Key Design Considerations for Concrete Topping Slabs in Split-Slab Construction

Split-slab assemblies for plaza decks
use a concrete topping slab over
insulation, waterproofing, a drainage
board, and a structural concrete
deck. The topping slab primarily
functions as a wearing layer and
protects the underlying components. Concrete
topping durability can be adversely affected
by improper subsurface drainage, inadequate
resistance to freezing and thawing, inadequate
joint design, and deformation of the insulation
used in the assembly beneath the topping.
Improperly designed concrete topping slabs
can adversely affect the waterproofing system
and the structural deck.
The authors have firsthand experience in
investigating issues with split-slab construction
and waterproofing directly linked to the concrete
topping slab. This article discusses key
considerations for designing and specifying
concrete topping slabs in split-slab construction,
and it provides two case studies of representative
projects.
BACKGROUND
Open versus Closed Wearing Layers
In most plaza decks, the waterproofing system
is protected by either an open- or closed-system
wearing layer. The most popular open-system
wearing layer consists of pedestal-supported
pavers. However, other open wearing systems
such as granular supported pavers are also
used. In an open wearing system, water drains
through the joints of the wearing system down to
the waterproofing layer. This is typically referred
to as “subsurface drainage.”
Open wearing systems offer many advantages,
including ease of construction (particularly
when concrete placement is not practical,
as is the case for plaza decks on high-rise
building roofs), flexibility in achieving aesthetic
effects, and ease of access to the waterproofing
system for maintenance or repairs.
Furthermore, open wearing systems can be
constructed to be level because the drainage
occurs primarily below the wearing layer. In
most open wearing layers, the drains can be
concealed below the pavers, and a conventional,
single-stage drain can be used.
However, there are limitations to open
systems. They cannot support concentrated
loads, and there may be issues with paver displacement.
Pedestal-supported pavers can also
be problematic when used in ramps or level
surfaces that are subject to horizontal thrust
from vehicle tires due to turning, acceleration,
or deceleration of the vehicles. It should be
noted that many plaza decks that are primarily
intended for pedestrian traffic are periodically
subject to vehicular loads. For example, in cold
climates, large snowplow vehicles may be used
to remove snow from plaza decks. In some
cases, plaza decks must be designed to provide
access to fire trucks.
A closed wearing layer consists of a continuous
surface that does not provide for positive
drainage through its upper surface. Although
some water does drain through activated control
joints in a closed system, the primary path
of drainage is surface drainage. Positive drainage
of a closed system is provided by the wearing
surface profile being sloped to direct runoff
toward drains. These drains are two-stage
drains with the ability to remove water from the
wearing surface as well as any water entering
through joints that collects at the waterproofing
layer. The subsurface drainage system also
includes a drainage layer to facilitate lateral
movement of water to the drain.
The most common closed-system wearing
layer consists of a cast-in-place concrete topping
slab. However, other closed systems such
as mortar-set pavers or stone slabs are also used
in many areas. These alternative closed systems
should be used with caution in climates where
repeated freezing-and-thawing cycles can cause
deterioration of the setting mortar.
The term “split slab” generally refers to
a plaza deck assembly with a cast-in-place
concrete closed-system wearing layer. Such
November/December 2021 IIBEC Interface • 19
assemblies incorporate a concrete topping slab
over a structural concrete deck, which is separated
by a waterproofing system (Fig. 1).
The waterproofing system typically consists
of several components, including rigid
insulation, one or more drainage mats, a protection
layer, and a waterproofing membrane. The
configuration of the assembly can vary based
on the designer’s preferences and specifications
from the waterproofing system manufacturer.
The most significant variable is the placement
of the drainage mat. It can be installed below
or over the insulation layer, or mats can be
installed at both locations.
The most significant advantage of a cast-inplace
concrete topping slab is that the topping
slab can spread concentrated loads from vehicles
over larger areas. This enables split slabs
with properly designed cast-in-place topping
slabs to resist much higher plaza loads than
plaza decks using open systems such as pedestal-
supported pavers. Cast-in-place concrete
toppings also have better resistance to horizontal
forces (thrust) imposed by turning, accelerating,
or decelerating vehicles, or by sloped
wearing surfaces. However, they still need to
be restrained against horizontal movement in
some cases.
Although many aesthetic features, such as
pigmented concrete or stamped concrete, can
be incorporated into concrete topping slabs,
the slabs are generally more limited than other
options with regard to aesthetics. In addition,
concrete topping slabs are typically heavier and
more difficult to place at project sites than open
systems such as pedestal-supported pavers.
Function of Topping Slabs
The topping slab’s primary function is to
provide protection of all the underlying components.
The topping slab does not contribute to
the load-carrying capacity of the slab; therefore,
it is not considered to be a structural member.
However, it is required to resist loads imposed
by vehicles, maintenance operations, pedestrians,
and impact of various objects. Those
loads are then distributed by the topping slab
over a larger area and transferred through the
insulation, drainage mat, and other components
to the structural slab. For this reason, the
structural behavior of the concrete topping slab
must not be overlooked.
Because cast-in-place concrete topping
slabs are exposed to elements and traffic, they
must be durable. Their durability will primarily
depend on their resistance to freezing and
thawing, which can be measured through laboratory
testing of the proposed concrete mixture
design in accordance with ASTM C666,
Standard Test Method for Resistance of Concrete
to Rapid Freezing and Thawing.1
The topping slabs should also be resistant
to abrasion posed by vehicular traffic. Abrasion
resistance of concrete mixture designs can be
determined in accordance with ASTM C779,
Standard Test Method for Abrasion Resistance
of Horizontal Concrete Surfaces.2 However, this
test is not commonly performed because most
durable concrete mixtures have sufficient abrasion
resistance for typical topping-slab applications.
In cold climates, the topping slab may
also be exposed to deicing salts. Resistance of
concrete mixtures to deicing salts
can be evaluated through ASTM
C672, Standard Test Method for
Scaling Resistance of Concrete
Surfaces Exposed to Deicing
Chemicals.3 It should be noted
that this standard was withdrawn
without replacement in 2021.3
The topping slab could be
composed of normalweight concrete
or lightweight structural concrete.
