Page 224 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
223
FOOTINGS, FOUNDATION WALLS, BASEMENTS, AND SLABS

masonry are absorptive materials with a fine pore structure, the water
rising through the soil by capillary action would be picked up by the
concrete or masonry and continue its capillary migration.
To prevent the capillary rise of water into a slab-on-grade or below-
grade slab, an intervening layer of material must be added which is
either impervious to moisture penetration or has a pore structure large
enough to prevent capillary suction. Gravel and crushed rock are the
materials most commonly used to provide a capillary break under a
slab-on-grade or below-grade slab. The aggregate should be mostly sin-
3
gle graded and of 4 in. maximum size. Capillary water penetration
can also be prevented by installing dampproofing or membrane water-
proofing as a barrier against capillary movement (Figure 6-32).

6.5.2 Water Vapor Movement in Soils

Below-grade vapor pressures within the soil, particularly if capillary
moisture is present, are usually higher than vapor pressures within
buildings. This pressure differential creates a flow of vapor from the
soil toward the structure, regardless of season or interior heating or
cooling cycles (Figure 6-33). Vapor can then migrate through a con-

DAMPPROOF
DAMPPROOFING OR
COATING TO
SHEET MEMBRANE
FILL CAPILLARY
AS CAPILLARY BARRIER
PORES

GRANULAR FILL
AS CAPILLARY
BREAK
FIGURE 6-32
Capillary barrier. (from Beall, Christine, Thermal and Moisture Protection Manual,
McGraw-Hill, New York).

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