Unit 3 - Hardened concrete

Introduction
• The ability of concrete to resist load is called strength, The
strength of concrete depends upon water/cement ratio,
degree of compaction, quality and quantity of and other
concreting operations.
• Compressive strength- It may be defined as the resistance
of concrete to crushing.
• It is very strong in compression and resists very high
compressive loads.
• The compressive strength is generally measured in N/mm².
• Cubes of 15 cm size are generally used to determine the
compressive strength of concrete. The compressive
strength of concrete is measured in the Compression
Testing Machine (CTM).
• Compressive Strength =
Maximum Load Resisted by concrete
sample/Cross Sectional area of Concrete Sample
• Lower W/C ratio, lower air content, longer
curing period and greater age improve the
strength concrete.
• Percentage strength of concrete at various ages:
• The strength of concrete increases with age.
• Table shows the strength of concrete at different ages
in comparison with the strength at 28 days after
casting.
Age Strength percentage
1 Day 16%

3 Days 40%

7 Days 65%

14 Days 90%

28 Days 99%
 Tensile or flexural strength-
• The concrete has low tensile strength. Hence, it is weak in tension.
• Its tensile strength is approximately 8-12% of its compressive
strength in bending.
• The strength of concrete in bending is termed as flexural strength.
• The flexural strength is measured by the term modulus of rupture
which is maximum tensile or compressive strength at rupture.
• IS 456-2000 code of practice for plain and reinforced concrete has
specified relationship between compressive strength and some
other properties of concrete as under:-
Flexure strength, fcr = 0.7 √fck
Modulus of elasticity, Ec = 5000 √fck
Where fck is the characteristic cube compressive strength of concrete
in N/mm².
 Durability
• Durability is the property of concrete by virtue of which it is capable
of resisting the disintegration and decay.
• The concrete should be durable with proper regard to the various
weathering conditions such as action of atmosphere gases,
moisture changes and temperature variotions.
• A durable concrete is one that performs satisfactorily in the working
environment during its anticipated exposure conditions during
service. The materials and mix proportions specified and used
should be such as to maintain its integrity and, if applicable, to
protect embedded metal from corrosion.
• 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. Impermeability is governed by the constituents and
workmanship used in making the concrete.
• With normal-weight aggregates, a suitably low
permeability is achieved by having an adequate cement
content, sufficiently low free water/cement ratio by
ensuring complete compaction of the concrete and by
adequate curing.
• The factors influencing durability include:
a) the environment
b) the cover to embedded steel
c) the type and quality of constituent materials
d) the cement content and water/cement ratio of the
concrete
e) workmanship, to obtain full compaction and efficient
curing; and
f) The shape & size of the member
 Impermeability
• Impermeability is the resistance of concrete to
the flow of water into the pore-space in it.
Impermeability of concrete should be high.
• Impermeability in concrete can be achieved by:
a) Proper curing
b) using concretes with low W/C ratio
c) Proper and uniform compaction
d) selecting well graded aggregate
 Elasticity
• -The elasticity is the property by virtue of which a
material deformed under the load is enabled to return
to its original dimension when load is removed. The
maximum stress that can be applied on the material
without resulting in non-elastic or permanent
deformation when unloaded is called elastic limit.
• Elastic properties of concrete : In the theory, it is
assumed that concrete is elastic, isotropic and
homogeneous and obeys Hooke's law. Actually none of
these assumptions is strictly true and concrete is not
perfectly elastic material.
• Elastic modulus of concrete is the measure of stiffness (or
rigidity) of a concrete structure. The stiffness of a structural
component means how much it deflects under a given load.
Stiffness is important in designing structures which can only
be allowed to deflect by a certain amount. A stiff material
requires high loads to elastically deform i.e., it has high
modulus of elasticity.
• [Note: A stiff material has a high Young's modulus and
changes its shape only slightly under elastic loads (e.g.
diamond). A flexible material has a low Young's modulus and
changes its shape considerably (e.g. rubbers).]
• Modulus of elasticity of concrete- It can be defined as the
slope of the relation between the stress and strain. It can
also be defined as the change of stress with respect to the
elastic strain and may be computed by the following
relation.
• Modulus of Elasticity = Stress/ Strain
