CHAPTER 1: DESIGN CONCEPT OF

REINFORCED CONCRETE Hours=8

Marks=8
CLASS 12 – CIVIL ENGINEERING
Prepared by: Er Nilesh Kumar Jha

Contents:
1. Properties of concrete and steel reinforcement
2. Behavior of Reinforced concrete in Bending
3. Design of a Reinforced Concrete section
4. Modular Ratio, Permissible and Ultimate Stress
5. Describe Ultimate Load and Limit State Method of Design
 INTRODUCTION
1) Plain Cement Concrete (PCC):
➢ Plain Cement Concrete is a hardened mass obtained from a mixture of cement, sand, gravel and water
in definite proportions. The hardening of this mixture is caused by a chemical reaction between cement
and water.
➢ PCC has good compressive strength but very little tensile strength, thus limiting its use in construction.
➢ PCC is used where good compressive strength and weight are the main requirements and tensile
stresses are very low.

2) Reinforced Cement Concrete(RCC):

➢ Reinforced Cement Concrete is a composite material which is made up of Concrete and steel
reinforcement.
➢ PCC has very low tensile strength. To improve the tensile strength of concrete, some sort of
reinforcement is needed which can take up the tensile stresses developed in the structures.
➢ The reinforcing steel is placed in the forms and fresh concrete is poured around it. This solidified
composite mass is called as RCC.
 Reinforced Cement Concrete (RCC)

Plain Cement Concrete PCC

 1.1 PROPERTIES OF CONCRETE AND STEEL REINFORCEMENT
R.C.C. consists of concrete and reinforcing material. The strength of an R.C.C. section depends upon
the kind of concrete and reinforcement used.
(a) Properties Of Concrete: (Clause 6.2 of IS 456:2000)
The properties of concrete depend upon the properties and proportions of its ingredients. The following
are the important properties of concrete which are used in the design.

i. Compressive Strength: The characteristic compressive strength of concrete is defined as the

compressive strength of 150 mm cubes at 28 days in N/mm2, below which not more than 5% of the
test results are expected to fall. It is represented by fck. The minimum grade of concrete for plain
and reinforced concrete shall be as per Table 5 of IS 456:2000.
ii. Increase of Strength with Age: There is normally a gain of strength beyond 28 days. The
quantum of increase depends upon the grade and type of cement, curing, environmental
conditions, etc. The design should be based on 28 days characteristic strength of concrete unless
there is an evidence to justify a higher strength for a particular structure due to age.
iii. Tensile Strength Of Concrete: The tensile strength of the concrete can be correlated with the
characteristic compressive strength of concrete. IS 456:2000 gives the following correlation which
can be used in the design: ftc=0.7 𝑓𝑐𝑘
iv. Elastic Deformation or Modulus of Elasticity: The modulus of elasticity is primarily influenced
by the elastic properties of the aggregate and to a lesser extent by the conditions of curing and
age of the concrete, the mix proportions and the type of cement. The modulus of elasticity is
normally related to the compressive strength of concrete. The modulus of elasticity of concrete can
be assumed as follows: Ec=5000 𝑓𝑐𝑘 ; where Ec is the short term static modulus of elasticity in
N/mm2
v. Shrinkage: The total shrinkage of concrete depends upon the constituents of concrete, size of the
members and environmental conditions. For a given humidity and temperature, the total shrinkage
of concrete is mostly influenced by the total amount of water present in the concrete at the time of
mixing and, to a lesser extent, by the cement content.
vi. Creep of Concrete: The plastic deformation of concrete under the application of constant load is
called Creep. Creep of concrete depends, in addition to the factors listed in (v), on the stress in the
concrete, age at loading and the duration of loading. Age at loading Creep coefficient
7 days 2.2
28 days 1.6
1 year 1.1
vii. Thermal Expansion: The coefficient of thermal expansion depends on nature of cement, the
aggregate, the cement content, the relative humidity and the size of sections. The value of
coefficient of thermal expansion for concrete with different aggregates may be taken as follows:
Type Of Aggregate Coefficient of Thermal Expansion for Concrete/℃
Quartzite 1.2 to 1.3 * 10-5
Sandstone 0.9 to 1.2 * 10-5
Granite 0.7 to 0.95 * 10-5
Basalt 0.8 to 0.95 * 10-5
Limestone 0.6 to 0.9 * 10-5

