Product Limitations – Waterproofing Moving Joints with Hybrid Sealants
Historically, the waterproofing industry has had many product options for
joint sealing. Among these are liquid sealants and impregnated-foam
Liquid sealants suffer numerous shortcomings in
achieving the high performance needed in dynamic joints; and
impregnated-foam sealant alternatives require careful sizing to ensure
proper density to maintain a seal.
As the limitations of individual technologies
in any field become apparent efforts are made to enhance positive traits
while eliminating or reducing negative traits by combining different
species, materials, or technologies. Such hybridization results in new
products, or hybrids that preserve the best features of the component
materials while removing the weaknesses that limited performance in the
Liquid sealants are supplied in cartridges
(tubes), as well as in bulk form (pails or sausages), or in other ways
convenient for shipping. The installer first places into the joint, to a
prescribed depth, a foam backing. This backing is of a prescribed size and
shape to suit the joint size. The liquid sealant is extruded through a
nozzle into the joint over the backing material. In joints in which
movement cycling is expected, the backing material is typically rounded in
shape. The sealant is
tooled against the backing material to expel entrapped air and to achieve an
hourglass cross-sectional shape. The
achievement of this hourglass shape is critical to the performance of the
sealant in moving joints
(see Figure 1).
Figure 1: Shape and positioning of liquid
sealant foam backing material and tooling of liquid sealant achieves a
geometry in the cured elastomer that reduces bond-line stresses during joint
In contrast to sealants that
are placed into joints in a liquid state and whose functional form is
created through a manual process executed in the field by a mechanic,
preformed sealants are shipped to site in their finished or substantially
finished, functional state. Examples of preformed sealants include:
extruded overlay or bridge-seals (strips of silicone or other elastomeric
materials adhered over joints in a band-aid fashion); compression seals
(multi-cellular rubber extrusions adhered onto joint faces with adhesives);
strip seals (extruded rubber seals pressed into extruded metal rails that
are mechanically anchored to substrates); and impregnated expanding foam
Developed in the 1950’s in Europe, the components of impregnated-foam
sealants are: 1) open-cell polyurethane or polyether foam, and 2) a
water-repelling adhesive impregnation.
The custom-manufactured foam
matrix is produced to meet performance values of indentation force
deflection, and compression stress relaxation, which provide resilience
sufficient to resist the damping effect of the adhesive impregnation.
Coating the entire cellular
structure of the open-cell foam with non-drying, water-repelling adhesive
agents produces impregnated-foam sealants. The combination of this
impregnation treatment followed by compression of a certain volume of
impregnated-foam down to a specific compression level creates a sealant
material that is always in compression.
Limitations of Liquid Sealants
The performance of field-applied liquid sealants in movement joints is
limited by numerous factors. These include improper installation and flaws
in the sealant system concept.
The Sealant Waterproofing and Restoration Institute publishes a 50-page
manual, “Applying Liquid Sealants, An Applicator Training Program,” which
culmination of shared ideas, experience, and technical knowledge of liquid
sealant contractors, manufacturers, and scientists over the course of
fifteen years and three publication revisions. Its purpose is to “facilitate
quality applicator training and understanding of the liquid sealant
The manual covers the process of installing liquid sealants in a manner that
is likely to ensure that they will perform as intended. In summary, eight
steps are required in ensuring successful liquid sealant performance. They
are: 1) Joint Preparation, 2) Joint Taping, 3) Priming, 4) Sealant Backing,
5) Mixing a Sealant, 6) Applying a Sealant, 7) Tooling, and 8) Testing
Step 1-Joint Preparation, is
cited as “a leading cause of joint failure.”
Joint width, irregularity of joint size, defects in the joint face or edges,
and cavities in the joint face caused by mullion design, begin a long list
of joint preparation conditions. Protrusion of window or stone setting
shims into the joint area, available depth of joint face, temperature of
substrate, effect of temperature on joint size and the subsequent
implications on movement capability of the installed sealant and joint
cleaning are among the factors that must be addressed by designers and in
the field by the mechanic at the time of sealant placement.
