13 CONCRETE CONSTRUCTION
CONCRETE
CONSTRUCTION
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
HISTORY
Roman Concrete
• Earliest concrete, 300 BC
• Pozzolanic ash: Volcanic
ash originally found at
Pozzuoli, Italy; high in
silica and alumina content
• When pozzolanic ash was
mixed with lime, water,
sand, and rock, the
resulting mixture would
harden to an unusually
strong material, even
when fully submerged
under water.
• Hydraulic cement:
Cement that when
hardened, is not water
soluble
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
HISTORY
Modern Concrete
• Knowledge of hydraulic
cements is lost with the
fall of the Roman
Empire.
• Hydraulic cements are
rediscovered in late 18
c. England.
• 1824: Joseph Aspdin
patents Portland
Cement, an artificial
hydraulic cement made
from a finely ground
mixture of limestone
and clay. This name for
hydraulic cement
remains in use to today.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
HISTORY
Modern Concrete
• The embedding of steel
reinforcing in concrete,
to increase concrete's
tensile strength and
resilience, is the other
key component of
modern concrete.
• Steel reinforced
concrete first appears in
the 1850's.
• Right: Francois
Hennebique's 1892
patent for reinforced
concrete construction
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
13 CONCRETE CONSTRUCTION
CEMENT AND
CONCRETE
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Concrete ingredients
• Fine aggregate (sand)
• Coarse aggregate
(gravel)
– Coarse and fine
aggregate provide the
structural mass of the
concrete and constitute
the majority of the
concrete volume.
• Portland cement
– Cement binds the
aggregate.
• Water
– Water is necessary for
the chemical hydration
of the cement and the
hardening of the
concrete.
In the finished concrete, aggregate particles are surrounded by
the cement and bound into a solid mass.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Portland Cement
• Primary ingredient:
Calcium silicates
• Lesser amounts of
compounds of aluminum,
iron, and magnesium
• Raw materials can be
extracted from a variety of
sources depending on the
locale, such as limestone,
chalk, marl, marble, sea
shells, clays, shales,
industrial waste materials,
iron ore, bauxite, and
others.
• Materials are crushed,
ground, proportioned and
blended into a very fine
powder.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Portland Cement
• The blended materials are
burn or sintered in a hightemperature, rotating kiln,
creating clinker.
• The clinker is cooled and
ground to a fine powder.
• Small quantities of gypsum
are added to adjust the
cement's setting rate.
• In the U.S., a standard bag
of cement contains 1
cubic foot of material and
weighs 94 lb.
The cement kiln is gently sloped, so that as it rotates, the
cement ingredients travel down the slope at a controlled rate.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Air-Entraining Cement
• Ingredients added to the
cement generate bubbles
during concrete mixing that
create small, distributed voids
in the finished concrete.
• Air content in the range of 2
to 8 percent of the total
concrete volume is typical.
• Air-entrained concrete has
greater resistance to freezethaw damage. It is use for
concrete exposed to wet,
freezing conditions.
• Air-entrained concrete also
has improved workability
when wet.
• Air entraining reduces
concrete strength, unless the
proportions of other
ingredients in the concrete
mix are adjusted to
compensate.
Enlarged section of air-entrained concrete: Note the air voids
within the cement paste matrix.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
ASTM C150 Cement
Types
• Type I: General purpose
• Types II and Type V: For
concrete in contact with
soils or water with high
sulfate concentrations
• Type III: For cold weather
construction, concrete
precast in plants,
accelerated construction
schedules
• Type IV: For massive
structures, such as dams,
where the heat generated
during hydration must be
limited to avoid excessive
temperatures
ASTM C150
Description
Type I
Normal
Type IA
Normal, air entraining
Type II
Moderate resistance to sulfate
attack
Type IIA
Moderate resistance to sulfate
attack, air entraining
Type III
High early strength
Type IIIA
High early strength, air
entraining
Type IV
Low heat of hydration
Type V
High resistance to sulfate
attack
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
ASTM C150 Cement
Types
• Ninety percent of U.S.
manufactured cements are
Type I, Type II, or Type
I/II (cement that meets all
the requirements of both
types).
