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Hardened Concrete
• Some of the important properties of hardened concrete are as follows:
– Strength
– Elasticity
– Water-tightness
– Resistance to destructive agencies
– Volume changes
– Creep
– Extensibility
– Thermal properties
9/25/2008 2Concrete Methods and Principles, (c)
Al Nasra
Concrete Strength
• Compressive strength
• Tensile strength
• Splitting Strength
• Flexural Strength
• Shear Strength
9/25/2008 3Concrete Methods and Principles, (c)
Al Nasra
Compressive Strength
• Strength is defined as unit force (stress)
required to cause rupture
• Rupture may be caused by:
– Applied tensile stress – failure in cohesion
– Applied shearing – sliding stress
– Compression – crushing stress
9/25/2008 4Concrete Methods and Principles, (c)
Al Nasra
Typical Failure of Concrete in
CompressionTypes of compression failure
There are three modes of failure.
[1] Under axial compression
concrete fails in shear.
[2] the separation of the
specimen into columnar pieces
by what is known as splitting or
columnar fracture.
[3] Combination of shear and
splitting failure.
9/25/2008 5Concrete Methods and Principles, (c)
Al Nasra
Stress-Strain CurveUniaxial Stress versus Strain Behavior in Compression
c
Ec
o u
0.45f’c
fc
f’c12”
6”
9/25/2008 7Concrete Methods and Principles, (c)
Al Nasra
Concrete PropertiesThe standard strength test generally uses a cylindrical
sample. It is tested after 28 days to test for strength, fc.
The concrete will continue to harden with time and for a
normal Portland cement will increase with time as follows:
Age Strength
Ratio
Age Strength
Ratio
7 days 0.67 6 months 1.23
14 days 0.86 1 year 1.27
28 days 1.0 2 years 1.31
3 months 1.17 5 years 1.35 8
Concrete Properties
– Compressive Strength, f’c
• Normally use 28-day strength for design strength
– Poisson’s Ratio,
• ~ 0.15 to 0.20
• Usually use 0.17
c
Ec
o u
0.45f’c
fcf’c
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Al Nasra
Concrete Properties– Modulus of Elasticity, Ec
• Corresponds to secant modulus at 0.45 f’c
• ACI 318-99 (Sec. 8.5.1):
where w = unit weight (pcf)
90 pcf < wc <155 pcf
For normal weight concrete
(wc 145 pcf)
)('33)( 5.1 psifwpsiE cc
)('000,57)( psifpsiE cc
9/25/2008 11Concrete Methods and Principles, (c)
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Concrete Properties
– Concrete strain at max. compressive stress, o
• typical curves in compression
•o varies between 0.0015-0.003
• For normal strength concrete, o ~ 0.002
Ec
o u
0.45f’c
fc
f’c
9/25/2008 12Concrete Methods and Principles, (c)
Al Nasra
Concrete Properties
– Maximum useable strain, u
• ACI Code: u = 0.003
• Used for flexural and axial compression
Ec
o u
0.45f’c
fc
f’c
9/25/2008 13Concrete Methods and Principles, (c)
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Tensile Strength of Concrete
• The Tensile strength
of concrete is roughly
10% of its
compressive strength
or, perhaps more
precisely
)(5 / psifFct
9/25/2008 15Concrete Methods and Principles, (c)
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Flexural Strength
• When concrete is subjected to bending, tensile and compression stresses and in many cases direct shearing stresses are developed. The most common plain-concrete structure subjected to flexure is a highway pavement, and the strength of concrete for pavement is commonly evaluated by means of bending tests on 6X6 inch beam specimens. Flexural strength is expressed in terms of modulus of rupture. Though the modulus of rupture is a fictitious value, it is convenient for purposes of evaluation and is commonly used. It ranges from 60% to 100% higher than the direct tensile strength. The modulus of rupture ranges from 11 to 23 % of the compressive strength.; for concrete of compressive strength of 3,500 to 4,000 psi it is the order of 550 psi, or about 15% of the compressive strength.
9/25/2008 16Concrete Methods and Principles, (c)
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Flexural Strength
Indirect Tensile Strength
– Tensile strength ~ 8% to 15% of f’c
– Modulus of Rupture, fr
• For deflection calculations, use:
– Test:
2
6
bh
M
I
Mcf r
)('5.7 psiff cr ACI Eq. 9-9
P
fr
Mmax = P/2*a
unreinforced
concrete beam
9/25/2008 17Concrete Methods and Principles, (c)
Al Nasra
Splitting Test
Indirect Tensile Test (cont.)
– Splitting Tensile Strength, fct
– Split Cylinder Test
P
Concrete Cylinder
Poisson’s
Effect
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Indirect Tensile Test (cont.)
)(')75(
2
psiftof
ld
Pf
cct
ct
(Not given in ACI Code)
9/25/2008 20Concrete Methods and Principles, (c)
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Shear Strength
• Shear is the action of two equal and
opposite parallel forces applied in planes a
short distance apart. Since concrete is
weaker in tension than in shear, failure in
torsion invariably occurs in diagonal
tension.
