Lec02 Material Properties of Concrete and Steel (Reinforced Concrete Design I & Prof. Abdelhamid Charif)

24/2/2013
GE201: Dr. N. A. Siddiqui 1
CE 370
REINFORCED CONCRETE-I
Material Properties of Concrete and Steel
Revision from CE 306 (Properties and
Testing of Structural Materials)
CE370: Prof. A. Charif 1
CONCRETE
SBC 304 Definition (Section 2.1):
Mixture of Portland cement or any other hydraulic
cement, fine aggregates, coarse aggregates, and
water, with or without admixtures.

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Aggregates
Cement
Concrete
 Rocklike Material
 Ingredients
– Portland Cement
– Coarse Aggregate
– Fine Aggregate
– Water
– Admixtures (optional)

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Setting, Hydration and Hardening
- When cement is mixed with sufficient water, it loses its
plasticity and slowly forms into a hard rock-type material; this
whole process is called setting.
- Initial set: Initially the paste loses its fluidity and within a
few hours a noticeable hardening occurs - Measured by
Vicat’s apparatus
- Final set: Further to building up of hydration products is the
commencement of hardening process that is responsible for
strength of concrete - Measured by Vicat’s apparatus
- Gypsum retards the setting process
- Hot water used in mixing will accelerate the setting process
MAKING OF CONCRETE
Mixing, placing and finishing of concrete
Mixing: Involves weighing out all the ingredients for a batch of
concrete and mixing them together
Pumping and placing: Concrete is conveyed to the construction site
in wheel barrows, carts, belt conveyors, cranes or chutes or
pumped (high-rise building) - Concrete should be placed as near
as possible to its final position - Placed in horizontal layers of
uniform thickness and consolidated before placing the next layer
Finishing: The concrete must be leveled and surface made
smooth/flat

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Transit Mix Truck
(Ready-Mix Truck)
Placement Today
Direct From Transit Mixer

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Improperly consolidated Concrete
Concrete Mixing and Proportioning
Three principal requirements:
• Quality
• Workability
• Economy

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Quality of concrete:
• Measured by strength and durability
• Principal factors affecting strength are W/C
ratio and quality of hydration
Durability of concrete:
• Its ability to resist environmental effects (heat,
freezing, chemical attacks, sulfate…)
Workability (consistency) of concrete:
• Ease with which mass of plastic material may be deposited in
its final place and form without segregation
• Concrete should be such that it can be transported, placed,
compacted and finished without harmful segregation - The
mix should maintain its uniformity and not bleed excessively
• Bleeding is movement and appearance of water at the surface
of freshly-placed concrete, due to settlement of heavier
particles

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Workability
 Workability measured by slump test
(mm)
1. Layer 1: Fill 1/3 full. 25 stokes
2. Layer 2: Fill 2/3 full. 25 stokes
3. Layer 3: Fill full. 25 stokes
4. Lift cone and measure slump (typically 50-150mm.)
1 2 3 4
300
slump

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Slump test - The measurement of the consistency of the
mix is done with the slump-cone test. The recommended
slump values for various classes of concrete structures are:
Economy:
• Effective use of materials, effective production
of concrete
• Cost of producing quality concrete is very
important in the overall cost of the project

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Influence of ingredients on concrete properties
Ingredient Quality Workability Economy
Aggregate Increases Decreases Increases
Cement Increases Increases Decreases
Water Decreases Increases Increases
Water-to-cement ratio
Effect of W/C on concrete strength
ConcreteCompressive(psi)

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Effect of Curing on
Concrete Strength
• The condition in which concrete cures affects
the ultimate strength of the hardened
concrete f’c.
• Allowing the freshly-placed concrete to have
continuous moisture applied will significantly
increase the strength
• Conversely, subjecting the freshly-placed
concrete to constant air will decrease strength

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Aggregates
• SBC 304 Definition (Section 2.1): Granular material, such as
sand, gravel, crushed stone, and iron blast-furnace slag, used
with a cementing medium to form a hydraulic cement
concrete or mortar.
• The aggregates occupy about 75% of the concrete volume.
• Fine Aggregate: Any aggregate that passes a No. 4 sieve is said
to be fine aggregate (e.g. sand)
 Coarse Aggregate: Aggregate not passing a No. 4
sieve is considered to be coarse aggregate (19 mm
most common). Example: Gravel or crushed stone.
A No. 4 sieve has wires spaced
¼ in. (6 mm) in each direction.
SBC limits on maximum aggregate size
• The maximum size aggregates that can be
used in reinforced concrete are specified in
Section 3.3.2 of the SBC code :
• One-fifth (1/5) of the narrowest dimension
• One-third (1/3) of the depth of slabs
• Three-quarters (3/4) of the minimum clear
space between reinforcement

