REINFORCED CONCRETE STRUCTURE CHAPTER ONE GENERAL INTRODUCTION

ARBA MINCH UNIVERSITY
INSTITUTE OF TECHNOLOGY
REINFORCED CONCRETE I
1

Content
• General introduction
• Mechanical Properties of concrete
• Mechanical Properties of Reinforcing steel
2

General introduction
Concrete is the most frequently used building material in
the world. This is because the basic ingredients are available
everywhere and it is relatively easy and cheap to produce.
Concrete's inherent strength, fire resistance, energy
efficiency* and durability helps to build safe, secure and
comfortable buildings and other structures.
*energy efficiency The amount of energy required for the production of concrete is low compared with steel
3

Properties of reinforced concrete
Concrete is a solid, hard material produced by combining
Portland cement, coarse (gravel) and fine (sand) aggregates,
and water in proper proportions.
4

Concrete strength is affected by many factors, such as
quality of raw materials
water/cement ratio
coarse/fine aggregate ratio
age of concrete
compaction of concrete
temperature
relative humidity and
curing of concrete
5

A wide range of strength properties can be obtained for
concrete by;
 appropriate adjustment of the proportions of the
constituent materials,
using different degree of compaction and
the conditions of temperature and moisture under which
it is placed and cured.
6

Water-cement ratio is the main factor affecting the strength of
concrete, as shown in figure below. The lower the water-cement ratio,
the higher is the compressive strength.
7

Stress–strain relations
The loads on a structure cause distortion of its members with resulting
stresses and strains in the concrete and the steel reinforcement.
To carry out the analysis and design of a member it is necessary to have
a knowledge of the relationship between these stresses and strains.
This knowledge is particularly important when dealing with reinforced
concrete which is a composite material;
for in this case the analysis of the stresses on a cross-section of a
member must consider the equilibrium of the forces in the concrete
and steel, and also the compatibility* of the strains across the cross-
section.
*Strain Compatibility. The essence of strain compatibility is that plane sections remain plane and
the steel strain is the same as the concrete strain at all locations.
8

Stress–strain relations-concrete
Concrete is a very variable material, having a wide range of strengths
and stress–strain curves. A typical curve for concrete in compression
is shown in figure below
Figure; Stress–strain curve for concrete in compression
9

Stress–strain relations for concrete
As the load is applied, the ratio between the stresses and strains is
approximately linear at first and the concrete behaves almost as an
elastic material with virtually full recovery of displacement if the load
is removed.
Eventually, the curve is no longer linear and the concrete behaves
more and more as a plastic material.
If the load were removed during the plastic range the recovery would
no longer be complete and a permanent deformation would remain.
The ultimate strain for most structural concretes tends to be a
constant value of approximately 0.0035
10

Compressive strength of concrete
Compressive strength refers to the strength of hardened concrete
when measured by a compression test, which entails crushing cube or
cylindrical concrete in a compression testing machine.
It tests the capacity of concrete to withstand a load before experiencing
failure.
There are many tests applied to concrete, but the compressive strength
test is one of the more crucial tests as it provides contractors
information on the strength characteristics of concrete.
11

Compression strength of concrete
Standard test specimens of 150mm cube are taken at the age of 28days
to determine the compressive strength of concrete according to
Ethiopian standard institution (ESI).
At age of 7days, concrete may attain approximately about 2/3 of the
full compressive strength of concrete.
In some national standard cylinder specimens of 150mm diameter by
300mm high are taken.
12

Figure; Compression strength testing of a capped test cylinder.
 For normal strength concretes, the cylinder
strength is, on average, about 0.8 the cube
strength.
 All design calculations to EC2 are based on the
characteristic cylinder strength 𝑓𝑐𝑘
13

for example strength class C35/45 concrete has a characteristic
cylinder strength of 35 N/𝑚𝑚2
and a characteristic cube strength
of 45 N/𝑚𝑚2.
It will be noted that there is some ‘rounding off’ in these values,
which are usually quoted in multiples of 5 N/𝑚𝑚2 for cube
strength.
14

