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Facing The Challenges Of Plaza Deck Waterproofing Rehabilitation

May 15, 2006

INTRODUCTION
Rehabilitation of older plaza decks over
occupied spaces poses several challenges.
Typically, plaza decks that are subject to
rehabilitation were constructed years ago
and do not include drainage composite or
any provisions for proper subsurface
drainage slope at the waterproofing membrane
layer. Correcting these deficiencies
can result in increased system thickness
and additional dead loads, making the
rehabilitation a challenge.
In addition, every plaza
project has unique challenges
that can be related
to design or construction.
Typical Challenges
Rehabilitation of existing
plaza decks poses several
challenges that can
typically be divided into
two categories – design
and installation challenges.
Design Challenges
When tasked with
design of a new waterproofing
system for a
plaza deck, a designer is
typically forced to address
several issues. Many of
the issues are commonly
related to anticipating the
condition of the existing building components
and installation details as well as limitations
posed by the its construction. These
issues include:
• The condition of the concrete deck.
• Drainage slope.
• Overall system thickness and flashing
heights at boundary conditions.
• Structural capacity of the slab.
• Conditions that do not lend themselves
to proper detailing.
• Selection of an appropriate assembly.
• Membrane selection.
Condition of the Existing Concrete Deck
Before designing a new waterproofing
system, a designer should become thoroughly
familiar with the condition of the
components that will impact performance of
the new waterproofing system. For example,
if the waterproofing system has been leaking
for an extended period of time, it would
be reasonable to expect some reinforcing
steel corrosion in both the concrete deck
and planter walls (Photo 1).
While evaluation of the extent of corrosion
damage in many parking garage and
other exposed concrete decks is relatively
simple, it is very difficult in plaza decks.
This is due to the presence
of the waterproofing system
and the overlaying
materials that make it
impractical to examine the
upper deck surfaces. The
structural decks can
sometimes (not often) be
viewed from the bottom.
However, in most cases,
the deterioration begins at
the top surface of the concrete
deck. Therefore, a
relatively flawless soffit
surface on a plaza deck
does not necessarily indicate
that no corrosion
damage exists on the top
surface. In cases where
the entire soffit surface
can be examined, nondestructive
testing (i.e.,
half-cell potential, impact
echo, etc.) of the slab from
the soffit surfaces may yield some useful
results. But, such testing is typically expensive
and may not be practical in all cases.
Selective sounding of the structural slab
A U G U S T 2006 I N T E R FA C E • 1 3
Photo 1: Extensive deterioration of planter walls due to corrosion
of reinforcing steel is a common phenomenon.
top surfaces at exploratory openings is a
practical method of obtaining information
at a limited number of locations. Since
exploratory openings are almost always
needed to determine the deck slope and
system configuration, it is typically beneficial
to carefully examine the concrete deck
surfaces at the exploratory openings.
Designers should note, however, that such
openings represent only a small area of the
structural deck, and extrapolation from
exploratory openings findings should only
be relied upon to obtain an “order-of-magnitude”
estimate on repair quantities.
In cold climates, the designer should
also consider the potential for freeze-thaw
damage of the structural slab, particularly
along the outer edges of non-insulated
decks.
Ultimately, the actual condition of the
structural deck and the extent of deterioration
(if any) will not be known with certainty
until construction. As such, the design
documents should anticipate various types
of repairs that may be needed to remedy the
structural deck condition, and unit prices
for such repairs should be incorporated into
the contract documents.
Unfortunately, repair of the concrete
decks typically causes substantial changes
in the construction schedule and can sometimes
require leaving the concrete deck
open for several weeks. (See the
“Installation” section for a discussion on
logistical requirements.)
Drainage Slope
One critical factor in evaluating existing
conditions is to determine the drainage
slope at the surface where the new waterproofing
membrane will be installed.
The 2003 version of the International
Building Code (and most prior model building
codes) requires that “roofs” have a minimum
slope of 1/4-in per foot. While one
may argue that these requirements may not
apply to waterproofing systems, it is my
opinion that, where possible, such slope
should be designed in new waterproofing
systems placed over existing plaza decks.