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Properties of Aggregates
• Compressive strength
• Should be higher than concrete strength
• Strength of concrete is dependent on the strength of
aggregate particles and the strength of hardened paste
• Voids
• Represent the amount of air space between the aggregate
particles - Coarse aggregates contain 30-50% of voids and fine
aggregate 35-40%
• Moisture content
• represents the amount of water in aggregates: absorbed and
surface moisture - Coarse aggregates contain very little
absorbed water while fine aggregates contain 3-5% of
absorbed water and 4-5% surface moisture
Gradation of Aggregates
• Grading refers to a process that determines the
particle size distribution of a representative sample
of an aggregate
• Measured in terms of fineness modulus
• Sieve sizes for coarse aggregates are: 3/4”, 1/2”,
3/8”, #4 and #8
• Sieve sizes for fine aggregates are #4, #8 , #16, #30,
#50 and #100

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Types of Portland Cement
 Type I – common, all-purpose cement
 Type II – low heat of hydration and some resistance
to sulfates
 Type III – high, early strength; high heat of
hydration
 Type IV – low heat of hydration
 Type V – used for concrete with exposure to high
concentration of sulfates
Notes
• Concrete made with Type I Portland cement
must cure about two weeks to achieve
sufficient strength to permit removal of forms
and application of small loads.
 Concrete made with Type I Portland cement
reaches design strength in about 28 days.
 Concrete made with Type III Portland cement
reaches design strength in three to seven days.

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Water
• For proper chemical action, the amount of water
required is about 25% of the weight of cement
used.
• The water used for both mixing and curing should
be free from injurious amount of oils, acids,
alkalis, salts, organic materials or other
substances that may be harmful to concrete or
reinforcing steel.
• Potable water is considered satisfactory for
mixing concrete.
• The pH value of water should be more than 6.
Admixtures
• SBC 304 Definition (Section 2.1). Material
other than water, aggregate, or hydraulic
cement, used as an ingredient of concrete
and added to concrete before or during its
mixing to modify its properties.
• The concrete properties which are generally
modified using admixtures are:
– Workability
– Durability
– Time of hardening.

24/2/2013
• Need and types
• Admixtures are materials that are added to plastic concrete to
change one or more properties of fresh or hardened concrete.
• To fresh concrete: Added to influence its workability, setting
times and heat of hydration
• To hardened concrete : Added to influence the concrete’s
durability and strength
• Types: Chemical admixtures and mineral admixtures
• Chemical: Accelerators, retarders, water-reducing and air-
entraining
• Mineral : Strength and durability
Admixtures
Accelerating admixtures: Compounds added to cement to decrease
setting time and improve early strength developments - Used in
cold-weather concreting - A 25% of strength gain observed at the
end of three days
Retarding admixtures: Added to concrete to increase delay setting -
Used in hot weather applications
Problem: Early strength of concrete reduced
Water-reducing admixtures and super plasticizers : Used to reduce
amount of water - Added to improve the consistency/workability
of concrete and increase the strength
Air-entraining admixtures: Allow dispersal of microscopic air
bubbles (diameters ranging from 20 to 2000 μm) in concrete –
Decrease freeze-thaw degradation – Reduce weight of concrete
Chemical Admixtures

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• Mineral Admixtures: Used in concrete to replace part of cement
or sand . They are added in larger quantities compared to
chemical admixtures.
• Pozzolans: Raw and calcined natural materials – Siliceous and
aluminous with no cementing property, but in fine pulverized
form and in presence of water can react with lime in cement to
form concrete.
• Fly ash: By-product of coal from electrical power plants - Finer
than cement - Consists of complex compounds of silica, ferric
oxide and alumina – Increases strength of concrete and
decreases heat of hydration - Reduces alkali aggregate reaction.
• Silica fume: By-product of electric arc furnaces - Size less than
0.1μm – Increases compressive strength by 40-60%
Mineral Admixtures
Properties of Hardened Concrete
• Compressive Strength
• Modulus of Elasticity
• Tensile Strength / Modulus of Rupture
• Shrinkage and Creep

24/2/2013
Compressive Strength
SBC code:
 The specified compressive strength
of concrete is denoted by the symbol
 Compressive strength is determined
by testing a 150×300 mm cylinder at
an age of 28 days
'
cf
For most applications, the range of concrete
strength is 20 to 35 MPa.
Concrete Properties
The standard strength test generally uses a cylindrical
sample. It is tested after 28 days. The concrete will
continue to harden with time and for a normal Portland
cement will increase with time as follows:

24/2/2013
Required average compressive
strength (According to SBC 304)
Required Average Compressive strength when data are not
available to establish a standard deviation
Specified compressive
strength, in MPa
Required average compressive
strength, in MPa
20 to 35
Over 35
'
cf '
crf
5.8'
cf
0.510.1 '
cf
Compression Test Setup for
'
cf