• Most air-entrained
concrete is made with
admixtures added
separately to the concrete
mix rather than with airentraining cements.
Separate admixtures allow
greater control over
amount and quality of air
entraining in the final
concrete.
ASTM C150
Description
Type I
Normal
Type IA
Normal, air entraining
Type II
Moderate resistance to sulfate
attack
Type IIA
Moderate resistance to sulfate
attack, air entraining
Type III
High early strength
Type IIIA
High early strength, air
entraining
Type IV
Low heat of hydration
Type V
High resistance to sulfate
attack
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Portland Cement and
Carbon Dioxide
Emissions
• Cement production is a
significant source of
emissions of carbon
dioxide (CO2), a
greenhouse gas.
• The burning of fuel to heat
the cement kiln generates
CO2.
• The chemical conversion
of limestone (calcium
carbonate) within the kiln
releases additional large
quantities of CO2.
• In total, manufacturing
1000 pounds of portland
cement generates on
average roughly 900
pounds of carbon dioxide.
Simplified chemistry of the cement kiln:
Limestone + silica (2700 deg F) = portland cement + carbon dioxide
5CaCO3 + 2SiO2  (3CaO, SiO2) (2CaO, SiO2) + 5CO2
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Portland Cement and
Carbon Dioxide
Emissions
• Cement production
accounts for approximately
5 percent of worldwide
CO2 emissions and 1.5 to
2 percent of U.S.
emissions.
• The cement industry is
working to substantially
reduce its CO2 emissions.
Worldwide CO2 emissions from cement production
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Portland Cement and
Carbon Dioxide
Emissions
• The substitution of
industrial waste products
with cementing properties
(fly ash, silica fume, blast
furnace slag) for some
portion of the cement in
concrete reduces the total
CO2 emissions associated
with concrete production.
• When concrete is crushed
and recycled, it is capable of
absorbing up to half of the
CO2 originally generated in
the cement manufacture.
• New, experimental cements
aim to reduce CO2
emissions, even to the extent Cement clinker after emerging from the cement kiln
that concrete becomes
carbon-negative, absorbing
more CO2 during its lifetime
than is emitted during its
manufacture.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Aggregates
• Coarse and fine aggregate make up
60 to 80 percent of the total
concrete volume.
• Most aggregate comes from natural
sand and gravel deposits or is made
from crushed rock.
• Artificial aggregates may come from:
–
–
–
–
Blast furnace slag
Fly ash
Recycled concrete
Thermally treated clay, shale, and
other minerals
• Strength of concrete is heavily
dependent on the quality of the
aggregates:
–
–
–
–
–
–
Porosity
Size distribution
Moisture absorption
Shape and surface texture
Strength, elasticity, density,
soundness
Contamination or detrimental
substances
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Maximum Aggregate Size
• Generally, with larger aggregates,
less cement is required in the
concrete mix. Reducing the
quantity of cement reduces cost,
since cement is the most
expensive ingredient in the mix.
• The largest aggregate must fit
comfortably between reinforcing
bars and within the overall
thickness of the concrete. Largest
aggregate size should not exceed:
– 1/5 distance between form faces
– 3/4 of the space between
reinforcing bars
– 1/3 the depth of a slab
• Common maximum aggregate size
for concrete used in buildings
ranges from 3/8 to 1-1/2 in.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Aggregate Size Distribution
• Grading: Distribution of aggregate
particle sizes
• Generally, a lower percentage of
void space between particles
results in a stronger concrete that
requires less cement.
• When identically-sized particles
are closely packed, the void space
remaining between particles is
approximately 30 to 35 percent of
the total volume.
• To reduce void volume, a graded
range of aggregates varying in size
is used so that smaller particles fill
the spaces between larger particles
and a denser packing is achieved.