Good drainage slope will minimize water
ponding, reduce the rate of membrane deterioration,
minimize hydrostatic pressure on
membranes, and reduce the potential for
deterioration of other plaza deck components
from extensive exposure to moisture.
Given the critical nature of drainage, it
is important that slope of the existing concrete
deck be evaluated. Many older concrete
plaza decks have been constructed
with a level deck. This is due to the difficulty
of casting a concrete structural slab with
proper slope. Considering normal construction
tolerances, creep and elastic deflections
in concrete slabs, and normal building settlement,
it is not unusual to find that certain
areas of a structural plaza deck that
were intended to be level, vary in elevation
up to two or more inches. If the drains are
located at the high points of the slab, this
will cause excessive ponding.
In order to find the existing drainage
patterns on a plaza deck, exploratory openings
will be required. When selecting locations
of exploratory openings, one should
consider the anticipated drainage pattern
and select locations next to drains and
areas farthest from the drains. The elevation
measurements using conventional surveying
tools can yield relatively accurate
results regarding the contour of the structural
slab.
Once the existing elevations and
drainage slopes are determined, the designer
can assess the need for improving
drainage slope and consider various methods
to improve it. However, in many cases
involving older plaza decks, providing a
1/4-inch-per-foot drainage slope is not
practical, since the adjacent construction
such as doors and curtain walls are usually
placed near the surface of the existing
plaza finishes. Providing a 1/4-inch-perfoot
drainage slope may require that those
components be modified.
The various strategies for providing
drainage slope are well known in the industry.
These include the following:
1. Placement of a bonded and
tapered cementitious concrete
topping over the existing structural
deck. This option is only practical
when the additional dead loads
imposed by such topping can be
safely supported by the structural
deck and the additional thickness of
the topping can be accommodated at
boundary conditions (Photo 2).
2. Placement of tapered, rigid insulation
below the waterproofing system.
This option will limit the
designer in the choice of a waterproofing
membrane. If exercised, the
changes in condensation potential
in the plaza deck assembly should
be considered. In addition, the insulation
below the waterproofing membrane
should be carefully selected to
avoid compressive failure of the
insulation under plaza loads.
14 • I N T E R FA C E A U G U S T 2006
Photo 2: A cementitious tapered topping can be installed in some cases to
improve subsurface drainage. Note the previously placed areas to the right
are being moist cured with burlap and plastic sheets.
3. Addition of drains. While this may
be a seemingly simple solution,
adding drains over occupied spaces
is not always a practical option.
Also, in some cases, several new
drains will be needed to substantially
reduce ponding potential over the
waterproofing membrane.
Overall System Thickness and Flashing Heights at
Boundary Conditions
One of the most common challenges in
designing new plaza deck assemblies for
existing plaza decks is the limitation posed
by flashing and penetration heights.
Typically, existing adjacent masonry wall
weep holes, drainage pans for adjacent curtain
walls, doors, and other penetrations
are located within a few inches of the structural
deck surfaces. This will make it difficult
to modify the plaza assembly to provide
subsurface drainage or tapered topping
slabs. For example, take the following case:
A curtain wall adjacent to the plaza
deck is located nine inches above
the structural slab. The structural
slab is constructed level (i.e., no
drainage slope), and the deck
drains are located 24 feet away
from the curtain wall. If the designer
aims to provide a tapered topping
slab having a slope of 1/4-inch per
foot, the rise in the tapered topping
slab from the drains to the curtain
wall will be 6 inches Considering
that the tapered topping slab will
need to have a finite thickness at
the drains (say 1 inch), the total
thickness of the topping slab will be
7 inches along the curtain wall.
This leaves only 2 inches for the
flashing height at the curtain wall.
That dimension will also have to
accommodate the thickness of the
drainage layer and wearing course.