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Concrete Stress-Strain

24/2/2013
Concrete Stress-Strain (Contd.)
The relationship between
stress and strain is initially
roughly linear up to 50% of
the ultimate strength.
Beyond this range the
relationship is non-linear
Regardless of
compressive
strength , all
concretes reach
their maximum
strength at a strain
of about 0.002

24/2/2013
Regardless of the
strength, the ultimate
strain is of the order
0.003 to 0.004
Lower strength
concrete reaches
higher ultimate strains
than higher strength
concrete.
Static Modulus of Elasticity
• Concrete has no clear-cut modulus of
elasticity. Its value varies with different
concrete strengths, concrete age, type of
loading, and the characteristics and
proportions of the cement and aggregates.
• Furthermore, there are several different
definitions of the modulus.

24/2/2013
Static Modulus of Elasticity
• Initial Modulus: Slope of the stress-strain
diagram at the origin.
• Tangent Modulus: Slope of a tangent to the curve
at any point along the curve.
• Secant Modulus: The slope of a line drawn from
the origin to a point on the curve
• Apparent (Long-term) Modulus: It is determined
using stresses and strains obtained after the load
has been applied for a certain length of time
(including creep effects).
Modulus of Elasticity
(Various definitions)

Einitial
o
fc
f’c 300
150
cu
Esecant
Etangent

24/2/2013
(SBC Section 8.5.1)
• Modulus of elasticity Ec for concrete shall be
permitted to be taken as
MPa)(in043.0 '5.1
cc fwE c

MPa)(in4700astakenbe
topermittedbeshallconcrete,weightnormalFor
.kg/m2500and1500betweenoffor valuesvalidisequationaboveThe
kg/minconcretetheoftunit weightheiswhere
'
3
3
cc
c
c
c
fE
E
w
w

(SBC Section 8.5.1)
Note: The previous modulus is actually a secant
modulus with the line (whose slope equals the
modulus) drawn from the origin to a point on the
stress-strain curve corresponding approximately to
the stress (0.45 )'
cf
MPa)(in4700 '
cc fE 

24/2/2013
(SBC Section 8.5.1)

Ec (SBC)
o
0.45f’c
fc
f’c 300
150
cu
Poisson’s Ratio
 As a concrete cylinder is subjected to compressive
loads, it not only shortens in length but also
expands laterally.
 The ratio of this lateral expansion to the
longitudinal shortening is defined as Poisson’s ratio.
Its values (no unit) are:
 About 0.16 to 0.20 for normal strength concrete
 About 0.12 for high strength concrete

24/2/2013
Tensile Strength of Concrete
 Tensile strength of concrete is about 8 to 15% of its
compressive strength
 Tensile strength varies with the square root of the
 Concrete is filled with micro-cracks
 Micro-cracks affect tensile strength, but not
 While tensile strength is small, it nevertheless has a
significant impact on deflections, bond strength,
shear strength and torsional strength
Tensile Strength Determination
• The tensile strength of concrete is quite difficult
to measure with direct axial tension loads
because of problems in gripping test specimens
and because of difficulties in aligning the loads.
As a results of these problems, two indirect tests
have been developed to measure concrete’s
tensile strength. These are:
• Modulus of rupture Test (Indirect Flexure Test)
• Split Cylinder Test.

24/2/2013
Modulus of Rupture
 Modulus of rupture: It is defined as the flexural
tensile strength of concrete. This strength is quite
important when considering beam cracks and
deflections.
 It is measured by loading 150 × 150 × 750 mm plain
(i.e. unreinforced) rectangular concrete beam up to
failure with equal concentrated loads at its one-
third points.
L/3 L/3
P/2 P/2
Modulus of Rupture Test
MPa7.0:formulaSBC '
cr ff  For Normal concrete !
 Load is increased until failure occurs by cracking on the tensile
face. The modulus of rupture fr is then determined from the
flexure formula. b is the beam width, h its depth, and M is PL/6
which is the maximum computed moment:
2
3
12
1
26
bh
PL
f
bh
hPL
I
My
f
r
r














L/3 L/3
P/2 P/2

24/2/2013
Split Cylinder Test
In this test a cylinder is placed on its side in
the testing machine, and a compressive load
is applied uniformly along the length of the
cylinder, with support supplied along the
bottom for the cylinder’s full length.
The cylinder will split in two halves from end
to end when its tensile strength is reached.
The tensile strength at which splitting occurs
is referred to as the SPLIT CYLINDER
STRENGTH and can be calculated using:
LD
P
ft

2

P = Maximum compressive force
L = Length of the cylinder
D = Diameter of the cylinder.
Tensile Strength of Concrete
• The modulus of rupture is more used to
represent tensile strength of concrete

