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Evaluating Directly Bonded Stucco

May 15, 2012

EVALUATING DIRECTLY BONDED STUCCO
BRIAN LEE COPE, PE
WISS, JANNEY, ELSTNER ASSOCIATES, INC.
2915 Premiere Parkway, Ste. 100, Duluth, GA 30097
Phone: 770-923-9822 • Fax: 770-232-9044 • E-mail: lcope@wje.com
JOHN FRACZEK, PHD, PE, SE
WISS, JANNEY, ELSTNER ASSOCIATES, INC.
330 Pfingsten Road, Northbrook, IL 60062
Phone: 847-272-7400 • Fax: 847-291-9599 • E-mail: jfraczek@wje.com
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ABSTRACT
Wiss, Janney, Elstner Associates, Inc. (WJE) has evaluated several high-rise buildings
clad with stucco applied directly to concrete or concrete masonry in coastal environments.
Large areas of stucco delamination or poor bond between the stucco and the substrate were
frequent concerns with these buildings. WJE observed several conditions that caused these
issues, including substrate preparation, placement of stucco accessories, and installation
techniques. The authors will discuss the significance of delaminated stucco and the conditions
that affect the bond. They will also discuss field and laboratory test methods (destructive
and nondestructive) used to evaluate the conditions and present quantitative values for
the bond strength for consideration.
SPEAKERS
BRIAN LEE COPE, PE — WISS, JANNEY, ELSTNER ASSOCIATES, INC. DULUTH,
GA
LEE COPE, PE, is a licensed professional engineer who has extensive experience in
detailing and proper installation of Portland cement plaster (stucco) façade systems, air barriers,
window systems, waterproofing, and the interfaces of various envelope components.
He has evaluated a variety of structures relating to the cause and/or distress of the buildings’
exterior façade/curtain wall systems and building envelope systems. He is a voting
member of ASTM Committee C11 on Gypsum and Related Building Materials and Systems.
JOHN FRACZEK, PHD, PE, SE — WISS, JANNEY, ELSTNER ASSOCIATES, INC. NORTHBROOK,
IL
JOHN FRACZEK, PhD, PE, SE, is a licensed professional and structural engineer
engaged in the resolution of design and construction-related structural and materials problems.
He has investigated low-strength concrete problems and various other concrete materials
issues. Dr. Fraczek is an established author and previously served as a principal
speaker in a nationwide seminar series on concrete repair and restoration. He has presented
numerous lectures on structural failures, repair techniques, materials performance, and
nondestructive testing. His professional affiliations include the American Concrete Institute
(ACI), the Precast/Prestressed Concrete Institute (PCI), and the Structural Engineers
Association of Illinois (SEAOI).
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Why Stucco Can Be Potentially
Problematic for High-Rise Construction
ACI Tolerances for Formed-Concrete
Surfaces
EVALUATING DIRECTLY BONDED STUCCO
INTRODUCTION
There are hundreds of high-rise structures
in Florida and other coastal regions
that are clad with directly applied Portland
cement plaster (stucco). From a brief observation,
the stucco façade appears to be in
good condition. However, a close inspection
of virtually any of these structures will likely
reveal regions of delaminated stucco.
Therefore, using directly applied stucco as
the cladding of a high-rise structure can
potentially be problematic and lead to costly
repairs.
In the last several years, Wiss, Janney,
Elstner Associates, Inc. (WJE) has evaluated
several buildings in coastal environments
that are clad with stucco and has
found large areas of delamination or poor
bond between the stucco and the substrate.
This paper discusses the significance of
delaminated stucco and the conditions we
have found that affect the bond strength,
including substrate preparation, placement
of stucco accessories, and installation techniques.
Furthermore, WJE’s research into
these bond failures has revealed a significant
lack of quantitative requirements for
the bond strength of directly applied stucco.
ASTM C926, Standard Specification for
Application of Portland CementBased
Plaster, provides guidelines for the application
of directly applied plaster to solid
bases, including requirements to achieve an
adequate bond between the plaster and
substrate, but does not provide specific
requirements to evaluate the bond nor the
minimum bond strength necessary.
Therefore, this paper also discusses field
and laboratory test methods (both destructive
and nondestructive) that WJE has used
to evaluate bond conditions and presents
quantitative values for necessary bond
strength for consideration.
