Most above-grade materials are breathable in that they allow for negative vapor transmission.
This is similar to human skin; it is waterproof, allowing you to swim and bathe but also to perspire, which is negative moisture transmission. Most below-grade materials will not allow negative transmission and, if present, it will cause the material to blister or become unbonded.

Breathable coatings are necessary on all above-grade wall surfaces to allow moisture condensation from interior surfaces to pass through wall structures to the exterior. The sun causes this natural effect by drawing vapors to the exterior. Pressure differentials that might exist between exterior and interior areas create this same condition.

Vapor barrier (nonbreathable) products installed above grade cause spalling during freeze–thaw cycles. Vapor pressure buildup behind a nonbreathable coating will also cause the coating to disbond from substrates. This effect is similar to window or glass areas that are vapor barriers and cause formation of condensation on one side that cannot pass to exterior areas.

Similarly, condensation passes through porous wall areas back out to the exterior when a breathable coating is used, but condenses on the back of nonbreathable coatings. This buildup of moisture, if not allowed to escape, will deteriorate structural reinforcing steel and other internal wall components.

Below-grade products are neither ultraviolet-resistant nor capable of withstanding thermal movement experienced in above-grade structures. Whereas below-grade materials are not subject to wear, above-grade materials can be exposed to wear such as foot traffic.

Below-grade products withstand hydrostatic pressure, whereas above-grade materials do not. Waterproofing systems properties are summarized in Table 3.1.

Waterproofing Systems Differences

Since many waterproofing materials are not aesthetically acceptable to architects or engineers, some trade-off of complete watertightness versus aesthetics is used or specified. For instance, masonry structures using common face brick are not completely waterproof due to water infiltration at mortar joints. Rather than change the aesthetics of brick by applying a waterproof coating, the designer chooses a dampproofing and flashing system. This damp-proofing system diverts water that enters through the brick wall back out to the exterior.

Application of a clear water repellent will also reduce water penetration through the brick and mortar joints. Such sealers also protect brick from freeze–thaw and other weathering cycles.

Thus, waterproofing exposed vertical and horizontal building components can include a combination of installations and methods that together compose a building envelope. This is especially true of buildings that use a variety of composite finishes for exterior surfacing such as brick, precast, and curtain wall systems. With such designs, a combination of several waterproofing methods must be used. Although each might act independently, as a whole they must act cohesively to prevent water from entering a structure. Sealants, wall flashings, weeps, dampproofing, wall coatings, deck coatings, and the natural weathertightness of architectural finishes themselves must act together to prevent water intrusion (Fig. 3.1).

This chapter will cover vertical waterproofing materials, including clear water repellents, elastomeric coatings, cementitious coatings, and related patching materials. It will also review horizontal waterproofing materials including deck coatings, sandwich slab membranes, and roofing.

All envelope waterproofing applications must act together to prevent water intrusion.
FIGURE 3.1 All envelope waterproofing applications must act together to prevent water intrusion.

0 comentarios:

Post a Comment