Engineering and Construction

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As previously mentioned, the geotechnical engineer often provides the design and construction specifications that are used to prevent both water vapor and capillary rise through floor slabs. An example of moisture barrier recommendations below the slab-on-grade foundation is as follows (WFCA, 1984):

Over the subgrade, place 4  to 8 in. (10  to 20 cm) of washed and graded gravel. Place a leveling bed of 1 to 2 in. (2.5  to 5 cm) of sand over the gravel to prevent moisture barrier puncture. Place a moisture barrier over the sand leveling bed and seal the joints to prevent moisture penetration. Place a 2 in. (5 cm) sand layer over the moisture barrier.

The gravel layer should consist of  an open graded gravel. This means that the gravel should not contain any fines and that all of the soil particles are retained on the No. 4 (4.75-mm) sieve.

This will provide for large void spaces between the gravel particles that will help prevent capillary rise of water through the gravel (Day, 1992c). In addition to a gravel layer, the installation of a moisture barrier (such as visqueen) will further reduce the moisture migration through concrete (Brewer, 1965).

In some areas, there may be local building requirements for the construction of moisture barriers.

For example, Fig. 12.20 indicates the County of San Diego requirements for below slab moisture barriers when constructing on clays. Note in this figure that 4 in. (10 cm) of open-graded gravel or rock is required below the concrete slab-on-grade foundation. A 6-mil. visqueen moisture barrier is required below the open-graded gravel. These recommendations are similar to the example listed earlier, except that the sand layers have been omitted. The purpose of the sand layer is to prevent puncture of the visqueen moisture barrier. When using specifications similar to Fig. 12.20, it is best to use rounded gravel or add a geotextile so that the visqueen is not punctured during placement and densification of the gravel.



Inadequate surface drainage can be an important factor in moisture migration through concrete slab-on-grade foundations. For example, water ponding adjacent to a foundation can weep in through the perimeter footing and/or seep underneath the perimeter footing and up through the slab. The infiltration of ponding water can also raise the groundwater table, which could subject the foundation to hydrostatic water pressures. Figure 12.21 shows an example of poor surface drainage and ponding of water at a condominium complex. In Fig. 12.21, the condominium is visible along the right side of the photograph. In order to prevent such ponding of water at the site, it is important to have proper drainage gradients around the structure. In addition, the surface runoff must be transferred to suitable disposal systems, such as storm drain lines.


Using a below slab moisture barrier system as described previously and providing positive surface drainage around the foundation will not be enough to resist a naturally occurring high ground-water table or artesian groundwater condition. In these cases, a more extensive waterproofing system will need to be installed. One common approach is to provide a below foundation waterproofing system and a sump equipped with a pump to catch and dispose of any water that seeps through the waterproofing system. A sump is defined as a small pit excavated in the ground or through the basement floor to serve as a collection basin for groundwater. A sump pump is used to periodically drain the pit when it fills with water. When the water level reaches a certain level in the collector box, the submerged pump will be activated, and the collection box will be emptied of water.

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Introduction
This last section of  this chapter will discuss the moisture migration through concrete slab-on-grade foundations. Moisture migration into buildings is one of the major challenges faced by engineers, architects, and contractors. Many times, the project architect provides waterproofing recommendations, but in other instances, the geotechnical engineer may need to provide recommendations.

For example, in southern California, it is common practice for the geotechnical engineer to provide the moisture barrier requirements located below the slab-on-grade foundation.

Moisture can migrate into the structure through the foundation, exterior walls, and through the roof.

Four ways that moisture can penetrate a concrete slab-on-grade foundation are as follows (WFCA, 1984):

1. Water vapor. Water vapor acts in accordance with the physical laws of gases, where water vapor will travel from one area to another  whenever there is a difference in vapor pressure between the two areas.
2. Hydrostatic pressure. Hydrostatic pressure is the buildup of water pressure beneath the foundation that can force large quantities of water through cracks or joints.
3. Leakage. Leakage refers to water traveling from a higher to a lower elevation due solely to the force of gravity, and such water can surround or flood the area below the slab.
4. Capillary action. Capillary action is different from leakage in that water can travel from a lower to a higher elevation. The controlling factor in the height of capillary rise in soils is the pore size (Holtz and Kovacs, 1981). Open-graded gravel has large pore spaces and hence very low capillary rise. This is why open-graded gravel is frequently placed below the floor slab to act as a capillary break (Butt, 1992).

Moisture that travels through the concrete slab-on-grade foundation can damage floor coverings
such as carpet, hardwood, and vinyl. When a concrete slab-on-grade foundation has floor coverings,
the moisture can collect at the top of the slab, where it weakens the floor-covering adhesive.

Hardwood floors can be severely affected by moisture migration through concrete slab-on-grade foundations because they can warp or swell from the moisture. Moisture that penetrates the slab-on-grade foundation can also cause musty odors or mildew growth in the space above the slab. Some people are allergic to mold and mildew spores and they can develop health problems from the continuous exposure to such allergens. In most cases, the moisture that passes through the concrete slab-on-grade contains dissolved salts. As the water evaporates at the slab surface, the salts form a white crystalline deposit, commonly called efflorescence. The salt can build up underneath the floor covering, where it attacks the adhesive as well as the flooring material itself.

As an example of damage due to moisture migration through a concrete slab-on-grade foundation, Fig. 12.17 shows damage to wood flooring installed on top of the concrete slab. Only 6 months after completion of the house, the wood flooring developed surface moisture stains and warped upward in some places as much as 0.5 ft (15 cm). Most of the moisture stains developed at the joints where the wood planking had been spliced together. The joints would be the locations where most of the moisture penetrates the wood flooring. In Fig. 12.17, the arrow points to one of the moisture stains. The asterisk in Fig. 12.17 shows the location of the upward warping of the wood floor.



Oliver (1988) believes that it is the shrinkage cracks in concrete that provide the major pathways for rising dampness. Figures 12.18 and 12.19 show examples of damage due to moisture intrusion through shrinkage cracks in slab-on-grade foundations. Sealing of slab cracks may be necessary in situations of rising dampness affecting sensitive floor coverings. In other cases, the concrete may be porous or there may be an inadequate moisture barrier beneath the foundation, in which case a floor seal may be required in order to reduce the migration of moisture through the concrete slab-on-grade foundation (Floor Seal Technology, 1998).

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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.