Modulus of elasticity of concrete
As per Hooke’s law, up to the proportional limit, “for small
deformation, stress is directly proportional to strain.”
Mathematically, Hooke’s Law expressed as:
Stress α Strain
σ = E ε
In the formula as mentioned above, “E” is termed as Modulus
of Elasticity. σ is the Stress, and ε denotes strain.
E =
stress
strain
 The modulus of elasticity is required when investigating the deflection and
cracking of a structure.
15

A number of alternative definitions exist, but the most commonly
adopted is E = 𝐸𝑐𝑚 where 𝐸𝑐𝑚 is known as the secant or static modulus.
This is measured for a particular concrete by means of a static test in
which a cylinder is loaded to just above one-third of the corresponding
mean control cube stress 𝑓𝑐𝑚,𝑐𝑢𝑏𝑒 or 0.4 mean cylinder strength, and
then cycled back to zero stress. This removes the effect of initial
‘bedding-in’ and minor stress redistributions in the concrete under
load.
The load is reapplied and the behavior will then be almost linear; the
average slope of the line up to the specified stress is taken as the value
for 𝐸𝑐𝑚.
16

Figure; Modulus of elasticity of concrete
𝑓𝑐𝑚= 𝑓𝑐𝑘 + 8 (MPa)
The dynamic modulus of elasticity, 𝐸𝑑 , is sometimes
referred too since this is much easier to measure in the
laboratory and there is a fairly well-defined relationship
between 𝐸𝑐𝑚 and 𝐸𝑑.
17
𝐸𝑐𝑚 = 22[(𝑓𝑐𝑚) 10]
0.3

Table ; Short-term modulus of elasticity of normal-weight gravel concrete
The actual value of E for a concrete depends on many factors related to the mix, but a general
relationship is considered to exist between the modulus of elasticity and the compressive strength
18

Shrinkage and thermal movement
As concrete hardens there is a reduction in volume. This shrinkage is liable
to cause cracking of the concrete, but it also has the beneficial effect of
strengthening the bond between the concrete and the steel reinforcement.
Shrinkage begins to take place as soon as the concrete is mixed, and is
caused initially by the absorption of the water by the concrete and the
aggregate.
Further shrinkage is caused by evaporation of the water which rises to the
concrete surface.
During the setting process the hydration of the cement causes a great deal
of heat to be generated, and as the concrete cools, further shrinkage takes
place as a result of thermal contraction.
Even after the concrete has hardened, shrinkage continues as drying out
persists over many months, and any subsequent wetting and drying can
also cause swelling and shrinkage.
19

Thermal shrinkage may be reduced by restricting the temperature
rise during hydration, which may be achieved by the following
procedures:
1. Use a mix design with a low cement content or suitable cement
replacement e.g. fly ash (pulverised fuel ash) or ground
granulated blast furnace slag.
2. Avoid rapid hardening and finely ground cement if possible
3. Keep aggregates and mixing water cool , or may be need to keep
them under shade.
4. Maintaining the temperature & evaporating water by proper
curing.
20

• When the tensile stresses caused by shrinkage or thermal
movement exceed the strength of the concrete, cracking will
occur.
• To control the crack widths, steel reinforcement must be
provided close to the concrete surface;
• The codes of practice specify minimum quantities of
reinforcement in a member for this purpose.
21

Creep
Creep is the continuous deformation of a member under sustained
compressive stress over a considerable length of time (under long-
term loading).
Creep deformation depends on the stress in concrete, duration of
loading and water-cement ratio.
A typical variation of deformations with time can be obtained for
concrete member subjected to axial deformation under constant load
over considerable length of time, as shown next slide.
22