• Secant Modulus of Elasticity of concrete is the
most practical and is in the most general use as it
represents the actual deformation at the selected
point and no uncertainties are involved in its
determination.
• IS 456-2000 code of practice for plain and
reinforced concrete has specified relationship
between compressive strength and Elastic
properties of concrete as under:-
Modulus of elasticity, E= 5000 √fck
• Where fck is the characteristic cube compressive
strength of concrete in N/mm².
 Creep
• When concrete is subjected to compressive
loading it deforms instantaneously. This
immediate deformation is called
instantaneous strain. Now, if the load is
maintained for a considerable period of time,
concrete undergoes additional deformations
even without any increase in the load. This
time-dependent strain is termed as Creep.
• Creep coefficient = Ultimate creep
strain/Elastic strain at age of loading.
• Values of Creep Co-efficient at the age of
loading (As per IS 456-2000)
Age at loading Creep Coefficient
7 Days 2.2
28 Days 1.6
1 Year 1.1
 Factors affecting creep
• A Constituents and mix proportion of concrete - The amount of
cement paste content and its quality is one of the most important
factors influencing creep.
a) A poorer paste structure undergoes higher creep. Therefore it can
be said that the creep increases with increase in W/C ratio or creep is
inversely proportional to strength.
b) Aggregate properties- Aggregate undergoes very little creep. The
modulus of elasticity of aggregate is one of the factors influencing
creep. Higher elastic modulus of aggregate the less is the creep.
c) Age at loadings- Age at which a concrete member is loaded will have
a predominant effect on the magnitude of creep. The quality of gel
(givens strength) improves with time. Such gel creeps less, whereas a
young gel under load being not so stringer Greeps more. The moisture
content of the concrete being different at different ages als
 Shrinkage
• Volume change is one of the detrimental properties of
concrete, which affects the long term strength and
durability.
• The term Shrinkage is loosely used to describe the various
aspects of volume changes in concrete due to loss of
moisture at different stages due to different reasons.
• Shrinkage can be classified in the following: -
a) Plastic Shrinkage
b) Drying Shrinkage
c) Autogenous Shrinkage
d) Carbonation Shrinkage
e) Thermal Shrinkage
• Plastic Shrinkage: Plastic shrinkage develops
on the surface of concrete (fresh) soon after
the concrete is placed in the forms while the
concrete is still in the plastic state.
• Loss of water by evaporation from the
concrete surface or absorption of water by
aggregate or subgrade may be the reason of
plastic shrinkage.
• The Loss of water results in the reduction in
volume and consequently cracks may appear
at the surface or internally around the
aggregate or reinforcement.
• Drying Shrinkage : Drying shrinkage develops in hardened
concrete due to loss of water held in gel pores.) This loss
of water causes the change in volume. The loss of gel
water is progressively continued over a long time, as long
as concrete is kept in drying conditions.
• Autogenous ShrinkageIn a conservative system i.e.,
where no moisture movement to or from the paste is
permitted, when temperature is constant some shrinkage
may occur. It explains the shrinkage of concrete in sealed
conditions, without moisture exchange between concrete
and environment. The shrinkage of such a conservative
system is known as Autogenous shrinkage. It is of minor
importance and it not applicable in practice to many
situations except that of mass concrete in
• Carbonation Shrinkage : Carbon dioxide present in the
atmosphere reacts in the presence of water with
hydrated cement. Calcium hydroxide gets converted to
calcium carbonate and also some other cement
compounds are decomposed. This phenomena is called
Carbonation and it is accompanied by an increase in
weight of the concrete (decrease in volume) and by
shrinkage. This type of shrinkage is known as
Carbonation Shrinkage.
• Thermal Shrinkage : Shrinkage of concrete may also
occurs because of volumetric changes due to change in
temperature. For example, the roof slab or road
pavement expands during the day and undergoes
thermal shrinkage during night.
 Factor affecting shrinkage
• As per IS 456-2000, the total shrinkage of
concrete depends upon the constituents of
concrete, size of the member and environmental
conditions. For a given humidity and temperature
the total shrinkage of concrete is most influenced
by the total amount of water present in the
concrete at the time of mixing and to a lesser
extent by cement content.
a) Water/Cement ratio - Higher the W/C ratio, more
will be shrinkage of concrete
b) Aggregate - Aggregate particles restrain the shrinkage
of the paste. The harder aggregate results in low
magnitude of total shrinkage and softer aggregate shows
