CRREL REPORT 95-11

Sealants and Cold Regions

Pavement Seals

A Review

Stephen A. Ketcham May 1995
Abstract

A pavement joint seal prevents the passage of liquids into the pavement

base and the intrusion of solids into the joint. The primary mechanical

requirements of a pavement seal are that it respond elastically or viscoelastically

to any movement of the joint without failure and that it withstand indentation of hard objects like rocks. Because pavement joint movements and

seal deformations can be large, elastomeric sealants are often used to form

seals. Winter conditions are recognized as the most critical for a seal because of the possibility that failure stresses will be reached as the joint

opens to a maximum and the material stiffens in response to the temperature reduction. This report reviews the specific problems and requirements

that cold climates create for the performance of elastomeric seals. Emphasis is placed on the material response behavior that can lead to failure of a

seal. In an attempt to clarify the mechanics of sealant and seal performance associated with low-temperature pavement applications and to address the issue of low-temperature stiffening that should be a dominant

factor in the selection of a sealant, this report presents background information on the formulation and mechanical properties of elastomeric seal

materials and the structural behavior of field-molded joint and crack seals.

For conversion of SI units to non-SI units of measurement consult ASTM

Standard E380-93, Standard Practice for Use of the International System

of Units, published by the American Society for Testing and Materials,

1916 Race St., Philadelphia, Pa. 19103.
CRREL Report 95-11

Sealants and Cold Regions

Pavement Seals

A Review

Stephen A. Ketcham May 1995

Prepared for

OFFICE OF THE CHIEF OF ENGINEERS

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
PREFACE

This report was prepared by Stephen A. Ketcham, Research Civil Engineer, Civil and

Geotechnical Engineering Research Division, Research and Engineering Directorate, U.S.

Army Cold Regions Research and Engineering Laboratory. Funding for this research was

provided by DA Project 4A762784AT42, Cold Regions Engineering Technology; Task CP;

Work Unit K01, Maintenance of Surfaced and Unsurfaced Areas in Cold Regions. The author

thanks Vincent Janoo and Piyush Dutta of CRREL for their technical reviews of this report.

The contents of this report are not to be used for advertising or promotional purposes.

Citation of brand names does not constitute an official endorsement or approval of the use

of such commercial products.

ii
CONTENTS

Page

Preface ................................................................................................................................. ii

Introduction ........................................................................................................................ 1

Elastomers and sealant formulations.............................................................................. 2

Mechanical behavior of sealants ...................................................................................... 3

Phenomenological behavior of rubbers and elastomers ......................................... 3

Hyperelastic constitutive model ................................................................................. 6

Examples of sealant behavior...................................................................................... 7

Mechanical response of seals ........................................................................................... 8

Basic structural geometry and loading configurations of seals ............................. 9

Conventional performance testing for studying the load and deformation

response of joint and crack seals ............................................................................ 10

Response of seals to joint movements ....................................................................... 11

Summary and recommendations .................................................................................... 17

Literature cited ................................................................................................................... 18

Abstract .............................................................................................................................. 21

ILLUSTRATIONS

Figure

1. Results from three simple tension tests of vulcanized natural rubber ............. 4

2. Schematic depiction of load and extension responses of a typical polymer

tested in simple tension tests at different temperatures............................... 5

3. Real part of the complex Young’s modulus plotted against the temperature

of the material and the frequency of the harmonic loading in a three-axis

plot for the synthetic rubber Buna N .............................................................. 5

4. Variation of the simple tension stress–strain curves of an elastomer with

strain rate and temperature, and an envelope of failure points .................. 6

5. Schematic illustration of the variation of the coefficient of volume expansion

α with temperature T for polymers, and the glass transition

temperature Tg ................................................................................................... 6

6. Examples of the real part of the complex shear modulus G and the loss factor

tan δ as functions of temperature from low-strain harmonic

loading tests ........................................................................................................ 7

7. Butt joint seal cross sections in joint and crack locations with different

practical variations of joint preparation, uses of auxiliary materials

and seal geometries............................................................................................ 9

8. Ratio of apparent Young’s modulus to Young’s modulus of the material,

Ea/E, of a rubber block in the configuration of a long butt joint seal,

in relation to the shape factor d/w ................................................................... 12

 9. Approximation of the nominal stress–strain response of a butt joint seal in

 extension and compression ............................................................................. 13

iii
Page

10. Results of numerical analysis of a 1.2- × 1.2-cm silicone butt joint seal ........... 13

11. Karpati’s (1972a) results showing the effect of temperature on the

nominal stress–percentage elongation response of silicone butt

joint seal specimens............................................................................................. 14

12. Karpati’s (1972b) results showing the effect of deformation rate on the

nominal stress–percent elongation response of silicone butt joint seal

specimens ............................................................................................................. 15

13. Karpati’s (1972b) results showing the effect of deformation rate on the

nominal stress–percentage elongation response of polysulfide

butt joint seal specimens .................................................................................... 15

14. Results from constant rate of extension tests of butt joint seal specimens

formed from various sealants, at –7°C (20°F) and 22°C (72°F) ..................... 16

15. Cook’s (1965a) results from step deformation tests of polysulfide

butt joint seal specimens .................................................................................... 16

16. Results from step deformation tests of butt joint seal specimens formed

from various sealants .......................................................................................... 17

TABLES

Table

1. Example sealant formulations ................................................................................ 3

2. Tensile failure and extension data from simple tension tests of various

silicone joint sealants .......................................................................................... 8

iv
INTRODUCTION

ASTM standard C 717 defines a seal as a barrier against the passage of liquids, solids or gases

and defines a sealant as a material that has the

adhesive and cohesive properties to form a seal

(ASTM 1991a). For applications to buildings and

pavements, a field-molded seal can be formed in

a joint or a crack by the curing of a sealant that is

applied in liquid, viscous form. The primary mechanical requirements of the seal are that it respond elastically or viscoelastically, over a reasonable design life, to any thermal- or load-induced movement of the joint or crack without adhesive or cohesive failure, and that it cure to a

sufficient hardness and remain sufficiently hard

so that it will not be damaged by indentation of

harder objects. Such a seal is not meant to transfer

significant forces of the structure across the joint.

On the contrary, for large joint movements, sealants with lower load resistance are most able to

deform without failure of the seal or of the structure to which it is bonded.

Joints and cracks in building and pavement

structures undergo movements that are dominated

in amplitude by the seasonal movements caused

by the thermal expansion and contraction of the

structural materials. Expansion at higher temperatures and contraction at lower temperatures of the

building or pavement materials are realized at the

joint as closure during summer and opening during winter, respectively. The result is that a seal in

a building or pavement joint must be able to extend during cold periods and compress during

warm periods. Cold-period loading of a seal is

recognized as the most critical loading period because of the possibility that failure stresses will be

reached in the seal or at the adhesive interface as

the joint opens and the sealant hardens in response

to the temperature reduction. A seal will also be

subjected to shorter duration loads that may be

critical. For example, joint movements associated

with stick–slip motions, daily temperature variations and traffic can induce short duration loads

that will be superimposed on existing longer

period loads. For each specific application and

loading configuration, the cured sealant must have

mechanical properties that are appropriate for

the structural response and climatic requirements

of the seal. In addition, these properties must not

degrade significantly when the sealant is exposed

to weathering processes, loading cycles, chemical agents and other conditions that are present in

its working environment.

