mat (right) is easier and often more economical to install.

Figure 2.61 For a high degree of security against substructure 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 waterproo 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.



Figure 2.63 Waterproo ng in progress on a concrete foundation. Leftmost: the

bare foundation wall remains exposed. Middle: bentonite waterproo 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 waterproo ng qualities of the panel.

Right: drainage mat has been installed over the waterproo ng. The mats outer

face of 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 propertys 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).



Figure 2.64

Blind-side waterproo 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).

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 at against

the formwork where another wall will join

the one being poured. A fter the concrete

has been poured and cured and the

formwork has been removed from the

rst 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.



Categories: BUILDINGS, Waterproofing and Drainage

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, A labama)

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 walls

exterior, also protecting the reinforcing

from moisture and corrosion. However,

because of bentonites 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.

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