greater magnitude of total shrinkage.
c) Relative humidity and ambient temperature- Shrinkage
increases with reduction in relative humidity, rate of
shrinkage decreases rapidly with time.
d) Member geometry and size- Shrinkage decreases with
an increase in the size of the specimen. But above some
value, size effect is no longer apparent. If Volume to
surface ratio is small, the initial drying shrinkage will be
higher and vice-versa.
e) degree of hydration
f) age of paste
g) moisture content
h) Admixtures in concrete
i) Level of applied stress
j) duration of load
k) age of loading
l) cement types
 Test on Hardened Concrete
• The compressive strength of concrete is measured by
breaking concrete specimens in a compression testing
machine or a universal testing machine.
• The compressive strength of concrete is calculated
from the failure load divided by the cross-sectional
area of specimens which is resisting the load.
• The compressive strength of concrete is measured at
28 days in units of MPa (Mega Pascal) or N/mm².
• The compression test is commonly carried out on 150
mm cube specimens.
 Compressive strength Test of Concrete
(As per IS 516-1959)
• Compressive Strength Test of Concrete is one of the most
important tests done on concrete.
• This single test is enough to do a quality check on the type
of concreting that is being done at site.
• The compressive strength of concrete is given in terms of
the characteristic compressive strength of 150 mm size
cubes tested at 28 days (fck) as per Indian Standards.
• Apparatus Required - Compression Testing Machine, Cube
mould of 150 mm, Weighing balance, Vibrating machine,
Trowel, Tray.
• Materials required - Cement, sand, gravel (stone chips) and
water
 Procedure
1) First of all, 150 mm cube mould are properly cleaned, assembled
and oiled prior to mixing of concrete.The dry ingredients are weighed
and taken for mixing based on given concrete mix proportions. The
quality water is determined based on given water/cement ratio. For
nominal mix w/c = 0.4-0.6 range is taken.
3) The dry ingredients are mixed properly and measured quantity of
water is added and mixed properly to make concrete.
4) The freshly prepared concrete is poured in cube mould; three cubes
for each test. Moulds are preferably compacted using vibrator.
Otherwise Cube moulds are filled in three layers and each layer is
tapped 25 times using a standard tamping rod.
5) Concrete at the top of the mould is leveled by means of trowel and
given proper identification mark of the specimen.
• 6) Filled moulds are kept in damp condition in laboratory for
24 hours.
• 7) After 24 hours, concrete cubes are carefully removed from
the mould. Immediately after removal from mould, the
concrete cubes are immersed in water in the curing tank.
• 8) The concrete cubes are taken out of curing tank on the day
of test (3, 7, and 28 days) and are tested using Compression
Testing Machine (CTM). The load shall be applied without
shock and increased continuously at a rate of approximately
140 kg/sq cm/min until the resistance of the specimen to the
increasing load breaks down and no greater load can be
sustained. (Test day is counted from the day of mixing).
• 9) The ultimate load at failure or crushing load in KN of
concrete cube is noted down. The compressive strength in
(N/mm²) is determined by dividing the crushing load by cross
sectional area of cube (150x150 mm²).
10) Avg result of three is calculated and reported as compressive
strength of concrete for each day of test.
 Observation & calculation
Sr. Date of Cube ID Date of Crushing Cross Compres Average
No Casting testing Load (N) Sectional sive Strength
Area Strength
(mm2) of Cube
(N/mm2)
 Importance of NDT
• It is often necessary to test concrete structures
after the concrete has hardened to determine
whether the structure is suitable for its designed
use.
• Ideally such testing should be done without
damaging the concrete.
• The non-destructive test is the one in which there
is no damage to the concrete.
• The test can be performed quickly and properties
of hardened concrete can be assessed.
 Methods of NDT1
• Rebound Hammer Test (as per IS 13311 Part 2-1992):
The rebound hammer test could be used for assessing the
compressive strength of concrete with the help of suitable co-
relations between rebound index and compressive strength,
assessing the uniformity of concrete, assessing the quality of
the concrete in relation to standard requirements, and
assessing the quality of one element of concrete in relation to
another.
Working Principle of Rebound Hammer: When the plunger of
rebound hammer is pressed against the surface of the
concrete, the spring- controlled mass rebounds and the extent
of such rebound depends upon the surface hardness of
concrete. The surface hardness a Scroll for details efore the
rebound is taken to be related
 Factor affecting the Rebound Index