• Aggregate grading also affects
workability of wet concrete. An
optimum balance of characteristics
is achieved in concrete mixes with
void volumes somewhat greater
than the least possible.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Lightweight Aggregates: Used to make lighter-weight concrete
• Structural lightweight concrete:
– Roughly 80 percent or less of the weight of ordinary concrete
– Reduced structure weight saves costs
– Lower thermal conductivity increases resistance to building fires
• Nonstructural lightweight concrete:
– Roughly 60 percent or less of the weight of ordinary concrete
– Insulating roof toppings
– Fill material
• Expanded shale, clay, slate, slag: Minerals or industrial waste products thermally treated
such that they expand and take on a less dense, cellular structure
• Cinders and other volcanic rocks: Naturally occurring light-weight volcanic materials
• Vermiculate: Thermally expanded mica
• Perlite: Thermally expanded volcanic glass
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Water
• Water is an essential ingredient in
concrete, that combines
chemically with the cement as the
concrete hardens.
• Water must be free of
contaminants.
• Water that is potable is acceptable
for use in making concrete.
• ASTM C 1602 permits the use of
other sources such as waste water
from washing out concrete trucks
or storm water runoff at concrete
production facilities that may
contain limited amounts of
concrete waste material.
• The quantity of water in the
concrete mix must be controlled
as closely as any other ingredient:
Adding unneeded water dilutes
the cement paste, weakening the
hardened concrete.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Supplementary Cementitious Materials
(SCM):
• Materials added to concrete as a partial
substitute for portland cement to achieve
various benefits:
–
–
–
–
–
Increased concrete strength and durability
Higher early strength
Improved workability or wet concrete
Reduced concrete drying shrinkage
Reduced reliance on portland cement
• Pozzolans: Materials that react with
hydration byproducts in wet concrete to
form additional hydraulic cementing
compounds
– Pozzolans require the presence of a other
cementing ingredients with which to react.
They are not, on their own, hydraulic
cements.
Dumping waste molten blast furnace slag
• Hydraulic cements: Materials with intrinsic
hydraulic cementing properties
– Hydraulic cements do not require the
presence of other cementing materials with
which to react so as to function as hydraulic
cement.
Granulated blast furnace slag, an hydraulic cement
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Supplementary Cementitious Materials (SCM):
• Over half of all North American concrete includes SCMs
in the mix.
• SCMs can replace from 5 to 70 percent of the portland
cement in the concrete mix (see below).
• Use of SCMs reduces reliance on portland cement in
concrete, lessening the high energy consumption and
greenhouse gas emissions associated with portland cement
manufacturing.
• Use of SCMs can reduce the embodied energy of finished
concrete by as much as one-third.
• Use of industrial waste SCMs diverts these materials from
landfills.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
CEMENT AND CONCRETE
Admixtures
• Other ingredients in the concrete mix
used to alter or improve concrete
properties in various ways:
–
–
–
–
–
–
–
Air-entraining
Water-reducing
Cure accelerating or retarding
Workability modifying
Shrinkage-reducing
Corrosion inhibiting
Freeze protecting (for cold weather
concreting)
– Coloring
• E.g., High-strength concrete for very
tall buildings:
– SCMs, for greater strength
– Water reducers, to increase concrete
strength while maintaining workability
– Admixtures to improve pumpability
– Retarding admixtures, to allow adequate
time for placing
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
Burj Dubai, world's tallest
concrete framed building
(800+ meters, 2600+ feet)
13 CONCRETE CONSTRUCTION
MAKING AND
PLACING CONCRETE
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Concrete Quality
• Workability: Ease of placing,
consolidating, and finishing wet
concrete
• Structural properties when
hardened: Strength, stiffness,
durability
• Many other important properties:
–
–
–
–
–
–
–
–
–
–
–
–
Ease of placement
Rate of early strength gain
Degree shrinkage during curing
Flatness, for slabs and paving
Surface hardness, for industrial
slabs
Porosity
Density
Surface appearance, for
architectural concrete
Resistance to freeze/thaw and
weather, for exterior concrete
Watertightness, for dams, tanks,
exterior walls
Cost
and more…
Concrete for an exterior planter can be of relatively low strength, but
must have good resistance to weathering and, after formwork is
removed, a reasonably smooth and pleasing surface appearance.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Concrete Strength
• Concrete strength varies with design
of the concrete mix.