viii. Durability: It is the ability of concrete to resist weathering action, chemical attack and abrasion
while maintaining its desired engineering properties. A durable concrete is one that performs
satisfactorily in the working environment during its anticipated exposure conditions during service.
One of the main characteristics influencing the durability of concrete is its permeability to the ingress
of water, oxygen, carbon dioxide, chloride, sulphate and other potentially deleterious substances.

ix. Unit Weight: Generally, for PCC, the unit weight is taken as 24 kN/m3 and that of RCC is taken as
25 kN/m3
x. Poisson’s Ratio: Poisson’s ratio of concrete is the ratio of lateral strain (transverse strain) to
longitudinal strain in concrete specimen subjected to axial loads. In rich mix, the cement content
will be more and hence, more cement paste will be formed as compared to lean mix. This binds
all the aggregates together due to which the compressive strength will be increased which
results the lower transverse strains and hence, the lower value of Poisson’s ratio. Poisson’s
ratio decreases with rich mix and vice-versa.
• Poisson’s ratio for concrete varies from 0.1 to 0.2.
• The value 0.15 corresponds to high strength concrete or Richer mix.
• The value 0.2 corresponds to Serviceability criteria of concrete.
 (b) Properties of Reinforced Steel:
1. It possesses high tensile strength and elasticity.
2. Its thermal coefficient is nearly equal to that of concrete.
3. It develops good bond with concrete.
4. It is cheaply and easily available in bulk.
5. It is economical comparing all the aspects.
6. The modulus of elasticity of steel is taken as 200 KN/mm2 or 200000 N/mm2.
 Deformed Bars
Plain bars
 1.2 Behavior of Reinforced Concrete in Bending:

• Plain cement concrete is capable for taking compressive load only, but have minimum capacity to take tensile
load, while reinforced concrete structures have capability to take compressive as well as tensile loads. The plain
cement concrete structures fail in tension but reinforced concrete structures are less liable to fail under safe
design loads.
❑ To understand the behavior of beam before designing, let us consider that a RC beam is subjected to
transverse loading and the load is gradually increased in magnitude until the beam fails.
❑ The beam will go through three stages before any type of collapse:
(A) Uncracked concrete stage,
(B) Cracked concrete - elastic stress stage and
(C) Ultimate strength stage.

(A)Uncracked Concrete Stage:

➢ At small loads, when the tensile stresses are less than the modulus of rupture (the bending tensile
stress at which the concrete begins to crack), the entire cross section of the beam resists the bending with
compression on one side and tension on the other. And, this stage is known as uncracked concrete stage
(stage I).
 (B) Cracked concrete – Elastic stress stage:
➢ At loads beyond modulus of rupture, cracks begin to develop in the bottom fiber of the beam.
➢ The moment developed at that loads when the tensile stress in the bottom of the beam equals the
modulus of rupture is termed as cracking moment Mcr.
➢ On further increase in the load intensity, the moment will become greater than the cracking moment and
cracks quickly spread up to the concrete surrounding the neutral axis and neutral axis begins to move
upwards.
➢ Now concrete in the cracked zone cannot resist tensile stresses, so, the effective concrete section is reduced.
Steel reinforcement is required to take care of tensile stresses.
➢ The stresses now developed will be transferred to steel reinforcement.
➢ This stage is the stage II, which is known as the cracked concrete stage.
 (C) Ultimate Strength Stage – Yielding Of Tension reinforcement & Collapse
➢ If the loads are increased further, the tensile stress in the reinforcement and the compression stress in the
concrete increases further.
➢ The stresses over the compression zone will become non linear. However, the strain distribution over the
cross section is linear.
➢ At one point either the steel or concrete will reach its respective capacity; steel will start to yield or the
concrete will be crushed and this is called the ultimate strength stage or stage of collapse.
Fig. Moment-Curvature relationship for Reinforced Concrete
Structure in Bending.
 1.3 Design Of A Reinforced Concrete Section:
Design of any RC section comprises of the following:
1. To decide the size (dimensions) of the members and the amount of Reinforcement required.
2. To check whether the adopted portion will perform safely and satisfactorily during the lifetime of
the structure.

