Masonry Magazine December 1964 Page. 11
Strain along parabolic curve in
250
200
De
(depth of
sealant)
(joint width
150-when filled)
100
50
0
0
50
100
Linear expansion
D-0
150
-2
FIG. 4. Maximum strain on
joint plotted
against percentage of linear expansion, ap-
plicable to any joint and any sealant.
mended limit for elongation, 25 per-
cent. The thermosetting plastic seal-
ants are two-component liquid sys-
tems, which react after installation to
form elastomers. These include epoxy
resins, polysulfide rubber and epоху
polysulfide mixtures. These materials
will not soften appreciably in applica-
tions up to 225 deg F.
Types of strains classified
A joint sealant may be subjected to
extension (or shear) or compression.
Knowledge of the type of strain to
which a sealant will be subjected by
joint movement will assist in design-
ing a joint successfully. If the maxi-
mum strain on any part of the sealant
from the total movement exceeds its
physical limits, the joint will fail either
in extension (or shear) or in compres-
sion in any one of six ways.
Extension or shear failures four types
Adhesive failure. All sealants, re-
gardless of formulation, decrease in
extensibility as the temperature falls.
Generally, all sealants perform better
at higher ambient temperatures. When
Allowable joint expansion, %
120
100
80
60
Mastic (gas),
gun grade
((calking gun use)
Mastic (gas),
knife grade
for temp, range
of 10 to 110° F)
20
0
Ероку (Colma) joint sealer
Shore A 15
Mastic (gas), pressure grade
(slow movement,
50 to 150 F
0 10 2.0 30
Ratio,
40
FIG. 5. Maximum extension of joint at failure
vs. ratio of joint depth to width for different
calking materials, for an extension of is in.
per hour at 0 deg F. This temperature is
critical, using the formula of Egop Tons.
the internal cohesive capabilities of a
sealant exceed its adhesive capabilities
(bond to other materials), the joint
will fail in adhesion when overex-
tended. Mastic sealants particularly of-
ten fail in adhesion when the tempera-
ture falls and the internal cohesion in-
creases. The increase in cohesion of
elastomeric-type sealants at lower
temperatures is much less pronounced
than that of mastics.
Cohesive failure. When the ad-
hesive (bond) capabilities of a sealant
exceed its cohesive capabilities, the
sealant will tear apart as the joint ex-
pands. Failure of this type is more
common with elastomeric sealants
than with mastics because of their in-
herently high adhesive strength. Mas-
tic-type sealants generally exhibit co-
hesion failure at higher ambient tem-
peratures.
Internal failure. Under exten-
sion, a scalant may slowly tear apart
internally. This type of failure is more
common with elastomeric-type seal-
ants than with mastics. However, mas-
tics also exhibit these internal failures.
Si plate
Anchor bolt or in
Epoxy
Concrete
Concrete
Foam
FIG. 6. A joint with well-proportioned sealant,
covered for protection. This type of joint is
well suited for a bridge or other structure
where large movements are anticipated.
--- Colma joint sealer
Joint filler
"thick
Primer
Bond breaker
Bond to one side only
FIG. 7. A horizontal joint not subject to any
traffic can be cheaply protected by a in
thickness of sealer.
These are usually caused by fatigue
or by air bubbles inside the sealant.
Substrata failure. Where the
concrete surfaces of the joint slot are
weak, sealants of the elastomeric type
may, with their high tensile strength,
rip off concrete or mortar surface ma-
terial at the joint face.
Compression failures- two types
Compression set. When elasto-
meric-type sealants are compressed
too far, their internal structure will be
partially or completely destroyed.
They will no longer possess memory
to assume their previous shape and
will remain permanently deformed.
This is referred to as compression set.
Usually 25 percent compression is con-
sidered the maximum to which any
elastomeric material should be sub-
jected.
Extrusion. When a mastic type
of sealant is forced too far out of its
joint slot, it will not be pulled back
when the slot expands. Dirt will ac-
cumulate on the extruded material
(continued on page 12)
High strength
Concrete Base
(a)
Angle added
Colma joint sealer,
Bond
breaker
Sika seal
Concrete Base
(b)
Concrete Base
(c)
FIG. 8. Improperly designed mastic joint, (a), failed after a -in, movement. Redesigned joint, (b), in which elastomeric sealant was placed at time
of maximum travel, provides for -in movement.
MASONRY. December, 1964
11