TM 5-822-11/AFP 88-6, Chap. 7
(2) Improper web design can produce a
To maintain adequate adhesion and to reduce the
vertical component to the web stress which is
internal strains of a sealant, a shape factor
known as vertical stress. If this stress is not
between 1.0 and 1.5 is recommended (1.0#S#1.5,
balanced, the seal can work itself out of or deeper
where S = DJW) for most field-poured sealants.
into the joint as the joint opens and closes.
Some single-component cold-applied sealants
c. As joints open and close, field.poured
require a shape factor of less than 1 to perform
sealants change in shape but not in volume.
properly; therefore, particular care in designing
Therefore, the stresses that occur are primarily a
the joints and selecting the depth to install backer
function of the shape of the sealant at the time it is
material should be taken when these materials are
poured. The shape of the field-poured sealant is
used. The shape factor of the preformed
expressed in terms of the depth (D) to width (W)
compression seal is determined by the
ratio as shown in figure 2-1. The depth to width
manufacturer.
ratio (DJW) is known as the shape factor.
d. The depth of a field-poured sealant can be
Providing a small shape factor, small depth versus
controlled by the use of a backer material if the
a large width for an in-place sealant generally
joint reservoir depth is deeper than the depth
reduces the internal strains, hence improving the
required to maintain the proper shape factor. The
elasticity of the field-poured sealant. However, a
backer material helps support the sealant during
relatively large sealant depth is desirable to ensure
curing and prevents sagging of the sealant into the
adequate bonding of the sealant to the joint face.
lower portion of the joint. The backer material
2-8
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