Engineering and Construction

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).