REINFORCED CONCRETE

DESIGN
CIVIL-354 CONCRETE TECHNOLOGY AND RCC DESIGN

BY: ENGR. ERAJ MEHMOOD

•Commonly used all over the world
•Two component material: concrete and steel

–Concrete resists compression

–Steel resists tensile forces

•Wide applications in buildings and other structures because of its:

–Durability
–High resistance to static and dynamic loads
–Resistance to fire and weathering
–Availability of raw materials
–Low maintenance costs
• Long service life because the strength actually increases with time
provided steel is protected from corrosion.

• The joint behaviour of steel and concrete is based on the following

properties.

–Bond between steel and concrete is maintained after concrete hardens.

Use of deformed bars greatly improves bond and both materials deform
together under load.

–Coefficient of thermal expansions for concrete (10 to 15 x 10-6 /oC) and

steel (12 x 10-6 /oC) are very close. Differential strains under temperature
changes of 80 oC are not observed.
• Concrete protects steel reinforcement against corrosion and improves the
fire resistance of the whole structural member.

• Concrete is made of cement, aggregate and water proportioned in such a

way as to produce the required design strengths.
• Used by ancient Romans in the construction of walls and roofs.

• First modern record as early as 1760 in Britain

• In 1824, Joseph Aspdin manufactured Portland cement in Wakefield,

Britain.
• Relatively high compressive strength
• Better resistance to fire than steel
• Long service life with low maintenance cost
• In some types of structures, such as dams, piers and foundations, it is the
most economical structural material.
• It can be cast to take the shape required, making it widely used in pre-cast
structural components.

• Rigid members with minimum apparent deflection.

• Low tensile strength

• Requires mixing, curing, casting, all of which affect the final strength.

• Cost of forms is high.

• Low compressive strength compared to steel.

• Cracks develop due to shrinkage and live loads.

• Design engineers are usually guided by specifications called codes of
practice.

• Most codes specify design loads, allowable stresses, material quality,

construction types and other requirements for building construction.

• Building code requirements for structural concrete ACI 318, or the ACI code.

• British Standard (BS) code of practice for reinforced concrete, CP110 and
BS 8110

• CEB code (European code)

•The design is a process of selecting proper materials and
proportioning the different elements of the structure
according to state- of-the-art engineering science and
technology.

•Following conditions must be satisfied.

–Safety
–Serviceability
–Economy
–Functionality
1) Visualizing the building into a structural
model of load bearing frames and elements

2) Estimating the different types of loads.

3) Performing Structural Analysis to calculate

maximum moments, shears, torsion and axial
forces

4) Proportioning different structural members

and calculating the reinforcement needed

5) Producing structural drawings and

specifications
Factors that affect strength of
concrete:

1. Water – Cement Ratio

2. Properties and Proportions of Concrete Constituents
3. Method of Mixing and Curing
4 . A g e of Concrete
5. Loading conditions
6. Shape and Dimensions of the tested specimen
• The water–cement ratio is one of the most important factors affecting the strength of concrete.
• For complete hydration of a given amount of cement, a water–cement ratio (by weight) equal to
0.25 is needed.
• A water–cement ratio of about 0.35 or higher is needed for the concrete to be reasonably
workable without additives. This ratio corresponds to 4 gal of water per sack of cement (94 lb)
(or 17.8 litre per 50 kg of cement). Based on this cement ratio, a concrete strength of about 6000
psi may be achieved.
• A water–cement ratio of 0.5 and 0.7 may produce a concrete strength of about 5000 and 3000
psi, respectively.
• Concrete is a mixture of cement, aggregate, and water. An increase in the cement
content in the mix and the use of well-graded aggregate increase the strength of
concrete. Special admixtures are usually added to the mix to produce the desired
quality and strength of concrete.
• The use of mechanical concrete mixers and the proper time of mixing both have
favorable effects on strength of concrete. Also, the use of vibrators produces dense
concrete with a minimum percentage of voids. A void ratio of 5% may reduce the
concrete strength by about 30%.