• Normal strength concrete
– Up to 6000 psi compressive
strength
– Made with conventional ingredients
• High-strength concrete
– Greater than 6000 psi to roughly
20,000 psi
– Supplementary cementitious
materials are required to reach
higher strengths.
– Lower water content, required for
higher strength, results in a stiff,
unworkable mixture when wet. To
compensate, water reducing
admixtures or high-range water
reducing admixtures
(superplasticizers) are used to
improve workability.
• Specially formulated, ultra-high
performance concretes have
compressive strengths as high as
30,000 psi.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Water-Cement Ratio
• The water-cement ratio (w/c ratio)
is the most important determinant
of concrete strength. Lowering the
proportion of water to cement:
– Increases concrete strength and
durability
– Decreases workability
– Increases cost
• W/C ratio is measured by weight,
not volume.
• When supplementary
cementitious materials are added
to the mix, the ratio is measured
as the water-cementitious materials
ratio.
• Why use higher-strength concrete:
– Reduce column dimensions in
tall buildings.
– Achieve higher earlier strength,
allowing construction to proceed
more quickly.
– Satisfy more stringent structural
requirements.
(A-E Concrete: Air-entrained concrete)
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Mixing Concrete
• Most concrete is prepared at batch
plants and delivered to the
construction site in transit-mix
(ready mix) trucks.
• The concrete ingredients are
mixed in the rotating drum of the
truck so that the concrete is ready
to pour on arrival at the
construction site.
• Smaller batches of concrete may
be prepared on site by hand or
with the aid of portable power
mixers.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Slump Test
• The slump test provides a rough
measure of the workability of
concrete while wet.
• Concrete is placed into a conical
cylinder; the cylinder is removed,
and the loss in height of the
concrete mass is measured.
– Concrete with too low slump
may be difficult to place.
– Concrete with too high slump
may have had too much water
added.
• Specified maximum slump is
usually in the range of 3 to 5
inches.
• Slump tests are performed on
batches of concrete as the arrive
on the concrete site.
• Slump that varies excessively from
one batch to the next may indicate
quality control problems in the
concrete mixes.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Strength Test Cylinders
• Test cylinders may be cast from
each batch of concrete delivered
to the construction site.
– The most common cylinder size
is 6 in. diameter x 12 in. high.
– Concrete is placed in a cylindrical
mold and consolidated to
eliminate voids.
Test cylinders prepared and labeled on site.
• Cylinders are returned to the
laboratory, cured under controlled
conditions, and then strengthtested at appropriate times. Test
results for laboratory-cured
cylinders verify the quality of the
concrete as delivered to the site.
• Cylinders can also be cured on
site in conditions similar to that
for the cast concrete. Results from
these tests can be used to
determine when it is safe to
remove formwork or subject cast
concrete to construction loads.
Laboratory cylinder testing apparatus
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Placing Concrete
• Avoid delays, during which
concrete can stiffen and become
difficult to place. Depending on
conditions, concrete can placed
up to 90 minutes after mixing
commences.
• Concrete that does stiffen can
have water added prior to placing,
provided that:
– Maximum w/c ratio is not
exceeded
– Maximum slump is not exceeded
– Agitation limits are not exceeded
• Concrete may be placed on site
directly from the discharge chute
of a transit-mix truck, or by
combination of wheelbarrows,
power buggies, crane-lifted
buckets (right), conveyor belts,
pumpers, or other devices.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Placing Concrete
• Segregation, separation of large
•
•
•
•
aggregate from the finer
portions of the mix, must be
avoided.
