Lec 1-2-3-introduction

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Plain & Reinforced
Concrete-1
CE-313
Lecture # 1
31th Jan 2012
Introduction

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Plain & Reinforced Concrete-1
Plain Concrete
 Constituent material of concrete and their properties.
 Hydration of cement.
 Properties of fresh and hardened concrete and factors effecting them.
 Curing of concrete and its significance.
 Testing of concrete for various properties including physical tests, strength tests.
 Crushing or ultimate strain.
 Modulus of elasticity of concrete, types, tests. Determination and significance.
 Design of normal concrete mixes, factors affecting the workability of the fresh
concrete and strength & durability of the hardened concrete.
 Alkali aggregate reaction, carbonation and sulfate attack.
 Additives and admixtures for concrete.
 Cracks in concrete.

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Mechanics of Reinforced Concrete
 Basics of composite action of steel and concrete.
 Stress-strains curves of steel and concrete.
 Actual, simplified and equivalent stress blocks.
 Behavior of reinforced concrete members including columns, beams and slabs at
working and ultimate loads.
 Specifications, codes of practice and design loads.
 Analysis, design and detailing of
 Simply supported rectangular and T-beam by ultimate strength design
method
 Simply supported and continuous one way and two way slabs.
 Reinforced concrete members for axial compression and tension.
 Tied and spiral columns.
 ACI code provisions for design of columns.

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Mechanics of Reinforced Concrete (contd…)
 Shear and diagonal tension in concrete, design and detailing of flexural
members for shear.
 Corner reinforcement in slabs.
 Assessment of crack width in flexural members.
 Introduction to alternate method of design with applications
Practical
 Physical testing of constituent material for concrete.
 Acceptance test for cement.
 Test on fresh and reinforced concrete for workability, compressive strength,
tensile strength, modulus of rupture and modulus of elasticity.
 Casting of different types of beams and columns and testing to study the effects
of various factors.
 Detailing of designed elements.

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 Text Book
 Design of Concrete Structures (13th Edition)
by Arthur H. Nilson, David Darwin & Charles W. Dolan
 Concrete Structures (Part-I) by Zahid Ahmad Siddiqi
 References
 Reinforced Concrete (5th Edition) by Edward G. Nawy
 Building Code Requirements for Structural Concrete (ACI
318-08)

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Concrete
Concrete is a mixture of cement, fine and coarse aggregate.
Concrete mainly consists of a binding material and filler material. If
filler material size is < 5mm it is fine aggregate and > 5mm is coarse
aggregate.
Plain Cement Concrete (PCC)
Mixture of cement , sand and coarse aggregate without any
reinforcement is known as PCC.
PCC is strong in compression and week in tension. Its tensile strength
is so small that it can be neglected in design.
Reinforced Cement Concrete (RCC)
Mixture of cement , sand and coarse aggregate with
reinforcement is known as RCC. (Tensile strength is improved)

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Reinforced Cement Concrete (RCC) contd..
Mix Proportion
Cement : Sand : Crush
1 : 1.5 : 3
1 : 2 : 4
1 : 4 : 8
Water Cement Ratio (W/C)
W/C = 0.5 – 0.6
For a mix proportion of 1:2:4 and W/C = 0.5, if cement is 50 kg
Sand = 2 x 50 = 100 Kg
Crush = 4 x 50 = 200 Kg Batching By Weight
Water = 50 x 0.5 = 25 Kg

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Mechanism of Load Transfer
Load
Roof Surface
Roof Slab
Beams
Column
Foundation
Sub Soil
Function of structure is
to transfer all the loads
safely to ground.
A particular structural
member transfers load
to other structural
member.

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Merits of Concrete Construction
1. Good Control over cross sectional dimensions and Shape
One of the major advantage of concrete structures is the full
control over the dimensions and structural shape. Any size and
shape can be obtained by preparing the formwork accordingly.
2. Availability of Materials
All the constituent materials are earthen materials (cement, sand,
crush) and easily available in abundance.
3. Economic Structures
All the materials are easily available so structures are economical.
4. Good Insulation
Concrete is a good insulator of Noise & heat and does not allow
them to transmit completely.

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Merits of Concrete Construction (contd…)
5. Good Binding Between Steel and Concrete
there is a very good development of bond between steel and
concrete.
6. Stable Structure
Concrete is strong in compression but week in tension and steel as
strong in tension so their combination give a strong stable
structure.
7. Less Chances of Buckling
Concrete members are not slim like steel members so chances of
buckling are much less.
8. Aesthetics
concrete structures are aesthetically good and cladding is not
required

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Merits of Concrete Construction (contd…)
9. Lesser Chances of Rusting
steel reinforcement is enclosed in concrete so chances of rusting are
reduced.
Demerits of Concrete Construction
1. Week in tension
Concrete is week in tension so large amount of steel is required.
2. Increased Self Weight
Concrete structures have more self weight compared with steel
structures so large cross-section is required only to resist self
weight, making structure costly.
3. Cracking
Unlike steel structures concrete structures can have cracks. More
cracks with smaller width are better than one crack of larger width.