9/25/2008 21Concrete Methods and Principles, (c)
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Factors Affecting Results of
Strength Tests
• The following factors are most important:
– Size and shape of specimen
– Condition of casting
– Moisture content of specimen
– Temperature of specimen
– Bearing condition
– Rate of loading
9/25/2008 22Concrete Methods and Principles, (c)
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Concrete Properties
Shrinkage and Creep
– Shrinkage: Due to water loss to atmosphere (volume loss).
• Plastic shrinkage occurs while concrete is still “wet” (hot day, flat work, etc.)
• Drying shrinkage occurs after concrete has set
• Most shrinkage occurs in first few months (~80% within one year).
• Cycles of shrinking and swelling may occur as environment changes.
• Reinforcement restrains the development of shrinkage.
9/25/2008 23Concrete Methods and Principles, (c)
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Concrete PropertiesShrinkage of an Unloaded Specimen
• * 80% of shrinkage occurs in first year
Time
Shr.
Strain
T=α
9/25/2008 24Concrete Methods and Principles, (c)
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Concrete Properties
• Shrinkage is a function of
– W/C ratio (high water content reduces
amount of aggregate which restrains
shrinkage)
– Aggregate type & content (modulus of
Elasticity)
– Volume/Surface Ratio
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Concrete Properties
• Shrinkage is a function of
– Type of cement (finely ground…)
– Admixtures
– Relative humidity (largest for relative humidity of
40% or less).
– Typical magnitude of strain: (200 to 600) * 10-6
(200 to 600 microstrain)
9/25/2008 26Concrete Methods and Principles, (c)
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Free Shrinkage,
causes volume change, but no stresses
before shrinkage After Shrinkage
9/25/2008 28Concrete Methods and Principles, (c)
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Restrained Shrinkage- creates stresses,
which may cause cracking
9/25/2008 29Concrete Methods and Principles, (c)
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Restrained shrinkage cracking
Parallel cracking perpendicularto the direction of shrinkage
9/25/2008 30Concrete Methods and Principles, (c)
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Concrete Properties– Creep
• Deformations (strains) under sustained loads.
• Like shrinkage, creep is not completely reversible.
P
P
L
L, elastic
L, creep
= L/L
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Concrete Properties
• Magnitude of creep strain is a function of all the
above that affect shrinkage, plus
– magnitude of stress
– age at loading
9/25/2008 33Concrete Methods and Principles, (c)
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Concrete Properties
• Creep strain develops over time…
– Absorbed water layers tend to become thinner
between gel particles that are transmitting
compressive stresses
– Bonds form between gel particles in their
deformed position.
9/25/2008 34Concrete Methods and Principles, (c)
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Concrete Properties
– Tri-axial Compression
• Confined Cylinder
• Improved strength and ductility versus uniaxial
compression
• Example: spiral reinforced
where,
F1 = longitudinal stress at failure
F3 = lateral pressure
31 1.4'cf
F1
F1
F3
9/25/2008 35Concrete Methods and Principles, (c)
Al Nasra
Concrete Curing
• Tests show that improper curing can easily cut the strength of even the best concrete mix by 50%. Curing simply means keeping the water in the concrete where it can do its job of chemically combining with the cement and turning it into strong “glue” that will help make strong, durable concrete. Recommended practice calls for at least 7 days of curing.
9/25/2008 36Concrete Methods and Principles, (c)
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Concrete Curing
• All concrete must be cured to get the max.
strength of the concrete mix.
• Start curing as soon as possible after it has
hardened.
9/25/2008 37Concrete Methods and Principles, (c)
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Concrete Curing
Poor curing can cut the strength of
concrete by 50%
9/25/2008 38Concrete Methods and Principles, (c)
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Methods of Curing
• Water spay
• Waterproof papers holds moisture in
concrete by preventing evaporation
• Damp burlap
• Membrane curing compounds seal moisture
in the concrete
• Plastic sheets
9/25/2008 39Concrete Methods and Principles, (c)
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Concrete Curing
• Cure concrete longer when the temperature
is below 70 F
• Good curing results:
– More durable concrete
– More wear-resistance concrete
– Less cracking, crazing and spalling of the
concrete
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Durable Concrete DO’S
• Do specify 5 to 7% entrained air
• Do specify high-strength concrete
• Do specify quality and tested materials
• Do specify good workmanship
• Do specify proper curing
• Do consider of a surface sealer
• Do specify Pozzolith Admixture; improves the workability, placeability, and finishability of concrete; it reduces permeability, absorption and shrinkage cracking, and it increases strength
9/25/2008 41Concrete Methods and Principles, (c)
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Durable Concrete DON’TS
• Don’t assume that air entrainment alone is enough
• Don’t permit addition of “extra” water
• Don’t permit the use of materials of questionable
quality
• Don’t permit overworking of the concrete
• Don’t allow concrete to dry during curing
• Don’t allow application of salt to new concrete
9/25/2008 42Concrete Methods and Principles, (c)
Al Nasra
Cold Weather Concreting
• At low temperature concrete sets slowly and development of strength is delayed. Therefore, job planning should include one or more of the following:
• Heating the water and concrete materials
• Heating the area in which the concrete is placed
• Use additional cement or high early strength cement (Type III)
• Addition of calcium chloride to the mix
• Special provision for curing
9/25/2008 43Concrete Methods and Principles, (c)
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Tips on Cold Weather Concreting
• Temperature of all surfaces to be in contact
with the new concrete should be raised to as
close as practical to the temperature of the
new concrete.