The designer is often faced with difficult
choices. In the above example, the only
remedy will be to raise the curtain wall
assembly to provide sufficient height to
properly accommodate the tapered topping
slab and other plaza deck assembly components.
Alternatively, the designer can “compromise”
and reduce the slope of the
tapered topping slab to provide for
improved flashing heights. Striking a balance
between good subsurface drainage
and flashing height is often a dilemma faced
by the designer. Experience and judgment
will play a key role in striking that balance.
Structural Capacity of the Slab
In the example discussed above, the
designer’s dilemma can be further complicated
by the existing structural slab’s structural
capacity. Many older plaza decks may
have been designed for lower live loads than
currently required by the applicable building
code. Additionally, installing the proper
plaza deck assembly components (such as a
tapered topping slab) can add significant
dead load to the structural slab. As such, in
any situation where the existing load capacity
of the plaza slab is questionable, or if the
new plaza assembly is expected to impose
additional dead load, a structural evaluation
of the existing plaza slab should be performed.
If the analysis indicates structural
deficiencies, expensive and complicated
repairs may be needed. Alternatively, the
designer can examine all of his/her options
for the new plaza assembly to assess potential
means of reducing dead loads so that
the slab can comply with the building code
requirements.
Designers should be cautious of situations
where a structural slab originally
designed as a roof deck has been converted
to a sundeck or plaza. In almost all situa-
A U G U S T 2006 I N T E R FA C E • 1 5
tions, a structural slab designed to serve as
a roof deck is not designed to safely carry
the code-prescribed live loads for an assembly
area (a plaza deck).
Existing Conditions That Do Not Lend Themselves
to Proper Detailing
In many projects, there are existing conditions
that do not promote proper detailing
of the waterproofing system. For example,
some plaza decks will include exterior
columns that may be clad
with stone or precast panels.
Most plaza decks abut
adjacent curtain wall systems
and other walls.
Often, these structures
that interface with the waterproofing
system were
originally constructed after
installation of the waterproofing
system. However,
providing a new
waterproofing system may
be impractical without
removing the adjacent
materials such as curtain
walls and column claddings
(Photo 3).
In some cases, these
conditions can be addressed
through a combination
of design compromises
and creative details.
In other instances, construction
materials and
building systems will have
to be removed to accommodate the proper
detailing of the waterproofing system.
Selection of an Appropriate Assembly
A plaza deck is often designed very differently
than a roofing system. The deck
surface will often need to accommodate
heavy traffic and abuse. For that reason,
the designer should separate the waterproofing
membrane from the wearing surface.
Plaza deck assemblies can be divided
into two categories. These are typically
referred to as “open joint systems” and
“closed joint systems.”
The closed joint systems are the most
traditional types of plaza assemblies. In the
“closed joint system,” a vast majority of the
stormwater drains onto the plaza wearing
surface, necessitating the use of a two-tier
deck drain assembly. The wearing surface
is typically constructed of cast-in-place concrete
or mortar-set pavers. The pavers can
be stone, brick, or precast concrete. The
joints between the pavers (or the control
joints in the cast-in-place concrete wearing
slab) will then be sealed with sealant or
mortar. These joints will inevitably crack or
deteriorate over time.
The waterproofing system in a closed
joint system is placed below the wearing
surface (i.e., the mortar setting bed or concrete
wearing slab). In earlier generations of
waterproofing systems, no drainage composite
was typically provided over the membrane.
However, more recently, closed joint
systems typically include a drainage composite
over the membrane to facilitate
drainage. Another advantage of placing a
drainage composite over the membrane is
that it reduces the potential for critical saturation
of the mortar setting bed or the concrete
wearing slab.
In cold climates, critical saturation of
the mortar setting bed or the concrete wearing
slab can lead to freeze/thaw damage
and deterioration. For this reason, closed
joint systems for cold regions should be
selected with caution. Careful attention to
the selection of the mortar or concrete mix,
and quality control will be needed. Another
consideration is that the salts and lime from
the concrete or mortar setting bed will tend
to be washed out and clog the drainage
composite or its filter fabric. Application of
deicing salts can also lead to damage to the
mortar setting beds and concrete surfaces
due to crystallization pressure.