In recent years elastomeric sealants have been

introduced commercially that are formulated to

have relatively low resistance to load at low temperatures compared to conventional and traditional seal materials. The lower resistance at lower

temperatures is a result of a more flexible material formulation and of a lower temperature of the

material’s transition to harder behavior. This report is concerned with the behavior of these elastomeric sealants, which are often simply called “low

modulus sealants,” for sealing joints and cracks

in asphalt and portland cement concrete pavements in cold regions and with the specific problems and requirements that cold climates create

for seal performance. Emphasis is placed on the

material response behavior that can lead to failure of a seal, rather than on strength of sealants

and failure of seals. Several low modulus elastomeric compounds have been marketed as pavement sealants. A significant concern for pavement seal designers should be the transition temperatures below which these sealants in pavement

seal configurations perform inadequately because

of the stiffening of the material. In an attempt to

Sealants and Cold Regions Pavement Seals

A Review

STEPHEN A. KETCHAM
clarify the mechanics of sealant and seal performance associated with low temperature pavement

applications, and to address the issue of low temperature hardening that should be a dominant factor in the selection of a sealant, this report presents

background information on the formulation and

mechanical properties of elastomeric seal materials

and the structural behavior of field-molded joint

and crack seals.

ELASTOMERS AND

SEALANT FORMULATIONS

Commercially available sealants are produced

from a variety of materials. Because the ideal behavior of a pavement joint seal is that of an elastomer, it is revealing to describe sealant formulations in the context of elastomers and elastomeric

compounds.

“Elastomer” is a term often used for rubber and

polymers that have properties similar to those of

rubber (e.g., ASTM 1991b). Elastomers are polymers with a particular type of molecular structure.

Specifically they are linear, amorphous (i.e., noncrystalline), high molecular weight polymers in

which the chains of the molecular structure are

flexible and cross-linked (e.g., Hearle 1982). The

flexibility and cross-linking of the polymer chains

allow elastomers to behave elastically, as the name

implies, such that they are capable of recovering

from large deformations. Mark (1982) has listed

three molecular requirements for a material to exhibit this rubber-like elastic behavior:

• The material must consist of polymeric

chains.

• The chains must have a high degree of flexibility.

• The chains must be joined into a network

structure.

In this context the terms elastomer and rubber are

used as synonyms, and the broad range of natural and synthetic rubbers and rubber compounds

that behave like elastomers can be studied in a

single context because the behavior can be attributed to this molecular structure. It is of interest to

note that when the molecular chains lose their flexibility in response to a temperature reduction, there

is a transition in the material behavior of the polymer from a elastomeric behavior to that of a hard

material, e.g., glass. The transition is referred to

as the “glass transition.”

Elastomeric compounds are marketed commercially as formulations that consist of the base

elastomer or elastomers together with additives.

Various formulations with the same base elastomer can be developed for very different applications. For example, polyurethane formulations

can be used in elastomeric applications ranging

from shoe soles to joint sealants. Although elastomeric formulations can be designed or advanced

by theoretical modeling techniques, the development of most is primarily empirical.

Additives are used primarily to improve specific properties of a formulation. A “plasticizer” is

an additive that increases the flexibility of a compound. More specifically a plasticizer reduces the

stiffness of the base elastomer at a given temperature, lowers the temperature for which elastomeric

behavior can occur, increases the strain at which

failure occurs in the elastomer, and increases its

impact strength at lower temperatures (Immergut

and Mark 1965). In some sealant formulations,

plasticizers are used in this way, i.e., to lower the

modulus of the cured material and to improve its

low temperature capabilities. “Filler” refers to material that is used primarily to add bulk to the base

elastomer, although the term is often used to designate any additive other than plasticizers that has an

impact on the properties of the compound. At times

fillers have been used to improve the profit margin of the manufacturer at the expense of the desired elastic material properties. Panek and Cook

(1984) have suggested that such economic-based

compounding has been the cause of the inadequate

performance of some asphalt rubber highway joint

seals in the past. Other additives that might be

used include coloring agents, curing ingredients,

adhesive agents, and ultraviolet light and oxygen

stabilizers. Usually exact formulations of elastomeric compounds are kept as trade secrets of the

manufacturer. Table 1 includes several nonspecific sealant formulations that have been published by Panek and Cook (1984).

Elastomers are used as the base polymer of commercially available sealants so that seals formed

by these sealants will respond elastically to service loads and deformations. The base polymers of

sealants include natural rubber, styrene-butadiene

rubber, neoprene, silicone, polyurethane, polysulfide and polyvinyl chloride (Panek and Cook

1984). The sealant formulations range from asphalt

and rubber compounds to coal tar and polyvinyl

chloride compounds to silicone sealants. Often categorical distinctions are made in the applications

literature between asphalt rubber sealants, coal

tar rubber sealants and other polymeric sealants.

In the context of elastomeric compounds, however,

2
Table 1. Example sealant formulations, percentages by weight. (After Panek and Cook 1984.)

Urethane sealant: urethane polymer, 35–45%; fillers, 30–40%; colorants, 2–3%; thixotropic agents, 1–2%;

adhesion additives, 1–3%; plasticizers, 15–25%; solvent, 0–4%

Silicone sealant, structural grade: silicone polymer, 65–75%; silicone oil plasticizer, 5–15%; silica filler, 15–

25%; curing agent, 3–5%; adhesive additive, 1–3% (lower modulus silicone sealants would have a higher

amount of silicone oil plasticizer and less cross-linking trifunctional silane complex)

Polysulfide sealant: polysulfide polymer, 30–40%; various fillers, 30–40%; plasticizers, 20–25%; curing agents, 2–

5%; adhesion additives, 1–3%; miscellaneous, 1–3%; solvent, 3–5%

Hot-poured rubber asphalt sealant: ground rubber (e.g., styrene-butadiene rubber ground to a 30-mesh

size), at least 25%; asphalt, remainder

Hot-poured polyvinyl chloride (PVC) and coal tar: PVC powder dispersed in a coal tar base

in which additives are blended and reacted with

the base elastomer(s) and cured to form the final

product, the asphalt and coal tar in such products

can be categorically regarded as fillers or plasticizers and adhesive agents, and the chemical and

physical reactions between these materials and the

rubber polymer can be viewed as a typical curing

process that yields an elastomeric material. Indeed,

it would be advantageous to consider all elastomeric-based sealants, including asphalt and coal

tar rubbers, in a single context with respect to formulation, so that engineers who select and use these

materials will have a mechanical framework that

can be utilized to compare and contrast them.

Different elastomers and elastomeric formulations can attain their application configuration,

cross-linking and mechanical properties through

a variety of curing reactions and mechanisms. Sealants can be formulated to cure by chemical and

physical reactions and solvent release. The reactions can be triggered, for example, by heat, as in

the case of the hot-poured sealants, or by catalysts,

as in the case of moisture-induced cure of silicone

sealants.

MECHANICAL BEHAVIOR

OF SEALANTS

In general, the commercially available, low

modulus sealants that are used for sealing joints

and cracks in pavements are formulated to behave

with rubber-like characteristics. That is, these materials are formulated so that a seal made from a low

modulus sealant, loaded to a relatively large deformation, will return to its original shape and size

upon unloading. For a description of the mechanical properties of sealants it is helpful to consider

sealant behavior in the context of the behavior of

rubbers and elastomers since the material response

features of practical interest for sealing joints and

cracks are clearly revealed, and since the elastomeric qualities that a seal displays when subjected

to field loadings and conditions can be used to

judge the effectiveness of the sealant. In addition,

if a seal formed by an elastomeric-based sealant

does not behave elastically, e.g., if the seal deforms

plastically or if it behaves with excessive viscous

behavior, the deviation from ideal elastomeric behavior could be quantified and an assessment of

the sealant could be made. This section describes

the mechanical behavior of rubber-like materials

and low modulus sealants from a phenomenological perspective. The section also describes the

hyperelastic material model, which is conventionally used as a constitutive model for elastomeric

materials, and its potential application to sealants.

Phenomenological behavior of

rubbers and elastomers

In unconfined loading configurations the deformations of a rubber structure will generally be

dominated by shear distortions. This is because

the volume compressibility of elastomers and

elastomeric formulations of rubber-like materials

is, relative to the shear deformability, very low.

Typically a rubber will have a bulk modulus that

is orders of magnitude greater than its shear modulus. As a result, the material is constrained geometrically by its own response to deform primarily by distortions. This is particularly true for the

base elastomer, and, depending upon the additives, is likely to be the case for an elastomeric formulation.

The shear deformability of rubbers has been investigated using different loading and test speci3
Test programs that incorporate these experimental configurations are sometimes devised to investigate material behavior, to calibrate phenomenological constitutive models of rubber elasticity behavior that can be used for material and structural

response calculations, and to verify predictions

made using the constitutive models.

Simple tension tests of elastomers yield response curves like those shown in Figure 1. The

figure relates force per unit of unstrained area to

extension ratio, which is defined as the deformed

specimen length normalized by the undeformed

specimen length. The data in the figure are for a

vulcanized natural rubber tested at room temperature, and are published by Treloar (1975).

Figure 1 includes the force-extension ratio responses measured in three simple tension tests.

Curve (a) illustrates the response of a specimen

that was loaded to failure, whereas curves (b) and

(c) illustrate the responses of specimens that were

loaded and unloaded without failure. Based on the

curves (b) and (c) Treloar (1975) noted that up to

an extension ratio of about 5.5 the material response

was substantially reversible, but that loading to a

higher extension resulted in significant hysteresis.

Curve (c) illustrates clearly the low resistance, high

extension, nonlinear elastic response that is the

characteristic feature of the behavior of rubber materials. For example, it is roughly the behavior of

the material of a rubber band when stretched.

Implicit in this description of an elastomer is a

temperature range for which rubber-like behavior

occurs. The transition from rubbery to glassy behavior (the glass transition) is often characterized

by a single temperature, although, unlike a crystalline material, the mechanical behavior changes

gradually over a temperature range. The transition

occurs because the chains of the amorphous molecular network become less flexible with decreasing temperature until the structure is rigid. Figure 2

illustrates the changes in the load and extension

response that are typically observed in polymers

that are tested in simple tension tests at different

temperatures. Only above the temperature range

of the glass transition will a polymer in simple tension tests deform with homogeneous strains and

behave like an elastomer (Ward 1983). Within and

below the transition range the deformation will not

be homogeneous, and ductile or brittle failure can

occur. The figure illustrates that a material intended for use as an elastomer will actually behave as a stiff and perhaps brittle material when

its temperature is below the glass transition temperature.

12345678

8

6

4

2

0

Extension Ratio

Tensile Force Per Unit of Unstrained Area (N mm–2)

8

6

4

2

0

8

6

4

2

0

Figure 1. Results from three simple tension tests of vulcanized natural rubber. (After Treloar 1975.)

men configurations. Traditionally, for obtaining material stress-strain data under homogeneous stress

and deformation conditions, load and deformation responses have been measured in simple tension, equibiaxial tension and pure shear tests. Torsion tests, which inherently generate inhomogeneous stress and deformation conditions, can provide material stress–strain data as well (Penn and

Kearsley 1976), by using a technique for calculating material stress–strain data from the measured

torque and axial force of the torsion sample (Kearsley and Zapas 1980). For standard comparative

measures of material properties the simple tension

configuration is often used (e.g., ASTM 1991d).

a.

b.

c.

4
rial can be calculated for the material’s

response at various frequencies and temperatures. Because the responses of polymers can have a viscous component, the

moduli that are calculated are complex

quantities. Figure 3 depicts the real part

of the complex Young’s modulus plotted against the temperature of the material and the frequency of the harmonic

loading for the synthetic rubber Buna N.

The data were originally published by

Nolle (1950). This figure illustrates that

the glass transition occurs primarily between 0 and –20°C for lower frequency

loading, but that the transition occurs at

higher temperatures for higher frequency loading. These features, i.e., that

glass-like behavior can occur both at low

temperatures and at high frequency

loading rates, are typical of elastomers.

The distinction between rubbery and

glassy behavior is made here to clearly illustrate the impact that temperature can

have on the behavior of sealants. A distinction between rubbery and viscous behavior should be

made as well, however, since the effects of the viscous response of predominantly rubber-like materials can be more than subtle, and since compounding of elastomers with additives can yield

overly viscous formulations that behave with excessive creep and stress relaxation for sealant applications. ASTM (1991b) provides a definition for

rubbery behavior by suggesting that a specimen

made from rubber, when stretched to twice its

length for a duration of one minute, will return

upon release to 1.5 times its original length within

one minute. This definition implies that a distinction can be made between predominantly rubbery

and viscoelastic behaviors, while recognizing that

rubber behavior incorporates some viscoelasticity.

Failure of elastomers is typified by brittle fracture that occurs only after large, nonlinear elastic

deformations, with any plastic deformation being

restricted to a very small volume of the material

around the fracture (Williams 1984). Bueche and

Berry (1959), however, have suggested that, for predictions of tensile failure, a critical stress criterion

rather than a fracture mechanics-based criterion

is preferable. For simple tension specimen configurations, Smith and Stedry (1960) have made measurements that demonstrate the influence that

strain rate and temperature have on tensile failure, and have shown that a tensile failure envelope connecting rupture points of the stress–strain

Figure 3. Real part of the complex Young’s modulus

plotted against the temperature of the material and the

frequency of the harmonic loading in a three-axis plot

for the synthetic rubber Buna N. (After Hearle 1982 and

Nolle 1950.) Modulus (N/mm

2)

 10–2 100 102

 Frequency (Hz)

 104 106 108

40

20

0

Temperature (°C)

–20 –40 

103

102

101

100

The glass transition temperature is often

quantified by the measurement of the load and

deformation response of a material specimen

during low strain amplitude harmonic loading

experiments, creep tests and stress-relaxation

tests. From the dynamic measurements of harmonic loading tests, elastic moduli of the mateExtension Load

a

b

c

d

Figure 2. Schematic depiction of load and extension responses of a

typical polymer tested in simple tension tests at different temperatures, from temperature well below the glass transition temperature, i.e., curve a, to temperature above the glass transition temperature, i.e., curve d. (a) brittle fracture, (b) ductile failure, (c) colddrawing, and (d) rubber-like behavior. (After Ward 1983.)

5
Stress

0

Strain

Decreasing Temperature

Increasing Strain Rate

Figure 4. Variation of the simple tension

stress–strain curves of an elastomer with

strain rate and temperature, and an envelope of failure points. (After Ward 1983 and

Smith and Stedry 1960.)

data can be formed that reflects this influence. Figure 4 shows a schematic illustration of their data.

Brittle fracture of an elastomer after large deformations should be distinguished from brittle

fracture of the same material in a glassy state. The

use of the “brittle fracture” to describe rubber failure can be misleading and is not often used. In

addition, the transition from rubber-like behavior

to glassy behavior should be distinguished from

a ductile to brittle transition. The terms ductile and

brittle describe primarily the failure response at

large strains, whereas the terms rubbery and glassy

encompass mechanical response behavior of a

greater extent.

Although elastomers generally behave as incompressible materials in a mechanical loading

context, thermally induced volumetric strains can

be significant and, if constrained, can have a significant influence on the stresses in the material.

The thermal coefficient of volume expansion of a

polymer is another property that varies with temperature such that the glass transition temperature is revealed. Figure 5 illustrates this variation

schematically.