With an appropriate degree of safety, the structures should have the following basis;
1. Sustain all loads expected on it.
2. Sustain deformations during and after construction.
3. Should have adequate durability.
4. Should have adequate resistance to misuse and fire.
5. Structure should be able to have alternate load paths to prevent overall collapse under accidental
loadings.

Correct estimation of all types of loads (dead loads, live loads, wind loads, Earthquake loads, etc.)
should be made before the design. Design of reinforced concrete section shall ensure the following
design considerations:

……………to be continued
1. Safety is the basic requirement of the reinforced concrete structures. The concrete sections should be
designed in such a way that it’s all dimensions are liable to take applied loads safely.
2. The concrete structure should be stable enough to take expected load safely.
3. Serviceability requirement is related to the utility of the structure. It means that the structure should
satisfactorily perform under service loads without discomfort to the user due to excessive deflection,
cracking, vibration and so forth.
4. Reinforced concrete structure should be economical as much as possible. The economy of reinforced
concrete section is determined by the size of sections and quality of materials to be used.
5. A RC structure is said to be durable if it functions up to its design period properly. Durability of
concrete is determined by the quality of materials and workmanship during construction.
6. Aesthetic consideration of the reinforcement concrete structure includes selection of shape,
geometrical proportions, symmetry, surface texture, color and harmony. The structural engineer must
work in close coordination with the architects, planners and other design professionals to design
aesthetic structures that are elegant and at the same time economical and functional.
7. The reinforced concrete structure shall be designed to meet the requirement of environmental
friendliness. An environment friendly structure is the such structure, which requires least degradation
during construction process.
8. A structure must always be designed to serve its intended functions as specified by the owner and
architecture.
9. The ability of reinforced concrete material to sustain a large permanent deformation under a tensile
load up to the point of fracture is called ductility. The reinforced concrete structure shall be designed to
withstand the compressive as well as tensile load safely.
 1.4 Modular Ratio, Permissible and Ultimate Stress:
1. Modular Ratio: Modular ratio (m) is defined as the ratio between Modulus of Elasticity of
Es
Steel and Modulus of Elasticity of Concrete. m=
Ec
Where, Es=Modulus of Elasticity of Steel which is 200000 N/mm2
Ec= Modulus of Elasticity of Concrete which is 5000 𝑓𝑐𝑘
But this value of m cannot be used for design purpose as it doesn’t take into account the long-term effects
𝟐𝟖𝟎
such as Creep. Hence, the modular ratio m has the value: m= as per ANNEX B, IS 456:2000
3σcbc
where, σcbc is the permissible compressive stress in the concrete in Bending.
2. Ultimate Stress: The maximum stress that a given material can withstand under an applied
load/force is known as ultimate stress.

3. Permissible Stress: Stress in structures are not allowed to exceed a certain proportion of
the yield stress of the material, which is known as permissible stress. Permissible stresses are
obtained by dividing the ultimate strength of concrete or yield strength of steel by appropriate factors
of safety. Permissible stresses for the various grades of concrete shall be taken as those given in
Table 21 and 23. Permissible stresses in steel reinforcement shall not exceed the values specified in
Table 22 of IS 456:2000
 1.5 Describe Ultimate Load and Limit State Method of Design
(a) Ultimate Load Method:
❑ It is also known as load factor method.
❑ In this method, ultimate or collapse load is used as design load.
❑ The ultimate loads are obtained by increasing the working or service loads suitably by some factors. These
factors which are multiplied by the working loads to obtain ultimate loads are called as load factors.
❑ These load factors give the exact margins of safety in terms of load.
❑ This method uses the real stress-strain curve of concrete and steel and takes into account the plastic behavior
of these materials. This method was given in detail in IS 456:1964.
❑ This method gives very thin sections which result in excessive deformation and cracking thus, making the
structure almost unserviceable.
❑ This method is not at all used by designers.