In either case, the concrete
mixture design for the topping slab
should be carefully formulated in
accordance with the American
Concrete Institute’s Specifications
for Concrete Construction (ACI
301).4 A detailed discussion of considerations
for a durable concrete
mixture is beyond the scope of this
article. However, specifiers should
be aware that many factors such
as the water-cementitious materials
ratio (w/cm), air content, and proper use
of admixtures and pozzolans are critical in
achieving a durable concrete mixture.
Bonded concrete topping slabs are bonded
to the structural slab below, so that the topping
and the slab behave compositely (without
slippage). It should be noted that bonded concrete
topping slabs are also used in plaza deck
construction, but for different reasons such
as creating sufficient slope for drainage at the
waterproofing layer. The design of such topping
slabs is also beyond the scope of this article.
The design of concrete wearing slabs for a
split-slab assembly is often based on empirical
methods and prior experience of the designer,
and many designers do not carefully consider
the topping slab design because they consider
topping slabs to be merely “sacrificial.”
However, improper design of topping slabs can
result in adverse effects. For example, problems
can arise if the designer does not consider
how surface drainage on a closed system such
as a concrete topping differs from that of an
open system such as a paver system. Whereas
paver systems allow water to drain into the
assembly below through each joint, split slabs
require sufficient slope and adequate drainage
systems to properly shed water from their
exposed surface. In addition, concrete topping
durability can be adversely affected by improper
subsurface drainage, improper resistance to
freezing and thawing, inadequate joint design,
and deformation of the insulation used in the
assembly below. More importantly, improperly
designed concrete topping slabs can adversely
affect waterproofing and the structural deck.
20 • IIBEC Interface November/December 2021
Figure 1. Example of split-slab construction.
KEY CONSIDERATION
FOR CLOSED SYSTEMS
Resistance to Freezing and Thawing
One cause of deterioration in closed systems
is cyclical freezing and thawing between
materials or within cavities of a particular
material. Because concrete materials are
porous, they can absorb moisture. When moisture
fills most of the pores in the concrete, that
water can freeze, expand, and cause microcracks
in concrete. When the ice in the crack
thaws, microcracks are left behind that can
allow more water to accumulate within the
concrete. Thus, deterioration escalates with
each cycle of wetting, freezing, and thawing. In
some climates, the freezing-and-thawing cycle
can repeat daily. Topping slabs are particularly
affected by these cycles because their surfaces
are nearly horizontal, they have a low thermal
mass due to their relatively thin section, and
they are insulated from below, which prevents
the conduction of heat from inside the building
into the topping slab.
The quality of the concrete mixture affects
the extent of the cracking and deterioration
caused when concrete is subjected to repeated
cycles of wetting, freezing, and thawing.
Deterioration from freezing-and-thawing
effects of properly proportioned air-entrained
concrete made with aggregate susceptible to
freezing-and-thawing damage is referred to as
“D-cracking.”5 According to ACI 201.2R-16,
Guide to Durable Concrete,5 young concrete
(concrete that has not yet attained a compressive
strength of at least 500 psi) can be damaged
by a single freeze. Resistance to freezing and
thawing refers to the concrete’s ability to resist
such deterioration when subject to repeated
cycles of freezing and thawing.
ACI 318-19, Building Code Requirements
for Structural Concrete,6 defines classes of various
conditions of concrete exposure to freezing-
and-thawing cycles. These are designated as
Exposure Classes F0 through F3. Based on the
Exposure Class, maximum w/cm provisions are
established in Table 19.3.2.1 of ACI 318-19. For
example, Exposure Class F0—which is defined
as exposure with no freezing-and-thawing
cycles—does not have a restriction on w/cm. In
contrast, Exposure Class F3—which is defined
as exposure to freezing-and-thawing cycles
along with frequent exposure to water and
exposure to deicing chemicals—is restricted to
a maximum w/cm of 0.40. As explained in ACI
318-19 Commentary R19.3.2, the purpose of
limiting the w/cm is to achieve low permeability
and the intended durability.
Air entrainment is also typically used to
resist effects from freezing and thawing. It
entails introducing entrained air bubbles of
diameters larger than 3 μm (0.0001 in.), which
are distributed in the cement paste at a spacing
not greater than 0.2 mm (0.008 in.), by use
of air-entraining admixtures or air-entraining
hydraulic cement. These closely spaced air bubbles
provide relief from the pressure built up by
the freezing of water in capillary cavities in the
cement paste, which minimizes damage to the
hardened paste.7 Table 4.2.3.2.4 in ACI 201.2R-
165 provides recommended air contents for various
maximum aggregate sizes and Exposure
Classes. These values range from 5.5% through
7.5%. It should be noted that overfinishing or
hard-troweling concrete with air content of
more than 3% can lead to problems and should
be avoided.4
ASTM C6661 provides two procedures
for determining resistance of concrete specimens
to rapidly repeated cycles of freezing
and thawing. Procedure A is Rapid Freezing
and Thawing in Water. Procedure B is Rapid
Freezing in Air and Thawing in Water. Both
procedures determine the effects of variations
in the properties of concrete on the resistance
to freezing-and-thawing cycles. Neither
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procedure is intended to provide a quantitative
measure of the length of service that may be
expected from the concrete. Testing involves
subjecting a specimen to (a) freezing-and-thawing
cycles until its relative dynamic modulus of
elasticity reaches 60% of its initial modulus or
(b) 300 freezing-and-thawing cycles, whichever
comes first. Based on test data, a durability
factor can be calculated. If the test indicates
that the concrete is relatively unaffected by
freezing and thawing, it can be assumed that it
was made with sound aggregates and a proper
air-void system, and that it was allowed to cure
properly.