Volume changes in rubbers can also be caused

by exposure to organic liquids such as hydrocarbons (Treloar 1975). Rubber materials can swell and

contract with the exposure and expulsion of the

liquids, and the volume compressibility can increase with exposure such that an assumption that

the rubber is incompressible is no longer valid

(Treloar 1975). Swelling does not imply that any

chemical interaction between the rubber and the

liquid has occurred, however. For rubbers the process is simply a physical mixing process in which

the molecules of the liquid diffuse into the molecular structure of the rubber (Treloar 1975). Volume changes due to shrinkage during curing of

the rubber formulation can be significant, particularly if the curing process includes evaporation of

solvents (Panek and Cook 1984). As mentioned

above, if the volumetric strains are constrained,

the material stresses will be affected.

Hyperelastic

constitutive model

To characterize the stress–strain response of an

elastic body undergoing large deformations, it is

sufficient to specify the form of the strain energy

function W as a function of the deformation or current strain (e.g., Rivlin 1956, Green and Adkins

1960). The derivative of the strain energy function

with respect to a strain component gives the corresponding stress component. The term hyperelastic has been used to describe such an ideal elastic

material (Malvern 1969).

The form of strain energy functions of rubbers

has been deduced from both molecular considerations and phenomenological experiments, and

similarities between the forms have been found

(Treloar 1975, Green and Adkins 1960). Based on

experimental measurements, Rivlin and Saunders

(1951) have suggested that, for vulcanized natural

rubber,

Figure 5. Schematic illustration of the variation of the coefficient

of volume expansion α with temperature T for polymers, and the

glass transition temperature Tg. (After Eisenberg 1984.)

1

V

δV

δT p

= α

T

Tg

6
Because a response predicted by a hyperelastic

model is independent of the previous state of

strain, the predicted stresses are independent of

the strain history and deformation rate, and the

response is that of a conservative system. As such,

hyperelasticity cannot model viscous or plastic behavior, such as hysteresis, creep, stress relaxation

and permanent deformation (Green and Adkins

1960). However, for rubber-like time-dependent

materials, McKenna and Zapas (1986), citing Rivlin

(1956), describe the validity of using isochronal

stress–strain data from stress–relaxation tests, measured at relatively short times after a rapid strain

application, as an estimate of the equilibrium data

for calculating strain energy functions for use in

finite elasticity calculations. Thus, if the data of

the time-dependent material response are treated

correctly, the time dependence can be ignored for

purposes of the finite deformation calculations, and

the results could be viewed as the response at a

given time, e.g., the long-term response.

Examples of

sealant behavior

Previous experimental studies of the mechanical behavior of joint and crack sealants have relied primarily on structural configuration tests

rather than material property tests to obtain measures of the sealant response. Although structural

configuration tests can reveal “apparent” system

properties (Gent and Lindley 1959), material properties cannot readily be found from these tests. Results from structural configuration tests will be

reviewed in the following section.

Catsiff et al. (1970a) have described material test

data from several sealant materials, including polysulfide, silicone and asphalt-polyurethane sealants.

They presented measurements indicating that an

incompressibility assumption is valid for analyzing structures formed from these materials, and

emphasized the validity of the incompressibility

assumption for the sealant formulations, not just

the base-elastomer. They further suggested that

stress and elongation data in an isochronous form,

e.g., stress and extension data for a given time after the loads are applied in stress-relaxation tests,

are appropriate for structural analysis techniques

in which time-dependent behavior is not incorporated. This suggestion is consistent with the method suggested by McKenna and Zapas (1986) for

calculating the strain energy function for quasielastomeric materials, as mentioned above. Catsiff

et al. (1970a) presented data illustrating the validity of this technique for sealants.

W = C(I1 – 3) + f (I2 – 3) , (1)

where I1 and I2 are strain invariants for an isotropic,

incompressible material, C is a constant, and f denotes “a function of.” Specifically, 

I1 1

2

2

2

3

2

=++ λλλ

and I2 1

2

2

2

3

2

=++ 111 /// λλλ , where the λs are the

extension ratios, i.e., ratios of the current length to

the original length, in the principal directions. The

first term of eq 1 has been found by relating the

change in molecular dimensions to macroscopic

strain measurements from homogeneous strainloading experiments, and the last term has been

called a “correction” term (Treloar 1974). A large

body of work has been conducted to establish improved or alternative forms of the strain energy

function for isotropic rubber materials from a phenomenological perspective (see Treloar 1975, for

a review). For example, the following polynomial

form of the strain energy density function (e.g.,

Rivlin and Saunders 1951) is often used for phenomenological modeling:

 

W CI I C i j = − ( ) ( ) − =

= =

∞

∑ ij

i 0, j 0

1 2 00 33 0 , . (2)

The experimental determination of the strain energy function is typically a detailed task. Rivlin

and Saunders (1951) described a technique using

biaxial loading experiments of rubber sheets that

has been adapted by many workers. In this technique ∂W/∂I1 and ∂W/∂I2 are calculated from measurements as functions of I1 and I2, and the functions are used to define eq 1. The conditions during the experiments dictate the thermodynamic nature of the strain energy function that is found,

e.g., that the strain energy function is the function

for isothermal and constant pressure conditions

(Ward 1983). Other techniques have been developed as well. For example, Penn and Kearsley

(1976) describe a data reduction technique for torsion experiments that allows ∂W/∂I1 and ∂W/∂I2

to be calculated from torque and normal force measurements at different torsion angles.

Implementations of strain energy functions for

large deformation response predictions of rubber

structures are described for several homogeneous

and inhomogeneous deformation problems by

Rivlin (1956). Numerical implementations of hyperelastic models, using the finite element method

with large deformation capabilities, are described,

for example, by Hibbitt et al. (1989) and Finney

and Kumar (1988). The effect of temperature on

the structural response can be incorporated in the

analysis by measuring the strain energy as a function of temperature as well as strain.

7
Table 2. Tensile failure and extension data from simple

tension tests of various silicone joint sealants. (From

Spells 1987.)

Force per unit

unstrained area at

Sealant extension ratio = 1.5 Extension ratio at

classification (kPa) breaking failure

High modulus 690 5

Medium modulus 276–690 5–12

Low modulus 276 12

Tensile failure and extension data from simple

tension tests of various silicone joint sealants have

been described in the literature by Spells (1987) and

are reproduced in Table 2. The sealants were designated by Spells as high modulus, medium moduFigure 6. Examples of the real part of the complex shear

modulus G (G = G1 + iG2) and the loss factor tan δ

(tan δ = G2/G1) as functions of temperature from lowstrain harmonic loading tests of (a) a polysulfide sealant, (b) a polysulfide elastomer, (c) a urethane elastomer,

and (d) a natural rubber, all tested at three loading frequencies in beam tests. (After Nashif and Lewis 1991.)

Temperature (°C)

105

106

107

108

109

1010

105

106

107

108

109

1010

105

106

107

108

109

1010

105

106

107

108

109

1010

10–3

10–2

10–1

100

101

10–3

10–2

10–1

100

101

10–3

10–2

10–1

100

101

10–3

10–2

10–1

100

101

Tan δ G1

(Pa)

Tan δ G1

(Pa)

Tan δ G1

(Pa)

Tan δ G1

(Pa)

–80 –40 0

0 40 80 120

0 40 80 120

–80 –40 0 40 80 120

–80 –40 

–80 –40

40 80

50


500


5000

Frequency


(Hz) lus, and low modulus sealants in order to demonstrate their range of deformation capabilities, and

were tested according to standard procedures

(ASTM 1991d) at near room temperature. For each

category the table lists the force per unit of the

original, unstrained area at an extension ratio of

1.5, and the extension ratio at breaking failure.

Very few data exist in the literature regarding

the effect that temperature has on sealant properties, although in the rubber and polymer industry

such material property data are routinely generated for evaluating the effectiveness of the material, and standard test methods are suggested for

obtaining the data (e.g., ASTM 1991c). One example of shear modulus vs. temperature data from

low-strain harmonic loading tests of a sealant is

shown in Figure 6. The data are for a polysulfide

sealant tested at three loading frequencies using a

beam test, from a publication of Nashif and Lewis

(1991). The real part of the shear modulus G(G =

G1 + iG2) and the loss factor tan δ (tan δ = G1/G2)

are shown. Also shown from this publication, for

purposes of comparison, are corresponding data

sets for a polysulfide elastomer, a urethane elastomer and a natural rubber.

More general reviews of the characteristics of

sealants have been given by Panek and Cook (1984)

and the American Concrete Institute (ACI 1993).

MECHANICAL

RESPONSE OF SEALS

The current state of knowledge of the mechanical response of joint and crack seals comes primarily from field and laboratory experimental studies of the deformation capabilities of pavement and

building seals. As indicated by the extent of the

experimental work that is documented in the literature and by the concerns of the practitioners

who are charged with sealing joints and cracks,

a.

b.

c.

d.

8
revealed by a selection of analysis results and experimental measurements, is reviewed.

Basic structural geometry and

loading configurations of seals

In general, portland cement concrete pavements

are constructed with regular joint patterns and

spacing to avoid cracking of the pavement from

thermal response and shrinkage of the concrete,

whereas asphalt concrete pavements are rarely constructed with built-in joints. When cracks are sealed

in asphalt concrete pavements, where crack patterns can vary from regular spacings of relatively

straight transverse and longitudinal cracks to irregular patterns of meandering cracks, it is conventionally recommended that butt joint reservoirs be formed along the cracks using sawing or

routing construction techniques (e.g., Lynch 1990).

Some authors, however, without presenting analyses to support their contentions, have suggested

that seal performance is not improved by this technique (Panek and Cook 1984). Further, in the case

of reflection cracks in asphalt concrete overlays of

portland cement concrete pavements, where the

crack patterns typically follow the regular joint

pattern of the portland cement concrete pavement, it is also conventional practice to construct

butt joint reservoirs along the cracks by sawing or

routing the cracks. For these reasons little distinction is made in this section between seals formed at

joints and at cracks and between seals in portland

cement concrete joints and in asphalt concrete

joints, although the term crack is used in reference to asphalt concrete pavements.

In an engineering design context, a pavement

joint or crack seal formed from a sealant can be

described as a long structure with a butt joint cross

section such as one of those depicted in Figure 7.

This figure shows typical seal cross sections in joint

and crack locations with variations of joint preparation, uses of backing rod and separating tape,

and seal geometries. Often in practice the constructed cross sections will deviate from the ideal configurations illustrated due to construction irregularities, backing rod installation or irregularities

in the crack or joint. For crack seals in particular,

it is also likely that there will be variations along

the length of the seal as a result of the meander of

the fracture in the pavement surface, and it may

be that a joint reservoir is not prepared during the

sealing operation and the seal is formed in the cavity of the crack. Although not quantified by realistic structural analyses of seals, it is generally recognized that when there is significant deviation from

the structural performance of seals is of great interest to civil and pavement engineers. Although

it is generally recognized that the low-temperature extreme of the seasonal temperature cycle can

lead to failure of the seal if the material hardens

significantly and the seal is extended by the movement of the joint or crack (e.g., ACI 1993, Beech

1985, Catsiff et al. 1970a), very little work has been

conducted to quantify the effect of low-temperature variations on the structural response of seals.

In this section the basic geometry of seal structures

and their loading is discussed; conventional experimental techniques for studying pavement joint

and crack seals are described; and the mechanical

response of seal structures to joint movements, as

Figure 7. Butt joint seal cross

sections in joint and crack locations with different practical

variations of joint preparation,

uses of auxiliary materials and

seal geometries.





















































































Separating Tape

Backing Rod











9
the butt joint configuration, there can be premature failure of a seal because the load distribution

results in increased stresses in the seal (e.g., ACI

1993, ASTM 1991f).

A pavement seal can be subjected to many types

of loadings. These include loadings caused by seasonal, daily and traffic-induced movements of the

joint or crack opening; the direct loading of the

seal itself by tires or hard objects such as small

rocks under the weight of tires; the forces of constraint caused by the volume change response of

the seal material that results from the curing process, temperature variations or the interaction of

the seal material with a foreign substance such as

jet fuel; and the forces of constraint caused by

changes in the stress–strain behavior of the seal

material induced by temperature variations or the

aging of the material. The distribution of loads on

pavement joint seals as a result of long- and shortduration movements of the joints is not a directly

measurable quantity, whereas joint movements can

be measured directly. As a consequence the “movement capability” (see e.g., Panek and Cook 1984,

ASTM 1991f) of a model seal in terms of either a

safe-working elongation or an ultimate, failure

elongation, rather than a load capability in terms

of the sealant’s strength, is commonly used to

comparatively describe sealants. Examples of horizontal joint movement measurements are the data

of Minkarah et al. (1982), which are from portland

cement concrete highway test sections in Ohio with

regular joint spacings. Minkarah et al. mentioned

that short-term measurements of ±25% were typical for joints formed by 6.4- and 12.2-m highway

sections, and were greater than longer cyclic movements. Such movements can be roughly estimated

with manageable structural analysis calculations.

For example, the American Concrete Institute (ACI

1993) has suggested an analysis technique for predicting horizontal movements of joints in portland

cement concrete pavements due to thermal contraction and expansion of the concrete material.

Conventional performance testing for

studying the load and deformation

response of joint and crack seals

As mentioned above, it is typically a “movement capability” of a model seal, in terms of either a safe or ultimate extension, compression or

other deformation, that is used to summarize the

load and deformation performance of the corresponding sealant product. In general, although an

apparent modulus is often reported for a model

seal, conventional experimental techniques for

characterizing a seal’s response to load sometimes

adopt the movement capability concept without

concern of the load. These techniques are intended

to provide performance data so that a seal designer

can ensure that the working movement capability

of a seal is greater than the movement that the joint

will experience.

Tests of model seals are generally conducted

using block-shaped seal specimens constrained between parallel surfaces of a given substrate. Performance-based standard tests of these model seals

(Beech 1985), for both the pavement and building

seal industries, have been developed primarily to

provide a basis for quantifying and indexing movement capability in the context of the movements

associated with seasonal joint opening and closing. As suggested by the standards, these tests are

typically conducted at a specific low temperature

in recognition of the severity of the low temperature extreme. None of the standard methods incorporate test series for measuring the possible stiffening of the response as the temperature is lowered,

however. Many other laboratory and field tests that

are not standard have been performed to measure

the movement capability of model seals. These include laboratory tests of a model seal’s response

to transverse movements (e.g., Shisler and Klosowski 1990), and long-duration field tests using thermally designed loading fixtures that allow a model

seal to be subjected to environmental weathering

and temperature variation-induced joint movements (e.g., Karpati et al. 1977). Regarding the latter, although an extensive amount of research on

sealant movement capability has been performed

using outdoor strain cycling exposure racks (e.g.,

Karpati 1980, 1989), continuous load and deformation measurements with strain cycling have not

been made, and thus quantitative structural response information has not been obtained during

the exposure. Such measurements could provide

significant quantitative data regarding temperature-induced hardening and its effect upon the load

and deformation response of seal specimens. However, typically only the joint movement and temperature have been measured while the seals are

on the rack, and changes of the model seals have

been noted visually (Karpati 1980).

