Objectives
Steel
-

and Methods of Analysis
and Design,
Properties
of Concrete and Steel

and Properties

of Concrete

and

1.2.2 Properties
of Concrete:
plain concrete
is prepared by mixing cement, sand
(also known as fine aggregate),
gravel
(also known as coarse aggregate)
and
water with specific
proportions.
Mineral
admixtures
may also be added to improve
certain
properties
of concrete.
Thus, the properties
of concrete
regarding
its
strength
and deformations
depend on the individual
properties
of cement, sand,
gravel,
water and admixtures.
Clauses 5 and 6 of IS 456:2000 stipulate
the
standards
and requirements
of the individual
material
and concrete,
respectively.
Plain concrete
after
preparation
and placement needs curing
to
attain
strength.
However, plain
concrete
is very good in compression
but weak in tension.
That is why steel is
used as reinforcing
material
to make the composite sustainable
in tension
also.
Plain concrete,
thus when reinforced
with steel bars in appropriate
locations
is
known

as

reinforced

concrete.

The strength
and deformation
characteristics
of concrete
thus depend on the
grade and type of cement, aggregates,
admixtures,
environmental
conditions
and
curing.
The increase
of strength
with its age during curing is considered
to be
marginal
after
28 days. Blended cements (like
fly ash cement) have slower rate
of strength
gain than ordinary
Portland
cement as recognized
by code,
Depending on several
factors
during its preparation,
placement and curing,
concrete
has a wide range of compressive
strength
and the material
is graded on
the basis of its compressive
strength
on 28 day also known as "characteristic
strength"
as defined
below while discussing
various
strength
and deformation
properties.

(a) Characteristic
strength
property:
Characteristic
strength
is defined
as
the strength
below which not more than five per cent of the test results
are
expected to fall.
Concrete is graded on the basis of its characteristic
compressive
strength
of 150 mm size cube at 28 days and expressed in N/mm2. The
grades are designated
by one letter
M (for mix) and a number from 10 to 80
indicating
the characteristic
compressive
strength
(fck)
in N/mm2. As per IS 456
(Table 2), concrete
has three groups as (i) ordinary
concrete
(M 10 to M 20),
(ii)
standard
concrete
(M 25 to M 55) and (iii)
high strength
concrete
(M 60 to
M 80). The size of specimen for determining
characteristic
strength
may be
different

in

different

countries.

(b) other strengths
of concrete:
In addition
to its good compressive
strength,
concrete
has flexural
and splitting
tensile
strengths
too. The flexural
and
splitting
tensile
strengths
are obtained
as described
in IS 516 and IS 5816,
respectively.
However, the following
expression
gives an estimation
of flexural
strength
(fa) of concrete
from its characteristic
compressive
strength
(cl.
6.2.2)
fa = 0.7,ifCk
in N/mmA2
(c)

Elastic

curve

of

deformation

of concrete

(formula

Fig.

Figure 1.2.1 shows a typical
stress-strain
curve
where E0 = initial
tangent modulus at the origin,
static

1.2.1

Strain

of concrete
in compression,
also known as short term

modulus

Es secant

modulus

inelastic

strain

at A; E, tangent

modulus

at A;

39 = elastic

EC in

strain

atA

g,-

atA

It is seen that the initial
tangent modulus is much higher
modulus at A). Near the failure,
the actual
strain
consists
(elastic
and inelastic
respectively)
components of strain.
modulus

; Stress

concrete

N/mm2 is

estimated

from

E = 5000

fck,

where

than E; (tangent
of both 39 and s,The initial
tangent
fck

= characteristic

compressive
strength
of concrete
at 28 days
The initial
tangent modulus EC is also known as short term static
modulus
elasticity
of concrete
in N/mm2 and is used to calculate
the elastic

of

deflections.

(d) Shrinkage
of concrete:
Shrinkage
is the time dependent deformation,
generally
compressive
in nature.
The constituents
of concrete,
size of the
member and environmental
conditions
are the factors
on which the total
shrinkage
of concrete
depends. However, the total
shrinkage
of concrete
is most influenced

=

by the total
amount of water present in the concrete
at the time of mixing for a
given humidity
and temperature.
The cement content,
however, influences
the
total
shrinkage
of concrete
to a lesser
extent.
The approximate
value of the
total
shrinkage
strain
for design is taken as 0.0003 in the absence of test data
(cl.

6.2.4.1).