24/2/2013
Shrinkage and Creep of Concrete
Shrinkage: Due to water loss to atmosphere
• Plastic Shrinkage: Occurs while concrete is still wet
(especially in hot days)
• Drying shrinkage: Occurs after concrete has set
• Most shrinkage occurs in first few months (80% of
creep occur in first year)
• Environment changes may cause cycles of shrinkage
and swelling
• Range of shrinkage strain : 200 to 600 microstrains
• Steel reinforcement restrain development of shrinkage
Shrinkage (continued)
Shrinkage is affected by:
• W/C ratio (higher water content increases
shrinkage)
• Relative humidity (largest shrinkage for
relative humidity of 40% or less)
• Type of cement and admixtures

24/2/2013
Creep
• Creep = Deformations under sustained loads
• Creep affected by same parameters as
shrinkage plus:
Magnitude of stress
Age at loading
• Suppression of sustained loads causes:
Elastic recovery
Partial creep recovery
Some permanent strains remain
Creep
 Deformations (strains) under sustained loads.
 Like shrinkage, creep is not completely reversible.
P
P
L
dL, elastic
dL, creep
 cr = dLcr /L

24/2/2013
Steel Reinforcement
• Because concrete is weak in tension, it is
reinforced with steel bars (or wires) that resist
the tensile stresses.
• Steel reinforcing bars are basically round in cross
section and can be plain or deformed (with lugs
or deformations rolled into the surface to aid in
anchoring the bars in the concrete).
• Plain bars are not used very often except for
wrapping around longitudinal bars, primarily in
columns.
Deformed Rebars
Ribs

24/2/2013
Specifying Bar sizes
• Plain round bars are indicated by their
diameters in fractions of an inch as 3/8”ø,
1/2”ø and 5/8”ø or in mm (SI)
• Deformed bars are round and specified using
Bar Number (#). Their sizes vary from #3 to
#11, with two very large sizes, #14 and #18.
• For bars up to an including #8, the number of
the bar coincides with the bar diameter in
eighths of an inch. For example, a #7 bar has a
diameter of 7/8 in. and a cross sectional area
of 0.60 in2 [=π/4× (7/8)2]
Specifying Bar sizes (Contd.)
KSA: Bars identified by diameters in mm
ly.respectivebarssquare.in
4
1
1.in
4
1
1bars,square.in
8
1
1.in
8
1
1
bars,square1in.in1oldtheofareasthetoequalareas
providethatdiametershavebars#11and#10,#9,The


ly.respectivebarssquare.in2in.-2andbarssquare
.in
2
1
1.in
2
1
1oldtheofareasthetoequalareasprovidethat
diametershavebars#18and#14theSimilarly,


Bar
No
Diame
ter (in)
Area
(in2)
3 0.375 0.11
4 0.500 0.20
5 0.625 0.31
6 0.750 0.44
7 0.875 0.60
8 1.00 0.79
9 1.13 1.00
10 1.27 1.27
11 1.41 1.41
14 1.70 2.25
18 2.26 4.0

24/2/2013
Grades of Reinforcing Steel
• There are several types of reinforcing bars which
are available in different grades as Grade 40,
Grade 50, Grade 60, and Grade 75.
• There is only a small difference between the
prices of reinforcing steel with yield strengths of
40 ksi and 60 ksi. As a result, 60-ksi bars are the
most commonly used in reinforced concrete
design.
• Grade 60 means the steel has a specified yield
point of 50 ksi (or 50, 000 psi). 1 ksi ≈ 7 MPa
• Grades 40, 50 , 60 and 75 approximately
corresponds to 300, 350, 420 and 520 MPa.
Steel Reinforcement
Stress
Strain
0.20
GR 300
GR 420 (less ductile)
Es
1
Es = Initial tangent modulus
Es = 200,000 MPa = 200 GPa (for all grades)
Same stress-strain curve in tension and compression
Note: GR300 has a longer yield plateau

24/2/2013
Reinforcing bars are placed a certain minimum distance
away from the edge of the member to ensure that it will
not be susceptible to water/salt infusion. This is referred
to as cover distance.

24/2/2013
Bar arrangement in layers
The bars in successive layers must be directly above the bottom
bars.
Reinforcement bar arrangement for two layers

24/2/2013
Minimum Cover Dimension
Bar Spacing, Layer Spacing
SBC 3.3.2 :
Nominal maximum
aggregate size :
- 3/4 of clear bar spacing
- 1/3 of slab depth
- 1/5 of narrowest dim.
Casting of a two-way slab floor using a concrete pump. Note the
green epoxy coating used to protect steel bars from corrosion

24/2/2013
Thank you

Lec02 Material Properties of Concrete and Steel (Reinforced Concrete Design I & Prof. Abdelhamid Charif)

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