Creep
Figure; Typical increase of deformation with time for concrete
23

Creep
The characteristics of creep are
1. The final deformation of the member can be three to four times the short-
term elastic deformation.
2. The deformation is roughly proportional to the intensity of loading and to
the inverse of the concrete strength.
3. If the load is removed, only the instantaneous elastic deformation will
recover – the plastic deformation will not.
4. There is a redistribution of load between the concrete and any steel present.
24

Creep
 The effects of creep are particularly important in beams, where the
increased deflections may cause the opening of cracks, damage to
finishes, and the non-alignment of mechanical equipment.
 Redistribution of stress between concrete and steel occurs primarily in the
un-cracked compressive areas and has little effect on the tension
 The provision of reinforcement in the compressive zone of a flexural
member often helps to restrain the deflections due to creep.
25

Durability
Concrete structures, properly designed and constructed, are long
lasting and should require little maintenance. The durability of the
concrete is influenced by;
1. the exposure conditions;
2. the cement type;
3. the concrete quality;
4. the cover to the reinforcement;
5. the width of any cracks.
26

Durability
Concrete can be exposed to a wide range of conditions such as the soil,
sea water, de-icing salts, stored chemicals or the atmosphere.
The severity of the exposure governs the type of concrete mix required
and the minimum cover to the reinforcing steel.
Whatever the exposure, the concrete mix should be made from
impervious and chemically inert aggregates.
A dense, well-compacted concrete with a low water–cement ratio is all
important and for some soil conditions it is advisable to use a sulfate
resisting cement.
27

Durability
Air entrainment is usually specified where it is necessary to cater for
repeated freezing and thawing.
Adequate cover is essential to prevent corrosive agents reaching the
reinforcement through cracks and pervious concrete.
The thickness of cover required depends on the severity of the exposure
and the quality of the concrete.
The cover is also necessary to protect the reinforcement against a rapid
rise in temperature and subsequent loss of strength during a fire.
28

Reinforcing Steel
Reinforcing Steel is a steel bar or mesh of steel wires used as a tension
device in reinforced concrete and reinforced masonry structures to
strengthen and aid the concrete under tension.
Concrete is strong under compression, but has weak tensile strength.
Rebar significantly increases the tensile strength of the structure.
29

Reinforcing Steel
Reinforcing steel is capable of resisting both tension and
compression.
Compared with concrete, it is a high strength material
The shape of the stress-strain curve is similar for all steel, and differs
only in the value of strength of steel,
The modulus of elasticity, ES being for all practical purposes constant.
ES is taken as 200GPa.
30

Reinforcing Steel
The most commonly used bars have projected ribs on the surface of
bar. Such bars are called deformed bars.
The ribs of deformed bar improve the bond between steel and the
surrounding concrete in RC members by providing mechanical keys.
A wide range of reinforcing bars is available with nominal diameter
31

Composite action
33
Concrete and steel have widely differing properties, some of which
are listed below:
Concrete steel
Strength in tension poor good
Strength in compression good good, but slender bars will buckle
Strength in shear fair good
Durability good corrodes if unprotected
Fire resistance good poor, suffers rapid loss of strength at high
temperature

Composite action
34
when steel and concrete are combined, the steel is able to provide
the tensile strength and probably some of the shear strength
Concrete, strong in compression, protects the steel to give it
durability and fire resistance
The tensile strength of concrete is only about 10 per cent of the
compressive strength. Because of this, nearly all reinforced concrete
structures are designed on the assumption that the concrete does not
resist any tensile forces.
Reinforcement is designed to carry these tensile forces, which are
transferred by bond between the interface of the two materials.

Composite action
35
Figure below illustrates the behavior of a simply supported beam
subjected to bending and shows the position of steel reinforcement
to resist the tensile forces, while the compression forces in the top of
the beam are carried by the concrete.
Figure; composite action

REINFORCED CONCRETE STRUCTURE CHAPTER ONE GENERAL INTRODUCTION

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REINFORCED CONCRETE STRUCTURE CHAPTER ONE GENERAL INTRODUCTION