It is also clear that there are many aspects of
a number of the steps in successful liquid sealant installation on which
there is not general agreement. For example, Step 3-Priming, is an actively
debated issue. Whether or not primer is needed on all substrates or just
non-porous substrates; whether to prime before or after installation of
backing materials; what application device to use; and determination of
appropriate primer thickness followed by assurance that the entire sealing
surface has been properly covered, are among the contentious aspects of this
one of eight installation steps.
In the world of adhesives (into which liquid building sealants fall), a
fundamental concept is that adhesives are at their best when used in
conditions where the basic stresses in the material are shear stresses and
not tensile or ‘peel” stresses. This fundamentally desirable feature is
missing in a typical construction sealant joint when the joint is opening.
To mitigate tensile stresses at the bond line, the liquid sealant is tooled
against a rounded backing material in order to reduce the seal thickness at
its center and maximize sealant contact area at the bond line.
The negative effect of these
tensile stresses in moving joints is aggravated by installation of the
liquid sealant in a geometry other than this hourglass configuration
(see Figure 2). Alteration of the geometry as
the result of improperly installed backing material combined with changes in
the joint size and sealant-material state as the result of movement in the
joint prior to full cure, further limits the functionality of the finished
Figure 2: Effect of positioning and sizing
of backing material and correct shaping of sealant on system performance
during joint opening.
Impregnated-foam sealants historically utilized
asphalt-based or paraffin-wax-based impregnations. These impregnations
resulted in the possibility, under conditions of high temperature, of
staining of sensitive substrates (certain marbles and other natural
stones). Incompatibility with liquid sealants was also a limitation from
the standpoints of staining and achieving adhesion between the
impregnated-foams and liquid sealants.
Developments in recent years of modified,
water-based asphalt impregnations, combined with developments for asphalt
compatibility in liquid sealant technology, have converged to allow the
development of asphalt impregnated-foam sealant and silicone hybrid
sealants. However, it was the shift to acrylics from asphalt and wax-based
impregnations in the1980’s that precipitated early hybrid sealant
Acrylic impregnations are water-based, do not
bleed, and are free of any components that stain substrate materials or
liquid sealants with which the acrylics come in direct contact. Acrylic
impregnated-foam sealants have been tested for chemical compatibility and
adhesion by many manufacturers of liquid sealants. These tests, as well as
those of hybrid sealant developers proved chemical compatibility with a wide
range of liquid sealants, and demonstrated tenacious adhesion between
acrylic-impregnated-foam sealants and certain liquid sealants, thus allowing
the development of hybrids to begin.
Another limitation of impregnated-foam sealants
concerns color selection. Asphalt, the most extensively used impregnation
base for many years, resulted in the only color choice being black.
Paraffin wax, and even the advent of acrylic-based impregnations, barely
broadened the color selection to include dark grey.
As with any sealant intended for moving joints,
joint preparation and sizing imposes limitations on successful performance.
Impregnated-foam sealants require similar attention to joint face
cleanliness as liquid sealants. However because they utilize compression in
combination with adhesion for their performance, impregnated-foam sealants
are less susceptible than liquid sealants to improperly prepared or moist
Because impregnated-foam sealants exert a
backpressure, it is important that joint faces be parallel so as to
eliminate the possibility of the foam jacking itself out of a wedge-shaped
Impregnated-foam sealants, like liquid
sealants, require careful attention to sizing. Material must be sized to
match changes in joint size in order to ensure that it remains at a level of
compressed density to allow it to resist water penetration.
Composition of Hybrids
Figure 3 shows acrylic-impregnated-foam sealant
combined with factory-applied ultra-low modulus silicone liquid sealant in
the form of a bellows.
The key property in liquid sealant performance
as a component of a hybrid sealant is modulus. Development trials revealed
that in order to consistently form a regular bellows shape after
compression, ultra-low modulus (in the range for 15 Shore A) was required.
comparison with polyurethane chemistries, high-performance,
ultra-low-modulus silicones have been shown to better retain their modulus
properties over time as well as over temperature change.