If subsurface drainage is ineffective, water
will saturate the concrete topping slab and
cause premature deterioration in the form of
extensive cracking and spalls. If a drainage mat
is only provided below insulation in the assembly,
the topping slab can become saturated and
be prone to freezing-and-thawing effects. In
many cases, designers specify a drainage mat
above insulation in the assembly to mitigate
freezing-and-thawing effects. However, drainage
mats can get clogged with lime deposits
from the topping slab above. Although it may
diminish thermal values, the authors recommend
providing a double layer of drainage,
with one drainage mat placed directly beneath
the topping slab and another placed between
the insulation and structural deck below. An
open drainage mat directly beneath the topping
slab will provide drying potential for the
insulation.
Deicing Salt Resistance
The most common damage from freezing-
and-thawing effects in exposed concrete
slabs is surface scaling, which is loss of paste
and mortar from the surface of the concrete.
Scaling is accelerated considerably by deicing
salts, which are used to remove ice from pavements.
5 Alkali-aggregate reaction (AAR) can
cause damaging expansion in concrete structures.
Two types of AAR have been recognized:
alkali-carbonate reaction (ACR) and alkali-silica
reaction (ASR). ACR causes rapid expansion
and extensive cracking of concrete, with
cracking usually exhibited within five years of
concrete placement. ASR is a major cause of
premature concrete deterioration.5 Table 5.2 of
ACI 201.2R-165 provides examples of rock types
and minerals susceptible to ASR. Some examples
include shale, sandstone, and chert. ACI
201.2R-16 also notes that research has shown
resistance to D-cracking is reduced for concrete
containing susceptible aggregates when the
concrete is exposed to deicing salt. Chemical
deicers may also contribute to advanced deterioration
of joints in exterior flatwork when subsurface
drainage is not functioning properly.
In addition to lowering the freezing point
of water, deicing chemicals change physical
and chemical properties of solutions that fill
joints, such as their viscosity, surface tension,
and sorption. These changes create a higher
degree of saturation and increase the frequency
of cracking and microcracking under certain
temperature cycling conditions.5
Strategies to mitigate the effects of chemical
deicers include minimizing the permeability
and controlling the reactivity of concrete.
5 According to ASTM C33, Standard
Specification for Concrete Aggregates,8
cement-aggregate combinations that have an
expansion greater than 0.05% at three months
or 0.10% at six months are reactive. Aggregates
have different transport properties from those
of cement paste, and the water permeability of
concrete with low-permeability aggregates is
approximately one or two orders of magnitude
lower than that of cement paste.5 The use of
supplementary cementitious materials such as
slag can significantly reduce the permeability
and diffusivity of concrete. The material may
not reduce the total porosity significantly, but
it refines and subdivides the pores so they are
less continuous.5
Proper curing of concrete is important for
mitigating the effects of deicers. Care should
be taken to avoid exposing concrete to deicers
during the first year of service. Mitigation of
chemical deicers’ effects also requires adequate
surface drainage to minimize the duration of
exposure.5 During service, annual cleaning and
washing to remove excess surface deicing salts
may also be helpful in reducing the potential for
deicing salt damage.
Slip and Abrasion Resistance
Topping slabs must be designed to minimize
the risk of slipping by pedestrians and
vehicles, especially when the slabs are wet.
Finishes, textures, and sealers can all affect slip
resistance, as follows:9
• Rougher textures generally provide
more slip resistance than smoother
ones.
• Sealers that fill the surface and form
a film generally reduce slip resistance
when the surface is wet. Slip-resistant
additives can be added to the sealer to
counteract this effect.
• Stamped and broom finishes tend to
provide similar slip resistance.
• Broom finishes provide more slip resistance
across the grain rather than along
it. Therefore, broom finishing should
be performed perpendicular to the
usual direction of movement, if possible.
• Abrasive blast finishes provide variable
slip resistance because the removal of
surface mortar is uneven.
• When using stamped and stenciled finishes,
sufficient surface texture should
be provided to ensure adequate slip
resistance.
Abrasive grains in the surface of concrete
can be added to provide slip resistance.10 When
embedded in concrete, silicone carbide and
aluminum oxide cause an effective abrasive
action that produces long-lasting slip resistance.
These materials are typically introduced
into the concrete surface after it has been floated
and troweled once.
Polyurethane coatings provide a long-lasting
thin topping for concrete surfaces and permit
evaporation of water vapor through the
topping.10 However, such coatings should be
avoided in a split-slab configuration because
they will entrap moisture in the assembly. Acidetching
to achieve surface texture is not recommended
because it can lead to damage to the
concrete topping and the waterproofing below.
Resistance to abrasion helps concrete topping
slab maintain durability. ASTM C7792
describes three testing methods to determine
variations in surface properties of concrete.
Each method simulates abrasion conditions to
evaluate their effects on the abrasion resistance
of concrete, concrete materials, and curing or
finishing procedures. Because the equipment
used in each testing method can be portable,
the methods are suitable for field testing.
Each procedure determines the relative wear
of concrete surfaces. Procedure A uses a revolving-
disk machine that operates by sliding and
scuffing of steel disks in conjunction with abrasive
grit. Comparison of measurements of average
depth of wear of representative surfaces at
30 and 60 minutes of exposure to abrasion indicates
the relative abrasion resistance. Procedure
B uses a dressing-wheel machine that operates
by impact and sliding friction of steel dressing
wheels. The apparatus depends on the abrasive
action of three sets of steel dressing wheels riding
in a circular path over a horizontal concrete
surface. Similar to Procedure A, comparison
of measurements of average depth of wear of
representative surfaces at 30 and 60 minutes of
exposure to abrasion indicates the relative abrasion
resistance. Procedure C uses a ball-bearing
machine that operates by high-contact
stresses, impact, and sliding friction from steel
balls. The apparatus uses abrasive action of a
22 • IIBEC Interface November/December 2021
rapidly rotating ball bearing under load on a
wet concrete test surface. Water is used to flush
out loose particles from the test path, bringing
the ball bearing in contact with sand and stone
particles still bonded to the concrete surface.
This provides impact as well as sliding friction.
Comparison of depth of wear versus time for
each surface tested indicates the relative abrasion
resistance.
As previously noted, in most conventional
applications, a durable concrete mixture typically
provides sufficient resistance to abrasion.