• The rebound index/numbers are influenced by

a number of factors like
a) types of cement and aggregate
b) surface condition and moisture content
c) age of concrete and
d) extent of carbonation of concrete.
 Determination of rebound index & compressive strength of
concrete by rebound hammer test as per I.S. 13311(Part 2)-1992

1) For testing, smooth, clean and dry surface is to be selected. If loosely

adhering scale is present, this should be rubbed off with a grinding wheel or
stone. Rough surfaces resulting from incomplete compaction, loss of grout,
spalled or tooled surfaces do not give reliable results and should be avoided.
2) The point of impact should be at least 20 mm away from any edge or shape
discontinuity.
3) For taking a measurement, the rebound hammer should be held at right
angles to the surface of the concrete member. The test can thus be conducted
horizontally on vertical-surfaces or vertically upwards or downwards on
horizontal surfaces. If the situation demands, the rebound hammer can be
held at intermediate angles also, but in each case, the rebound number will
be different for the same concrete.
4) Rebound hammer test is conducted around all the points of observation on
all accessible faces of the structural element. Concrete surfaces are
thoroughly cleaned, before taking any measurement. Around each point of
observation, six readings of rebound indices are taken and average of these
readings after deleting outliers as per IS-8000-1978 becomes the rebound
index for the point of observation
N/mm2
• The rebound hammer method provides a convenient and rapid
indication of the compressive strength of concrete by means of
establishing a suitable correlation between the rebound index and
the compressive strength of concrete.
• It is also pointed out that rebound indices are indicative of
compressive strength of concrete to a limited depth from the
surface. If the concrete in a particular member has internal micro
cracking, flaws or heterogeneity across the cross-section, rebound
hammer indices will not indicate the same.
• As such, the estimation of strength of concrete by rebound hammer
method cannot be held to be very accurate and probable accuracy
of prediction of concrete strength in a structure is & 25 percent. If
the relationship between rebound index and compressive strength
can be checked by tests on core samples obtained from the
structure or standard specimens made with the same concrete
materials and mix proportion, then the accuracy of results and
confidence thereon are greatly increased.
Average Rebound Number Quality of Concrete
>40 Very Good hard layer
30-40 Good Layer
20-30 Fair
<20 Poor Concrete
0 Delaminated
Non – Destructive Test on Concrete
Ultrasonic Pulse Velocity Test (as per IS 13311 Part 1-1992)
• The ultrasonic pulse velocity method could be used to
establish:
(i) the homogeneity of the concrete,
(ii) the presence of cracks, voids and other imperfections,
(iii) changes in the structure of the concrete which may occur
with time,
(iv) the quality of the concrete in relation to standard
requirements
(v) the quality of one element of concrete in relation to
another, and
(vi) the values of dynamic elastic modulus of the concrete.
• The ultrasonic pulse is generated by an electro-
acoustical transducer.
• When the pulse is induced into the concrete
from a transducer, it undergoes multiple
reflections at the boundaries of the different
material phases within the concrete.
• A complex system of stress waves is developed
which includes longitudinal (compressional),
shear (transverse) and surface (Rayleigh)
waves. The receiving transducer detects the
onset of the longitudinal waves, which is the
fastest.
• Because the velocity of the pulses is almost independent of
the geometry of the material through which they pass and
depends only on its elastic properties, pulse velocity method
is a convenient technique for investigating structural concrete.
• The underlying principle of assessing the quality of concrete is
that comparatively higher velocities are obtained when the
quality of concrete in terms of density, homogeneity and
uniformity is good.
• In case of poorer quality, lower velocities are obtained. If
there is a crack, void or flaw inside the concrete which comes
in the way of transmission of the pulses, the pulse strength is
attenuated and it passes around the discontinuity, thereby
making the path length longer. Consequently, lower velocities
are obtained.
 Determination of quality of concrete
by ultrasonic pulse velocity test
• During the test, the transducer held in contact with one surface of
concrete and it traverse a known path length in the concrete and then an
electrical signal passed the second transducer held in contact with the
other surface of the concrete member and the transit time (T) of the pulse
to be measured. The pulse velocity (V) is given by:
V = L/T
• Once the path is discovered by transducer the pulse velocity is transmitted
at a right angle to the surface of the concrete to get the best result. It is
essential that pulse velocity propagated or transmission by the transducer
is detected by receiving transducer. To ensure that they keep sufficient
coupling between the concrete and the face of each transducer. Generally,
copulates are petroleum jelly, grease, liquid soap, and kaolin glycerol
paste.
• In case, if there is a very rough concrete surface it is essential to smoothen
it for placing transducer.
 Specification for deciding the quality of concrete by ultrasonic
pulse velocity as per I.S. 13311 (Part – 1) – 1992

