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Roof Decks A to Z Part VII: Cast-In-Place Structural Concrete

May 15, 2015

Abstract: This is the seventh in a series
of articles examining various deck types.
Among the numerous considerations when
selecting a roof system, the type of decking is
among the most important. With the variety
of decks to be encountered (both new and
old), it is incumbent upon roofing experts to
be the authority on these matters. This article
will explore features of cast-in-place structural
concrete decks. Aspects of placement
techniques and mix proportioning are not
examined in this text. Confusion abounds regarding
this seemingly straightforward
type of roof deck. To
simply refer to it as a “concrete
deck” can mean any
number of things, as there are
numerous types of concrete, components,
and assemblies that are commonly used in
the construction of roof decks. Cast-in-place
concrete deck systems (also referred to as
job-cast; Figure 1) are explored here, along
with a definition of differences between
insulating and structural concrete.
To understand how a “lightweight” deck
(seen in some approval-guide directories)
functions, some clarification may be in
order. If a roof deck is termed merely “lightweight,”
that begs the question: Does this
mean lightweight insulating concrete, or
does it mean lightweight structural concrete?
Lightweight insulating concrete is
made using manufactured lightweight
aggregates such as perlite, vermiculite, or
polystyrene beads that provide insulating
properties. Depending upon the design mix,
the dry density of the insulating concrete
can vary from 20 to 40 pounds per cubic
foot and attain a compressive strength of
125 to 500 pounds per square inch.
Another type of lightweight insulating
concrete is commonly referred to as “cellular”
concrete. This concrete is produced
by mixing Portland cement and water with
an air-entraining agent or pregenerated
foam. As the Portland cement and water
slurry are combined with the preformed
foam, the foam bubbles become coated with
the cement paste. As the mix hardens, the
bubbles remain and produce concrete with
air cells; thus, the name “cellular” concrete.
The dry density of these mixes varies from
24 to 32 pounds per cubic foot, yet insulating
concrete is not a structural component.
This configuration of roof deck was explored
in an earlier installment of this series.
There are two general types of structural
concrete: “normal-weight” and “lightweight.”
Structural concrete can be mixed from any
number of designs to yield desired physical
8 • I n t e r f a c e A p r i l /Ma y 2 0 1 5
Figure 1 – The network of internal reinforcing steel for a project that will soon receive the
concrete pour. Note the temporary forms that will later be stripped from below.
and performance properties. The terminology
on a set of construction drawings that
refers to “concrete” typically implies that the
concrete is normal-weight.
Normal-weight structural concrete
(NWSC) is made with natural stone or
crushed gravel (complying with ASTM C33)
that is mixed with Portland cement, sand,
water, and various chemical admixtures
used to enhance physical properties of the
mix. Dry density of normal-weight concrete
typically ranges from 145 to 155 pounds
per cubic foot, and typical compressive
strengths range from 3,000 to 6,000 pounds
per square inch. For reference, there is
nothing at all lightweight about this material.
As a matter of fact, it’s the heaviest roof
deck that comes to mind.
Lightweight structural concrete (LWSC)
is similar to NWSC, except the former
is approximately two-thirds the weight of
the latter. LWSC is made with ingredients
similar to normal-weight concrete except
that lightweight aggregates, complying with
ASTM C330, are utilized instead. These lightweight
aggregates are made from naturaloccurring
products such as shale, clay,
and slate. These products are crushed and
some products are heated to high temperatures,
causing a small amount of water that
is naturally embedded in the aggregate to
turn to steam, and causing the particles to
expand in volume. The expanded particles
are lighter than crushed gravel, and when
used in a concrete mix, result in a density
of 85 to 120 pounds per cubic foot; compressive
strength values are comparable to
those of NWSC.