Place concrete as close to final
position as possible.
Do not push concrete over
large horizontal distances.
Avoid dropping concrete from
high heights or discharging
against obstacles (use drop
chutes if needed).
Right: Concrete delivered to its
final location by pumping.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Consolidating Concrete
• Consolidation (compaction):
The elimination of voids and
air pockets within the concrete
pour.
–
–
–
–
Hand rodding or tamping
Screeding (top)
Internal vibration (bottom)
External vibration
• Consolidation is especially
critical with stiff concrete
mixes or when concrete is
placed around densely packed
reinforcing arrays.
• Over-consolidation must be
avoided, as it can lead to
segregation of aggregate as
larger particles descend and
finer components rise to
toward the surface.
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Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Curing Concrete
• Concrete hardens by hydration, the
chemical bonding of water and
cement.
• If concrete dries out prematurely,
the hydration process stops and
maximum strength is not achieved.
• Hydration, along with increasing
strength and durability, can continue
for a very long time, even years.
Concrete strength is normally
specified at 28 days.
• Exposed surfaces of newly poured
concrete must be protected from
evaporation and drying. Concrete
may be regularly misted, covered
with moisture-retaining materials, or
treated with a chemical surface
sealer.
• In very hot or cold weather, steps
may be taken to moderate the
temperature of the concrete, such as
pre-heating of ingredients, adding
water as ice, or the use of retarders
or accelerators to adjust cure rate.
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Copyright © 2009 J. Iano. All rights reserved.
MAKING AND PLACING CONCRETE
Formwork
• Formwork: Construction, usually
temporary, to hold freshly poured
concrete in the desired shape until
the concrete achieves sufficient
strength to support itself.
• Formwork also frequently helps to
protect newly poured concrete
from drying too quickly.
• Formwork must be strong and stiff
enough to support the weight and
fluid pressure of the concrete.
• In conventional concrete
construction, formwork can
account for half or more of total
concrete construction costs.
• Form release compounds are oils,
waxes, or plastic coatings applied
to formwork surfaces to prevent
adhesion of the formwork to the
concrete and ease formwork
removal.
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Copyright © 2009 J. Iano. All rights reserved.
13 CONCRETE CONSTRUCTION
REINFORCING
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
REINFORCING
The Concept of Reinforcing
• Concrete has no useful tensile strength—ability to resist pulling forces. (See table
below.)
• Steel bars or wires are laid into concrete along lines of tension, to provide
resistance to these forces.
• Steel and concrete work well together.
– The two materials have similar rates of thermal expansion/contraction.
– The alkaline chemistry of the concrete protects the steel from corrosion.
– The two materials bond well, allowing them to work as a single structural composite.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
REINFORCING
Steel Reinforcing Bars
• Steel reinforcing bars (rebar): The
most common concrete reinforcing
material. Rebar are hot-rolled steel,
deformed with surface ridges so as to
better bond with concrete.
• Bar sizes are numbered, with numbers
generally corresponding the diameter
of the bar in 8ths of an inch. E.g.,
– #4 bar is 1/2 in. diameter
– #8 bar is 1 in. diameter
• Bars are made with steel grades of
varying strength.
• Higher strength bars are used to
reduce rebar congestion, where
reinforcing becomes crowded and
concrete placement becomes more
difficult. Use of higher strength steel
allows for bars smaller in diameter or
with greater spacing.
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Copyright © 2009 J. Iano. All rights reserved.
REINFORCING
Welded Wire Reinforcing
• Welded wire reinforcing
(WWR) or welded wire fabric
(WWF): Prefabricated, welded
grids of reinforcing bars or
wires
• Especially common for
concrete slab reinforcing.
• Smooth wire designations:
From W1.4, 0.134 in.
diameter , to W20, 0.505 in.
diameter
• Deformed wire designations:
From D4, 0.226 in. diameter,
to D20, 0.505 in. diameter.