Most high-rise structures that are clad
with directly applied stucco consist of a
cast-in-place concrete frame with concrete
masonry unit (CMU) infill walls. Table 1 in
ASTM C926 specifies the total nominal
thickness for two-coat and three-coat systems
over solid substrates. For a three-coat
system, the total nominal stucco thickness
over unit masonry or cast-in-place concrete
is 5/8 in. For a two-coat system, the total
nominal stucco thickness over unit masonry
is 1/2 in., and 3/8 in. over cast-in-place
concrete.
Since the stucco is directly applied to
the substrate, there is no weather barrier
behind the stucco. Therefore, for these
structures, the stucco itself is intended to
function as a water barrier. For it to function
as a water barrier, it is critical that the
stucco be properly placed and bonded to its
substrate. However, if proper bond is not
achieved, directly applied stucco can be
problematic, and delamination and cracking
will likely occur. The delaminations can
occur between the stucco and its substrate
and/or between the various layers (lifts) in
the stucco.
When delaminations occur in the stucco,
the stucco becomes unsupported within
the delaminated regions. When the stucco
becomes unsupported, it cannot transfer
thermal or wind loads to its substrate and
cracking typically occurs in the stucco. The
cracking can allow moisture to migrate
through the stucco and possibly into the
building. From performing a number of
investigations over the last several years,
WJE has observed several conditions that
affect the bond between the various layers
in the stucco or between the stucco and the
substrate. These conditions are as follows:
• American Concrete Institute (ACI)
tolerances for formed-concrete surfaces
do not correspond to the tolerances
for solid substrates to receive
stucco provided in ASTM C926.
• Poor surface preparation
• Poor placement of stucco
• Build-out materials
• Stucco accessories
• Incompatible substrates
One of the main conditions that affects
stucco installation on a high-rise structure
is access to the exterior façade. For highrise
construction, access to the façade is
typically obtained by using swing stages,
mast climbers, or a combination thereof.
Therefore, access is limited and can contribute
to the conditions that cause delamination
in stucco. The limited access likely
results in poor supervision between the
general contractor and the stucco subcontractor
and likely contributes to the conditions
related to poor surface preparation,
poor placement, and poor curing.
The following discuss each of these conditions
in detail and summarize their effects
on the bond between the stucco and the
substrate and between the various stucco
layers.
ACI tolerance for formed-concrete surfaces
are not consistent with the tolerances
for solid substrates to receive stucco provided
in ASTM C926. When constructing a
high-rise structure that consists of a castin-
place concrete frame, the tolerances for
plumbness and horizontal variances are
controlled by ACI 117, Specification for
Tolerances for Concrete Construction and
Materials. Section 4.1 of ACI 117 provides
plumbness requirements for cast-in-place
concrete building heights less than or equal
to 83 ft., 4 in. The permissible out-of-plumb
dimension is the lesser of 0.3% of the total
height or 1 in. Therefore, for a building with
a 10-ft. floor-to-floor height, the allowed
out-of-plumbness in a building surface
between the top and bottom of the floor is
0.36 in., or approximately 3/8 in. Similarly,
the maximum allowed tolerance over two
stories is 0.72 in., or approximately 3/4 in.,
and 1 in. for three or more stories.
Section 4.2 of ACI 117 provides tolerances
for horizontal variances in the vertical
edge of an opening in a floor, wall, or beam.
The permissible out-of-plumb dimension for
floor edges, beams, or walls is +/-1 in.
Therefore, for a typical building with multiple
bays, the first bay is allowed to be outof-
variance by +1 in. compared to the specified
plan dimension, and the adjacent bay
is allowed to be out-of-variance by -1 in.,
which equates to an offset in the framed
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Poor Surface Preparation
floor edges of 2 in. across the two bays.
When placing stucco on a high-rise
structure that consists of a cast-in-place
concrete frame with CMU infill walls, the
columns, edges of floors, and block walls
are to be within ¼ in. of the same vertical
plane. This tolerance is based on Section
5.2 of ASTM C926, Standard Specification
for Application of Portland CementBased
Plaster, which requires the substrate to be
“straight and true within 1/4 inch in 10
feet.” Since the stucco is intended to be relatively
thin (3/8 to 5/8 in., depending on
the substrate), if the concrete frame is out
of plumb or if its horizontal variances
exceed ¼ in. in 10 ft., it is almost guaranteed
that the thickness of the stucco will
exceed the nominal thicknesses specified in
ASTM C926.