Beech (1985) has suggested that tests of model

seals yield superior results for measuring movement capability, compared to tests of actual seals

in field service, because the results from in-service tests reflect a specific set of circumstances and

do not lend themselves to statistical analysis. Indeed, when the goal of the test is to determine a

10
capability of a sealant to form a seal, independent

of the workmanship and other conditions peculiar to a given application of the sealant, such an

isolation of variables is required to improve the

degree of control of the experiment. Beech stated

that the long-duration exposure and seasonal cycling tests noted above (Karpati et al. 1977) are the

“closest practicable approach” to in-service tests.

There is confusion in the sealant literature regarding the nature of tests on model seals, however. In particular, although the block-shaped seal

specimens do not respond with a true plane-strain

response because of their typically short length, the

models are often assumed to represent long joint

seal structures. Additionally, research test results

of the model structure are often reported as if a

homogeneous material response rather than an inhomogeneous structural response has been measured. For example, when model seals are tested in

material testing apparatuses, the nominal stress

and strain are often used to calculate the apparent

modulus exhibited by the structure, yet the value

is typically reported simply as a “modulus” as if

the structural response could be considered as homogeneous. Indeed, a leading textbook for sealants (Panek and Cook 1984) has incorrectly suggested that the modulus of elasticity of the sealant

can be calculated from experiments on bonded

model seals with 1.3- × 1.3-cm cross sections, without alerting the reader that the result is the apparent

modulus of the structural system rather than the

material modulus. Furthermore, “creep” and

“stress-relaxation” tests have been conducted using

model seal specimens, without following the conventional approach of using a structural solution

to calculate the corresponding material response

(Cook 1965a, Sandberg and Rintala 1990), even

though creep and stress-relaxation refer to material responses and not system responses. The

implication is that, in general, researchers in this

field do not distinguish between the response of

the material and the response of the structure

formed from the material. Evidently this lack of

distinction has led to a conventional practice in

which butt joint seal structures are tested not only

to ascertain the behavior of a particular structure,

but also to specify the behavior of the sealant material, and to do so without suggesting that the

behavior is the apparent material behavior for the

particular structure. For the practicing engineer

this distinction may not be of concern, but it is

important that the researchers who establish design criteria do so with a firm understanding and

clear communication of such basic mechanics.

Regarding the performance testing of model seal

structures as the conventional technique for specifying the behavior of the corresponding sealant

material, Catsiff et al. (1970a) state

Relevance of the measurements obtained to actual

use is likely to require an almost intuitive appreciation of past experience. Special-purpose tests may

be devised, but the multiplicity of end-use requirements can be expected to yield an equal multiplicity of such tests. In any case, correlation of field

failures or successes with numerous laboratory tests

and controlled field tests is bound to be a tedious

and time consuming operation.

Their remarks underscore the need for more engineering mechanics-based specifications of sealant

behavior, such that practical decisions can be made

competently by an engineer who does not have

years of experience with special-purpose performance tests.

In addition to movement capability experiments, other load and deformation response tests

are incorporated in standard test methods to mimic

the load associated with indentation of hard objects. For example, in ASTM D 3407 (1991e), which

is a standard for testing joint sealants for concrete

and asphalt pavements, these tests are penetration

index tests of cured sealant specimens.

Response of seals to

joint movements: Results from

analyses and experiments

In the field of building and pavement sealants,

the most widely used and referenced technique

for the analysis of the response of seals to joint

movements is the nominal strain calculation approach of Tons (1959). Tons used observations of

elastomer-based seals to suggest that the deformed

top and bottom surfaces of a long butt joint seal

are constrained to have a parabolic shape, and to

assume that the material of the seal is incompressible. His analysis consisted of calculating an apparent, nominal strain along “the parabolic curvein line” so that the movement capability of seals

with different shape factors could be compared.

He used his observations, constraint assumptions

and calculations to suggest guidelines for shape

factor design, stating that “for like conditions, the

greater the width of the joint, the less the sealer

will be strained for the same percentage of joint

opening,” and that “the shallower the joint is

sealed, the less the sealer will be strained when

the joint opens, other conditions being the same.”

Tons observed that at large deformations the de11
012345

d/w

1

2

3

4

5

6

7

8

9

10

Ea/E

Figure 8. Ratio of apparent Young’s modulus to Young’s modulus of

the material, Ea/E, of a rubber block in the configuration of a long

butt joint seal, in relation to the shape factor d/w. (From Gent and

Lindley 1959.)

formation pattern deviated from the parabolic

shape and the nominal strain distribution along

the parabolic surface varied slightly from a “uniform” nominal strain. The calculation of a nominal strain that Tons suggested, and the use of the

calculations to compare the movement capability

of seals formed with different shape factors, is an

approach that has been incorporated into design

manuals (ACI 1993) and used extensively by practicing engineers.

Another publication from 1959 describes a simple analytical solution for shape factor design that

is based on structural mechanics theory. Gent and

Lindley (1959) presented results of elasticity analyses and experiments designed to assess the effect

of a shape factor on the apparent modulus of rubber blocks, i.e., the stiffness of the blocks, in butt

joint seal configurations under compression loads.

They showed that the apparent Young’s modulus

Ea of a rubber block in the configuration of a long

butt joint seal, in relation to the Young’s modulus

of the material E, the width of the joint w and the

depth of the seal d, can be approximated by

 

E

E

d

w

a = + 



 



 4

3

1

3

2

. (3)

The relation is illustrated in Figure 8. It was derived for volume-incompressible rubbers at small

deformations, and was shown to compare well to

the experimental behavior of compressed rubber

blocks. Applying the relation to joint seals, for ratios of d/w less than 1, the apparent modulus of a

seal can approach a value that is 4/3 the sealant

modulus, yet the apparent modulus can increase

dramatically with increasing d/w ratios. This information could be used in the design choice of

d/w to ensure that stresses in the seal and at the

adhesive interface will not be excessive, i.e., to ensure that a seal formed from a low modulus sealant will not respond with a high stiffness. The relation provides a clear structural rational for the

need to maintain control over shape factors in

crack-sealing jobs through such means as rout and

seal or saw and seal techniques. Gent and Mienecke (1970) presented similar solutions for bending and apparent shear joint movements, which

could also be used for joint seal shape design. Gent

and Lindley (1959) further showed that the eq 3

relation might not apply directly to volume-compressible rubber formulations, although they suggested that the relation could be modified to predict the response of such compounds. Chalhoub

and Kelly (1991) presented an instructive derivation of the eq 3 relationship, and also considered

volume compressibility.

For an approximate analysis of the larger deformation compression or extension responses of

the rubber in a butt joint seal configuration, Gent

and Lindley (1959) utilized an elasticity-based solution (Treloar 1975) of the stress–strain response

of a rubber in pure homogeneous compression or

extension, with the Young’s modulus replaced with

the apparent Young’s modulus. From the solution

12
Extension

Compression

0

–0.1

–0.2

–0.3

–0.4

0.8 0.9 1.0 1.1 1.2

0.1

0.2

0.3

λ

σ/Ea

Figure 9. Approximation of the nominal stress–strain response of a

butt joint seal in extension and compression. (From Gent and Lindley

1959.)

the ratio of the nominal stress to the apparent

Young’s modulus can be written as

 

σλ λ

Ea

= − –2

3 , (4)

where σ is the nominal stress acting on the seal

and λ is the extension ratio of the current joint

width to the original joint width. The relation is

depicted in Figure 9. Gent and Lindley illustrated

with experimental results that the approximation

reasonably represents the compression of vulcanized natural rubber blocks with d/w ratios from

0.36 to 1.7, and with lengths equal to w, to approximately 25% compression. These results, by the influence of the apparent modulus on the large deformation response, provide further elucidation of the

influence of the shape factor on the stress of a seal.