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Demerits of Concrete Construction
4. Unpredictable Behavior
If same conditions are provided for mixing, placing and curing
even then properties can differ for the concrete prepared at two
different times.
5. Inelastic Behavior
concrete is an inelastic material, its stress-strains curve is not
straight so its behavior is more difficult to understand.
6. Shrinkage and Creep
Shrinkage is reduction in volume. It takes place due to loss of
water even when no load is acting over it. Creep is reduction in
volume due to sustained loading when it acts for long duration.
This problem is not in steel structures.
7. Limited Industrial Behavior
Most of the time concrete is cast-in-situ so it has limited industrial
behavior.

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Specification & Codes
These are rules given by various organizations in order to
guide the designers for safe and economical design of
structures
Various Codes of Practices are
1. ACI 318-05 By American Concrete Institute. For
general concrete constructions (buildings)
2. AASHTO Specifications for Concrete Bridges. By
American Association of State Highway and
Transportation Officials.
3. ASTM (American Standards for Testing and
Materials) for testing of materials.

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Specification & Codes (contd…)
No code or design specification can be construed
as substitute for sound engineering judgment in
the design of concrete structures. In the structural
practice, special circumstances are frequently
encountered where code provisions can only serve
as a guide, and engineer must rely upon a firm
understanding of the basic principles of structural
mechanics applied to reinforced or pre-stressed
concrete, and the intimate knowledge of nature of
materials

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Design Loads
 Dead Load
“The loads which do not change their magnitude and
position w.r.t. time within the life of structure”
Dead load mainly consist of superimposed loads and self load of
structure.
 Self Load
It is the load of structural member due to its own weight.
 Superimposed Load
It is the load supported by a structural member. For
instance self weight of column is self load and load of
beam and slab over it is superimposed load.

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Design Loads (contd…)
 Live Load
“Live loads consist chiefly of occupancy loads in buildings
and traffic loads on bridges”
 They may be either fully or partially in place or not
present at all, and may also change in location.
 Their magnitude and distribution at any given time are
uncertain, and even their maximum intensities throughout
the life time of the structure are not known with precision.
 The minimum live loads for which the floor and roof of a
building should be designed are usually specified in the
building codes that governs at the site construction.

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Densities of Important Materials
Material Density (Kg/m3)
PCC 2300
RCC 2400
Brick masonry 1900-1930
Earth/Sand/Brick ballast 1600-1800
Intensities of Live Loads (Table 1.1, Design of concrete structures by Nilson)
Occupancy / Use Live Load(Kg/m2)
Residential/House/Class Room 200
Offices 250-425
Library Reading Room 300
Library Stack Room 730
Warehouse/Heavy storage 1200

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Basic Design Equation
Applied Action x F.O.S = Max. Internal Resistance
Factor of Safety
F.O.S. = Max. Failure load/Max. Service Load
Following points are relevant to F.O.S
1. It is used to cover uncertainties due to
1. Applied loads
2. Material strength
3. Poor workmanship
4. Unexpected behavior of structure
5. Thermal stresses
6. Fabrication
7. Residual stresses
2. If F.O.S is provided then at service loads deflection and cracks are
within limits.
3. It covers the natural disasters.

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Ultimate Strength Design (USD)/LRFD Method
Strength design method is based on the philosophy of
dividing F.O.S. in such a way that Bigger part is applied on
loads and smaller part is applied on material strength.
Material Strength ≥ Applied Load x F.O.S.1 x F.O.S.2
{1 / F.O.S.2} Material Strength ≥ Applied Load x F.O.S.1
F.O.S.1 = Overload factor or Load Factor {greater than 1}
1/F.O.S.2 = Strength Reduction factor or Resistance Factor {less than 1}

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Ultimate Strength Design (USD)/LRFD Method
(contd...)
ΦSn ≥ U
Where
Sn = Nominal Strength
ΦSn = Design Strength
Φ = Strength Reduction Factor
U = Required Strength, calculated by applying load factors
For a member subjected to moment, shear and axial load:
ΦMn ≥ Mu
ΦVn ≥ Vu
ΦPn ≥ Pu

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Allowable Strength Design (ASD)
In allowable strength design the whole F.O.S. is applied on
material strength and service loads (un-factored) are taken
as it is.
Material Strength / F.O.S. ≥ Service Loads
In both Allowable strength design and Ultimate strength
design analysis carried out in elastic range.
fc’
fc’/2
Concrete Steel
fy
fy/2
fu
Strain Strain
Stress
Stress

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Plastic Design
In plastic design, plastic analysis is carried
out in order to find the behavior of
structure near collapse state. In this type
of design material strength is taken from
inelastic range. It is observed that
whether the failure is sudden or ductile.
Ductile failure is most favorable because it
gives warning before the failure of
structures

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Capacity Analysis
In capacity analysis size, shape, material strengths and
cross sectional dimensions are known and maximum
load carrying capacity of the structure is calculated.
Capacity analysis is generally carried out for the
existing structures.
Design of Structure
In design of structure load, span and material
properties are known and cross sectional dimensions
and amount of reinforcement are to be determined.