Section 5.2.3 of ASTM C926 states that
“where total plaster thickness will exceed
the total thickness specified in Table 1 for
types of solid bases, furred or self-furring
metal plaster base shall be installed in
accordance with Specification C 1063”.
However, when a furred or self-furring
metal plaster base (lath) is used, the total
nominal thickness in Table 1 (ASTM C 926)
is 7/8 in. However, whether or not a metal
plaster base is used, depending on the
plumbness and horizontal variances of the
concrete frame, the total thickness of the
stucco can easily exceed 7/8 in. and typically
does. In our experience, when investigating
façades that consist of directly
applied stucco, the stucco is typically
always thicker than the nominal thickness
provided by ASTM C926. In most cases, we
have measured the stucco to be at least ¾-
in. thick, with some measurements extending
up to 3 in. In these cases, the stucco is
placed in several thin layers, which increases
the possibility of a delamination occurring
between the various coats. Figure 1
illustrates an example in which the thickness
of the stucco on a project was measured
to be 2 in.
Therefore, for high-rise construction,
due to less stringent tolerances for the concrete
frame, it is very likely that the stucco
will be thicker than the required nominal
thicknesses specified in ASTM C926, and
sometimes substantially so. This condition
in itself can be problematic. However,
excessively thick stucco combined with
poor surface preparation, poor placement of
the stucco, improper build-out materials,
improperly installed stucco accessories,
and incompatible substrates will exacerbate
the potential for poor bond in the stucco
and can result in very costly repairs.
As mentioned above, for directly applied
stucco to perform as a proper cladding system
for high-rise construction, the stucco
must be properly bonded to its substrate.
To help ensure there is proper bond
between the stucco and the cast-in-place
concrete frame and CMU infill walls,
Section 5 of ASTM C926 provides requirements
for solid bases such as masonry,
stone, cast-in-place, or precast concrete to
receive stucco. These requirements discuss
substrate preparation. Section 5.2 states
that solid surfaces to receive plaster “shall
be free of…oil or other elements which
would interfere with bonding.” Section 5.2.1
states that “solid surfaces shall have the
suction (ability to absorb water) or have
surface roughness, or both, to provide the
bond required for the plaster.”
Luckily, CMU provides an adequate
substrate for proper bond. Therefore, CMU
infill walls typically have the ability to
absorb water and have a surface roughness
that requires little surface preparation.
However, since the cast-in-place concrete
frame consists of a smooth, nonabsorbent
surface, proper surface preparation is
required to help ensure there is proper
bond between the stucco and the concrete
frame. Section 5.2.2 provides guidelines on
how to prepare the concrete surface. It
states that “smooth or nonabsorbent solid
surfaces, such as cast-in-place or precast
concrete, shall be prepared to receive
Portland cement plaster by one of the following
methods:”
• Section 5.2.2.1, “Sandblasting, wire
brushing, acid etching, or chipping,
or a combination thereof,”
• Section 5.2.2.2, “Application of a
dash bond coat applied forcefully
against the surface, left untroweled,
undisturbed, and moist-cured for at
least 24 hours,” or
• Section 5.2.2.3, “Application of a
bonding compound suitable for exterior-
or interior-exposure solid surfaces
in accordance with manufacturer’s
written directions.”
In addition to these methods, Section
5.2.3 states that where bond cannot be
obtained over the entire surface to receive
plaster by one of the methods discussed
above, “furred or self-furring metal plaster
base shall be installed in accordance with
Specification C 1063.”
While ASTM C926 is clear on how to
prepare a solid substrate, we typically find
that the concrete frame has not been properly
prepared, and the requirements in
ASTM C926 are often ignored (Figures 2 and
Figure 1 – View of stucco that is approximately 2 inches thick. Note: arrow
indicates void between stucco and substrate.
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Poor Placement of Stucco


Figure 3 – An exploratory obtained over the
opening in the stucco façade entire surface to
revealing a smooth cast-in- receive plaster.
place concrete substrate. (Note While WJE typicalthat
the stained area is “liquid ly does not find
nails used to attach the furred or self-furcorner
bead and not the ring metal plaster
bases used in
coastal environments,
we have
remnants of a bonding agent.)
