TM 5-805-6
Some example coefficients of solar absorption are:
cially for silicone sealants) or according to the
Almost any flat black color:
0.95
general guidelines described above.
Bare galvanized steel:
0.90
f. Example. Following is an example of the
Tarnished copper:
0.80
above calculations in inch-pound units:
Bare concrete:
0.65
Determine the joint size for expansion joints
Clear-finish aluminum:
0.60
Aluminum paint:
spaced at 20-ft. intervals for a concrete masonry
0.40
Clear glass:
unit wall (C = 5.2 x 10-6 inch/inch/degree F) in
0.15.
(3) The maximum substrate material tempera-
which vertical joints are to be sealed. An elastome-
ture T(max) minus the application temperature
ric sealant with a movement capacity of plus or
T(app) can then be used to estimate the joint
minus 25 percent is to be used and a maximum
movement in compression due to temperature in-
temperature gradient of 100 degrees F is
creases as follows:
anticipated.
MC = L x C x [T(max) - Ttapp)]
(eq. 2)
Joint movement M = (20 ft. x 12 in./ft.) x (5.2
Where MC = Estimated joint compression in inches
x 10-6
L = Length in inches of substrate material
inch/inch/degree F) x 100 degrees F = 0.125
C = Coefficient of expansion
inch.
T(max) = Maximum substrate material temperature
Joint width W = (1/.25) x M = 0.125/.25 = 0.5
T(app) = Application temperature
inch.
(4) When the material is thin or readily ab-
Therefore, assuming zero construction toler-
sorbs solar radiation, and solar radiation is not a
ance, the joint width should be inch and the
major factor, the maximum material temperature
joint depth should be -inch.
can be approximated by adding 20 degrees C (30
degrees F) to the maximum ambient temperature.
8. Materials.
Ambient predicted temperature ranges may be
a. Bond-breaker. If sealant adheres to the bot-
obtained from table 1, chapter 24, of the ASHRAE
tom of the joint, subsequent movement of the joint
Handbook of Fundamentals.
will cause substantial distortion of the sealant
c. Extension. The temperature range that will
cross-section as shown in figure 3. Consequently,
cause the maximum amount of joint extension (or
when movement is anticipated, a bond breaker
joint-expansion due to contraction of the substrate
must be applied to the bottom of the joint. A liquid
material) is the difference between the highest
breaker should not be used because it will inter-
application temperature and the material's lowest
fere with adhesion of the sealant to the sides of
service temperature. The lowest ambient tempera-
the joint. A typical bond breaker is a self-adhesive
ture Ta is usually nearly the same as the lowest
polyethylene or polytetrafluoroethylene tape which
material temperature T(min). Some joints experi-
is placed, adhesive-side down, in the bottom of the
ence lower temperatures, e.g., from artificial cool-
joint. A backing material may be treated to act as
ing or by radiation of an insulated surface to a
a bond breaker. However, a bond breaker is not
cold night sky. The material will change in tem-
required if the backing material is sofe enough to
perature from the application T(app) temperature
allow movement of the sealant. Figure 4 shows a
to the lowest ambient temperature; consequently,
typical bond breaker application in a shallow joint.
the expansion of the joint movement can be ex-
b. Backing. Joints are frequently deeper than
pressed as Me where:
the allowed depth of sealant as a result of struc-
Me = L x C x [T(app) - T(min)]
(eq. 3)
Where Me = Estimated joint expansion
tural requirements or convenience of construction.
L = Length of substrate material
If these joints will have any motion, a backing is
C = Coefficient of expansion
required to control the depth and shape of the
T(min) = Minimum material temperature
sealant and, sometimes, to act as a "secondary
T(app) = Application temperature
seal." When the sealant is "tooled," the backing
d. Joint width. The maximum joint width W is
provides support needed to assure uniform sealant
calculated as the greater of either the width for
depth and full wetting of the substrate surfaces.
compression WC or width for expansion We:
(1) Secondary seal.. When used as a secondary
WC = (1/MS) x MC
(eq. 4)
and
seal, the backing should be an elastomeric closed-
We = (1/MS) x Me
cell foam, installed in compression as specified by
Where MS = 1/2 of total movement capacity of the sealant
the manufacturer. Uncompressed closed-cell back-
in combined compression and tension. For example, if a sealant
ing should have a diameter 25 to 33 percent
is rated for a maximum total joint movement of 50 percent, MS
greater than the joint width, while open cell
is 25 percent or 0.25.
e. Joint depth. Determine the joint depth as
backing should be 40 to 50 percent greater. The
recommended by the sealant manufacturer (espe-
backing must be compatible with the substrate,
4
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