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Objectives of Designer
There are two main objectives
1. Safety
2. Economy
Safety
The structure should be safe enough to carry all the applied
throughout the life.
Economy
Structures should be economical. Lighter structures are
more economical.
Economy α 1/self weight (More valid for Steel Structures)
In concrete Structures overall cost of construction decides the
economy, not just the self weight.

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Load Combinations
To combine various loads in such a way to get a critical situation.
Load Factor = Factor by which a load is to be increased x probability
of occurrence
1. 1.2D + 1.6L
2. 1.4D
3. 1.2D + 1.6L + 0.5Lr
4. 1.2D + 1.6Lr + (1.0L or 0.8W)
Where
D = Dead load
L = Live load on intermediate floors
Lr = Live load on roof
W = Wind Load

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Strength Reduction Factor / Resistance Factor, Φ
Strength Condition Strength Reduction Factor
Tension controlled section
(bending or flexure)
0.9
Compression controlled section
Columns with ties 0.65
Column with spirals 0.75
Shear and Torsion 0.75

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Shrinkage
“Shrinkage is reduction in volume of concrete due to loss
of water”
Coefficient of shrinkage varies with time. Coefficient of shortening is:
 0.00025 at 28 days
 0.00035 at 3 months
 0.0005 at 12 months
Shrinkage = Shrinkage coefficient x Length
Excessive shrinkage can be avoided by proper curing
during first 28 days because half of the total shrinkage
takes place during this period

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Creep
“creep is the slow
deformation of material
over considerable lengths of
time at constant stress or
load”
Creep deformations for a given
concrete are practically
proportional to the magnitude of
the applied stress; at any given
stress, high strength concrete
show less creep than lower
strength concrete.
Compressive
strength
Specific
Creep
(MPa) 10-6 per MPa
20 145
30 116
40 80
55 58

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Creep (contd…)
How to calculate shortenings due to creep?
Consider a column of 3m which is under sustained load for
several years.
Compressive strength, fc’ = 30 MPa
Sustained stress due to load = 10 MPa
Specific creep for 28 MPa fc’ = 116 x 10-6 per MPa
Creep Strain = 10 x 116 x 10-6 = 116 x 10-5
Shortening due to creep = 3000 x 116 x 10-5
= 3.48 mm

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Specified Compressive Strength Concrete, fc’
“28 days cylinder strength of concrete”
 The cylinder has 150mm dia and 300mm length.
 According to ASTM standards at least two cylinders
should be tested and their average is to be taken.
ACI 5.1.1: for concrete designed and constructed in
accordance with ACI code, fc’ shall not be less than 17 Mpa
(2500 psi)

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Specified Concrete Compressive Strength, fc’
BS specifies the compressive strength in terms of
cube strength.
 Standard size of cube is 6”x6”x6”
 BS recommends testing three cubes and taking their
average as the compressive strength of concrete
Cylinder Strength = (0.75 to 0.8) times Cube Strength

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Concrete Cylinder Concrete Cube

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Relevant ASTM Standards
 “Methods of Sampling Freshly Mixed
Concrete” (ASTM C 172)
 Practice for Making and Curing Concrete
Test Specimens in Field” (ASTM C 31)
 “Test Methods for Compressive Strength of
Cylindrical Concrete Specimen” (ASTM C
39)

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Testing of Samples for Compressive Strength
Cylinders should be tested in moist condition because in
dry state it gives more strength.
ACI 5.6.2.1: Samples for strength tests of each class of concrete placed
each day shall be taken :
 Not less than once a day
 Not less than once for each 110m3 of concrete.
 Not less than once for each 460m2 of concrete.
Code allows the site engineer to ask for casting the test sample if he
regards it necessary.

