Engineering and Construction

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The main structural design and construction details to prevent moisture migration through basement walls is a drainage system at the base of the wall to prevent the build-up of hydrostatic water pressure and a waterproofing system applied to the exterior wall surface. Such a system for basement walls is shown in Fig. 11.24. A perforated drain is installed at the bottom of the basement wall footing. Open graded gravel, wrapped in a geofabric, is used to convey water down to the perforated drain. The drain outlet could be tied to a storm drain system.

The waterproofing system frequently consists of a high strength membrane, a primer for wall preparation, liquid membrane for difficult to reach areas, and mastic to seal holes in the wall. The primer is used to prepare the concrete wall surface for the initial application of the membrane and to provide long-term adhesion of the membrane. A protection board is commonly placed on top of the waterproofing membrane to protect it from damage during compaction of the backfill soil. Self-adhering waterproofing systems have been developed to make the membrane easier and quicker to install. For example, Fig. 11.25 shows a self-adhering waterproofing system that has been installed to the back side of a basement wall. In Fig. 11.25, the waterproofing system is located on the right side of the photograph and the perforated drain is visible in the center of the photograph.

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Moisture that travels through basement walls can damage wall coverings, such as wood paneling.

Moisture traveling through the basement walls can also cause musty odors or mildew growth in the basement areas. If the wall should freeze, then the expansion of freezing water in any cracks or joints may cause deterioration of the wall.

The moisture that is passing through the basement wall will usually contain dissolved salts. The penetrating water may often contain salts originating from the ground or mineral salts naturally present in the wall materials. As the water evaporates at the interior wall surface, the salts form white crystalline deposits (efflorescence) on the basement walls. Figure 11.23 shows photographs of the build-up of salt deposits on the interior surface of basement walls.

The salt crystals can accumulate in cracks or wall pores, where they can cause erosion, flaking, or ultimate deterioration of the contaminated surface. This is because the process of crystallization often involves swelling and considerable forces are generated. Another problem is if the penetrating water contains dissolved sulfates, because they can accumulate and thus increase their concentration on the exposed wall surface, resulting in chemical deterioration of the concrete (ACI, 1990).

The effects of dampness, freezing, and salt deposition are major contributors to the weathering and deterioration of basement walls. Some of the other common deficiencies that contribute to moisture migration through basement walls are as follows (Diaz et al., 1994; Day, 1994f):

1. The wall is poorly constructed (for example, joints are not constructed to be watertight), or poor-quality concrete that is highly porous or shrinks excessively is used.
2. There is no waterproofing membrane or there is a lack of waterproofing on the basement-wall
exterior.
3. There is improper installation, such as a lack of bond between the membrane and the wall, or deterioration with time of the waterproofing membrane.
4. There is no drain, the drain is clogged, or there is improper installation of the drain behind the basement wall. Clay, rather than granular backfill, is used.
5. There is settlement of the wall, which causes cracking or opening of joints in the basement wall.
6. There is no protection board over the waterproofing membrane. During compaction of the backfill, the waterproofing membrane is damaged.
7. The waterproofing membrane is compromised. This happens, for example, when a hole is drilled through the basement wall.
8. There is poor surface drainage, or downspouts empty adjacent the basement wall.

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Introduction
Water can penetrate basement walls due to hydrostatic pressure, capillary action, and water vapor. If a groundwater table exists behind a basement wall, then the wall will be subjected to hydrostatic pressure, which can force large quantities of water through wall cracks or joints. Figures 11.21 and 11.22 show two photographs of a basement wall subjected to hydrostatic water pressure that forced large quantities of water through the wall. A subdrain is usually placed behind the basement wall to prevent the buildup of hydrostatic pressure. For the drains to be effective, the backfill material should consist of granular (i.e., permeable) soil.

Another way for moisture to penetrate basement walls is by capillary action in the soil or the wall itself. By capillary action, water can travel from a lower to higher elevation in the soil or wall.

Capillary rise in walls is related to the porosity of the wall and the fine cracks in both the masonry and, especially, the mortar. To prevent moisture migration through basement walls, an internal or surface waterproofing agent is used. Chemicals can be added to cement mixes to act as internal water-proofs. More common are the exterior applied waterproof membranes. Oliver (1988) lists and describes various types of surface-applied waterproof membranes.

A third way that moisture can penetrate through basement walls is by water vapor. Similar to concrete floor slabs, water vapor can penetrate a basement wall whenever there is a difference in vapor pressure between the two areas.