Figure 2 – View of an
exploratory opening in the observed usage of
stucco façade revealing a PVC lath or nylon
smooth cast-in-place scrim, typically
concrete substrate. over floor lines. At
these locations,
the lath or scrim
spans the joint
between the CMU
infill walls and the
concrete floor slab
WJE has seen delaminations at the bond in an attempt to control cracking between
line between the various coats in the stuc- the two different substrates. From inspectco.
WJE attributes the poor bond between ing the stucco at these areas, it often
the various coats to poor workmanship sounds hollow when tapped with a sound-
(improper keying of the various stucco lay- ing hammer, indicating the presence of a
ers together and poor curing) and lazy possible delamination. However, on creating
3). From creating openings in the façade
after locating delaminations, we typically
find that the concrete frame is generally
smooth (nonabsorbent) and free of bonding
agents. Therefore, poor surface preparation
is a condition that can result in delamination
between the stucco and its substrate.
Another source of delamination in
directly applied stucco is poor application of
the stucco. As previously mentioned, directly
applied stucco is intended to be 3/8- to
5/8-in. thick, depending on its substrate
and whether it is a two-coat or a threecoat
system. The stucco should also be
fully compressed and bonded throughout
its thickness. In addition to a poor bond
due to an improperly prepared substrate,
installation (poor compression during the
stucco placement). This condition is exacerbated
when the thickness of the stucco
exceeds the specified nominal thicknesses
provided by ASTM C926 and when there are
multiple (more than three) coats present. It
is also exacerbated by delays between the
applications of the various coats.
As noted above, ASTM C926 states that
a furred or self-furring metal plaster base is
to be used when the stucco thickness
exceeds the nominal thicknesses provided
in ASTM C926 or when bond cannot be
Figure 5 – Void between backside
of stucco and cast-in-place
substrate due to lath s not being
fully encapsulated in the stucco.
Figure 4 – View of poor key between
the PVC lath and stucco.
exploratory openings to verify the delamination,
the apparent delamination is often
related to a void that is created between the
back of the PVC lath and the cast-in-place
concrete substrate (Figures 4 and 5). When
lath is used, the lath should be fully submerged
in the base or scratch coat. Section
7.2.1 of ASTM C926 states that the “first
coat shall be applied with sufficient material
and pressure to form full keys through,
and to embed the metal base, and with sufficient
thickness of material over the metal
to allow for scoring the surface.”
Another problem that is typically uncovered
with the use of PVC lath is improper
fasteners. From creating exploratory openings,
we often observe the PVC lath to be
anchored to the concrete substrate with
stub nails that are 3/8-in. long (Figure 6).
Figure 6 – View of 3/8-in.-long stub
nails used to attach PVC lath.
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Build-Out Materials
Stucco Accessories
Incompatible Substrates
Section 6.7.1.1 of ASTM C1063, Standard
Specification for Installation of Lathing and
Furring to Receive Interior and Exterior
Portland CementBased
Plaster, states that
“nails for attaching metal plaster bases to
solid substrates shall be not less than 3/4-
in. long.” Therefore, to help ensure that the
lath is properly anchored, the nails should
Figure 7 – Improper buildout
material after the
delaminated stucco was
removed.
Figure 8 – Close-up view of
improper build-out
materials with excessive
cracking associated with
expansion due to moisture.
be 3/4-in. long in lieu of 3/8-in., even
though PVC lath is used in lieu of metal
lath.
Rather than use stucco, contractors
often apply build-out material to the concrete
frame and CMU infill walls in order to
create a straight and plumb substrate for
the stucco. This is typically done when the
Figure 9 – View of vinyl corner bead
that is poorly encapsulated in the
stucco.
Figure 10 – Close-up view of
3-in. corner bead with
virtually no encapsulation.
concrete and CMU variances are so great
that very excessive (1½ in. or more) stucco
thicknesses will result. However, from our
experience, the build-out material is typically
applied directly to the concrete or CMU
substrate, and relatively thick sections are
not properly anchored or reinforced.
Build-out materials are typically proprietary,
prepackaged concrete repair mortars
intended for vertical and overhead trowel
application. As such, they contain additives
and/or admixtures to provide a
nonsag consistency and relatively rapid
set. We have found some of these materials
to be volumetrically unstable in moist
environments, resulting in delaminations
at the concrete/CMU or the stucco interface
(Figures 7 and 8).