However, in highly abrasive applications (such
as a topping slab used in a loading dock subjected
to frequent abrasion by garbage disposal
equipment), abrasion resistance should be
evaluated.
Control Joints
As concrete cures, it loses moisture and
undergoes drying shrinkage. This shrinkage
typically results in cracking of the concrete.
In reinforced concrete structures, cracking
is controlled using reinforcing steel bars, which
distribute cracks and control their width. In
unbonded topping slabs and most slabs on
grade, shrinkage cracking is controlled through
the use of control joints, which create weak
planes in the cross section of concrete to promote
cracks at “controlled” locations. These
joints can be tooled during finishing or cut into
recently placed concrete (typically within 42
hours of placement and before drying-related
shrinkage can cause cracks). Although reinforcing
steel can be used in concrete topping slabs,
the authors advise against it for the following
reasons:
• Reinforcing steel bars are prone to
corrosion-related issues, particularly
on exterior horizontal sources that are
subjected to deicing chemicals. Even
when epoxy-coated reinforcing is used,
discontinuities in the epoxy coating
can lead to corrosion. Noncorrodible
reinforcement is typically considered
too costly for such applications.
• Placement of the reinforcing before
concrete is cast may result in damage to
the underlying waterproofing, and this
damage may be difficult to detect and
correct after placement.
• The reinforcing (particularly welded
wire fabric) is often placed near the bottom
of the slab, where it is ineffective.
Typically, the design guidelines for locating
control joints are based on recommendations
provided in industry publications. For example,
both the Portland Cement Association11 and
ACI12 provide tables indicating recommended
control joint spacing for unreinforced concrete
slabs based on slab thickness. Notably, these
empirical recommendations are intended for
use at slabs-on-grade, which means they are
based on shrinkage of concrete restrained by
contact of soil below. However, topping slabs
behave differently than slabs-on-grade because
topping slabs are not restrained by friction to
soil. The only restraint is provided by the insulation
or drainage mat below the topping slab.
Therefore, topping slabs are susceptible to more
shrinkage cracking than slabs-on-grade.
Because insulation in split slabs does not
provide a thermal sink similar to slabs-ongrade,
topping slabs are also more prone than
slabs-on-grade to movement caused by thermal
changes. It is typically assumed that the
thermal movement of topping slabs will be
accommodated by the openings at each control
joint. Control joints are typically left open
because the system as a whole is designed to
handle water at the waterproofing layer, and
sealing the control joints, therefore, does not
do much to improve the system’s waterproofing
function. In addition, sealing the control joints
will create a maintenance issue. However, dirt
and debris can wash into and clog the unsealed
control joints, which prevents them from acting
as small expansion joints to relieve thermal
movement. Therefore, large sections of topping
slabs require accommodation for expansion in
addition to contraction/control joints.
Concentrated Loads
Topping slabs are susceptible to cracking
due to high concentrated loadings. Because
topping slabs are supported by rigid insulation
above the waterproofing, deformation of
the insulation may cause the topping slab to
crack. Distribution of loads depends on the
thickness of the topping slab and the stiffness
of the substrate (the drainage mat, insulation,
and waterproofing membrane). Consider this
simple example: A 4000-lbf (18-kN) wheel load
over a 2- × 2-ft (0.6- × 0.6-m) effective area of
topping slab creates 7 psi (48 kPa) of compressive
pressure on the rigid insulation below.
ASTM C578, Standard Specification for Rigid,
Cellular Polystyrene Thermal Insulation,13 stipulates
compressive resistance for various types
of rigid insulation at yield or 10% deformation,
November/December 2021 IIBEC Interface • 23
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INTRODUCTION
In evaluating building enclosure
problems, the author has encountered
many newly constructed, wood-framed,
low-slope roofs and exterior balconies
and decks that exhibit excessive/sustained
ponding of water (Figure 1). These
conditions can lead to interior water
damage through premature deterioration
of roof coverings and/or excessive
deflection of roof framing members. The
ponding (and associated creep of the
framing) can be so significant that it
may ultimately lead to failure of the roof
framing.
The purpose of this article is to provide
insight into the most likely causes
of these problematic ponding conditions
as they relate to commonly accepted
design and construction methods.
36 • IIBEC IntErfaCE OCtOBEr 2019
Figure 1 – Excessive ponding water
on a roof.
Figure 2 – Ponding typically occurs prior to reaching discharge points.
INTRODUCTION
The concept of building for resilience
has been increasingly adopted by various
organizations over the past five years.
Organizations use different definitions or
phrases to describe resilience and the hazards
that are included in resilient design.
These definitions from six sources are compared
and a single definition incorporating
these is developed.
RESILIENCE AS DEFINED BY SELECT
ORGANIZATIONS
Industry Statement
Twenty-one organizations, including the
U.S. Green Building Council (USGBC), the
American Society of Heating, Refrigerating,
and Air-Conditioning Engineers (ASHRAE),
the American Institute of Architects (AIA),
the American Society of Civil Engineers
(ASCE), the Building Owners and Managers
Association (BOMA), and the National
Institute of Building Sciences (NIBS) issued
an industry statement on resilience[1] that
stated (the bold or red text is theirs):
Representing more than 750,000
professionals, America’s design and
construction industry is one of the
largest sectors of this nation’s economy,
generating over $1 trillion in
GDP. We are responsible for the
design, construction, and operation
of the buildings, homes, transportation
systems, landscapes, and public
spaces that enrich our lives and
sustain America’s global leadership.
We recognize that natural and
manmade hazards pose an increasing
threat to the safety of the public
and the vitality of our nation. Aging
infrastructure and disasters result
in unacceptable losses of life and
property, straining our nation’s ability
to respond in a timely and efficient
manner. We further recognize
that contemporary planning, building
materials, and design, construction,
and operational techniques can
make our communities more resilient
to these threats.
Drawing upon the work of the
National Research Council, we define
resilience as the ability to prepare
8 • IIBEC IntErfaCE SEptEmBEr 2019
This article is reprinted with permission
from Advances in Civil Engineering
Materials, Vol. 7, No. 1, 2018, copyright
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www.astm.org.