The types of embedded steel reinforcing
can vary, depending upon structural
requirements for the particular project. Jobcast
concrete typically utilizes one of two
types of steel reinforcing that is embedded
into the concrete at the time the concrete
is cast at the project site. These are usually
mild-strength deformed steel bars, or
instead, high-strength steel strands (called
tendons) for when post-tensioning is to be
carried out. When tendons are utilized, the
steel strands are typically sheathed in steel
or plastic tubes to prevent the wet concrete
from bonding to them. Once the concrete
has cured to a certain compressive strength,
the tendons are then pulled (tensioned) with
a hydraulic jack, and the tension force is
permanently locked with anchors at the
tendon ends. This method provides an effective
means for concrete structures to carry
heavier loads than when the structures are
reinforced with mild-strength bars. The configuration
is different enough from ordinary
cast-in-place decks that it will be explored
in a separate installment of this series.
Job-cast structural concrete decks can
be cast to a dead-flat profile. Sometimes,
this flat profile is used in anticipation of
a future vertical expansion of a building,
whereby the former roof deck becomes a
new floor level. Alternatively, structural
concrete roof decks can be cast and finished
with a top surface that is sloped to drains.
Job-cast concrete can be cast into various
shapes in order to meet requirements
of the designer and can include unusual
shapes such as hyperbolic paraboloids
(known also as saddles; Figure 2) and folded
plates (Figure 3). The structural concrete
can be cast integrally with penthouse curbs,
A p r i l /Ma y 2 0 1 5 I n t e r f a c e • 9
Figure 2 – Cast-in-place concrete
may be configured to many
different profiles. Here, the
designer envisioned a hyperbolic
paraboloid saddle.
Figure 3 – Folded
plate concrete is a
less-common variety,
relegated more often to
older structures.
parapet walls, and even support columns
(Figure 4). The final shape of the concrete
is limited by the structural requirements of
the section and by the practicality of building
and disassembling temporary forms that
are used to hold the wet concrete.
There is a considerable variety in the
types of forms that are encountered in
job-cast structural concrete. Accordingly,
the underside appearance of the deck can
reveal how to properly identify and characterize
it. While most forming systems
today are temporary, it was common practice
in older buildings to utilize forms that
remained or partially remained in place
after the concrete was cast and cured. Stayin-
place forming systems in older buildings
were economical because they saved the
costs of stripping and cleaning the forms
after the concrete cured. Forming systems
commonly utilized for job-cast concrete roof
decks can include:
• Proprietary flat and modular panel
systems (i.e., Symons forms, etc.)
• Fiberglass or metal “pan” forms to
support one-way and two-way structural
concrete decks
• Hollow tiles serving as the permanent
form (Figure 5). This deck is
a “composite” arrangement, usually
placed on concrete beams that contain
the reinforcing steel.
• Fabric sheet or layers of building
felt reinforced with steel wire
grid placed over open-web steel bar
joists (Figure 6)
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Figure 4 – Here the concrete is cast monolithically with the column capitals. This
arrangement was common in multistory buildings in decades past.
Figure 5 – Concrete is often
found in a “composite”
arrangement; here, concrete
beams (containing the
reinforcing steel) and cinder
tiles are the permanent form
for a concrete topping.
Figure 6 – Fabric sheet
reinforced with steel wire grid
over open-web bar joists.
• Composite coldformed
steel
decking as the
permanent form.
In this arrangement,
deformations
in the ribs
enhance bond
between the concrete
to the steel
form. The castin-
place concrete
may also be reinforced
with bars
and/or welded
wire fabric and
possibly Nelson
studs such as
with floor mezzanines
(Figure
7). This assembly
should never
be confused with
ordinary fluted
steel decking.
• Expanded wire
lath used to support
a concrete
slab (Figure 8).
These often have
cinder concrete on top (Figure 9).
• Structural clay tile (also called “book
tile”), situated between concrete
beams and columns, supports concrete
roof decks (Figure 10). In such
an arrangement, the concrete topping
course may be quite thin, covering
little more than the top of the
tile units (Hogan, 2011).
At the risk of complicating matters, a
roof deck might be constructed as a composite
system having insulating concrete
placed on top of structural concrete. This is
often done to achieve insulation in the roof
and for creating slope contours for drainage.