Pulse Velocity Concrete Quality

(km/second) (Grading)
Above 4.5 Excellent
3.5 to 4.5 Good
3.0 to 3.5 Medium
Below 3.0 Doubtful

The ultrasonic pulse velocity of concrete primly depends on concrete density and
modulus of elasticity property. Also, depends on concrete ingredients quality such as
cement, sand, and aggregate, there mix proportion as well as the method of placing,
compaction, and curing of concrete.
 General Strength of Concrete
- Concrete strength is defined by its ability to
withstand loads without failure.
- Typically measured in terms of compressive
strength.
- Factors such as mix proportion, curing, and age
influence strength.
 Factors Affecting Strength
• - Water-Cement Ratio: Lower ratio = higher
strength.
• - Curing Conditions: Proper curing improves
strength.
• - Aggregate Quality: Stronger aggregates lead to
better strength.
• - Admixtures: Chemical and mineral admixtures
affect strength.
• - Compaction: Poor compaction leads to voids,
reducing strength.
 Micro-Cracking & Stress-Strain
Relationship
• - Micro-cracks develop due to shrinkage, load
application, and thermal effects.
• - Stress-Strain relation: Concrete exhibits
nonlinear behavior.
• - Under increasing load, micro-cracks
propagate, leading to failure.
 Other Strength Properties
• - Impact Strength: Ability to resist sudden
loads.
• - Resistance to Abrasion: Ability to withstand
surface wear and tear.
• - Toughness: Energy absorbed before failure.
 Tensile vs. Compressive Strength
Relation
• - Concrete is much weaker in tension than in
compression.
• - Tensile strength is about 1/10th of
compressive strength.
• - Splitting tensile test and flexural test used to
measure tensile properties.
 Elasticity, Creep, and Shrinkage
• - Elasticity: Determines concrete’s ability to
return to its original shape.
• - Creep: Time-dependent deformation under
constant load.
• - Shrinkage: Reduction in volume due to
moisture loss over time.