TM 5-822-11/AFP 88-6, Chap. 7
should not bond with the sealant. If the sealant
and result in loss of bond between the sealant and
adheres to a stiff backer material, the sealant will
joint or crack face, internal rupture of the sealant,
not be as free to move at the bottom as it is at the
extrusion of the sealant from the joint or crack,
top. This causes higher stresses in the sealant and
and intrusion of debris into the sealant. Defects
can lead to premature failure of the sealant.
can be reduced or eliminated by one or more of
Therefore, the backer material should be as soft
the following:
and flexible as possible, but stiff enough to stay in
(1) Reduce the strains in the sealant by using
position in the joint to maintain the correct shape
better shape factors either by sawing or forming
factor. Backer materials that absorb water and
the joint reservoir to the proper size and shape.
expand must not be used, since the moisture can
(2) Use the proper backer materials to
damage the effectiveness of the sealant.
support the sealant and prevent sagging, provide
a suitable shape factor, and provide a weak bond
along the bottom of the joint to prevent three-sided
2-7.
Common Sealant Defects.
adhesion.
a. Field-Poured Sealants. Figure 2-6 illustrates
(3) Reduce movement at the joints by using
common sealant defects in field-poured sealants
shorter joint spacings.
and lists some of their possible causes. The defects
(4) Ensure joint and crack faces are properly
result from excessive stresses as discussed earlier
cleaned before sealing.
2-9
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