In coastal environments, the stucco
accessories (control joints, corner beads,
etc.) typically consist of vinyl. While relatively
small accessories (e.g., 1-in. corner
beads) have relatively large openings that
permit encapsulating the accessory in stucco,
relatively large accessories (e.g., 3-in.
corner beads) are, for the most part, solid,
with relatively small holes to provide sufficient
stiffness for the accessory to remain in
position during application of the stucco.
This makes it very difficult for a contractor
to fully encapsulate the accessory in the
stucco (Figures 9 and 10). Therefore, corner
beads and other accessories are often
incompletely filled with stucco, resulting in
paths for moisture within the stucco
cladding and flexing of the accessory due to
temperature change. The flexing typically
results in cracking of the stucco.
A bonding agent can be applied to help
improve the bond between the stucco and
its substrate. However, if not properly
installed and if the stucco is not placed in a
timely manner, the bonding agent can set
prematurely and act as a bond breaker
instead of a bonding agent (Figure 11). This
is particularly true of epoxy-based bonding
agents. Bonding agents based on polyvinyl
acetate may reemulsify in a moist, high-pH
environment and serve to delaminate the
stucco from its substrate if moisture penetrates
to the substrate.
Figure 11 – View of stucco
that was poorly bonded
due to a bonding agent
that set prematurely.
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ASTM C926 provides methods for substrate
surface preparation and stucco application
to achieve proper bond. However, it
does not define proper bond. In addition,
there is currently no standard available that
defines the proper testing of the bond
between stucco and its substrate. To help
ensure that stucco is properly bonded to its
substrate, it is our opinion that a standard
should be developed and acceptable bond
strength levels should be defined.
To measure the bond strength between
directly applied stucco and its substrate,
WJE has performed direct tension tests on
several projects where bond was a concern.
The tension test we typically employ utilizes
a reaction frame, a load cell, and a 3- by 3-
in. square plate (Figures 12 and 13). The
stucco can be cut with a diamond blade on
a grinder to minimize stresses at the bond
line associated with drilling cores. In addition,
the plate must be sufficiently stiff to
resist flexing during the test. In performing
these tests, we have measured various bond
strengths ranging from 0 to 5 psi, to well
over 100 psi. It is our opinion that bond
strengths of 50 psi or more represent good
bond. It is also our opinion that bond
strengths of less than 30 psi are likely
indicative of poor surface preparation
and/or poor stucco installation.
Petrographic examination is another
method WJE has used to evaluate the
quality of stucco bond. This method can
assist in determining locations of poor
bond between the various layers in the
stucco and can help determine the overall
quality of the stucco (Figure 14).
CONCLUSIONS
If poor bond strength exists between
directly applied stucco and its substrate,
the performance of the stucco in service can
be problematic and lead to costly repairs. To
help ensure adequate bond between the
stucco and its substrate, we recommend
that the construction team (general contractor,
concrete forming subcontractor,
stucco subcontractor, architect, and structural
engineer) be qualified and aware of the
potential problems that can occur during
the application of directly applied stucco.
Therefore, the team should
• Be aware of the potential downfalls
with regard to building access, particularly
as it affects substrate
preparation and stucco application
and curing.
• Be aware of the varying tolerance
between ACI and ASTM C926 for
concrete construction.
• Ensure that the surface of the substrate
is properly prepared.
• Ensure that the stucco is properly
installed and cured.
• If build-out material is used, ensure
that the build-out material is suitable,
and ensure that it is properly
anchored and reinforced.
• If bonding agents are used, ensure
that they are of the appropriate type
and installed properly.
• Ensure that stucco accessories are
properly installed and that the
accessory is properly encapsulated
in the stucco.
We also recommend that the bond
strength of the stucco be tested during the
mock-up phase and during construction, as
a method of quality control. The testing
could include direct tension-bond testing
and petrographic examination.
Figure 12 – View of direct tension
test apparatus.
Figure 13 – View of stucco
sample after direct tension test.
Figure 14 – View of stucco sample
that has been subjected to
petrographic examination,
identifying various lifts, voids,
and potential poor bond between
layers.
What Is Proper Bond?
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