𝐶𝑜𝑙𝑙𝑎𝑝𝑠𝑒 𝐿𝑜𝑎𝑑
Load Factor=
𝑊𝑜𝑟𝑘𝑖𝑛𝑔 𝐿𝑜𝑎𝑑
 Advantages:
o This method is more realistic as compared to working stress method because ultimate load method takes into
account the non-linear behavior of the concrete.
o This method gives exact margin of safety in terms of load unlike working stress method which is based on the
permissible stresses which do not give any idea about the failure or collapse load.
o The sections designed by ultimate load method are thinner and require less reinforcement. Hence, the method
is economical as compared to WSM.

Limitations:

o This method gives very thin sections which leads to excessive deformations and cracking, thus making the
structure unserviceable.
o New factors of safety are used for material stresses.
 (b) Limit State Method
o This is the most rational method which takes into account the ultimate strength of the structures and also the
serviceability requirements.
o It is a judicious combination of working stress and ultimate load methods of design.
o The acceptable limits of safety and serviceability requirements before failure occurs is called a limit
state.
o This method is based on the concept of safety at ultimate loads (Ultimate Load Method) and serviceability at
working loads (Working Load Method).

The two important limit states to be considered in design are:

1. Limit state of collapse and
2. Limit state of serviceability

1. Limit state of Collapse: (Clause 35.2, IS 456:2000)

This limit state is also called as a strength limit state as it corresponds to the maximum load carrying
capacity i.e., the safety requirements of the structures. The limit state of collapse is assessed from collapse
of the whole or part of the structure. This limit state is categorized into following types:
a. Limit state of collapse: Flexure
b. Limit state of collapse: Shear and Bond
c. Limit state of collapse: Torsion
d. Limit state of collapse: Compression
2. Limit state of Serviceability: (Clause 35.3, IS 456:2000)
A structure is of no use if it is not serviceable. Thus, this limit state is introduced to prevent excessive
deflection and cracking. It ensures the satisfactory performance of the structure at working loads. This limit
state corresponds to the serviceability requirements i.e., deformation, cracking, etc. It is categorized into
following types:

a. Limit state of Serviceability: Deflection

b. Limit state of Serviceability: Cracking
c. Limit state of Serviceability: Vibration
 Advantages of Limit State Method:
1. It is a modern method of design which involves wide range of logical and technical considerations.
2. Limit state method uses a multiple safety factor format which attempts to provide adequate safety at
ultimate loads as well as adequate serviceability at working loads.
3. It involves separate consideration of different kinds of failure, types of material and types of loads.
4. It gives more economical section.

Disadvantages of Limit State Method:

1. It is based on probabilistic approach. Hence the structure may not provide sufficient strength and
serviceability at worst combination of loads and during extreme conditions.
 Bonus: Advantages and Disadvantages of RCC
Advantages:
1. RCC has very good strength in tension as well as in compression.
2. RCC structures are durable if designed and laid properly. They can last up to 100 years.
3. RCC sections can be given any shape easily by properly designing the formwork. Thus, it is more suitable for
architectural requirements.
4. The steel reinforcement imparts ductility to the RCC structures.
5. RCC is cheaper as compared to steel and pre-stressed concrete. There is an overall economy by using RCC
because its maintenance cost is low.
6. The raw materials which are required for RCC i.e., cement, sand, aggregate, water and steel are easily
available and can be transported easily.
7. RCC structures are more fire resistant than other commonly used construction materials like steel and wood.
8. RCC is almost impermeable to moisture.
9. Properly designed RCC structures are extremely resistant to earthquakes.

Disadvantages:
1. RCC structures are heavier than structures of other materials like steel, wood and glass etc.
2. RCC needs lot of formwork, centering and shuttering to be fixed. Thus, requires lot of site space and skilled
labor.
3. Concrete takes time to attain its full strength. Thus, RCC structures cant be used immediately after construction
unlike steel structures.