Recently, “open joint systems” have
gained more acceptance, since they offer
many advantages. These systems are also
commonly referred to as “pedestal paver
systems.” In open joint systems, the vast
majority of the stormwater is drained
through the wearing surface’s open joints,
down to the membrane level. As such, primary
waterproofing is provided by the membrane.
In many cases, the use of a drainage
composite is not required since the wearing
surface pavers are typically
supported on pedestals
or shims. This creates an
open cavity below the
pavers that facilitates
good drainage.
Another advantage
offered by open joint systems
is that the wearing
surface can be constructed
level for improved aesthetics.
There is typically
no need for the use of surface
drains. Therefore,
deck drains can also be
concealed below the
pavers.
However, like any
other alternative, open
joint systems have their
disadvantages. Without
careful installation, the
pavers can rock, crack, or
become displaced. Typically,
the perimeter confinement
of the pavers
must be carefully designed to minimize the
potential for paver shifting. Also, pedestalsupported
paver systems are not suitable
for plaza surfaces subjected to heavy vehicular
traffic.
Membrane Selection
The appropriate selection of a waterproofing
membrane deserves a long discussion.
For the purposes of this article, it
should be emphasized that one of the factors
in selecting a membrane is the practicality
of its installation. For example: selecting
a hot, rubberized asphalt or bituminous
roofing membrane may pose challenges
when the installation is expected to take
place in an environment with little or no tolerance
to odors. Another consideration is
the time required for curing of the substrate
and the waterproofing membrane. Coldapplied
membranes typically require longer
curing times and are more susceptible to
problems associated with moisture release
16 • I N T E R FA C E A U G U S T 2006
Photo 3: In many cases, original waterproofing system base flashings
are installed prior to many exterior components, such as column
cladding. In such cases, column claddings and curtain wall bases
will have to be modified so that new base flashing can be installed.
from the substrate concrete. If the construction
schedule does not allow sufficient
time for the substrate concrete to “dry out”
(see Reference 3), an alternative waterproofing
membrane such as a single-ply system
should be considered.
When selecting a membrane, the
designer should evaluate all of its pros and
cons, including installation limitation.
Installation Challenges
Ideally, most detailing and installation
issues should be anticipated during the
design process. In addition, designers are
often forced to consider logistical problems
associated with rehabilitating plaza decks
over occupied spaces. Every project can
have its own set of logistical and field problems.
However, in the author’s experience,
logistical challenges associated with environmental
issues, temporary weather protection,
and surface preparation are common
to most projects.
Environmental Issues
The installation of some waterproofing
systems (such as loose-laid thermoplastic
membranes) may have little or no impact on
the surrounding environment. Such systems
typically lend themselves well to
installation in areas where there are sensitivities
to odors, volatile organic chemicals
(VOCs), and fumes generated during installation
of chemically-cured or hot-applied
systems. However, in many instances,
VOCs, fumes, and odor issues can be managed
to acceptable levels through implementation
of controls and processes during
installation. Such controls and processes
can include:
• Installation can occur during hours
where there is less sensitivity to
environmental issues (i.e., off hours
for an office or school buildings).
• Fume, VOC, or odor generation can
be reduced significantly through the
use of alternative materials. For
example, when applying a trafficbearing
waterproofing membrane,
two-component materials with
fewer VOCs can be specified.
• The impact of the generated VOCs,
fumes, and odors can be minimized
by re-routing air intakes into occupied
spaces and ensuring that air
leakage paths in the building envelope
are sealed properly.
Temporary Weather Protection
Rehabilitating a plaza deck over occupied
spaces will almost always involve the
removal (or damaging of) the existing membrane.
While in many instances the existing
waterproofing membrane is already deteriorated
and leaks in several locations, its
complete removal can only exacerbate the
potential for leaks during construction.