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Acceptance Criteria for Concrete Quality
ACI 5.6.3.3: Strength level of an individual class
of concrete shall be considered satisfactory if
both of the following requirements are met:
 Every arithmetic average of any three consecutive
strength tests equals or exceeds fc’.
 No individual strength test (average of two
cylinders) falls below fc’
 by more than 3.5 MPa (500 psi) when fc’ is 35 MPa (5000
psi) or less; or
 by more than 0.10fc’ when fc’ is more than 35 MPa

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Acceptance Criteria for Concrete Quality (contd…)
Example
For Required fc’ = 20 MPa, if following are the test results of 7
samples
 19, 20, 22, 23, 19, 18, 24 MPa
Mean 1 = (19 + 20 + 22) / 3 = 20.33 MPa
Mean 2 = (20 + 22 + 23) / 3 = 21.67 MPa
Mean 3 = (22 + 23 + 19) / 3 = 21.33 MPa
Mean 4 = (23 + 19 + 18) / 3 = 20.00 MPa
Mean 5 = (19 + 18 + 24) / 3 = 20.33 MPa
1. Every arithmetic average of any three consecutive strength tests
equals or exceeds fc’.
2. Non of the test results fall below required fc’ by 3.5 MPa.
Considering these two point the quality of concrete is
acceptable

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Mix Design
 Ingredients of concrete are mixed together in order to
get a specified Required Average Strength, fcr’ .
 If we use fc’ as target strength during mix design the
average strength achieved may fall below fc’.
 To avoid under-strength concrete fcr’ is used as target
strength in-place of fc’.
fcr’ > fc’

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Mix Design (contd…)
ACI-5.3.2 Required Average Compressive Strength
Table 5.3.2.1-Required Average Compressive Strength when Data Are
Available to Establish a Sample Standard Deviation
Specified Compressive Strength,
fc’ (MPa)
Required Average Strength, fcr’
(MPa)
fc’ ≤ 35 Larger of value computed from Eq. (5-1) & (5-2)
fcr’ = fc’ + 1.34 Ss (5-1)
fcr’ = fc’ + 2.33 Ss – 3.5 (5-2)
fc’ > 35 Larger of value computed from Eq. (5-1) & (5-3)
fcr’ = fc’ + 1.34 Ss (5-1)
fcr’ = 0.9fc’ + 2.33 Ss (5-3)
Ss = Standard deviation of compressive strength test

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Mix Design (contd…)
Table 5.3.2.2-Required Average Compressive Strength when Data
Are Not Available to Establish a Sample Standard Deviation
Specified Compressive
Strength, fc’ (MPa)
Required Average
Strength, fcr’ (MPa)
fc’ < 21 fcr’ = fc’ + 7
21≤ fc’ ≤ 35 fcr’ = fc’ + 8.5
fc’ > 35 fcr’ = 1.10fc’ + 5

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Stress Strain Curve of Concrete
fc’ 0.85fc’
Stress
Strain
Crushing
0.0028 to 0.0045,
generally 0.003
•The first portion
of curve, to about
40% of the
ultimate strength
fc’, can be
considered linear.
•The lower the
strength of
concrete the
greater will be the
failure strain
0.4 fc’

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Modulus of Elasticity
Concrete is not an elastic material therefore it does not have a fixed
value of modulus of elasticity
Strain
Stress
Secant Modulus
Tangent Modulus
Initial tangent
Modulus
Tangent and Secant Moduli of Concrete
0.4fc’

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Modulus of Elasticity (contd…)
Secant modulus (Ec) is the one which is being used in design.
Ec = 0.043 wc
1.5√fc’
wc = density of concrete in kg/m3
fc’ = specified cylinder strength in MPa
For normal weight concrete, say wc = 2300 kg/m3
Ec = 4700√fc’

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Reinforcing Steel
Steel bars are:
 Plain
 Deformed (currently in use)
Deformed bars have longitudinal and transverse ribs. Ribs provide a good
bond between steel and concrete. If this bond fails steel becomes in
effective.
The most important properties for reinforcing steel are:
 Young's modulus, E (200 GPa)
 Yield strength, fy
 Ultimate strength, fu
 Size and diameter of bar

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Steel Bars

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Reinforcing Steel (contd..)
Stress Strain Curve for Steel
fy
fy/2
fu
Strain
Stress
Strain Hardeningyielding

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Reinforcing Steel (contd…)
Steel Grade Designation
 Grade 300, fy = 300 MPa Grade 40
FPS
Strain
Grade 300
Grade 420
Grade 520
Stress
For hot rolled
steel bars
Cold twisted
steel bars are
available in
grade 420
For hot rolled steel bars

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Reinforcing Steel (contd..)
For simplification the stress strain diagram is consider bilinear because after yielding
cracks appear and concrete becomes in effective.
Strain
Stress
Bilinear Curve

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Marks Distribution
 Mid Term Examination = 20 Marks
 Final Term Examination = 40 Marks
 Sessional = 20 Marks
 Quiz - I = 37.5 %
 Quiz – II = 37.5 %
 Assignments = 25 %
 Lab Work = 20 Marks

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Concluded

Lec 1-2-3-introduction

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