“Results indicate that polyurethane sealants show a significant increase in
modulus under cold conditions whereas silicone sealants show relatively
minor modulus change over a very broad temperature range.”
The hybrid is produced by partially compressing
the impregnated-foam. The silicone coating is then applied to the partially
compressed impregnated-foam in a uniform thickness after which it is cured
under controlled conditions free of dirt, temperature change and movement.
Once the silicone coating has cured, the composite material is compressed to
an installation dimension comfortably less than the field-measured joint
size. It is held in this pre-compressed state by its packaging until
immediately prior to insertion into the intended joint (see Photograph 1).
The result is a composite material that makes
the best possible use of the benefits of the two sealant material components
while eliminating the disadvantages of both.
Photograph 1: Hybrid
silicone/impregnated-foam sealant inserted into joint.
The hybrid is secured to joint faces in three
ways: 1) compression, 2) adhesion, and 3) through application of a fillet
bead of liquid sealant.
The stored strain energy of compression in the foam backing results in a
backpressure against the substrates.
of impregnation to substrates: The impregnation is an adhesive which, under
the pressure of compression exerted by the foam backing, bonds the entire
contact area of the foam to the substrate
Bead of Liquid Sealant: After installation of the hybrid into the joint,
and after the hybrid has expanded firmly against the substrates, a fillet
bead of liquid sealant is tooled between the substrate and the pre-cured
silicone bellows. The size of the fillet beads is determined by the size of
the bellows, which is in turn a function of the size of the joint being
filled. In general the bonding contact area of the fillet bead will vary
from approximately 3mm (1/8-inches) to 6mm (1/4-inches) depending on the
size of the silicone bellows.
The consistent use of fillet
beads is a development of the last six to eight years of hybrid installation
and the consequence of a broadening of applications for which hybrids have
proven themselves. In prior years hybrids were installed without fillet
beads. This practice was based on independent testing of the early hybrids’
abilities to resist water penetration under accelerated weatherometer
testing. Two thousand hours of accelerated weathering according to the ASTM
G26-77 standard were performed. The testing concluded that the seal “did
not deteriorate under the 2000 hrs of weathering and provided a continuous
Inspection of 10-year old
installations confirms that the fillet bead is not required to ensure the
performance of hybrid seals.
However the hybrid sealant’s increasing versatility in addressing larger,
more dynamic-movement joints (such as in seismic joints) resulted in the
precautionary requirement of installing fillet beads. For purposes of
consistency in installation, and to eliminate the need for users to know
when and when not to install fillet beads, the application of fillet beads
has become standard practice.
Figure 3: Composition of
silicone/impregnated-foam hybrid seal.
The opening and closing movement of the joint
(see Figure 4) results in the surface sealant folding and unfolding (rather
than stretching and compressing) thereby essentially eliminating substrate
bond-line stresses and failure or composition changes resulting from
pre-cure joint movements.
Figure 4: Hybrid sealant in extension.
Silicone bellows unfolds essentially free of tension either at the bond line
or within the material.
Installation involves removal of the sealant
from the hardboard and shrink-wrap packaging that holds it compressed to
less than the joint size. The sealant is inserted joint opening recessed to
the desired depth but at least deep enough to accommodate a fillet-bead of
sealant applied later. A pressure-sensitive mounting adhesive on one face
holds the material in space while it slowly expands to fill the joint. A
fillet-bead of liquid silicone locks the bellows to the substrate. The
fillet-bead, while field applied, is never in tension as in a conventional
liquid-sealant-and-backer-rod installation and is a redundant measure in
ensuring that the bellows is sealed to the substrates.