(e) Creep of concrete:
Fig. 1.2.1
: Stress
Strain
curve of concrete
Creep is another time dependent deformation
of concrete
by which it
continues
to deform, usually
under compressive
stress.
The creep strains
recover
partly
when the stresses
are released.
Figure 1.2.2 shows the creep recovery
in
two parts.
The elastic
recovery
is immediate and the creep recovery
is slow in
nature.
Thus, the long term deflection
will
be added to the short term
deflection
to get the total
deflection
of the structure.
Accordingly,
the long
term

modulus

the effect

E09

or

of creep

effective

is

obtained

so,

where so = short term strain
= ultimate
creep strain;

456)

as

the

loads.

of

concrete

will

The relationship

be

needed

between

to

include

E09 and EC

follows:

at the age of loading
6 = creep coefficient

at a stress value of fc;
8 (cl.
6.2.5.1
of IS

8 c (1.4)

The values
respectively.

of 6 on 7,

Then the total
strain
where E09 = effective
8cEc

modulus

due to permanent

28

and 365"

day of loading

= so + so, =
modulus of concrete

T E c9 8 +8 8 +8 1+0 c Cr c Cr

are 2.2,

1.6

and 1.1

(1.5)
From the above Eq. (1.5),

we have f;

T

(1.6)

The effective
modulus of E09 of concrete
is used only in the calculation
of
creep deecon.
It is seen that the value of creep coefficient
6 (Eq. 1.4) is
reducing with the
age of concrete
at loading.
It may also be noted that the ultimate
creep strain
60, does not include
short term strain
8c. The creep of concrete
is influenced
by
Properties
of concrete water/cement
ratio:
Humidity
and temperature
of
curing Humidity
during the period of use Age of concrete
at first
loading
Magnitude of stress
and its duration
Surfacevolume
ratio
of the member
(f) Thermal expansion of concrete:
The knowledge of thermal expansion of
concrete
is very important
as it is prepared and remains in service
at a wide
range of temperature
in different
countries
having very hot or cold climates.
Moreover,
concrete will
be having its effect
of high temperature
during fire.
The coefficient
of thermal expansion depends on the nature of cement, aggregate,
cement content,
relative
humidity
and size of the section.
IS 456 stipulates
(cl.
6.2.6)
the values of coefficient
of thermal expansion for concrete
/ °C for
different
types of aggregate.
1.2.3workability
and Durability
of Concrete:
workability
and durability
of
concrete
are important
properties
to be considered.
The relevant
issues are
discussed
in the following:
(a) Concrete mix proportioning:
The selected
mix proportions
of cement,
aggregates
(fine
and coarse) and water ensure:
o the workability
of fresh concrete,
0 required
strength,
durability
and
surface finish
when concrete
is hardened. Recently more than forty
per cent of
concrete
poured world over would contain
admixtures.
(b) workability:
It is the property
which determines
the ease and
homogeneity with which concrete
can be mixed, placed,
compacted and finished.
A
workable concrete will
not have any segregation
or bleeding.
Segregation
causes
large voids and hence concrete
becomes less durable.
Bleeding
results
in several
small pores on the surface
due to excess water coming up. Bleeding
also makes
concrete
less durable.
The degree of workability
of concrete
is classified
from
very low to very high with the corresponding
value of slump in mm (cl.
7 of IS
456).

(c) Durability
of concrete:
A durable concrete
performs satisfactorily
in
the working environment
during its anticipated
exposure conditions
during
service.
The durable concrete
should have low permeability
with adequate cement
content,
sufficient
low free water/cement
ratio
and ensured complete compaction
of concrete
by adequate curing.
For more information,
please refer
to cl. 8 of
IS

456.

(d) Design mix and nominal mix concrete:
In design mix, the proportions
of
cement, aggregates
(sand and gravel),
water and mineral
admixtures,
if any, are
actually
designed,
while in nominal mix, the proportions
are nominally
adopted.
The design mix concrete
is preferred
to the nominal mix as the former results
in
the grade of concrete
having the specified
workability
and characteristic
strength
(vide cl. 9 of IS 456).
(e) Batching:
Mass and volume are the two types of batching
for measuring
cement, sand, coarse aggregates,
admixtures
and water.
Coarse aggregates
may be
gravel,
grade stone chips or other man made aggregates.
The quantities
of
cement, sand, coarse aggregates
and solid
admixtures
shall
be measured by mass.
Liquid admixtures
and water are measured either
by volume or by mass (cl.
10 of
IS

456).