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whichever occurs first. For insulation having
60-psi (414 kPa) compressive resistance, 7 psi
(48 kPa) of compressive pressure will result in
1.2% deformation (assuming a linear relationship
between deformation and pressure). If
there is 10-in.-thick (250-mm) rigid insulation
under the topping slab, the insulation will compress
approximately ⅛ in. (3 mm). This localized
deformation of the insulation can cause
the topping slab to crack.
Clearly, the topping slab should be designed
such that it has sufficient flexural strength to
resist stresses that cause cracking. The design
analysis should consider the specified compressive
strength of the insulation, the type of
anticipated vehicular load, and the thickness
of the insulation. Reinforcing steel could be
provided within the topping slab to improve
flexural strength, distribute the loading over
a larger area, and assist in controlling cracks.
However, as mentioned previously, the authors
advise against using reinforcing steel where its
use can be avoided.
Vaca and colleagues14 used structural software
in a parametric study to provide an elastic
analysis of a topping slab supported on spring
supports above a structural floor. Based on
their findings, they recommend using a topping
slab that is at least 3 in. (75 mm) thick for
light traffic, or at least 5 in. (125 mm) thick for
heavy traffic. Unreinforced topping slabs can
be designed to adequately resist cracking from
anticipated concentrated loads.
Certain enhancements in concrete such as
addition of fibers can increase ductility of the
concrete and reduce cracking. Use of fibers in
concrete is a complex and often debated topic
that is beyond the scope of this article.
Subsurface Drainage
In a closed system, water drainage primarily
occurs over the upper surface of the
topping slab, but some water will penetrate
through cracks and control joints within the
concrete topping slab. Therefore, a subsurface
drainage system must be properly designed
and constructed to drain water. Specifically,
the waterproofing membrane must be sloped
toward the drain assembly, a capillary break
such as a drainage composite must be provided,
and two-stage drains are needed to allow water
drainage at the waterproofing layer and the top
surface.
Our experience has shown that the drainage
composites and two-stage drains
are prone to clogging due to concrete
leachate or accumulation of calcium
hydroxide. When water flows through
cracks, microcracks, and joints, it dissolves
free calcium hydroxide, which
is subsequently deposited elsewhere
within the system when the water
evaporates. In many cases, we have
observed substantial clogs in twostage
drain screens or the fabric layer
of the drainage composite due to this
phenomenon (Fig. 2).
To reduce potential clogging of
two-stage drain screens, the authors
use a granular fill in an area surrounding
the drain. This increases the
effective filter area and traps some of
the leachate before it reaches the drain
screen, where the water can evaporate.
To minimize the clogging of
drainage composite filter fabric, we
typically recommend the use of two
layers of drainage composite. One layer is
placed between the insulation and the concrete
topping slab, and the other is placed between
the insulation and the waterproofing system. In
this arrangement, if the top-layer drainage composite
fabric is clogged, water will pass through
joints of the drainage composite and insulation
and eventually reach the lower layer of drainage
composite, where it can freely drain.
This practice is not ideal because it does
not necessarily prevent critical saturation of
the topping slab if the top-layer drainage composite
fabric is clogged. It can also be argued
that allowing the water to freely drain to the
lower drainage composite layer will reduce the
effectiveness of the insulation during cold conditions
when melting snow at low temperatures
bypasses the insulation.
Figure 2. A filter screen around a two-stage drain is clogged due to decalcification of the concrete topping.
Figure 3. Example of a proprietary joint system using a raised flanged connection to the
structural deck below.
24 • IIBEC Interface November/December 2021
26 • IIBEC Interface November/December 2021
Thermal Expansion and Contraction
Materials expand and contract due to
increases and decreases in temperature. The
dimensional change due to thermal effects is
the product of a material’s length, its coefficient
of thermal expansion, and the change in temperature.
The coefficient of thermal expansion
for concrete is approximately 0.0000055 in.
per degree Fahrenheit. For a given length of
concrete, the larger the temperature change is,
the more thermal movement the concrete will
exhibit.
Shear Stress Transfer to Substrate
The interaction of topping slabs and substrate
below can create large stresses that will
damage adjacent materials. Forces due to friction,
thermal expansion and contraction, and
other effects can transfer to abutting members,
or they can transfer through components in the
assembly into the structural deck. A 50-ft-long
(15-m) portion of concrete topping subject to a
temperature change of 60°F (33°C) will theoretically
move 0.2 in. (5 mm). If this movement
is transferred to substrate, adjacent walls, or
other elements to which the topping connects,
it will impart incredibly large forces. Unless the
movement can be accommodated, failure of
materials is inevitable. Therefore, the thermal
movement of the topping slab should be carefully
considered and accommodated properly.
LESSONS LEARNED
FROM CASE HISTORIES
The following are two case histories that
illustrate some of the issues discussed in the
previous sections of this paper.
Plaza Deck Case Study
A four-story instructional center of a community
college included a long plaza deck over
occupied spaces along the side of the building.
The plaza deck was approximately 30 ft (9 m)
wide and 700 ft (213 m) long, and it sloped
away from the remainder of the building. It was
originally constructed in 1974 as a split-slab
assembly with two transverse building expansion
joints intersecting the plaza deck. The
expansion joints were located approximately
at the one-third points along the plaza’s length,
which aligned with expansion joint locations in
the building frame.
In 2011, the plaza deck was renovated, a
project that included removing existing materials
down to the concrete structural slab. The
new plaza deck assembly included a tapered
bonded concrete filler slab over the existing
structural slab to provide for better drainage,
a hot fluid-applied waterproofing membrane,
a drainage mat, rigid insulation, and a concrete
topping slab. The topping slab was constructed
with control joints saw-cut approximately 4½ ft
(1.4 m) apart.