Unlike roofing, where the same areas
torn off in a day are typically covered the
same day, rehabilitation of waterproofing
systems requires much more extensive surface
preparation. This will dictate the use of
temporary weather protection in many
cases (Photo 4).
Factors that influence how much time
elapses between the removal of the membrane
and installation of a new one include:
• Waterproofing membranes are typically
fully adhered and will require
power equipment in order to be
removed. In some instances, the
entire surface will have to be ground
or scarified to remove the existing
system. This is typically a time-consuming
process that does not allow
removal and installation of new
membrane in the same day.
A U G U S T 2006 I N T E R FA C E • 1 7
• In many cases, the existing concrete
structural slabs will require repairs.
These repairs will take a few days to
implement, and many more days to
properly cure before they can be
overlaid with a
new membrane. If
the new waterproofing
membrane
is fully adhered,
then moisture
release issues
from the newly
repaired areas
should also be
considered. Without
some sort of
forced drying and
moisture protection,
occasional
rainfall can result
during weeks of
delays. (Note that
forced drying can
only be performed
after moist curing
of concrete patches.)
• In some cases, a
new, tapered,
cementitious topping slab is required
to provide adequate drainage
slope.
Surface Preparation
Surface preparation can play a key role
in the long-term performance of the waterproofing
membrane. It is often believed that
surface preparation for loose-laid waterproofing
membranes is not critical. The
author has observed single-ply, loose-laid
membrane failures resulting from inadequate
surface cleaning prior to application
of a membrane. When small pieces of gravel
or large sand particles are left on the substrate
surface and the membrane is applied
directly over the substrate, punctures can
result (Photo 5).
When using a fully adhered waterproofing
membrane, surface preparation becomes
one of the most critical factors in
performance of the membrane. Fully adhered
membranes will depend on their bond
to the substrate for proper performance. In
addition, the structural integrity of the substrate
can impact them significantly, since
any unaccommodated movements in the
substrate can result in failures in the membrane.
Surface preparation problems can be
divided into several categories, including:
• Laitance, dust, and chemical contamination
on the substrate.
• Moisture emission through the substrate.
• Physical deficiencies in the substrate.
Laitance, Dust, and Chemical Contamination on
the Substrate
In many instances, dust, grease, or
other surface contaminants can result in
poor adhesion. Other factors that are often
overlooked include the presence of laitance
(particularly on newer
concrete surfaces), concrete
curing compounds,
or surface sealers.
When specifying new
concrete topping slabs
over existing concrete
decks, the designer should
carefully consider curing
methods and other potential
factors that can inhibit
the bond of the new
waterproofing membrane
to the substrate. In the
author’s opinion, wet curing
using burlap is one of
the best methods for curing
fresh concrete. However,
even in instances
where the use of sealers
and curing compounds is
avoided, laitance can
form on the surface.
Therefore, when the bond
of the membrane to the
substrate is critical (as is the case in almost
all bonded waterproofing membranes), surface
preparation using shot blasting or similar
methods would be beneficial.
Photo 5: Inadequate surface preparation can lead to problems. Small
pieces of gravel left on the surface can lead to punctures in the membrane.
18 • I N T E R FA C E A U G U S T 2006
Photo 4: In some cases, the work areas will have to be completely enclosed
temporarily to provide weather protection during plaza rehabilitation.
Slightly roughened concrete surfaces
promote better primer penetration and
mechanical bond between the waterproofing
membrane and the substrate. They also
help remove the weak laitance layer that
typically forms on fresh concrete surfaces.
When performing surface preparation
using mechanical methods, care should be
exercised to avoid over roughening the substrate,
as large peaks and valleys in the
substrate can result in undesirable membrane
thickness variations (Photo 6).
One good source for specifying the
required surface roughness for concrete
substrates is ICRI Guideline No. 03732 –
Selecting and Specifying Concrete Surface
Preparation for Sealants, Coatings, and
Polymer Overlays. This guide provides an
overview of various surface preparation
methods and establishes nine concrete surface
profiles (CSP) that can be specified.
