Structural Mechanics Solutions
for Butt Joint Seals in Cold Climates
STEPHEN A. KETCHAM
properties. When temperature and loading rate
INTRODUCTION
variations are expected, these properties are mea-
sured as a function of temperature and loading
An effective joint seal* that is formed in a build-
rate so that the effect on structural response can
ing or pavement joint by the curing of a sealant
be evaluated. This is in contrast to the current
will respond with elastic or viscoelastic behavior
practice for the design of building and pavement
over a reasonable design life to any movement of
seals, which, being based on the "movement ca-
the joint without adhesive or cohesive failure.
pability" of a model seal structure (e.g., ACI 1993,
Such a seal is not meant to transfer significant
Panek and Cook 1991), does not utilize structural
forces across the joint. On the contrary, for a given
analysis and does not incorporate measurements
joint movement, seals with lower stiffness are
of the stressstrain mechanical properties of seal-
most able to deform without cohesive or adhe-
ants. As such, the design practice is not compat-
sive failure of the seal or of the structure to which
ible with conventional thermal analysis measures
it is bonded. It is in recognition of this desirable
for rubber materials, such as the modulus of elas-
response feature that lower modulus, rubber-
ticity vs. temperature and the coefficient of ther-
based, elastomeric materials have been formu-
mal expansion vs. temperature.
lated and promoted as joint sealants. For a seal
As indicated by the shear modulus vs. temper-
formed from an elastomeric sealant, it should gen-
ature data in Figure 1, measurements of the modu-
erally be expected that the modulus of elasticity
lus of elasticity as a function of temperature can
will depend upon temperature and loading rate,
be very revealing to the designer of a rubber struc-
such that the modulus increases with a reduction
ture. The data in Figure 1 were published by
in temperature and an increase in loading rate,
and it should be expected that the seal stiffness
Nashif and Lewis (1991) as an example of a large
will depend upon the material modulus and the
database of the properties of rubbers and other
shape of the seal.
materials. The curves shown are of a natural rub-
In the field of rubber technology, conventional
ber and a polysulfide sealant tested at a 50-Hz
engineering design of rubber structures incorpo-
rates engineering mechanics-based structural
peratures, and were obtained using measurement
analysis techniques and corresponding material
techniques that are included in standard test meth-
ods (ASTM 1991b). The shear modulus variations
of the two materials illustrate the dramatic in-
* In this paper the standard terminology for "seal" and
crease in material stiffness that can occur over a
"sealant," given by ASTM C717-88c (ASTM 1991a) for
narrow, low temperature range in rubber materi-
buildings, is adopted. Specifically, "seal" describes a
als, as well as a more subtle increase that is pos-
barrier against the passage of liquids and solids, and
sible. By examining the data of the natural rub-
"sealant" describes a material that has the adhesive
ber, for example, a designer might suggest that
and cohesive capabilities to form a seal. These defini-
20C should be the lowest temperature at which
tions are used in an engineering mechanics sense to
this rubber is used for loading applications at the
allow distinction between the material properties of
50-Hz frequency. Although the data shown are
the sealant and the load-deflection behavior of the seal.
from high-frequency loading tests, a designer
The discussion here is limited to formed-in-place seals.
Previous Page