In this instance, core sampling into the roof
from above may reveal different materials
than interior observations reveal from
below. The investigating consultant should
always review any available construction
documents and then confirm conditions
by visual observations and coring when
practical.
When it is time to place the roof over
a concrete deck, there is frequent concern
about the amount of latent moisture in
recently placed concrete or regarding a deck
that has been subjected to water infiltration
from a failed roof covering. This con-
A p r i l /Ma y 2 0 1 5 I n t e r f a c e • 1 1
Figure 9 – Cinder concrete atop expanded wire lath (image courtesy of Donald Kilpatrick).
Figure 8 –
Expanded
wire lath
was a
common
permanent
form in
decades
Figure 7 – Composite cold-formed steel past.
decking as the permanent form. In
this arrangement, deformations in the
ribs enhance bond between concrete
and the steel form. The concrete may
also be reinforced with bars and/
or welded wire fabric and possibly
Nelson studs. This assembly should
never be confused with ordinary fluted
steel decking.
cern is usually associated
with floor slabs
that are to receive a
coating or final covering;
however, there
is sometimes equivalent
concern when
adhesives (especially
hot bitumen) are to be
used in a roof assembly.
There are four recognized
methods for
determining moisture
in concrete. The tests
are classified as qualitative
or quantitative,
with qualitative tests
providing a general
indication of moisture
and quantitative tests
providing a numerical
result (Schnell, 2014).
These are:
1. Plastic sheet
test (ASTM
D4263, Test
Method for
Indicating Moisture Content in
Concrete by the Plastic Sheet Method).
This is a qualitative test method that
relies on a dew point and enough
moisture in the concrete slab to
condense at the surface temperature
of the concrete. This method is
being dismissed as a poor indicator
because of sealing difficulties at the
sheet edges (Schwetz, 2014).
2. Electronic instruments: Electrical
resistance test (moisture meter),
electrical impedance test (comparative
measures of moisture in slabs),
nuclear moisture gauge (comparative
measures of moisture in slabs),
and nuclear magnetic resonance
3. Moisture vapor emission rate
(MVER): Calcium chloride test
(ASTM F1869, Standard Test Method
for Measuring Vapor Emission Rate of
Concrete Subfloor Using Anhydrous
Calcium Chloride). This is a quantitative
test method most commonly
recognized in the U.S. for the determination
of moisture conditions in
concrete slabs. The test method suffers
from several serious deficiencies,
and users should interpret test
results with caution.
4. Relative humidity measurement
(ASTM F2170, Standard Test Method
for Determining Relative Humidity in
Concrete Floor Slabs Using In-Situ
Probes). This is a quantitative test
method that uses electronic probes
to measure moisture in concrete.
Direct-to-deck adhesion of insulation
with mopping asphalt historically has greater
problems than that of excess moisture
in the deck. Asphalt was—and continues
to be—a plausible adhesive for insulation
on concrete decks. When components are
adequately bonded, astounding uplift values
can be achieved; however, the critical
1 2 • I n t e r f a c e A p r i l /Ma y 2 0 1 5
Figure 10 – Concrete roof decks may be poured over structural clay tile (also called “book tile”) situated between
concrete beams and columns.
Figure 11 – Hot bitumen was—and continues to be—a plausible adhesive for insulation on
concrete decks. This image, however, depicts poor (virtually nonexistent) adhesion of a roof
that was installed over an old vapor retarder surface following tear-off.
notion is the time window
for getting the boards situated
and properly “steppedin.”
Unfortunately, the hot
bitumen is often allowed to
chill (from “long-mopping”)
or does not gain full contact
under construction traffic
to develop the adhesion
needed. Poor embedment of
insulation has often been
observed during exploratory
coring of roofs. Figure 11
depicts virtually nonexistent
adhesion of insulation that
was adhered to an old vapor
retarder following tear-off. In
reroofing scenarios, there is
often a temptation to tear-off
down to the old vapor barrier.