Typically, a surface profile ranging from
CSP 1 to CSP 4 is used for waterproofing
membranes (Reference 1).
In some cases, roughening of substrates
can be performed using acid etching.
However, if not properly performed, acid
etching can result in damage to the concrete
substrates. Furthermore, acid etching
will require neutralizing and a thorough
wash after application of diluted acid solution.
This process introduces more moisture
into the substrate, which will necessitate
longer drying if an adhered waterproofing
system is used.
When performing any chemical cleaning
of the concrete substrates, the pH of the
substrate should be tested in accordance
with ASTM D-4262 prior to application of
the waterproofing membrane (see Reference
2).
Removal of grease can be performed
with detergent cleaning. Once again, the
final rinse will result in the introduction of
moisture into the substrate.
One of the most critical surface preparation
factors for adhered waterproofing
membranes is dust. Dust should be thoroughly
cleaned with oil-free, compressed
air. However, in many instances, blowing
the surface clean with compressed air will
simply result in redepositing the dust elsewhere.
In my opinion, the most suitable
method for removing dust from the substrates
is vacuum cleaning. To verify that
dust has been removed from a concrete surface,
it can be wiped with a clean, black
cloth. Any dust deposits on the concrete
surface will be readily observable on the
black cloth.
Moisture Emission Through the Substrate
One of the most prevalent modes of failure
in liquid-applied membranes (LAM) is
blistering and debonding due to substrate
moisture emission.
Photo 6: Scarifying is a good method for removal of an existing adhered waterproofing
membrane. However, care should be taken to avoid over-roughening of
the substrate.
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A U G U S T 2006 I N T E R FA C E • 1 9
The mechanism of moisture emission
from concrete substrates is a complex one
that requires thorough understanding of
concrete properties, moisture vapor pressure
in concrete, and environmental factors.
This phenomenon is discussed in
Reference 3. Portions of that discussion are
repeated in this article.
Concrete is a porous material. The
porosity of concrete greatly depends on its
quality and water-to-cement ratio (w/c). As
such, concrete always contains some moisture.
Depending on the relative humidity
and temperature of the concrete, and relative
humidity and temperature of the ambient
air, concrete either emits or absorbs
moisture in vapor form. In addition, concrete
can also absorb significant amounts of
liquid water when exposed to it.
In most cases, concrete surfaces that
appear to be dry are either emitting or
absorbing water vapor. If liquid water moves
through the concrete, as long as the rate of
evaporation from the surface is greater than
the rate of moisture emission, the concrete
surface appears dry. If moisture moves
through the concrete in vapor form, the
concrete surface will not have a wet appearance,
regardless of the evaporation rate.
When a membrane is applied to the surface
of concrete in fluid form, it creates a
vapor retarder at the concrete surface that
prevents evaporation or moisture emission.
Therefore, water vapor moving to the surface
of the membrane cannot escape, thus
causing a build-up of water vapor pressure
between the membrane and concrete surface.
This phenomenon can occur within
minutes of applying a membrane to concrete
surfaces.
In the case of hot-applied membranes,
the moisture emission mechanism is further
complicated by the heat transfer into
the concrete. Build-up of water vapor pressure
shortly after application can inhibit
development of a proper bond between the
membrane and the concrete substrate.
In some cases, the moisture being emitted
from the concrete surface works its way
to the outer surface of the membrane before
the membrane cures or cools. This typically
manifests itself as blisters or pinholes in the
membrane that can lead to leakage under
hydrostatic pressure (Photo 7). However, it
is important to note that other causes of
pinhole formation – such as entrained air
due to application and formation of gases
due to the membrane’s chemical curing
mechanism – do exist.
There are also some myths regarding
the causes of failure. For example, some
believe that moisture vapor emission long
after the membrane has cured can cause
debonding and failure. With the exception
of those few membranes that are susceptible
to alkali attack at the bond line, such
mechanism cannot cause debonding of the
membrane after it has cured and established
proper bond to the substrate. The
bond value of most membranes to concrete
is in excess of 200 pounds per square inch,
while the water vapor pressure differences
are less than 1 psi. As such, water vapor
pressure alone cannot cause a physical failure
at the bond line between a well-bonded
membrane and the concrete substrate.
