Structural Mechanics Solutions for Butt Joint Seals in Cold Climates

EXTENSION AND COMPRESSION LOADING

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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-

harmonic loading frequency and at several tem-

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.

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