The result is the installation of a system
proved by independent testing to the standards of ASTM G26-77,
and ASTM E330
as well as by field observation of in-place installations as old as ten
essentially free of bond line tensile stresses—The
impregnated-foam backing is in compression while providing support for the
silicone bellows. The field-applied fillet-bead at the silicone-to-substrate
interface is not in tension, and therefore does not suffer tensile stress as
do conventionally installed liquid sealants during joint opening.
essentially free of tensile stresses within the silicone bellows material—The
base material of the outer skin component is silicone sealant, but because
it is in the form of a bellows as part of the hybrid, it maintains a seal as
it moves through the gathering and releasing of folds in the bellows. The
silicone material, while experiencing minuscule bending stresses at the
folds, remains virtually unstressed through joint opening and closing
anchored positively by 3 means: its
mechanical backpressure; the pressure-sensitive adhesion of the impregnation
agent; and by the silicone fillet bead
anchored non-invasively without
drilling—The performance and suitability of impregnated-foam and silicone
sealant hybrids for sealing large movement joints (as large as 250mm
(10-inches)) make hybrids an alternative to extruded rubber and metal rail
strip seal systems widely used for these applications. In contrast to strip
seals which must be mechanically anchored to substrates after first drilling
pilot-holes, the hybrid sealants are secured non-invasively. This feature
offers significant advantages in sealing large joints at property lines, on
historic structures, and in applications such as at inside corner elevations
where access for drilling and anchoring is obstructed.
spalling of the substrate—In
consequence of its backpressure, hybrid sealants do not put undue stress on
less sturdy substrates such as exterior insulation and finish systems.
7) Resists the
effects of air-pressure differentials—In
consequence of its compressed density and secure attachment to the
substrates into which it is installed, the hybrid sealant system is capable
of resisting the forces of air pressure differentials applied to either its
positive or negative faces. Proof through testing of the system’s ability
to prevent water infiltration has been demonstrated by successful system
performance under the following standards: ASTM E283 – Rate of air leakage
through curtain walls;
ASTM E331-Water penetration of curtain walls by uniform static air pressure
and ASTM E330 – Structural performance of curtain walls by uniform static
air pressure difference.
insulates--With an R-value of
approximately 1.29cm (3.28 per inch) of depth, which varies from 30mm (1
¼-inches) to 150mm (6-inches) depending on joint size, the hybrid sealant
offers thermal insulation at joints.
difficult to vandalize—As
the result of combining two seals into one system, damage to the silicone
facing does not mean water will infiltrate the joint being sealed.
As part of weatherometer testing
to ASTM G26-77, several of the samples being tested were removed from the
weatherometer at the 810-hour mark.
The silicone coating was
punctured in numerous locations with a steel pin to a depth of 12mm
(1/2-inch) and testing was resumed. At 1500 hours, slicing longitudinal
cuts in the silicone bellows with a sharp knife further intentionally
damaged punctured samples. In addition, on one sample, a portion of the
silicone approximately 6mm x 6mm (1/4-inch x 1/4 –inch) was removed entirely
and weatherometer testing (light and water spray) was resumed. Observation
and moisture probe readings confirmed, “the intentional damage of the
primary RTV silicone seal did not change the performance characteristics of
the [seal] and a continuous watertight seal was maintained.”
cost-effective on an installed-cost basis—
Labor costs are generally the largest component of waterproofing costs. In
consequence of being preformed and shipped to the site pre-compressed to
less than the joint size, and because of its non-invasive and simple
installation process, hybrid sealants are quickly and efficiently installed.
cost-effective on a long-term performance basis—Because
of their excellent resistance to UV and other deleterious environmental
factors, silicones are offered by their manufacturers with warranties up to
twenty years. The hybrid removes from the use of silicones tensile stresses
that are the remaining cause of premature silicone failure. Consequently,
and as has been proved by over a decade of successful performance in the
field, the hybrid offers long-term performance advantages even in rigorous
large-and-high movement joints.
The enclosed chart (Figure 5) summarizes the
merits and shortcomings of both basic kinds of sealant and the advantages of
the hybrid sealant composite produced by combining the material types.