1.2.4 Properties
of Steel:
As mentioned earlier
in sec. 1.2.2,
steel is used as
the reinforcing
material
in concrete
to make it good in tension.
Steel as such
is good in tension
as well as in compression.
Unlike concrete,
steel
reinforcement
rods are produced in steel
plants.
Moreover,
the reinforcing
bars
or rods are commercially
available
in some specific
diameters.
Normally,
steel
bars up to 12 mm in diameter
are designated
as bars which can be coiled
for
transportation.
Bars more than 12 mm in diameter are termed as rods and they are
transported
in standard lengths.
Like concrete,
steel also has several
types or
grades. The four types of steel
used in concrete
structures
as specified
in cl.
5.6 of IS 456 are given below:
(i) Mild steel and medium tensile
steel bars conforming
to IS 432 (Part 1);
ii)
High yield
strength
deformed (HYSD) steel bars conforming
to IS 1786
(iii)
Harddrawn steel wire fabric
conforming
to IS 1566:
iv)
Structural
steel conforming
to Grade A of IS 2062.
Mild steel bars had been progressively
replaced by HYSD bars and subsequently
TMT bars are promoted in our country.
The implications
of adopting
different
kinds of blended cement and reinforcing
steel should be examined before
adopting.
Stress-strain
curves for reinforcement:
Fig.1.2.3
: Stress - Strain
curve for
mild steel
{idealised}
{Fe 250] with definite
yield
pdint.
Fig. 1.2.4
: Stress-Strain
Curve for cold worked deformed bar
Figures 1.2.3 and 1.2.4 show the representative
stress-strain
curves for steel
having definite
yield
point and not having definite
yield
point,
respectively.
The characteristic
yield
strength
fy of steel is assumed as the minimum yield
stress
or 0.2 per cent of proof stress
for steel having no definite
yield
point.
The modulus of elasticity
of steel is taken to be 200000 N/mm2.
For mild steel
(Fig.
1.2.3),
the stress
is proportional
to the strain
up to the
yield
point.
Thereafter,
post yield
strain
increases
faster
while the stress
is
assumed to remain at constant
value of fy.
For coldworked
bars (Fig.
1.2.4),
the stress
is proportional
to the strain
up
to a stress
of 0.8 fy. Thereafter,
the inelastic
curve is defined
as given
below:

Stress Inelastic
strain
0.80 fy Nil
0.85 fy 0.0001
0.95 fy 0.0007
0.975 fy 0.0010
1.00 fy 0.0020
Linear interpolation
is to be done for intermediate
values.

The two grades

coldworked

Fe

bars

used

as

steel

reinforcement

are

Fe

415

and

0.90

500

fy

with

0.0003
of

the

values of fy as 415 N/mm2 and 500 N/mm2, respectively.
Considering
the material
safety
factor
)/m (vide sec. 2.3.2.3
of Lesson 3) of
steel as 1.15, the design yield
stress
(fyd) of both mild steel and cold worked
bars is computed from
fyd = fy/ }/m
(1.7)

Accordingly,
the representative
stress-strain
curve for
by substituting
fyd for fy in Figs. 1.2.3 and 1.2.4 for
with or without
the definite
yield
point,
respectively.

the design is
the two types

obtained
of steel

1.2.5 other Important
Factors:
The following
are some of the important
factors
to be followed
properly
as per the stipulations
in IS 456 even for the design
mix concrete with materials
free from impurities
in order to achieve the desired

strength
and quality
of concrete.
mentioned as ready references
for

The relevant
clause
each of the factors.

numbers of IS 456 are also

(a) Mixing (cl.
10.3):
Concrete is mixed in a mechanical
mixer at least for two
minutes so as to have uniform distribution
of the materials
having uniform
colour and consistency.
(b) Formwork (cl.
11):
properly
designed formwork shall
be used to
maintain
its rigidity
during placing
and compaction of concrete.
It should
prevent the loss of slurry
from the concrete.
The stripping
time of formwork
should be such that the concrete
attains
strength
of at least
twice the stress
that the concrete
may be subjected
at the time of removing the formwork.
As a
ready reference
IS 456 specifies
the minimum period before striking
formwork.
There is a scope for good design of formwork system so that stripping
off is
efficient
without
undue shock to concrete
and facilitating
reuse of formwork.
(c) Assembly of reinforcement
(cl.
12):
The required
reinforcement
bars
for the bending moment, shear force and axial
thrust
are to be accommodated
together
and proper bar bending schedules shall
be prepared.
The reinforcement
bars should be placed over blocks,
spacers,
supporting
bars etc. to maintain
their
positions
so that they have the required
covers.
High strength
deformed
steel

bars

should

not

be

rebent.