New proprietary expansion joint assemblies
were provided in the renovated plaza deck
at the original locations. The new expansion
joint assemblies consisted of a rubber bellows
clamped between two L-shaped flanges with
integrated rubber glands. The horizontal legs
of the L-shaped flanges were anchored into
the new bonded concrete topping slab. Figure
3 depicts the configuration of the expansion
joints. Water leakage occurred through the plaza
deck at the new expansion joint assemblies
shortly after renovation was completed. An
evaluation was performed to assess the source
or sources of the water leakage. The evaluation
consisted of a review of the renovation drawings,
visual review of field conditions, water
penetration testing, and exploratory openings
through the plaza assembly.
The visual review of the expansion joints
indicated deformations around the retaining
caps of the expansion joint gland (Fig. 4). Water
penetration testing was performed along the
expansion joints in 6-ft (2-m) increments using
a calibrated spray bar (Fig. 5). At most locations,
water leakage was observed in the occupied
space below within 2 to 5 minutes after the
start of testing.
Figure 4. Deformation around a retaining cap.
Figure 5. Water penetration
testing along an expansion joint.
Exploratory openings were made to identify
the path of water leakage through the expansion
joints. Upon removal of the expansion
joint retaining caps, investigators observed that
flanges of the sealing gland were severed from
the body of the gland (Fig. 6). The investigators
also observed a large tear in the vertical leg of
the expansion joint flashing sheet at the bottom
edge of the expansion joint retaining cap (Fig.
7), and they found that an expansion joint
anchor bolt used to attach the horizontal leg of
the expansion joint flange to the slab had failed
in tension.
The damage to the waterproofing and the
expansion joint assembly was attributed to differential
movement between the topping slab
and the supporting structural deck. The tributary
spacing of these expansion joints was as
much as 245 ft (76 m), resulting in significant
thermal movements of the concrete topping relative
to the underlying waterproofing and the
structure. This differential movement resulted
in large thrust forces being exerted on the
expansion joint vertical legs. The thrust forces
caused rotation of the expansion joint flanges
and the failure of the anchors that secured
them to the deck below. This, in turn, caused
the expansion joint flanges to rotate and tear
the membrane along the horizontal leg of the
expansion joint flanges. The thrust forces also
caused damage to the expansion joint gland
clamping strips.
Assuming no thermal movement could be
accommodated by the small gaps at the control
joints, the topping slab could expand as
much as 1.9 in. (48 mm) if there were a 120°F
(67°C) temperature differential. In contrast, the
structure below, to which the expansion joint
assembly was attached, experienced only small
thermal movements because the structure was
conditioned, maintaining a relatively constant
temperature. This differential movement of the
topping slab relative to the structure below was
not accommodated by the expansion joints
because the entire assembly was attached to the
structure. The purpose of that expansion joint
was to allow building movements, not differential
movements between the topping slab and
the structure.
Even if the expansion joints had been
designed to allow differential movements of
the topping slab with respect to the structure,
the spacing and size of the joints would have
been entirely inadequate. Calculations indicated
each expansion joint assembly must be
capable of accommodating almost 2 in. (50
mm) of expansion.
To remedy the condition, the concrete topping
was removed adjacent to the existing expansion
joints so that waterproofing and expansion
joint repairs could be performed. Included in
the repairs were seven additional new expansion
joints in the topping slab only, as well as bilevel
expansion joints at existing expansion joint
locations. The repairs were performed from the
plaza deck level, and operations within the space
below were unaffected during repairs.
COMMERCIAL BUILDING CASE STUDY
This case history involves a two-story commercial
building. The first two floors of the
structure served as commercial spaces, and the
roof of the structure (the third level) served as
a parking deck above the second-floor spaces.
Access to the parking deck was provided via
ramps.
November/December 2021 IIBEC Interface • 27
Figure 6. Severed flange of an
expansion joint sealing gland.
Figure 7. A large tear in an
expansion joint flashing sheet.
28 • IIBEC Interface November/December 2021
The structure consisted of precast concrete
columns, double tees, and inverted T beams.
Typically, concrete columns supported inverted
T beams, which supported long-span precast
concrete double-tee planks.
Double-tee planks at the third-floor level
(the roof) were superimposed by a waterproofing
system and a concrete topping slab. The
waterproofing system consisted of a hot rubberized
asphalt waterproofing membrane applied
directly over the double tees’ top surfaces, a
drainage mat, insulation, and a cast-in-place
concrete topping slab. Before the waterproofing
was applied over the double-tee planks, the
joints between planks were sealed with sealant,
and an uncured neoprene flashing system was
used to strip in the joints.
Shortly after completion of construction,
leaks were reported below the roof parking
area. Several investigations of these leaks were
conducted, and notable findings included the
following:
• Several leaks were attributed to punctures
in the waterproofing membrane
caused by stakes used to form the concrete
topping slab sections. The stakes
were driven through the insulation
after nondestructive leak detection had
been completed during original construction
and the drainage mat and
insulation were installed. This finding
underscores the risks related to post-installation
activities over plaza deck
waterproofing systems.
• Some leaks were traced to localized
debonding of the waterproofing membrane
over concrete surfaces. The
examination of these areas revealed
moisture below the membrane and
the presence of a brown oily substance.
It appeared that the membrane
was adversely impacted by exposure
to moisture, although not all parties
agreed with this conclusion. Moisture
readings of substrate surfaces revealed
relatively high relative humidity levels
within the concrete when measured
in accordance with ASTM F2170,
Standard Test Method for Determining
Relative Humidity in Concrete Floor
Slabs Using in situ Probes.15 Most of the
high moisture readings were recorded
over the largest dimensions of the
inverted T beams. Such members
typically take far longer than thinner
members to sufficiently dry. Further
testing of the waterproofing membrane
would have been needed to assess this
issue. Detailed discussion of moisture
below waterproofing membranes and
the effect of water on some bituminous
membrane is beyond the scope of this
article.
• Some leaks were traced to failure of the
waterproofing membrane over side-lap
joints (flange-to-flange connections) of
the precast concrete double tees. These
joints were sealant joints intended to
support the waterproofing membrane
and the uncured neoprene flashing.
However, investigation indicated that
some had significantly widened (Fig. 8).