Figure 5: Summary of advantages and
limitations of liquid and impregnated-foam sealants and the resulting
combination of advantages in a silicone
liquid-sealant and impregnated-foam hybrid.
Design Considerations in the Use of Silicone
and Impregnated-Foam Hybrids
Depth of substrate:
Because the impregnated-foam backing of the
hybrid contains the stored strain energy of compression, the depth of the
foam is important in maintaining the hybrid’s stability and preventing
bowing of material. As joint sizes increase so must the depth of the foam.
Historically, impregnated-foam sealant manufacturers required a depth of
seal that was twice the joint width, example: if a joint measures 25mm
(1-inch) then the depth of the impregnated-foam seal was 50mm (2-inches).
Although it varies among manufacturers, advances in manufacturing technique
have allowed depth to be reduced to approximately one and a quarter to one
and a half times joint width. Nevertheless, it must be assured that
sufficient depth of substrate is available to properly support the full
depth of the hybrid.
While not generally a limitation because of the strength and rigidity of
most substrate materials, joint design must ensure that substrates are
capable of resisting without deflection the backpressure exerted by the
impregnated-foam backing of the hybrid. The backpressure is known, in the
instance of one manufacturer to be approximately 17 kPa (2.5 psi) at nominal
This means, for example, that in a joint that measures 50mm (2-inches) to be
sealed with a 60mm (2 ½-inch) deep hybrid sealant, 75 pounds of force per
foot of joint length will be exerted on the substrates.
Hybrid sealants are currently available in lengths of approximately 2m (6.56
feet). This is both an advantage and a limitation. The advantage lies in
the ability of manufacturers to custom produce material in varying widths
for installation into joints that taper as the consequence of construction
tolerance buildup or construction error. The supply of sizes to suit the
taper ensures that the joint is sealed with material with sufficient
movement capability to handle expected movement from thermal cycling, sway,
seismic activity, etc.
Joining of lengths in straight runs is
accomplished by butt joining. Joining to follow
architectural contours and changes in plane and direction is achieved by
various joinery techniques to suit the condition (see Photograph 2). At all
joins the silicone bellows is joined through the application of a light wipe
of liquid silicone on the faces to be joined. To ensure color matching,
silicone from the same batch as that from which the bellows was produced is
supplied for joining.
The limitation of joining is
largely one of aesthetics. Depending on the skill and diligence of the
installer, and from close-up viewing distances, the joins can be visible.
However the efficacy of the joins in preventing moisture ingress and in
handling movement is well proved in laboratory
as well as field conditions, and to the casual observer the aesthetics are
not likely to be an issue.
Photograph 2: Silicone
liquid-sealant and impregnated-foam hybrid in inside-corner, outside-radius
in dissimilar metal-panel and masonry substrates.
Hybrid sealants available today perform in many
1) Movement joints
(structural, expansion, seismic, settlement, etc.)
2) Large joints and
small joint over 8mm (3/8-inch) to 305mm (12-inch)
3) Where resilience or
the need to resist air-pressure and thermal differentials is essential
Anywhere a structural
or new-to-existing gap needs filling and sealing.
As the primary and secondary sealants in structural joints in air-barrier
wall design in the facade and structural
Because of their non-invasive anchoring,
watertightness, color choice, and wide range of essentially tensionless
movement, the hybrid bellows sealants outperform liquid sealants or
traditional impregnated-foam sealants alone; and in terms of ease and cost
of installation they excel over extruded-rubber compression seals and
particularly combination metal rail and rubber gland “strip seals”.
Small-size hybrids for mass production and for
use in window and panel perimeters are under development and promise to make
their use as cost-effective as current liquid-sealant and backer-rod
options. Under development throughout the world, other hybrids include:
1) Combinations of
chemically-resistant liquid sealants and impregnated-foam sealants for use
in wastewater, caustic and other harsh environments;
2) Combinations of
materials to provide
fully fire-rated, watertight movement joints;
3) Combinations of
chemical resistant coatings and impregnated-foam sealants to
handle below-grade and