The

reinforcement

bars

should

be

assembled

to

have proper flow of concrete without
obstruction
or segregation
during placing,
compacting and vibrating.
(d) Transporting,
placing,
compaction and curing
(cl.
13): Concrete should
be transported
to the formwork immediately
after
mixing to avoid segregation,
loss of any of the ingredients,
mixing of any foreign
matter or loss of
workability.
Proper protections
should be taken to prevent evaporation
loss of
water in hot weather and loss of heat in cold weather.
To avoid rehandling,
concrete
should be deposited
very near to the final
position
of its placing.
The
compaction should start
before the initial
setting
time and should not be
disturbed
once the initial
setting
has started.
while placing
concrete,
reinforcement
bars should not be displaced
and the formwork should not be moved.
The compaction of concrete
using only mechanical vibrators
is very important,
particularly
around the reinforcement,
embedded fixtures
and the corners of the
formwork to prevent honeycomb type of concreting.
Excessive vibration
leads to
segregation.
Proper curing
prevents
loss of moisture
from the concrete
and
maintains
a satisfactory
temperature
regime. In moist curing,
the exposed
concrete
surface
is kept in a damp or wet condition
by ponding or covering
with
a layer of sacking,
canvas, hessian etc. and kept constantly
wet for a period of
7-14 days depending on the type of cement and weather conditions.
Blended cement
needs extended curing.
In some situations,
polyethylene
sheets or similar
impermeable membranes may be used to cover the concrete
surface
closely
to
prevent

evaporation.

(e) Sampling and strength
of designed concrete
mix (cl.
15): Random samples of
concrete
cubes shall
be cast from fresh concrete,
cured and tested at 28 days as
laid down in IS 516. Additional
tests on beams for modulus of rupture
at 3 or 7
days, or compressive
strength
tests at 7 days shall
also be conducted.
The
number of samples would depend on the total
quantity
of concrete
as given in cl.
15.2.2 and there should be three test specimens in each sample for testing
at 28
days, and additional
tests at 3 or 7 days.
(f) Acceptance criteria
(cl.
16):
Concrete should be considered
satisfactory
when both the mean strength
of any group of four consecutive
test results
and
any individual
test result
of compressive
strength
and flexural
strength
comply
the limits
prescribed
in IS 456.
(g) Inspection
and testing
of structures
(cl.
17):
Inspection
of the
construction
is very important
to ensure that it complies the design.
Such
inspection
should follow
a systematic
procedure covering
materials,
records,
workmanship and construction.
All the materials
of concrete
and reinforcement
are to be tested following
the relevant
standards.
It is important
to see that
the design and detailing
are capable of execution
maintaining
a standard with
due allowance
for the dimensional
tolerances.
The quality
of the individual
parts of the structure
should be verified.
If needed, suitable
quality
assurance
schemes should be used. The concrete
should be inspected
immediately
after
the

removal of formwork to remove any defective
work before concrete
has hardened.
Standard core tests
(IS 516) are to be conducted at three or more points
to
represent
the whole concrete work in case of any doubt regarding
the grade of
concrete
during inspection
either
due to poor workmanship or unsatisfactory
results
on cube strength
obtained
following
the standard
procedure.
If the
average equivalent
cube strength
of cores is equal to at least
85 per cent of
the cube strength
of that grade of concrete
at that age and each of the
individual
cores has strength
of at least
75 per cent, then only the concrete
represented
by the core test is considered
acceptable.
For unsatisfactory
core
test results,
load tests should be conducted for the flexural
members and proper
analytical
investigations
should be made for nonflexural
members.
Such load tests should be done as soon as possible
after
expiry
of 28
days from the date of casting
of the flexural
members subjected
to full
dead
load and 1.25 times the imposed load for 24 hours and then the imposed load
shall
be removed. The maximum deflection
of the member during 24 hours under
imposed load in mm should be less than 40 LA2/D, where L is the effective
span
in m and D is the overall
depth of the member in mm. For members showing more
deflection,
the recovery
of the deflection
within
24 hours of removal of the
imposed load has to be noted. If the recovery is less than 75 per cent of the
deflection
under imposed load, the test should be repeated after
a lapse of 72
hours. The structure
is considered
unacceptable
if the recovery
is less than 80
per

cent.

There are further
provisions
of conducting
nondestructive
tests like
ultrasonic
pulse velocity
(UPV), rebound hammer, probe penetration,
pull out and
maturity,
as options
to core tests or to supplement the data obtained
from a
limited
number of cores. However, it is important
that the acceptance criteria
shall
be agreed upon prior
to these nondestructive
testing.
There are reports
that UPV tests conducted three days after
casting
after
removal of side formwork
give very dependable insight
about the quality
of concrete.