This finding prompted a more detailed
review of the joints and a visual review
from below. These reviews indicated
that some of the welded connections
at the double-tee side laps had failed
in tension. In addition, cracking and
delamination of the concrete around a
few of these connections provided evidence
of differential lateral movements
between the precast concrete double
tees (Fig. 9).
Figure 8. A failed sealant joint.
Figure 9. Cracked and delaminated concrete on
the underside of a precast concrete double tee.
30 • IIBEC Interface November/December 2021
The failure of welded connections at side
laps of the double tees prompted concerns
regarding the structural integrity of the building.
In particular, failure of side-lap connections
would diminish the ability of the double-
tee deck to act as an effective diaphragm to
resist wind loads.
Original calculations prepared by the
precaster’s engineer for flange-to-flange connections
between adjacent double tees were
reviewed. The review indicated adequate
design of the connections to resist diaphragm
loads. However, the original design criteria had
not required any consideration of the thermal
movement of the topping slab and its impact on
the double-tee deck.
The side-lap flange-to-flange connections
were located at approximately 10-ft (3-m) spacing
along double-tee sides (Fig. 10). Several
additional flange-to-flange connections should
have been provided to distribute loads more
uniformly to adjacent double tees. Flange-toflange
connections are typically spaced periodically
5 to 8 ft (1.5 to 1.8 m) along the length
of double tees, and sometimes they are spaced
even closer near midspan.16
Although thermal movement of the topping
slab is not typically considered by designers,
this movement will create tension stresses
in the double tees below due to frictional
forces. During our original investigation,
most of the parties involved, including the
authors, believed that the layers of insulation,
waterproofing, and drainage mat would provide
sufficient bond break between the topping
slab and the deck to avoid transfer of stresses
between the two. However, further evaluation
indicated that thermal movements of the concrete
topping slab could induce shear stresses
in the deck below.
Based on the weight of the topping slab, horizontal
movement of the topping slab transfers to
the double tees through shear friction between
the two surfaces. From our research, we estimated
that the lowest shear friction coefficient
between various components of the assembly
below the topping slab would be on the order
of 0.30. Based on this friction coefficient, a 60°F
(33°C) increase in the topping slab temperature
(a realistic assumption based on solar gain alone)
could lead to an expansion of 0.04 in. (1 mm)
over a 10-ft-wide (3-m) double-tee section. Using
a linear stress-strain relationship of the concrete,
this equates to more than 650,000 lbf (2890 kN)
of expansive forces applied to each double-tee
side connection plate when the plates are spaced
near the ends of 52-ft-long (16-m) double tees.
Such forces can readily overcome the design
capacity of the connections. In addition, these
forces are cumulative in nature and can proportionately
increase over multiple double-tee
widths (Fig. 11).
Obviously, there are many other factors that
can change these estimations. For example, the
control joints formed in the concrete topping
slab may be able to accommodate some of the
expansion joint. However, as previously indicated,
accumulation of debris in the control
joints will minimize their ability to accommodate
concrete expansion. In addition, although
the insulation and the waterproofing components
are somewhat elastic in nature and can
deform in shear, such deformations would still
result in transfer of shear stresses to the double
tees below.
To evaluate this phenomenon of transferred
forces, the investigative team installed
highly accurate displacement data loggers on
the underside of double tees at three separate
flange-to-flange connections. Each data logger
had three sensors, one for each direction of possible
displacement, which were rigidly mounted
onto custom-fabricated aluminum plates on
each adjacent double tee (Fig. 12).
The data loggers recorded displacements
every 15 minutes for more than 10 weeks. The
displacements were plotted on graphs with
the vertical axis indicating displacement and
the horizontal axis indicating time. Exterior
temperatures and sunny conditions, based on
weather reports, were also plotted on the same
graphs (Fig. 13).
When the data were analyzed, movements
were found to be small. Differential movements
tended to be inversely related to exterior
temperature change. For example, as exterior
temperature increased, the joints enlarged.
This reverse correlation with outdoor temperature
suggests that thermal movements of
the topping slab as well as friction between
the unbonded topping slab and underlying
components imparted stresses on the flangeto-
flange double-tee connections, as previously
hypothesized.
In some locations, the failed connection
plates were repaired using connection plates
that allowed more flexibility in the connection
to accommodate the small, anticipated movements.
This work involved removing the concrete
topping slab and underlying components
along rows of connection plates. In addition,
expansion joints were constructed through the
topping slab to minimize future movements.
Figure 10. Excerpt of a connection detail from the precaster’s engineer.
Figure 11. Expansive forces applied to a double-tee connection plate.
November/December 2021 IIBEC Interface • 31
Investigators also determined that some
yielding of the connection plates would occur
as a result of normal movements. Although
such movements would not compromise the
structural integrity of the building, they could
cause local cracking of the precast concrete
double-tee flanges around the connection
plates. For safety purposes, precautions were
taken to prevent cracked concrete from spalling
and falling. At some locations, cracked portions
of concrete were removed. At other locations,
steel plates were installed on the underside of
cracked areas to prevent possible spalls from
falling. Where connection plates could not be
readily accessed, a fall protection system was
installed to ensure falling concrete could not
pose any safety issues.
Periodic monitoring of the structure and the
connection plates has indicated that the development
of cracking around the precast concrete T
connection plates has decreased significantly. In
addition, other waterproofing repairs performed
on the top side of the deck have proven effective
in controlling water leakage.
SUMMARY
Topping slab design for split-slab construction
requires careful consideration to ensure
the concrete topping is durable and does not
cause damage to the structure below. Concrete
topping durability can be adversely affected
by improper subsurface drainage, insufficient
resistance to freezing and thawing, inadequate
joint design, and compression of insulation
used in the assembly below. Concrete topping
slab movements can transfer loads to the
structure below, even in the presence of several
layers between the topping slab and the structure.
These stresses can cause distress in the
structure, adjacent parapet walls and vertical
Figure 13. Data logger results.
Figure 12. Data logger assembly at a double-tee flange connection.