Another common myth in the industry
is that if the concrete is cured for 28 days,
it will be suitable for application of liquidapplied
membranes. Several membrane
manufacturers’ application instructions
indicate “fully cured” or “28-day-cured concrete”
as the only moisture criteria for
application of their membranes. The most
important factor to consider is service environment.
If the concrete has cured for 27
days and then is exposed to rain, the moisture
content in the concrete will be
increased to a level close to the initial moisture
content and will require a longer drying
time than concrete that is kept continuously
dry. Other factors such as ambient temperature
and humidity during curing will
affect the rate of drying. While the age of
concrete can be one factor, it does not correlate
well with its moisture vapor emission
rate (MVER).
Other manufacturers stipulate that the
concrete “shall be dry” prior to application
of their material. If “dry” implies completely
free of moisture, obtaining dryness in most
construction projects is impractical. The
term “dry” needs to be clearly defined by the
manufacturer, and specific acceptance criteria
should be provided.
Currently, the most widely used method
for evaluating surface moisture condition of
concrete substrates for application of waterproofing
membranes is ASTM D-4263
(Reference 4). This test method involves
installing a plastic sheet on the concrete
surface and monitoring it for formation of
visible moisture below the sheet. This test
method can provide useful information for
acceptance of a concrete substrate for
waterproofing system application. However,
it does not provide any qualitative results.
Furthermore, in the author’s experience, it
may indicate false results under certain
conditions.
Another method that can be used for
qualitative measurement of moisture emission
from a concrete substrate is ASTM F-
1869 (Reference 5). This test takes approximately
72 hours to complete, which may
make it impractical to use in many situations.
The results are expressed in pounds
of moisture vapor emitted through the sur-
20 • I N T E R FA C E A U G U S T 2006
Photo 7: Excessive moisture emission through the substrate can cause blistering
of waterproofing membranes.
face in 24 hours for 1,000 square feet of
concrete surface. The results obtained
reflect the condition of the concrete only at
the time of the test. Another drawback to
this test is that there are currently no
industry standards for threshold MVER values
obtained through ASTM F-1869 prior to
application of waterproofing membranes.
However, a value of 3 pounds in 24
hours/1,000 sf has been used by some as
the threshold for application of impermeable
membranes.
While concrete MVER can be remotely
related to its moisture content, other factors
such as ambient relative humidity and temperature
and concrete temperature play a
large role in determining MVERs from concrete
surfaces. Despite its drawbacks, this
test method is a good tool for evaluating the
MVER of concrete surfaces.
Other methods, such as measuring the
relative humidity gradients within the concrete
slabs, have been used with success.
This method involves drilling holes in the
concrete, placing relative humidity probes
at different depths, and monitoring relative
humidity profiles in the concrete. Experienced
operators are required to gather and
interpret the data. These methods are currently
somewhat too sophisticated for
everyday use at construction sites and have
not gained widespread acceptance.
Ultimately, the objective is to achieve a
good bond to the substrate without blister
formation. To that end, a simple patch test
(applying the waterproofing membrane to a
small area) may provide the best results. In
the case of hot, fluid-applied membranes,
this may be the most practical method.
However, in the case of chemically cured
membranes that take longer to cure, this
may not provide a practical solution for
evaluating substrate moisture conditions.
In the author’s opinion, more research
is required in this area to develop realistic
test methods and acceptance criteria for
substrate moisture emission rates.
Physical Deficiencies in the Substrate
For a waterproofing membrane to perform
properly, it will have to be placed over
a sound substrate. Physical deficiencies in
concrete substrates can result in premature
membrane failures. These physical deficiencies
can include cracking, delamination,
scaling, etc. These issues are more critical
in the case of fully adhered membranes.