• The temperature of freshly placed concrete
in cold weather should be at least 50F and
not more than 90F
9/25/2008 44Concrete Methods and Principles, (c)
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Cold weather Concreting (cont.)
• Use calcium chloride or high early strength
cement. Calcium chloride up to 2% by
weight of cement is often recommended.
9/25/2008 45Concrete Methods and Principles, (c)
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Cold Weather Concreting (cont.)
• Protect from wind and rapid moisture loss; provide heated
enclosure if necessary.
• Provide insulation or heated enclosure to maintain concrete
temperature for min. periods as shown
70 F 50F
Plain concrete 3 days 7 days
PC - calcium Chloride 2 days 3 days
Type III Cement 2 days 3 days9/25/2008 46Concrete Methods and Principles, (c)
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Cold Weather concreting (cont.)
• Curing and protection from start to finish
should be continuous.
9/25/2008 47Concrete Methods and Principles, (c)
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Hot Weather Concreting
• Before placing
– Concrete sets faster in hot weather, use retarder such as Pozzolith Retarder.
– Periodically spay the forms, reinforcing steel and subgrade with water.
– Erect sun shields and wind barriers to protect the fresh concrete from stiffening or crusting and to help minimize cracking, crazing, plastic shrinkage and rubber set
9/25/2008 48Concrete Methods and Principles, (c)
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Hot Weather Concreting
• During placing and finishing
– Don’t let ready mix trucks stand in the sun
– Promptly notify ready mixed concrete producer
of any delay in placing.
– Vibrate or screed without delay
– Protect test specimens by covering, and
maintaining at 60F to 80F
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Hot Weather Concreting
• After Finishing
– Start curing immediately
– Periodically spray water on the outside of forms
for effective cooling and curing.
9/25/2008 50Concrete Methods and Principles, (c)
Al Nasra
Materials Unfriendly to Hardened Concrete- How to Prevent Damage, From ACI
Chemical
Substance
Effect in Concrete How to minimize
the effect
Protective coating
suggested
Carbon dioxide Harmless to
mature concrete,
may dissolve in
water to produce
carbonic acid
Be sure
combustion
heaters are
properly vented
when placing
concrete in a
heated enclosure
Surface hardeners
and various
coatings, per ACI
515
Carbonic acid Highly corrosive
to lean, permeable
concrete, causes
slow disintegration
of better concrete
Use dense,
impermeable
concrete with high
cement content
Epoxy, neoprene,
vinyl and other
coating per ACI
515
Garbage Disintegrate
concrete slowly
Use good quality
concrete of low
permeability
Regular scraping
with metal blade is
necessary 51
Materials Unfriendly to Hardened Concrete- How to Prevent Damage, From ACI
Chemical
Substance
Effect in Concrete How to minimize
the effect
Protective coating
suggested
Gasoline Not harmful to
hardened concrete
Lactic acid Causes slow
disintegration
Antibacterial
cements. Dry mix,
w/c <0.44
Protective coating
Milk Fresh, not harmful,
sour see lactic acid
9/25/2008 52Concrete Methods and Principles, (c)
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Materials Unfriendly to Hardened Concrete- How to Prevent Damage, From ACI
Chemical
Substance
Effect in Concrete How to minimize
the effect
Protective coating
suggested
Nitric acid Continued contact
with strong
solution destroy
concrete. Weak
solution attack
slowly
Be sure
combustion
heaters are
properly vented
when placing
concrete in a
heated enclosure
Protective coating,
per ACI 515
Sea water Disintegrate
concrete of
inadequate sulfate
resistance and
attacks
reinforcement
Provide a high
quality air-
entrained mix with
3-inch cover over
reinforcement
Protective coating
Vinegar, 5% acetic
acid
Disintegrate
concrete slowly
Heavy duty
concrete
Protective coating
53, (c) Al Nasra
Materials Unfriendly to Hardened Concrete- How to Prevent Damage, From ACI
Chemical
Substance
Effect in Concrete How to minimize
the effect
Protective coating
suggested
Sewage Usually not
harmful to good
concrete. If
hydrogen sulfide
gas is present and
exposed to air,
sulfuric acid may
form and attack
concrete
Use carbonate
aggregates.
Prevent the gas
conversion
process. For
industrial waste,
use cement with <
8% C3A
Bituminous,
epoxy, vinyl
coating. Brick or
tile covering
Sugar and sugar
products
Dry, no effect on
hardened concrete.
In solution: attacks
concrete
Cure concrete
thoroughly, then
allow to dry >28
days.
Fluo-silicate
treatment or
bituminous, epoxy,
neoprene, vinyl
and other coating
per ACI 51554
Recommended