Masonry Magazine April 1971 Page. 32
WIND PRESSURE
POUNDS PER
SQUARE FOOT
20 25 30 35 40 45 50
ANNUAL
PRECIPITATION-INCHES
20-30 40 50
FIG. 2
terized by partially filled joints, deep furrowing
of the mortar beds, and improper execution of
flashing and caulking details will be more sub-
ject to rain penetration. This subject will be
discussed further in other Technical Notes in
this series.
Condensation
In addition to rain water and ground water
which may penetrate masonry walls, water may
accumulate within the wall as a result of con-
densation of water vapor. Frequently, efflores-
cence that appears on rain-resistant masonry
walls is due to this accumulation of water.
Condensation is usually due to moisture origi-
nating inside buildings. The moisture content of
the outside air, which enters the building and is
heated for comfort purposes, is invariably
increased by moisture released from cooking,
bathing, washing and other operations employing
water or steam, and by the moisture released by
exhalation and perspiration from the occupants.
This gain in moisture content of the air
increases the vapor pressure substantially above
that existing in the outdoor atmosphere. This
increased pressure tends to drive the vapor out-
wardly from the building through any vapor-
porous materials that may comprise the enclos-
ing surfaces.
When vapor passes through porous and homogeneous materials, which may be warm on one side and cold on the other, it may pass through the zone of its dew point temperature without condensing into water. But, if the flow of vapor is impeded by vapor-resistant surfaces at a temperature below the dew point temperatures, the vapor may condense upon such cold surfaces. This condensed moisture can contribute to efflorescence on the surface of the wall.
Construction
Another source of water which may cause "new-building bloom" and contribute to future occurrences of efflorescence in a building is the water which enters the assembly during construction. The improper protection of a building during construction phases may materially contribute to future problems, including efflorescence. It is at this stage, when interior assemblies are exposed, joints are open and foreign materials are present on the project, that the construction is highly vulnerable to the entry of considerable moisture. Also, in some cases, additional soluble salts from other sources may contaminate the wall assembly.
OTHER EFFLORESCENCE STAINS
There will occasionally occur on the surface of masonry structures stains other than the fairly common white efflorescence previously discussed. These are "green stains" and "brown stains."
Green Stain
Some structural clay products develop yellow and green efflorescence salts when they come in contact with water. These stains are usually vanadium salts. They can be found on red, brown or white clay products; however, they are more objectionable and usually noticed on the lighter color units. The vanadium salts responsible for these stains have their origin in the raw materials used for the manufacture of clay products. The yellow and green stains which appear as efflorescence on brick are usually vanadyl salts consisting of sulfates and chlorides, or hydrates of these salts.
The mechanisms of this type of efflorescence are as follows: as water travels through the brick it dissolves both the vanadium oxide and sulfates. In this process the solution may become quite acidic. As the solution evaporates from the surface of the product, the salts are deposited.