Masonry Magazine February 1977 Page. 12
Mortar Joints
Mortar joints of concave or vee shape
use where the masonry will be subject
to rain or freeze-thaw exposure. Beaded and weathered types have also
performed satisfactorily. The other
four types (flush, raked, extruded, and
struck) should be used only where
weathertightness is of no concern,
such as, indoors or protected.
Parapets
Because of their exposure,
parapets, Fig. 3, require special attention by the designer. Parapets are
unheated, undergo differential thermal
and moisture stresses and are difficult
to seal at their top surface. Note in
Fig. 3 that the coping atop the parapet
wall is sloped in a manner so as to
minimize water drainage onto the
masonry surface. For the low parapet
the slope is toward the roof; for the
tall parapet it is toward the outside.
Where water drains onto masonry,
penetration is more likely to occur.
Note also, Fig. 3, that the coping
should be provided with drip ledges on
both sides of the wall, again to reduce
or eliminate water drainage directly
onto the masonry surface.
Flashing
Finally and most important
note in Fig. 3 the thru-wall flashing at
the top of the parapet wall in all cases,
and a second impermeable flashing
thru-wall at the base of a tall (over 15
inches) parapet wall. Coping is set in a
mortar bed with the bed joint raked
back and caulked with a sealant. Cap
flashing slopes to the roof over the
pre-cut cant and is completely and
positively sealed at all joints.
Figure 3 also shows waterproofing
of the spandrel beam by means of
flashing which forms a cut off through
the masonry wall at each floor level
and at the roof. This flashing is most
important because the masonry is thin-
ner at the spandrel and more likely to
permit water penetration.
The placement of the various flash-
ings shown in Fig. 3 follows the rule or
principle mentioned earlier: "...the
designer must search each area of his
design to see if water can enter and
where it will flow or accumulate once
it has entered." Flashing is placed in
the wall where water can accumulate
once it has entered. This principle or
logic for placement of flashing will be
seen in further examples.
Figure 4 is typical of good thru-wall
flashing placement at a location where
water can accumulate once it has
penetrated a masonry wall. Prolonged
rain, driven by wind, is assumed to
eventually penetrate the outer mason-
ry wythe of a cavity wall. Once it has
entered, it will travel downward until
it encounters the solidly-filled con-
crete masonry lintel. Here the water
would accumulate and possibly enter
the structure through the inner wythe
were it not for the flashing. The flash-
ing is installed so as to turn the water
outward and weep holes are provided
to expedite its departure from the
wall. Similar flashing would be
installed in any multi-wythe masonry
wall that did not have the vertical col-
lar joint between the two wythes solid-
ly filled with mortar. In composite
walls with a filled collar joint, the
collar is sufficient barrier to the entry
of water. But, for all other multi-
wythe walls, flashing should be
provided.
Materials
The individual materials of masonry
construction have been more fully
researched with respect to weather-
tightness than the other two factors:
(1) Design, and (2) Workmanship. Past
research is valuable in that it shows the
range of performance characteristics
one can expect from a specific materi-
al in a given exposure. Example: The
ASTM Specifications for Concrete
Masonry Units list different require-
ments for units exposed to different
design conditions. There are units with
different strengths, units with dif-
ferent unit weights, loadbearing and
nonloadbearing units, etc. The same is
true of the ASTM Specification for
Mortars for Masonry.
Concrete Masonry Units
The precise
concrete masonry unit selected for an
exterior wall exposure will depend
upon: (1) geographic area or antici-
pated weather conditions, and (2) the
surface expression the architect desires
in the finished structure. If the exteri-
or walls are to be painted with a pro-
tective waterproof coating, the con-
crete masonry units should conform
with:
* ASTM Designation C 90 for hollow load-bearing units
* ASTM Designation C 129 for nonload-bearing units
* ASTM Designation C 145 for solid units
* ASTM Designation C 55 for concrete brick
If the exterior surface is to remain
unpainted, that is, architectural face or
customized concrete masonry, some-
what more stringent requirements are
necessary to assure durability and
weathertightness. Most manufacturers
of these premium units have developed
their own recommended specifications
which are usually more restrictive than
ASTM Specifications. In the absence
of a manufacturers recommended
specification, the strength and absorp-
tion requirements of ASTM C 55,
Grade N, Normal Weight Concrete
Brick may be employed for architec-
tural facing units or customized units
in exterior walls that are weathertight.
ASTM C 55, Grade N, will provide a
minimum compressive strength of
3500 psi and in the Normal Weight a