• Designation example:
6x12-W12xW5
– W12 wires spaced at 6 in.,
crossed by
– W5 wires spaced at 12 in.
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Copyright © 2009 J. Iano. All rights reserved.
REINFORCING
Reinforcing Fabrication
and Erection
• Concrete reinforcing
requirements are shown
on the structural
drawings.
• A reinforcing fabricator
prepares shop drawings
that are reviewed by the
structural engineer.
• Reinforcing is cut to
length, bent as needed,
and possibly partially
assembled before being
transported to the
construction site.
• Fabrication continues on
site. Eventually
reinforcing is assembled
in its final configuration.
• After inspection by the
engineer, concrete may
be poured.
Shop-fabricated spiral reinforcing, for drilled concrete piers.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
REINFORCING
Reinforcing Fabrication
and Erection
Heavy reinforcing for a concrete shear wall is being pre-assembled on site.
Once complete, the assembly will be lifted into place by a construction
crane.
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Copyright © 2009 J. Iano. All rights reserved.
REINFORCING
Reinforcing a Simple
Beam
• Top: In a simply
supported beam, the
greatest tension forces
occur at the bottom
middle of the beam.
• Where tension forces
cross compression
forces closer to the
beam ends, shear
forces also occur.
• Bottom: The
placement of
reinforcing in a
concrete beam
approximates the lines
of tension, but is
simplified to reduce
fabrication costs.
Theoretical lines of tension and compression in a simple beam.
Simple concrete beam reinforcing, including heavy bottom bars, lighter top
bars, and U-shaped shear stirrups
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REINFORCING
Reinforcing a Column
• Vertical or column bars: Largerdiameter bars placed vertically in
the column
• Ties or spirals: Wrap around the
vertical bars
• The vertical bars add to the
strength of the column in
compression, and resist tensile
forces that are introduced from
wind or seismic forces, or from
connections to beams.
• Ties prevent the vertical bars from
buckling outward.
• Note how, where the vertical bars
extend beyond the ties, they are
bent slightly inward toward the
center of the column. As the
column height is extended further,
the next section of reinforcing will
nest and overlap with these ends.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
REINFORCING
Fibrous Reinforcing
• Fibrous reinforcing: Short
fibers of glass, steel, or
polypropylene, added to the
concrete mix
• Microfiber reinforcing:
Relatively low amounts of fibers,
to aid concrete in resisting
plastic shrinkage cracking that
occurs during early curing
• Macrofiber reinforcing: Greater
concentrations of fibers, that
also resist longer-term cracking
due to drying and thermal
stresses
• Steel fiber reinforcing also
increases the durability of the
concrete surface
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
13 CONCRETE CONSTRUCTION
PRESTRESSING
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
PRESTRESSING
Prestressing
• Prestessing: Applying an initial
compressive stress to a
concrete member, so as to
improve its structural
efficiency.
• High-strength steel strands are
stretched tightly and then
restrained by the concrete,
putting the concrete into
initial compression.
• When normal service loads
are added, the concrete in
prestressed members is
subject to less tensile force.
• Prestressed members are
typically more slender and
lighter than comparable
conventionally reinforced
members.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.
PRESTRESSING
Prestressing
• Pretensioning: Steel
strands are tensioned
before concrete is
cast. This requires
heavy abutments to
restrain the strands
and is normally only
done with concrete
in precasting plants,
but not on the
construction site.
• Posttensioning: Steel
strands are tensioned
after concrete has
been cast and
reached adequate
strength. Concrete
cast on the
construction site can
be posttensioned.
Posttensioning strands, encased in green plastic sheaths, for reinforcing a slab.
The strands are purposely draped to more closely follow the natural lines of
tension in the slab.
Fundamentals of Building Construction, Materials & Methods, 5th Edition
Copyright © 2009 J. Iano. All rights reserved.