• The curing conditions exercise an important influence on the strength of concrete. Both
moisture and temperature have a direct effect on the hydration of cement. The longer
the period of moist storage, the greater the strength. If the curing temperature is higher
than the initial temperature of casting, the resulting 28-day strength of concrete is
reached earlier than 28 days.
• The strength of concrete increases appreciably with age, and hydration of cement
continues for months. In practice, the strength of concrete is determined from cylinders
or cubes tested at the age of 7 and 28 days. As a practical assumption, concrete at 28
days is 1.5 times as strong as at 7 days: The range varies between 1.3 and 1.7. The
British Code of Practice [2] accepts concrete if the strength at 7 days is not less than
two-thirds of the required 28-day strength. For a normal portland cement, the increase of
strength with time, relative to 28-day strength, may be assumed as follows:
• The compressive strength of concrete is estimated by testing a cylinder or cube to
failure in a few minutes. Under sustained loads for years, the compressive strength of
concrete is reduced by about 30%. Under 1 day sustained loading, concrete may lose
about 10% of its compressive strength. Sustained loads and creep effect as well as
dynamic and impact effect, if they occur on the structure, should be considered in the
design of reinforced concrete members.
• The common sizes of concrete specimens used to predict the compressive strength are
either 6 × 12-in. (150 × 300-mm) or 4 × 8-in. (100 × 200-mm) cylinders or 6-in. (150-mm)
cubes. When a given concrete is tested in compression by means of cylinders of like
shape but of different sizes, the larger specimens give lower strength indexes
• The relative strengths of a cylinder and a cube for different compressive strengths are
shown below:
• Compressive strength is criterion of concrete quality.
• It can be easily and accurately determined from tests.
• Specimen used to determine compressive strength may
be cylindrical, cubical or prismatic.
• Cubes of 6-inch or 8-inch size are used in Britain,
Germany and other parts of Europe.
• Prism specimens are used in France, Russia and other
countries.
• The rupture of the concrete specimen may be caused by
the applied tensile stress (failure in cohesion), the
applied shearing stress (sliding failure), the compressive
stress (crushing failure), or combinations of these
stresses.
• Stress–strain curves for concrete are obtained by testing a
concrete cylinder to rupture at the age of 28 days and recording
the strains at different load increments.

• All curves consist of an initial relatively straight elastic portion,

reaching maximum stress at a strain of about 0.002

• Rupture occurs at a strain of about 0.003

• Concrete is a brittle material, and it can not resist the high tensile
stresses

• Direct tension tests are not reliable for predicting the tensile
strength of concrete.

• An indirect tension test in the form of splitting a 6 by 12-inch cylinder is

used. The test is usually called the splitting test.
• Pure shear is not encountered very often in reinforced concrete
members because it is usually accompanied by the action of
normal forces.

• Shear strength may be taken as 20 to 30% greater than the tensile

strength of concrete, or about 12% of its compressive strength

• The ACI Code allows a nominal shear stress of 2𝜆√f ′c psi on plain
concrete sections.
• One of the most important elastic properties of concrete is its
modulus of elasticity, which can be obtained from a compressive
test on concrete cylinders

• The modulus of elasticity, Ec, can be defined as the change of

stress with respect to strain in the elastic range

• The modulus of elasticity is a measure of stiffness, or the resistance of

the material to deformation.

• In concrete, as in any elastoplastic material, the stress is not

proportional to the strain, and the stress–strain relationship is a curved
line
• The slope of the tangent to the curve at the origin under elastic
deformation

• This modulus is of limited value and cannot be determined with

accuracy
• For practical applications, the secant modulus can be used

• The secant modulus is represented by the slope of a line drawn

from the origin to a specific point (normally at f’c/2) on the stress–
strain curve.

• The ACI code gives a formula considering the secant modulus at a

level of stress equal to half the concrete ultimate strength f’c.
• Ratio of the transverse to the longitudinal strains under axial stress
within the elastic range

• This ratio varies between 0.15 and 0.20 for both normal and
lightweight concrete.

• An average value of 0.18 can be used for concrete.

• The modulus of elasticity of concrete in shear ranges from about 0.4 to 0.6 of the
corresponding modulus in compression. From the theory of elasticity, the shear modulus
is taken as follows:
• The modular ratio n is the ratio of the modulus of elasticity of steel to the modulus of
elasticity of concrete: n=E s/E c.

Because the modulus of elasticity of steel is considered constant and is equal to 29 ×

106 psi and Ec = 33𝑤1.5√f ′c
• Concrete undergoes volume changes during hardening.

• It loses moisture by evaporation and shrinks

• Causes of volume changes:

 Changes in moisture content

 Chemical reaction of the cement with water

 Variation in temperature

 Applied loads
• The unit weight, 𝑤, of hardened normal concrete ordinarily used in buildings and

similar structures depends on:

 the concrete mix

 maximum size and grading of aggregates

 water–cement ratio,

 strength of concrete
• Unit weight of plain concrete using maximum aggregate size of 34 in. (20 mm)

varies between 145 and 150 lb/ft3 (2320 to 2400 kg/m3). For concrete of strength

less than 4000 psi(280 kg/cm2), a value of 145 lb/ft3 (2320 kg/m3) can be used,

whereas for higher strength concretes, 𝑤 can be assumed to be equal to 150 lb/ft3

(2400 kg/m3).