It should be noted, however, that the use of eq 4

implies the applicability of a strain energy function with the form of the first term in eq 1. Although this term provides a good representation

of rubber constitutive behavior, it does not represent rubber behavior in general (Treloar 1974).

Cook (1965a, 1965b) later considered the homogeneous rubber elasticity relation of eq 4 for sealant materials, but inappropriately used the equation directly for the inhomogeneous deformation

of butt joint seal specimens without distinguishing between the material Young’s modulus and

the apparent Young’s modulus, as shown to be necessary by Gent and Lindley (1959).

10% 20%

0.220 in.


(5.59 mm)

0.198 in.


(5.03 mm)

0.198 in.


(5.03 mm)

0.275 in.


(6.99 mm)

0.250 in.


(6.35 mm)

0.250 in.


(6.35 mm)

0.300 in.


(7.62 mm)

0.300 in.


(7.62 mm)











173

a. Deformed quarter sections at 10% and 20%

joint extension.

b. Maximum principal stress distribution at 20%

joint extension.

Figure 10. Results of numerical analysis of a 1.2-

× 1.2-cm silicone butt joint seal. (From Catsiff et

al. 1970b.)

13
Rubber elasticity models have been used with

finite-deformation numerical analysis techniques

to model the large deformation behavior of seals.

Figure 10 shows results generated by Catsiff et al.

(1970b) using finite element approximations for

plane strain sections of silicone butt joint seals.

The stress distribution and deformed configurations in Figure 10 illustrate a quantitative prediction of the response, and demonstrate the insight

into the response behavior that can be obtained

through such analysis. In particular, the high stress

level at the corner reveals the critical zone within

the structure, and suggests that adhesive failure

would likely occur in this area if failure stresses

are reached. The use of these techniques requires

an appropriate calibration of the hyperelastic material model, however, which has not always been

the case. For example, Holland (1990) calibrated a

hyperelastic model with results from structural

property tests rather than material property tests,

evidently presuming that the structural responses

represented homogeneous behavior.

Other analysis techniques, particularly analytical solutions based on small-strain elasticity

theory, have been developed to investigate the effect of the material volume change behavior on

the stress distribution within a joint seal when the

seal is constrained to have zero width change. Such

techniques would apply to the response of a seal

to thermal and other volume change mechanisms.

Wu (1982) has reviewed these techniques.

Responses of model seals from experimental

work on building and pavement sealants from several research programs are illustrated in Figures

11 through 16. These examples from the literature

have been chosen to demonstrate the nature of the

responses of seals formed from a variety of materials under a range of conditions. The first example is Karpati’s (1972a) joint extension experiments showing the effect of temperature on silicone butt joint seal specimens with dimensions of

1.3 × 1.3 cm in the cross section and with a length

of 5.1 cm. The material of the supporting bars was

aluminum. The test results are depicted in Figure

11. As Karpati suggested, below 50% extension of

the joint and 350 kPa (50 psi) of nominal stress,

there was little effect of temperature upon the seal

responses, except that slightly stiffer responses

were observed at the –42°C (–44°F) and –51°C

(–60°F) temperatures. For this material the effect

of temperature was primarily evident at much

higher deformations of the seals. This observation

is consistent with the low glass transition temperature of silicone. The effect that the deformation

rate has on the response is shown in Figure 12 for

tests at –23°C (–10°F) and 22°C (72°F) (Karpati

1972b). Here is seen a more pronounced effect on

the stiffness below 50% extension, relative to the

negligible effect of temperature, with the higher

deformation rates resulting in stiffer specimens.

As a comparison, Karpati’s (1973) results from polysulfide model seals are illustrated in Figure 13.

Again results at two temperatures are shown

(–34°C and 23°C), at the different deformation

rates indicated, which in this case reflect a significant stiffening of the seal response from the higher

0 100 200 300 400

500

400

300

200

100

0

Elongation (%)

119

41

0

–29

–44

–60

76°F

Temperature


(°C)

0

8

16

24

32

Nominal Stress

(lb/in.2 (kg/cm ) 2)

Figure 11. Karpati’s (1972a) results showing the effect of temperature on

the nominal stress–percentage elongation response of silicone butt joint

seal specimens.

14
to the lower temperature. Karpati reported that

the polysulfide sealant material had a glass transition temperature of –46°C (–50°F). The results

also show stiffer responses for the higher deformation rates.

Karpati did not conduct material property tests.

She did, however, apply material property concepts of temperature and strain-rate dependence

from the polymers field to interpret the responses

of model structures (Karpati 1972c, 1973). Karpati

recognized that the model seals have “an extremely

complicated stress field” (Karpati 1972c), citing the

work of Catsiff et al. (1970b), and chose the geometry of a single structure for her work. Although

Karpati’s results provide insight into the material

behavior, as suggested by Catsiff et al. (1970a), it

0 0

4

8

12

16

20

40

80

120

160

200

240

280

0 100 200 300 400 0 100 200 300 400

Elongation (%)

Nominal Stress

(lb/in.2 (kg/cm ) 2)

5.0


0.5


0.2


0.005


0.00049


Break Point


Test Stopped

in./min

–10°F (–23°C) 72°F (22°C)

Figure 12. Karpati’s (1972b) results showing the effect of deformation rate on the

nominal stress–percent elongation response of silicone butt joint seal specimens.

0 0

4

8

12

16

20

40

80

120

160

200

240

280

0 100 200 300 400 0 100 200 300 400

Elongation (%)

Nominal Stress

(lb/in.2 (kg/cm ) 2)

in./min
 ––––––––– cm/min
 –––––––––

T = –30°F (–34.4°C) T = 73°F (22.8°C)

5.000


1.000


0.5000


0.1000


0.05000


0.01000


0.005000


0.0005000

1.969


0.394


0.1969


0.0394000


0.01969


0.00394


0.001969


0.0001969

Break Point


Test Stopped

Figure 13. Karpati’s (1972b) results showing the effect of deformation rate on the

nominal stress–percentage elongation response of polysulfide butt joint seal specimens.

15
is not possible to extrapolate such results to

quantitatively predict general structural behavior, i.e., the behavior of seals with different geometries and structural configurations.

Karpati’s results illustrated in Figures 11–13

indicate the nominal loads and joint extensions

at failure of the butt joint seals. A comparison of

the results with the schematic illustration in Figure 4 for material specimens demonstrates the

underlying influence of the strain rate and temperature dependence of the material on the butt

joint seal response. In general the failures take

place at high joint expansions relative to the requirements of a field joint. This is the desired

behavior of a low modulus sealant; i.e., the flexibility of the seal causes stresses to remain below failure levels for expected joint movements.

Other examples are the experiments by Collins et al. (1986), which demonstrate the effect

of temperature on the response of model seals.

Results from the experiments are shown in Figure 14. The tests were constant rate of joint extension experiments using seal specimens with

dimensions of 2.5 × 2.5 × 12.7 cm. The extension

rate was 1.2 cm/min, and the test temperatures

were –7°C (20°F), 22°C (72°F) and 54°C (130°F).

Materials marketed as pavement sealants were

used to form the model seals, and the material

of the supporting bars was portland cement concrete. The materials are indicated on the figure

together with the extension ratios at failure. Also

shown in the graphs are the responses of a preformed compression seal.

Figure 15. Cook’s (1965a) results from step deformation tests of polysulfide butt joint seal specimens.

0 20 40 60 80 100 120 140 160

Time (min)

0

10

20

20

16

12

8

4

0

30

40

50

Load

(kg) (lb)

500

400

300

200

100

0

250

200

150

100

50

0 0

0

40

80

120

160

200

20

40

60

80

100

0 234 1 8 (in.)

0 2 4 6 8 10 20 (cm)

0 2 1 9 (in.)

0 2 4 6 24 (cm)

Elongation

Elongation

(kg) (lb)

(kg) (lb)

Load Load

Rubberized


Asphalt (12.5%)

Preformed


(163%)

Polyurethane


(378%)

Polysulfide


(345%)

Silicone


(766%)

Rubberized Asphalt


(207%)

Preformed


(126%)

Polyurethane


(145%)

Polysulfide


(222%)

Silicone


(918%)

Figure 14. Results from constant rate of extension tests of

butt joint seal specimens formed from various sealants, at

–7°C (20°F) and 22°C (72°F). (From Collins et al. 1986.)

16
viscoelastically to any thermally or load-induced

movement of the joint without failure of the seal

or the adhesive bond, and that it cure to a sufficient hardness and remain sufficiently hard so that

it will not be damaged by indentation of harder

objects. Because joint movements are often large,

joint seals must be expected to withstand large deformations. Winter conditions are recognized as

the most critical for a seal because of the possibility that failure stresses will be reached in the seal

or at the adhesive interface as the joint opens and

the material stiffens in response to the temperature reduction. For all conditions the ideal behavior of a pavement joint seal is that of an elastomer.

As a result sealants have been formulated with a

rubber or elastomer as a base material in order to

have relatively low resistance to load at low temperatures.

Pavement joint sealants that include a rubber

or elastomer as a base material are, in essence, elastomeric compounds, and should be considered as

such for research and engineering applications. Although plastic behavior is observed in the response

of some sealant compounds containing elastomers,

in general these sealants are formulated and promoted to behave with rubber-like characteristics.

To describe the mechanical properties of these sealants it is helpful to consider sealant behavior in

the context of the behavior of rubbers and elastomers since the material response features of practical interest for sealing joints are clearly revealed,

and since the elastomeric qualities that a seal displays when subjected to field loadings and conditions can be used to judge the effectiveness of the

sealant. Implicit in any description of an elastoRubberized Asphalt

Polyurethane

Silicone

Polysulfide

Preformed

0 10 20 30 40 50 60

0

20

40

60

Time (min)

Stress Change (%)

Figure 16. Results from step deformation tests of butt joint seal

specimens formed from various sealants. (From Collins et al. 1986.)

The load and deformation responses shown

in Figure 14, as well as secant stiffness values of

the model seals for 50% extension reported by

Collins et al. (1986), show that significant stiffening of the responses occurred at –7°C for all of the

materials except the silicone sealant. The stiffening of the rubberized asphalt seal due to the temperature reduction was extreme.

Results from step deformation tests of polysulfide butt joint seal specimens performed by

Cook (1965) are shown in Figure 15. The load vs.

time curve in Figure 15 reflects the stress–relaxation material response and the viscous nature of

the polysulfide sealant. Results from similar experiments reported by Collins et al. (1986) are

shown in Figure 16. These tests were conducted

at 22°C (72°F) using the materials of Figure 14

and the model seal geometry described above for

the joint extension tests of Collins et al. (1986).

The measurements were made after extending

the model seals 50% at a deformation rate of 25

cm/min. The percent change in the nominal stress

vs. time is shown in Figure 16. It is interesting to

note that while all of the responses shown in this

figure reveal viscous behavior, as suggested by

Collins et al. some of the behavior is viscoelastic

and some viscoplastic. Without unloading curves

this distinction cannot be made, however.

SUMMARY AND

RECOMMENDATIONS

The primary mechanical requirements of a pavement joint seal are that it respond elastically or

17
meric compound, however, is a temperature range

for which rubber-like behavior occurs. As the

temperature of an elastomer is decreased, its mechanical behavior changes until it behaves like

glass. This is a phenomenon that can cause seals

to stiffen during cold periods. Very few data exist

in the literature or in manufacturers’ specifications concerning sealant properties as a function

of temperature, although in the rubber and polymers industry such material property data are

routinely generated for evaluating the effectiveness of a material. Without such data, or without

extensive practical experience, seal designers cannot independently know the temperature below

which a sealant will perform inadequately. For a

particular cold region application, performance

tests conducted at a single low temperature, which

is a typical condition implemented in standard

test methods, do not in general provide enough

information for an engineer to establish the effectiveness of a sealant.

Previous experimental studies of the mechanical behavior of joint and crack sealants have relied primarily on structural configuration performance tests rather than material property tests to

obtain measures of the sealant response. Although

structural configuration tests can reveal system

properties, properties reflecting the stress and

strain behavior of the material cannot be found

from these tests. Structural tests are generally conducted using block-shaped model seal specimens

constrained between parallel surfaces of a given

substrate. These tests have been developed primarily to provide a basis for quantifying the capability of the seal to withstand the movements

of seasonal joint opening and closing. There is confusion in the sealant literature regarding the nature of tests on model seals, however. Of greatest

concern is that research test results of model seals

are often reported as if a homogeneous material

response rather than an inhomogeneous structural response has been measured. This lack of distinction between the response of the material and

the response of the structure formed from the material has led to a conventional practice in which

butt joint seal structures are tested not only to ascertain the behavior of a particular structure, but

also to specify the behavior of the sealant material, and to do so without suggesting that the behavior is the apparent material behavior for the

particular structure. Structural and experimental

analysis conducted more than 30 years ago clearly showed that this practice can be misleading.

Structural analysis, for the most part, has been

ignored by sealant researchers, although there have

been recommendations within the sealant research

community that conventional practices of structural mechanics be used to study and specify the

behavior of sealants. In the related field of rubber

and elastomers it is conventional practice to conduct material property tests, as a function of temperature, and to use the information obtained to

conduct structural analysis. In all of structural engineering research it is also conventional to validate the results of analysis with tests of model structures. This classic framework should be applied

in the research of sealants and joint seals, and

should be followed to study sealants for cold climate applications. As indicated in this review, phenomenological theories of material behavior, material property tests, structural analysis methods

and structural testing capabilities are available for

adopting this framework. For the development of

selection and design criteria for cold climate sealants and seals, this approach, as compared to the

conventional performance testing approach, would

be most efficient and productive.

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Sealants and Cold Regions Pavement Seals: A Review PE: 6.27.84A

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Cold regions Pavement seals

Elastomers Sealants

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

A pavement joint seal prevents the passage of liquids into the pavement base and the intrusion of solids into the

joint. The primary mechanical requirements of a pavement seal are that it respond elastically or viscoelastically

to any movement of the joint without failure and that it withstand indentation of hard objects like rocks. Because pavement joint movements and seal deformations can be large, elastomeric sealants are often used to

form seals. Winter conditions are recognized as the most critical for a seal because of the possibility that failure

stresses will be reached as the joint opens to a maximum and the material stiffens in response to the temperature

reduction. This report reviews the specific problems and requirements that cold climates create for the performance of elastomeric seals. Emphasis is placed on the material response behavior that can lead to failure of a

seal. In an attempt to clarify the mechanics of sealant and seal performance associated with low-temperature

pavement applications and to address the issue of low-temperature stiffening that should be a dominant factor

in the selection of a sealant, this report presents background information on the formulation and mechanical

properties of elastomeric seal materials and the structural behavior of field-molded joint and crack seals.