32 • IIBEC Interface November/December 2021
projections, or waterproofing accessories such
as expansion joints. In the design of concrete
topping in split-slab construction, key considerations
include adequate surface drainage,
proper subsurface drainage, flexural strength
of the topping slab to resist cracking under load,
proper selection of insulation to resist excessive
compression, control joint placement, expansion
joints to accommodate thermal movements,
and concrete durability.
REFERENCES
1. ASTM International. 2015. Standard
Test Method for Resistance of Concrete
to Rapid Freezing and Thawing. ASTM
C666/C666M-15. West Conshohocken,
PA: ASTM International. doi: 10.1520/
C0666_C0666M-15.
2. ASTM International. 2019. Standard
Test Method for Abrasion Resistance of
Horizontal Concrete Surfaces. ASTM
C779/C779M-19. West Conshohocken,
PA: ASTM International. doi: 10.1520/
C0779_C0779M-19.
3. ASTM International. 2012 (withdrawn
2021). Standard Test Method for Scaling
Resistance of Concrete Surfaces Exposed
to Deicing Chemicals. ASTM C672/
C672M-12. West Conshohocken, PA:
ASTM International. doi: 10.1520/
C0672_C0672M-12.
4. American Concrete Institute (ACI).
2016. Specifications for Concrete
Construction. ACI 301-16. Farmington
Hills, MI: ACI.
5. ACI. 2016. Guide to Durable Concrete.
ACI 201.2R-16. Farmington Hills, MI:
ACI.
6. ACI. 2019. Building Code Requirements
for Structural Concrete and
Commentary. ACI 318-19. Farmington
Hills, MI: ACI.
7. ACI. n.d. “Technical Questions:
Resistance to Cycles of Freezing and
Thawing.” Accessed September 10,
2021. https://www.concrete.org/
tools/frequentlyaskedquestions.aspx?-
faqid=658.
8. ASTM International. 2018.
Standard Specification for Concrete
Aggregates. ASTM C33/C33M-18.
West Conshohocken, PA: ASTM
International. doi: 10.1520/C0033_
C0033M-18.
9. Cement and Concrete Aggregates
Australia. 2008. “Guide to Concrete
Flatwork Finishes.” https://www.
ccaa.com.au/imis_prod/documents/
Library%20Documents/CCAA%20
Technical%20Publications/CCAA%20
Guides/CCA AGUIDE2009-T59-
Flatwork%20WEB.pdf.
10. Aberdeen Group. 1966. “Non-Slip
Finishes for Concrete.” Publication
#C660351. https://www.concreteconstruction.
net/_view-object?
id=00000153-8b70-dbf3-a177-
9f7990b70000.
11. Portland Cement Association (PCA).
2021. Design and Control of Concrete
Mixtures. 17th ed. Skokie, IL: PCA.
12. ACI. 2010. Guide to Design of Slabs-on-
Ground. ACI PRC-360-10. Farmington
Hills, MI: ACI.
13. ASTM International. 2019. Standard
Specification for Rigid, Cellular
Polystyrene Thermal Insulation. ASTM
C578-19. West Conshohocken, PA:
ASTM International. doi: 10.1520/
C0578-19.
14. Vaca, E., N. K. Gosain, and G. A.
Jimenez. 2015. “Design Considerations
for Unbonded Concrete Topping
Slabs in Plaza Deck Systems Subjected
to Vehicular Traffic.” Concrete Repair
Bulletin. January/February 2015, pp.
18-23. https://cdn.ymaws.com/www.icri.
org/resource/resmgr/crb/2015janfeb/
CRBJanFeb15_Vaca-Gosain-Jime.pdf.
15. ASTM International. 2019. Standard
Test Method for Determining Relative
Humidity in Concrete Floor Slabs
Using In Situ Probes. ASTM F2170-
19a. West Conshohocken, PA: ASTM
International. doi: 10.1520/F2170-19A.
16. Lucier, G., C. Naito, A. Osborn,
M. Nafadi, and S. Rizkalla. 2017.
“Double Tee Flange Connections—
Experimental Evaluation.” Paper
98 presented at PCI Convention
and National Bridge Conference,
Cleveland, OH, Feb. 28–Mar. 4.
Please address reader comments to chamaker@
iibec.org, including “Letter to Editor” in the
subject line, or IIBEC, IIBEC Interface, 434
Fayetteville St., Suite 2400, Raleigh, NC 27601.
Michael F. Wiscons,
SE, PE, is an associate
principal with
Building Technology
Consultants Inc., a
forensic engineering
firm in Arlington
Heights, Ill. He is a
licensed structural
engineer in the
state of Illinois and
a licensed professional
engineer in
Illinois, Wisconsin,
and Minnesota.
Wiscons has managed more
than 300 structural and building facade projects.
These projects have included steel, concrete,
masonry, and timber building systems on
institutional, governmental, industrial, historic,
commercial, and residential buildings.
Michael F. Wiscons,
SE PE
Kami Farahmandpour,
PE, F-IIBEC,
FNAFE, RBEC,
CCS, CCCA, is
the principal of
Building Technology
Consultants Inc. He
is a fellow of IIBEC
and the National
Academy of Forensic
Engineers. He has
been involved in the
evaluation, testing,
and repair of construction
materials
and building enclosure performance since 1984.
Farahmandpour has managed hundreds of projects
involving multiple disciplines and complex
building enclosure issues. Many of these projects
have involved concrete assessment and plaza deck
construction.
Kami Farahmandpour,
PE, F-IIBEC, FNAFE,
RBEC, CCS, CCCA;
George Seegebrecht,
PE, FACI, is principal
with Concrete
Consulting Engineers
PLLC. He has
more than 40 years
of experience in the
construction industry,
and for the past
30-plus years, his
primary work has
been troubleshooting
concrete construction
problems concerning
design, materials, and workmanship issues.
Seegebrecht holds a B.S. in civil engineering from
Valparaiso University and is a licensed professional
engineer in multiple states. He has provided litigation
support and testimony in the United States
and Canada on various issues of construction
design, materials, and workmanship.
George Seegebrecht,
PE, FACI