In that event, however,
the entire new assembly (if
adhered) is dependent on the
bond between the concrete
deck and the vapor barrier;
that surface may very well
be the “limiting element” in
A p r i l /Ma y 2 0 1 5 I n t e r f a c e • 1 3
Figure 12 – Regardless of what the static dew point analysis may suggest, a vapor barrier should be
reconsidered in new construction or where an existing deck has been wet for extended periods.
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overall uplift resistance. Test it, carefully
evaluate it, or tear it off if there is any question
about this interface.
Scarifying an old concrete deck surface
is nice but seldom practical in reroofing.
Such work is tedious, and having the covering
off an occupied building for an extended
time is risky. If asphalt is to be used as
the adhesive, quick-dry bituminous primer
(i.e., ASTM D41) is highly recommended to
adhere insulation boards and ply sheets to
bare concrete, be it old, scarified, or freshly
placed.
Structural concrete is capable of very
good fastener engagement. On such decks,
fastener withdrawal testing will undoubtedly
result in extraordinary values—sometimes
well beyond 2,000 pounds. However,
just a few hours at the business end of a
rotary hammer will have the operator pondering
alternative ways to anchor the insulation.
There is also the risk of encountering
steel-reinforcing tendons when mechanical
fastening is implemented. Modern low-rise
foam adhesives have met this need and are
capable of yielding equivalent wind ratings
for modern roof assemblies.
SUMMARY REMARKS
The air impedance benefit of monolithic
cast-in-place concrete is recognized, even
by loss-prevention agencies that do not test
or evaluate concrete decks. When poured
integrally with parapet walls and penthouse
curbs, the time rate of air diffusion through
such a deck is quite low. Accordingly,
loosely laid roofs (ballasted with stone or
pavers where permitted by code) are a good
selection. Inverted roofing tiles and even
valve-equalized systems marry well on castin-
place concrete decks.
Finally, of recent interest and concern
are 1) unexplained moisture gain, 2)
the loss of insulation facer bond, and 3)
development of biological colonies in some
materials installed over concrete decks
(Figure 12). Even when no vapor retarder
was necessary (by psychrometric determination),
sensitive organic materials have
absorbed moisture, causing insulationfacer
distress (both cohesive and adhesive)
and development of mold (Capolino, 2014).
Cranking up interior heat shortly following
concrete placement can aggravate these
behaviors. Regardless of what the static
dew point analysis may suggest, a vapor
barrier should be reconsidered in new
construction or where an existing deck has
been wet for extended periods.
REFERENCES
Remo Capolino, “Entrapped Moisture …
But This Is a LEED Gold Building,”
Interface, July 2014, pp. 34-38.
Lyle Hogan, Don Kilpatrick, and Richard
Koziol, “Roof Decks A to Z; Part 3:
Structural Clay Tile and Plywood,”
Interface, November 2011, pp. 5-10.
Don Schnell, “Reducing the Moisture
Content in Poured Concrete Slabs,”
Durability + Design, May 2014, p. 18.
Joe Schwetz, “Risks of Roofing Over
Concrete Decks,” Interface, July
2014, pp. 20-32.
Lyle Hogan is
owner and principal
engineer of
Fincastle Engineering,
Inc., Greensboro,
North Carolina.
He is a registered
engineer
in five states, a
Registered Roof
Consultant, a Fellow
of RCI, and
an ICC structural
masonry inspector.
He has designed and administered roofing
projects in half of the U.S. using a variety of
systems. Hogan has received RCI’s Lifetime
Achievement Award, its William C. Correll
Award, and its Horowitz Award.
Lyle D. Hogan,
PE, RRC
Robert G. Kennerly
Jr., PE, is founder
and owner of
Engineered Concepts,
a structural
engineering and
building solutions
firm located in
Greensboro, North
Carolina. Kennerly
has more than 35
years of experience
in the design
of building structures
and in the investigation and remediation
of problems with building enclosures.
He is a registered professional engineer in
12 states across the Southeast and Mid-
Atlantic.
Robert G. Kennerly
Jr., PE
1 4 • I n t e r f a c e A p r i l /Ma y 2 0 1 5
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