Requirements for treatment of cracks
can vary significantly from one manufacturer
to another. In general, most manufacturers
of adhered membranes require that
moving or dynamic cracks must be treated
with a detail coat or additional reinforcing
layer (Photo 8). The additional reinforcing
layer is required to distribute the strain
caused by the movement over a wider area,
thus reducing stresses in the membrane.
The ultimate movement capability of the
membrane should not be relied upon to
design for movements over cracks, since its
movement capabilities can diminish over
time.
Consideration should also be given to
cracks that are too narrow to be identified
visually, but that can widen over time. A
crack measuring 2 mils wide can easily
widen to 20 mils, resulting in a theoretical
movement of 1,000%. This can happen with
freshly placed concrete substrates where
the concrete substrates can undergo drying
shrinkage. For this reason, initial wet curing
and allowing the concrete to undergo
shrinkage for some time is critical.
CONCLUSION
When designing a replacement waterproofing
system for an existing plaza deck,
the designer is faced with many challenges.
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A U G U S T 2006 I N T E R FA C E • 2 1
Ultimately, the designer should address all
of these challenges while maintaining
integrity of the design and considering the
impact of solutions on the durability of the
system.
In many instances, the designer is faced
with difficult decisions and is forced to compromise
on certain design principles that
can result in code violations, reduced durability,
or higher potential for future leaks.
The decision on where to compromise will
largely depend on the designer’s experience
and understanding of the particular
requirements of the project.
In addition to design considerations,
site factors and logistical issues should also
be considered. Proper consideration of these
factors will require a through understanding
of the construction and installation
process, and site limitations. Such limitations
should be carefully considered by the
waterproofing system designer.
REFERENCES
1. ICRI Guideline No. 03732, “Selecting
and Specifying Concrete Surface
Preparation for Sealants, Coatings,
and Polymer Overlays,” International
Concrete Repair Institute.
2. American Society for Testing and
Materials, ASTM D-4262, “Test
Method for pH of Chemically
Cleaned or Etched Concrete Surfaces.”
3. Farahmandpour, K. “How Dry
Should Concrete Decks Be…For
Application of Liquid-Applied Waterproofing
Membranes?” Interface,
October 2001.
4. American Society for Testing and
Materials, ASTM D-4263, “Test
Method for Indicating Moisture in
Concrete by the Plastic Sheet
Method.”
5. American Society for Testing and
Materials, ASTM F-1869, “Standard
Test Method for Measuring Moisture
Vapor Emission Rate of Concrete
Subfloor Using Anhydrous Calcium
Chloride.”
Superdome Roof Replacement
22 • I N T E R FA C E A U G U S T 2006
Kamran (“Kami”) Farahmandpour is the principal of Building
Technology Consultants, PC of Arlington Heights, Illinois. His
expertise is concentrated in the evaluation and repair of
building envelopes, including various types of exterior walls,
waterproofing systems, and roofs. Among his many professional
activities, he has served as the president of the Chicago
Area Chapter of RCI, the chair of RCI’s Building Envelope
Committee, and earned the Institute’s Richard M. Horowitz
Award for excellence in writing for Interface. He is also the coauthor
of a Practical Guide to Weatherproofing of Exterior Walls.
Kamran Farahmandpour, RRC, RWC, PE, CCS
Photo 8: Each manufacturer’s requirement for substrate crack treatment varies.
Designers should take the most conservative approach in addressing substrate
cracks to avoid future problems.
The 10-acre New Orleans Superdome, heavily damaged by Hurricane Katrina, is undergoing a rush reroofing. Brazos
Urethanes of College Station, Texas, was given 180 days to demolish half a million square feet of old roof decking and replace it
with new polyurethane-coated decking. The new roof is being built to withstand a Category 5 hurricane. The original polyurethane
roof lasted 26 years before its replacement in 2000 with a 60-mm, rubber-surfaced, “glued in place” roof. The new decking is
made of 3-ft x 16-ft sheets of 16-gauge, corrugated, galvanized steel sprayed with polyurethane foam and elastomeric urethane.
— ENR