• Unit weight of plain concrete of maximum aggregate size of 4 to 6 in. (100 to 150

mm) varies between 150 and 160 lb/ft3 (2400 to 2560 kg/m3). An average value of

155 lb/ft3 may be used.

• Unit weight of reinforced concrete, using about 0.7 to 1.5% of steel in the concrete

section, may be taken as 150 lb/ft3 (2400 kg/m3). For higher percentages of steel,

the unit weight, 𝑤, can be assumed to be 155 lb/ft3 (2500 kg/m3)

• Unit weight of lightweight concrete used for fireproofing, masonry, or insulation

purposes varies between 20 and 90 lb/ft3 (320 and 1440 kg/m3). Concrete of upper

values of 90 pcf or greater may be used for load-bearing concrete members.

• Reinforcement, usually in the form of steel bars, is placed in the concrete member,

mainly in the tension zone, to resist the tensile forces

• Reinforcement is also used to increase the member’s compression resistance

• Steel costs more than concrete, but it has a yield strength about 10 times the

compressive strength of concrete

• Longitudinal bars taking either tensile or compression forces in a concrete

member are called main reinforcement

• Additional reinforcement in slabs, in a direction perpendicular to the main

reinforcement, is called secondary, or distribution, reinforcement

• In reinforced concrete beams, another type of steel reinforcement is used,

transverse to the direction of the main steel and bent in a box or U shape. These are

called stirrups. Similar reinforcements are used in columns, where they are called

ties.
• The most important factor affecting the mechanical properties and stress–strain

curve of the steel is its chemical composition

• Carbon content and other alloys increases its strength but reduces its ductility

• The proportion of carbon used in structural steels varies between 0.2 and 0.3%

• The modulus of elasticity is constant for all types of steel. The ACI Code has

adopted a value of Es =29 × 106 psi (2.0 × 105 MPa).

• Used primarily to support axial compressive

• Have a ratio of height to the least lateral dimension of 3 or greater

• Perfect vertical alignment of columns in a multistory building is not possible,

causing loads to be eccentric relative to the center of columns

• The eccentric loads will cause moments in columns

• However, it can be assumed that axially loaded columns are those with relatively

small eccentricity, e, of about 0.1 h or less

1. Axially Loaded Columns

2. Eccentrically Loaded Columns

3. Biaxially Loaded Columns

4. Short Columns

5. Long Columns

6. Columns with different Shapes

7. Tied Columns

8. Spiral Columns

9. Braced Columns

10. Prestressed and Composite Columns

 The nominal load capacity of the column can
be written as follows:

A n and Ast are the net concrete and total steel compressive areas, respectively.
• For axially as well as eccentrically loaded columns, the ACI Code sets the
strength reduction factors at 𝜙=0.65 for tied columns and 𝜙=0.75 for
spirally reinforced columns

• The minimum longitudinal steel percentage is 1%, and the maximum

percentage is 8% of the gross area of the section

• At least four bars are required for tied circular and rectangular members
and six bars are needed for circular members enclosed by spirals

• Bars shall not be located at a distance greater than 6 in. clear on either side
from a laterally supported bar. The minimum concrete cover in columns is
1.5 in.
• The minimum ratio of spiral reinforcement, 𝜌s, according to the ACI Code is:

• The minimum diameter of spiral bar is 3/8 in. and their clear spacing according to
ACI Code, should not be more than 3 in. nor less than 1 in.

• Ties for columns must have a minimum diameter of 3/8 in. to enclose longitudinal
bars of no. 10 size or smaller and a minimum diameter of 1/2 in. for larger bar
diameters.
• Center to center spacing of ties shall not exceed the smallest of 48 times the tie bar
diameter, 16 times the longitudinal bar diameter, or the least dimension of the
member.
• The nominal load strength of an axially loaded column was given by:

• Because a perfect axially loaded column does not exist, some eccentricity occurs on the
column section, thus reducing its load capacity, P0.

• To take that into consideration, the ACI Code specifies that the maximum nominal load,
P0, should be multiplied by a factor equal to 0.8 for tied columns and 0.85 for spirally
reinforced columns
• Concrete will not crack as long as stresses are below its tensile strength; in this case, both concrete and
steel resist the tensile stresses, but when the tension force exceeds the tensile strength of concrete
(about one-tenth of the compressive strength), cracks develop across the section, and the entire
tension force is resisted by steel

• The nominal load that the member can carry is that due to tension steel only

• where 𝜙 is 0.9 for axial tension.

• Under working loads, the concrete cracks and the steel bars carry the whole tension force. The concrete
acts as a fire and corrosion protector
 Solution:

• Check Steel Percentage:

• Check Tie Spacing:
Solution: