Building Materials and Concrete Technology Unit 5

NRI INSTITUTE OF TECHNOLOGY DEPARTMENT OF CIVIL ENGINEERING
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B U I L D I N G M A T E R I A L S A N D C O N C R E T E T E C H N O L O G Y
UNIT V {CO 5,6}
Testing of Hardened Concrete-Factors affecting properties of Hardened concrete,
Compression tests, Tension tests, Flexure tests, Non-destructive testing methods –
Codal provisions for NDT – Rebound hammer and UPV method
Unit-V
S.No Long Answer Questions CO PO BTL Marks
1
Elaborate the factors affecting the strength of
concrete. 5,6 1,2,8 VI 7
2
Elaborate the Factors affecting properties of
Hardened concrete 5,6 1,2,8 VI 7
3
What the different tests are of hardened of
concrete? Explain in detail. 5,6 1,2,8 I 7
4
Explain the procedure to find Cube
compression test & split tensile strength test 5,6 1,2,8 II,V 7
5
Explain the procedure for finding out flexural
strength of concrete 5,6 1,2,8 II,V 7
6
Elaborate the different types of Non-destructive
testing methods on concrete 5,6 1,2,8 VI 7
7
Interpret the importance of NDT for concrete
and explain Rebound hammer test 5,6 1,2,8 V 7
8
Interpret the importance of NDT for concrete
and explain UPV test 5,6 1,2,8 V 7
9
Elaborate the Factors affecting compressive
strength of concrete 5,6 1,2,8 VI 7
10
Explain the procedure to find Cube
compression test & Flexural strength test 5,6 1,2,8 II,V 7
Concrete is widely used building material in the construction world. Concrete
is made up of various ingredients, and of course all of them have different role. The
properties of concrete generally rely on the mixing of concrete ingredients i.e.,
cement, coarse aggregates, fine aggregates (sand), and water.
The whole world wishes their structure to be strong and durable and for that,
they always design their structure according to the desired strength and service.
Strength gives an overall indication of quality of concrete; as it is directly related to
the lifelong performance of the concrete structure.
The strength of the concrete shows the ability of the structure to withstand
various loads (i.e., Dead Load, Live Load, Earthquake Load, Wind Load, etc..).

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The strength of the concrete can be measured with the different test that are
conducted on it such as, Compressive strength, Tensile strength and Flexural
strength. Apart from the above tests there are various factors that can also affects
the strength of the concrete, highlights of those factors are described below:

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Factors Affecting Compressive Strength of Concrete
01. Ratios of Ingredients
(a) Water/Cement Ratio
(b) Coarse / fine aggregate ratio
(c) Aggregate / Cement Ratio
02. Compaction of Concrete
03. Ingredients of Concrete
(a) Type and Quantity of Cement
(b) Types and Quantity of Aggregate
(c) Quality of Water
04. Curing of Concrete
05. The Shape of Aggregate
06. Maximum Size of Aggregates
07. Grading of Aggregate
08. Weather Condition
09. Temperature
10. The Rate of Loading
11. Age of Concrete
12.Relative humidity
01. Ratios of Ingredients
(a) . Water/Cement Ratio
The ratio of the weight of water to the weight of cement is called Water/Cement
ratio. It is the most important factor for gaining the strength of concrete. The lower
w/c ratio leads the higher strength of concrete. Generally, the water/cement ratio of
0.45 to 0.60 is used. Too much water leads to segregation and voids in concrete.
Water/Cement ratio is inversely proportional to the strength of concrete. As
shown in the chart below when the w/c ratio is increased the strength of concrete
gets decreased and when w/c ratio is decreased then the strength of concrete
increases.

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(b) Coarse / fine aggregate ratio
Following points should be noted for coarse/fine aggregate ratio:
 If the proportion of fines is increased in relation to the coarse aggregate, the overall
aggregate surface area will increase.
 If the surface area of the aggregate has increased, the water demand will also
increase.
 Assuming the water demand has increased, the water cement ratio will increase.
 Since the water cement ratio has increased, the compressive strength will decrease.
(c) Aggregate / Cement Ratio
Following points must be noted for aggregate cement ratio:
 If the volume remains the same and the proportion of cement in relation to that of
sand is increased the surface area of the solid will increase.
 If the surface area of the solids has increased, the water demand will stay the same
for the constant workability.
 Assuming an increase in cement content for no increase in water demand, the water
cement ratio will decrease.
 If the water cement ratio reduces, the strength of the concrete will increase.
The influence of cement content on workability and strength is an important one to
remember and can be summarized as follows:

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1) For a given workability an increase in the proportion of cement in a mix has little
effect on the water demand and results in a reduction in the water/cement ratio.
2) The reduction in water/cement ratio leads to an increase in strength of concrete.
3) Therefore, for a given workability an increase in the cement content results in an
increase in strength of concrete.
02. Compaction of Concrete
Compaction of concrete increases the density of the concrete because it is the
process in which air voids are removed from freshly placed concrete which makes
the concrete compact and dense. The presence of air voids in concrete greatly
reduces its strength.
Approximately 5 % of air voids can reduce the strength by 30 to 40 %. As we
can see in the above chart, even at the same water/cement ratio strength is different
with different compaction accuracies. In the fully compacted concrete, strength is
higher than the insufficiently compacted concrete.

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03. Ingredients of Concrete
The main ingredients of concrete are cement, sand, aggregate and Water.
Quality of each material affects the strength of the concrete. All materials, therefore,
should fulfil the standard criteria for use in concrete like,
a) Type and Quantity of Cement
The quantity of cement greatly affects concrete strength. The higher cement
content increases the tendency of shrinkage cracks when the concrete is getting
cured and hardened. Types of cement also have a great impact on the properties of
hardened concrete.
According to IS 456 2000, the minimum cement content specified ranges from
300 to 360 kg per cubic meter of concrete for various exposure conditions and for
various grades of concrete. Maximum cement content in concrete is also limited to
450 kg per cubic meter of concrete.
The grade of cement – i.e., 33 grade, 43 grade, 53 grades will also affect the
strength of concrete. The higher the grade, the higher strength particularly high
early strength

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(b) Types and Quantity of Aggregate
The strength of concrete depends upon the strength of aggregates. Low quality
of aggregate reduces the strength of concrete. The quantity of aggregate also affects
the properties of hardened concrete.

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At constant cement content, the higher amount of aggregate reduces the
concrete strength. The shape and grading of aggregate play a major role as far as
strength of concrete is concern.
(c) Quality of Water
Quality of water plays a significant role in the setting and hardening process
of concrete. Acidic, oily, silty, and seawater should not be used in concrete mix.
Impurities of water give an adverse effect on the strength of concrete.
Therefore, potable water is always used in concrete mix. Particularly the
impure water may lead to corrosion, carbonation or acid attack, therefore, reduces
the life of concrete.
04. Curing of Concrete
Curing of concrete is the most essential to prevent plastic shrinkage,
temperature control, strength development and durability. Curing provides the
desired moisture and temperature at the depth and near the surface after placing
and finishing of concrete for development of strength. In other words, curing
provides sufficient water to concrete for completing the hydration process without
interruption which is important for strength development.
Commonly 7-day curing corresponds to 70 % of compressive strength. Curing
period depends on the types of cement and the nature of work. Generally, it’s about
7 to 14 days for Ordinary Portland Cement. There are many methods of curing like
Ponding and immersion, Spraying and fogging saturated wet coverings etc.

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05. The Shape of Aggregate
There are many shapes of aggregate like angular, cubical, elongated, elongated
and flaky, flaky, irregular and rounded.
Angular aggregates are rough textured, and rounded aggregates are smooth
textured. Thus, the rounded aggregates, create the problem of lack of bonding
between cement paste and aggregate. Angular aggregates exhibit a better
interlocking effect in concrete, but the angular aggregate contains a larger number
of voids. For this, you needed well-graded aggregate.
The shape of aggregates becomes more important in case of high strength and
high-performance concrete where very low w/c ratio is used. In such cases, cubical
shape aggregates with uniform grading are required for better workability.
06. Maximum Size of Aggregates
Larger size aggregates give a lower strength because they have a lower surface
area for development of gel bond which is responsible for strength. Larger size
aggregate makes concrete heterogeneous. It will not distribute loading uniformly
when stressed.
Due to internal bleeding, the problem of development of the microcracks in
concrete happens when larger size aggregates are used in concrete.
07. Grading of Aggregate
Grading of aggregates determines the particle size distribution of aggregates.
It’s the most important factor for concrete mix. There are three types of graded
aggregate Gap Graded Aggregate, Poorly graded aggregate and Well-graded
aggregate.
Well-graded aggregate contains all size of particles of aggregate. So that, they
have a smaller number of voids. The use of well-graded aggregates gives higher
strength to the concrete.

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08. Weather Condition
Weather condition also affects the strength of concrete due to different
reasons. In cold climate, exterior concrete is subjected to repeated freezing and
thawing action due to the sudden change in weather.
It produces deterioration in concrete. With the change in moisture content,
materials expand and contract. It produced cracks in concrete.

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09. Temperature
With the certain degree of temperature increase, the rate of hydration process
increases in it which, it gains strength rapidly. Sudden temperature changes create
a thermal gradient, which causes cracking and spalling of concrete. So that, the final
strength of concrete is lower at the very high temperature.
The rate of hydration reaction is temperature dependent. If the temperature
increases the reaction also increases. This means that the concrete kept at higher
temperature will gain strength more quickly than a similar concrete kept at a lower
temperature.
However, the final strength of the concrete kept at the higher temperature will
be lower. This is because the physical form of the hardened cement paste is less
well-structured and more porous when hydration proceeds at faster rate. This is an
important point to remember because temperature has a similar but more
pronounced detrimental effect on permeability of the concrete.
10. The Rate of Loading
The strength of concrete increase with the increase in the rate of loading
because at the high rates of loading, there is less time for creep.
Creep produces permanent deformation in the structure at constant loading.
So that, the failure occurs at limiting values of strain rather than the stress.
In rapid loading, the load resistance is better than the slow loading.

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11. Age of Concrete
With increase in age of concrete, the degree of hydration would be more.
Hydration process is the chemical reaction of water and cement. Hydration produces
the gel which plays a significant role in the bonding of particles of the concrete
ingredients. Therefore, the strength of concrete increases with its age. Normally,
concrete strength gets doubled after 11 years provided there are no adverse factors.
The knowledge about factors which affect the concrete strength is helpful in
many ways particularly during designing the structure, choosing material for
concrete, observing precaution for different weather conditions, choosing different
methods for concreting, aiming better life of building structures, for low
maintenance of building after construction, longer durability and better
serviceability etc.
12. Relative humidity
If the concrete is allowed to dry out, the hydration reaction will stop. The
hydration reaction cannot proceed without moisture.
The three curves show the strength development of similar concretes exposed
to different conditions.

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Why do we test concrete compressive strength after 28 days?
Concrete gains strength with time after casting. It takes much time for
concrete to gain 100% strength and the time for same is still unknown. The rate of
gain of concrete compressive strength in higher during the first 28 days of casting
and then it slows down.
The table shows the compressive strength gained by concrete after 1, 3, 7, 14 and
28 days with respect to the grade of concrete we use.
From the table, we see that, concrete gains 16 percent strength in one day, 40
percent in 3 days, 65% in 7 days, 90% in 14 days and 99% strength in 28 days.
Thus, it is clear that concrete gains its strength rapidly in the initial days after
casting, i.e., 90% in only 14 days.
When, its strength has reached 99% in 28 days, still concrete continues to
gain strength after that period, but that rate of gain in compressive strength is very
less compared to that in 28 days.
Age Strength per cent
1 day 16%
3 days 40%
7 days 65%
14 days 90%

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28 days 99%
After 14 days of casting concrete, concrete gains only 9% in next 14 days. So,
rate of gain of strength decreases. We have no clear idea up to when the concrete
gains the strength, 1 year or 2 year, but it is assumed that concrete may gain its
final strength after 1 year.
So, since the concrete strength is 99% at 28 days, it's almost close to its final
strength, thus we rely upon the results of compressive strength test after 28 days
and use this strength as the base for our design and evaluation.
Though there are also some rapid methods of testing concrete compressive
strength which gives relation between rapid test methods and 28-day strength. This
rapid test is done where time is limited for construction and strength of structural
member must be known to carry out further construction work.
Gain of Strength with Age
The concrete develops strength with continued hydration. The rate of gain of
strength is faster to start with and the rate gets reduced with age. It is customary to
assume the 28 days strength as the full strength of concrete. Actually, concrete
develops strength beyond 28 days also. Earlier codes have not been permitting to
consider this increase of strength beyond 28 days for design purposes.
The increase in strength beyond 28 days used to get immersed with the factor
of safety. With better understanding of the material, progressive designers have been
trying to reduce the factor of safety and make the structure more economical. In this
direction, the increase in strength beyond 28 days is taken into consideration in
design of structures.
Some of the more progressive codes have been permitting this practice. Table
7.1 gives the age factors for permissible compressive stress in concrete, as per
British Code.

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Earlier IS code 456 of 1978 considered age factor and allowed the increase in
design stress in the lower columns in multistorey buildings. Earlier only one type of
cement i.e., cement governed by IS- 269 of 1976 was used in which case there was
appreciable increase in strength after 28 days.
After gradation of OPC the present-day cements particularly 53 grade
cements, being ground finer, the increase in strength after 28 days is nominal. Most
of the strength developments in respect of well cured concrete will have taken place
by 28 days.
Therefore, allowing age factor is not generally found necessary. Therefore, in
IS 456 of 2000, the clause is revised.
The clause states “There is normally a gain of strength beyond 28 days. The
quantum of increase depends upon the grade and type of cement, curing and
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”

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Many a time it may be necessary to estimate the strength of concrete at an
early age. One may not be able to wait for 28 days. Many research workers have
attempted to estimate the strength of concrete at 1, 3 or 7 days and correlate it to
28 days strength.
The relationship between the strength of concrete at a lower age and
28 days depends upon many factors such as compound composition of
cement, fineness of grinding and temperature of curing etc.

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Furthermore, mixes with low water/cement ratio gains strength,
expressed as a percentage of long-term strength, more rapidly than that of
concrete with higher water/cement ratio. This is presumably because the
cement particles are held at a closer interval in case of low water/cement
ratio than that of higher water/cement ratio, in which case there is a much
better possibility for the formation of continuous system of gel which gives
more strength.
Many research workers have forwarded certain relationships between 7 days
strength and 28 days strength.
The strength of concrete is generally estimated at 28 days by crushing field
test cubes or cylinders made from the representative concrete used for the structure.
Often it is questioned about the utility of ascertaining 28 days strength by which
time considerable amount of concrete will have been placed and the works may have
progressed. It is then rather too late for remedial measures, if the result of the test
cube at 28 days is too low.
On the other hand, the structure will be uneconomical if the result of the test
cube is too high. It is, therefore, of tremendous advantage to be able to predict 28
days strength within a few hours of casting the concrete so that we have a good idea
about the strength of concrete, so that satisfactory remedial measures could be
taken immediately before it is too late. There are many methods for predicting the
28 days strength, within a short period of casting.
Effect of Maximum size of Aggregate on Strength
At one time it was thought that the use of larger size aggregate leads to higher
strength. This was due to the fact that the larger the aggregate the lower is the total
surface area and, therefore, the lower is the requirement of water for the given
workability. For this reason, a lower water/cement ratio can be used which will
result in higher strength.

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However, later it was found that the use of larger size aggregate did not
contribute to higher strength as expected from the theoretical considerations due to
the following reasons. The larger maximum size aggregate gives lower surface area
for developments of gel bonds which is responsible for the lower strength of the
concrete.
Secondly bigger aggregate size causes a more heterogeneity in the concrete
which will prevent the uniform distribution of load when stressed
When large size aggregate is used, due to internal bleeding, the transition zone will
become much weaker due to the development of microcracks which result in lower
compressive strength.
Generally, high strength concrete or rich concrete is adversely affected by the
use of large size aggregate.
But in lean mixes or weaker concrete the influence of size of the
aggregate gets reduced. It is interesting to note that in lean mixes larger
aggregate gives highest strength while in rich mixes it is the smaller
aggregate which yields higher strength.
Fig. shows the influence of maximum size of aggregate on compressive
strength of concrete

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Fig. depicts the influence of size of aggregate on compressive strength of
concrete for different w/ c ratio.
Compressive Strength of Concrete Cube Specimens
Theory:
Compression test is the most common test conducted on hardened concrete,
partly because it is an easy test to perform, and partly because most of the
desirable characteristic properties of concrete are qualitatively related to its
compressive strength.
Cube, Beam and Cylinder moulding
The compression test is carried out on specimens cubical or cylindrical in
shape. Prism is also sometimes used, but it is not common in our country.
Sometimes, the compression strength of concrete is determined using parts of a

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beam tested in flexure. The end parts of beam are left intact after failure in flexure
and, because the beam is usually of square cross section, this part of the beam could
be used to find out the compressive strength.
Objective:
The test method covers determination of compressive strength of cubic
concrete specimens. It consists of applying a compressive axial load to moulded
cubes at a rate which is within a prescribed range until failure occurs.
Apparatus:
Testing Machine
The testing machine may be of any reliable type, of sufficient capacity for the tests
and capable of applying the load at the rate specified in 5.5. The permissible error
shall be not greater than ± 2 percent of the maximum load.
Cube Moulds - The mould shall be of 150 mm size conforming to IS: 10086-1982.
If the largest nominal size of the aggregate does not exceed 20 mm, 10 cm size
cubes may also be used as an alternative.
Cylindrical test specimens have a length equal to twice the diameter. They
are 15 cm in diameter and 30 cm long. Smaller test specimens may be used but a
ratio of the diameter of
the specimen to maximum size of aggregate, not less than 3 to 1 is maintained.
Weights and weighing device, Tools and containers for mixing, Tamper (square in
cross section) etc.
Age at Test - Tests shall be made at recognized ages of the test specimens,
the most usual being 7 and 28 days. Where it may be necessary to obtain the early
strengths, tests may be made at the ages of 24 hours ± ½ hour and 72 hours ± 2
hours. The ages shall be calculated from the time of the addition of water to the dry
ingredients.
Number of Specimens - At least three specimens, preferably from different
batches, shall be made for testing at each selected age.
Procedure:
1. Sampling of Materials - Samples of aggregates for each batch of concrete shall
be of the desired grading and shall be in an air-dried condition. The cement samples,
on arrival at the laboratory, shall be thoroughly mixed dry either by hand or in a
suitable mixer in such a manner as to ensure the greatest possible blending and
uniformity in the material.
2. Proportioning - The proportions of the materials, including water, in concrete
mixes used for determining the suitability of the materials available, shall be similar
in all respects to those to be employed in the work.

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3. Weighing - The quantities of cement, each size of aggregate, and water for each
batch shall be determined by weight, to an accuracy of 0.1 percent of the total weight
of the batch.
4. Mixing Concrete - The concrete shall be mixed by hand, or preferably, in a
laboratory batch mixer, in such a manner as to avoid loss of water or other materials.
Each batch of concrete shall be of such a size as to leave about 10 percent excess
after moulding the desired number of test specimens.
5. Mould - Test specimens cubical in shape shall be 15 × 15 × 15 cm. If the largest
nominal size of the aggregate does not exceed 2 cm, 10 cm cubes may be used as an
alternative. Cylindrical test specimens shall have a length equal to twice the
diameter.
6. Compacting – The concrete shall be filled into the mould in layers approximately
5 cm’s deep,
In placing each scoopful of concrete, the scoop shall he moved around the top
edge to the mould as the concrete slides from it, order to ensure symmetrical
distribution of the concrete within the mould. Each layer shall be compacted either
by hand or by vibration.
For cubical specimens, in no case shall the concrete be subjected to less than
35 strokes per layer for 15 cm cubes or 25 strokes per layer for 10 cm’s cubes. For
cylindrical specimens, the number of strokes shall not be less than thirty per layer.
The test specimens shall be made as soon as practicable after mixing, placed
in 3 layers with 25 blows per layer is done in and in such a way as to produce full
compaction of the concrete with neither segregation nor excessive laitance.
Vibrating table for cubes

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7. Curing - The test specimens shall be stored in a place, free from vibration, in
moist air of at least 90 percent relative humidity and at a temperature of 27° ± 2°C
for 24 hours ± ½ hour from the time of addition of water to the dry ingredients.
After this period, the specimens are marked and removed from the moulds
and unless required for test within 24 hours, immediately submerged in clean fresh
water or saturated lime solution and kept there until taken out just prior to test.
The water or solution in which the specimens are submerged, are renewed
every seven days and are maintained at a temperature of 27° ± 2°C. The specimens
are not to be allowed to become dry at any time until they have been tested.
8. Placing the Specimen in the Testing Machine –
The bearing surfaces of the testing machine shall be wiped clean and any loose
sand or other material removed from the surfaces of the specimen which are to be
in contact with the compression plates.
9. During the interval between testing the specimens as beams and testing the
broken portions as cubes, the specimens shall be stored in water at a temperature
of 24° to 30°C and shall be tested immediately on removal from the water and while
still in the wet condition. In the case of cubes, the specimen shall be placed in the
machine in such a manner that the load shall be applied to opposite sides of the
cubes as cast, that is, not to the top and bottom.
10. The axis of the specimen shall be carefully aligned with the centre of thrust of
the spherically seated platen. No packing shall be used between the faces of the test
specimen and the steel platen of the testing machine.
11. 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.
12. The maximum load applied to the specimen shall then be recorded and the
appearance of the concrete and any unusual features in the type of failure shall be
noted.
OBSERVATIONS:
Measured side of cube = ----- cm
Weight of the cube = ------ kg.
Load at first crack = ------- kg.
Load at ultimate failure = ------- kg.

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Sr. No. Age of
Cube
Weight
of
Cement
Cube
(gms)
Cross-
Sectional
area
(mm2)
Load (N) Compressive
strength
(N/mm2)
Average
Compressive
strength
(MPa)
CALCULATIONS:
Initial crack strength of concrete = Load at first crack
c/s area of the specimen
Ultimate cube compressive strength of the concrete = Maximum Load
c/s area of the specimen
Safe compressive strength of concrete = Ultimate strength
Factor of safety (3)
Failure of Compression Specimen
Compression test develops a rather more complex system of stresses. Due to
compression load, the cube or cylinder undergoes lateral expansion owing to the
Poisson’s ratio effect. The steel platens do not undergo lateral expansion to some
extent that of concrete, with the result that steel restrains the expansion tendency
of concrete in the lateral direction. This induces a tangential force between the end
surfaces of the concrete specimen and the adjacent steel platens of the testing
machine.
It has been found that the lateral strain in the steel platens is only 0.4 of the
lateral strain in the concrete. Due to this the platen restrains the lateral expansion
of the concrete in the parts of the specimen near its end. The degree of restraint
exercised depends on the friction actually developed. When the friction is eliminated
by applying grease, graphite or paraffin wax to the bearing surfaces the specimen
exhibits a larger lateral expansion and eventually splits along its full length.
With friction acting i.e., under normal conditions of test, the elements within
the specimen are subjected to a shearing stress as well as compression. The

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magnitude of the shear stress decreases and the lateral expansion increases in
distance from the platen.
As a result of the restraint, in a specimen tested to destruction there is a
relatively undamaged cone of height equal to √3 / 2 d (where d is the lateral
dimension of the specimen).10.1 But if the specimen is longer than about 1.7 d, a
part it of will be free from the restraining effect of the platen. Specimens whose length
is less than 1.5 d, show a considerably higher strength than those with a greater
length. (See Fig. Below).
Report:
The following information shall be included in the report on each specimen:
a) identification mark.
b) date of test,
e) age of specimen,
d) curing conditions,
e) nominal size of specimen.
f) maximum load,
g) equivalent cube strength, and
h) appearance of the concrete and type of fracture, if these are unusual
Reference:
IS: 516 - 1959, IS: 1199-1959, SP: 23-1982, IS: 10086-1982

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Percent of strength increases in days
Age Strength per cent
1 day 16%
3 days 40%
7 days 65%
14 days 90%
28 days 99%
Comparison between Cube and Cylinder Strength
It is difficult to say whether cube test gives more realistic strength properties
of concrete or cylinder gives a better picture about the strength of concrete. However,
it can be said that the cylinder is less affected by the end restrains caused by platens
and hence it seems to give more uniform results than cube. Therefore, the use of
cylinder is becoming more popular, particularly in the research laboratories.
Cylinders are cast and tested in the same position, whereas cubes are cast in
one direction and tested from the other direction. In actual structures in the field,
the casting and loading is similar to that of the cylinder and not like the cube. As
such, cylinder simulates the condition of the actual structural member in the field
in respect of direction of load.
The points in favour of the cube specimen are that the shape of the cube
resembles the shape of the structural members often met with on the ground. The
cube does not require capping, whereas cylinder requires capping. The capping
material used in case cylinder may influence to some extent the strength of the
cylinder.

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It is interesting to note that the restraining effect of the platens of the testing
machine extends over the entire height of the cube but leaves unaffected a part of
test cylinder because of greater height. It is, therefore, the strength of the cube made
from identical concrete will be different from the strength of the cylinder. Normally
strength of the cylinder is taken as 0.8 times the strength of the cube, but
experiments have shown that there is no unique relationship between the strength
of cube and strength of cylinder.
It was seen that the strength relation varies with the level of the strength of
concrete. For higher strength, the difference between the strength of cube and
cylinder is becoming narrow. For 100 MPa concrete the ratio may become nearly
1.00. Table 10.1 shows the strength pattern of cubes and cylinders.
Flexural Strength of Concrete
OBJECTIVE
To determine the Flexural Strength of Concrete, which comes into play when
a road slab with inadequate sub-grade support is subjected to wheel loads and / or
there are volume changes due to temperature / shrinking.
Concrete as we know is relatively strong in compression and weak in tension.
In reinforced concrete members, little dependence is placed on the tensile strength
of concrete since steel reinforcing bars are provided to resist all tensile forces.
however, tensile stresses are likely to develop in concrete due to drying shrinkage,
rusting of steel reinforcement, temperature gradients and many other reasons.
therefore, the knowledge of tensile strength of concrete is of importance.
A concrete road slab is called upon to resist tensile stresses from two principal
sources– wheel loads and volume change in the concrete.
wheel loads may cause high tensile stresses due to bending, when there is an
inadequate subgrade support. Volume changes, resulting from changes in
temperature and moisture, may produce tensile stresses, due to warping and due to
the movement of the slab along the subgrade.
Stresses due to volume changes alone may be high. The longitudinal tensile
stress in the bottom of the pavement, caused by restraint and temperature warping,
frequently amounts to as much as 2.5 MPa at certain periods of the year and the
corresponding stress in the transverse direction is approximately 0.9 MPa. These
stresses are additive to those produced by wheel loads on unsupported portions of
the slab.

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Determination of Tensile Strength
Direct measurement of tensile strength of concrete is difficult. Neither
specimens nor testing apparatus have been designed which assure uniform
distribution of the “pull” applied to the concrete.
While a number of investigations involving the direct measurement of tensile
strength have been made, beam tests are found to be dependable to measure flexural
strength property of concrete.
The value of the modulus of rupture (extreme fibre stress in bending) depends
on the dimension of the beam and manner of loading. The systems of loading used
in finding out the flexural tension are central point loading and third point loading.
In the central point loading, maximum fibre stress will come below the point
of loading where the bending moment is maximum. In case of symmetrical two-point
loading, the critical crack may appear at any section, not strong enough to resist the
stress within the middle third, where the bending moment is maximum.
It can be expected that the two-point loading will yield a lower value of the
modulus of rupture than the centre point loading. Figure below shows the modulus
of rupture of beams of different sizes subjected to centre point and third point
loading. I.S. 516-1959, specifies two-point loading. The details of the specimen and
procedure are described in the succeeding paragraphs.

29
REFERENCE STANDARDS
IS: 516-1959 – Methods of tests for strength of concrete
EQUIPMENT & APPARATUS
 Beam mould of size 15 x 15x 70 cm (when size of aggregate is less than 38
mm) or of size 10 x 10 x 50 cm (when size of aggregate is less than 19 mm)
 Tamping bar (40 cm long, weighing 2 kg and tamping section having size of
25 mm x 25 mm square)
Flexural Strength Test Arrangement
 Flexural test machine–
The bed of the testing machine shall be provided with two steel rollers, 38 mm in
diameter, on which the specimen is to be supported, and these rollers shall be so
mounted that the distance from centre to centre is 60 cm for 15.0 cm specimens or
40 cm for 10.0 cm specimens.
The load shall be applied through two similar rollers mounted at the third points of
the supporting span that is, spaced at 20 or 13.3 cm centre to centre.
The load shall be divided equally between the two loading rollers, and all rollers
shall be mounted in such a manner that the load is applied axially and without
subjecting the specimen to any torsional stresses or restraints.

30
PROCEDURE
1. Prepare the test specimen by filling the concrete into the mould in 3 layers of
approximately equal thickness. Tamp each layer 35 times using the tamping bar as
specified above. Tamping should be distributed uniformly over the entire cross
section of the beam mould and throughout the depth of each layer.

31
2. Clean the bearing surfaces of the supporting and loading rollers, and remove
any loose sand or other material from the surfaces of the specimen where they are
to make contact with the rollers.
3. Circular rollers manufactured out of steel having cross section with diameter
38 mm will be used for providing support and loading points to the specimens. The
length of the rollers shall be at least 10 mm more than the width of the test specimen.
A total of four rollers shall be used, three out of which shall be capable of rotating
along their own axes. The distance between the outer rollers (i.e., span) shall be 3d
and the distance between the inner rollers shall be d. The inner rollers shall be
equally spaced between the outer rollers, such that the entire system is systematic.
4. The specimen stored in water shall be tested immediately on removal from
water; whilst they are still wet. The test specimen shall be placed in the machine
correctly centred with the longitudinal axis of the specimen at right angles to the
rollers. For moulded specimens, the mould filling direction shall be normal to the
direction of loading.
5. The load shall be applied at a rate of loading of 400 kg/min for the 15.0 cm
specimens and at a rate of 180 kg/min for the 10.0 cm specimens.
CALCULATION
The Flexural Strength or modulus of rupture (fb) is given by
fb = pl/bd2 (when a > 20.0cm for 15.0cm specimen or > 13.0cm for 10cm specimen)
or
fb = 3pa/bd2 (when a < 20.0cm but > 17.0 for 15.0cm specimen or < 13.3 cm but >
11.0cm for 10.0cm specimen.)
Where,
a = the distance between the line of fracture and the nearer support, measured on
the centre line of the tensile side of the specimen
b = measured width of specimen (cm)
d = measured depth of specimen at point of failure (cm)
l = length of span on which specimen is supported (cm)
p = max. Load applied to specimen (kg)
Note:
If ‘a’ is less than 17.0 cm for a 15.0 cm specimen, or less than 11.0 cm
for a 10.0 cm specimen, the results of the test be discarded.
SAFETY & PRECAUTIONS:
 Use hand gloves while, safety shoes at the time of test.
 After test switch off the machine.
 Keep all the exposed metal parts greased.

32
 Keep the guide rods firmly fixed to the base & top plate.
 Equipment should be cleaned thoroughly before testing & after testing.
Discussion
As mentioned earlier, it is difficult to measure the tensile strength of concrete
directly. Of late some methods have been used with the help of epoxy bonded end
pieces to facilitate direct pulling. Attempts have also been made to find out direct
tensile strength of concrete by making briquette of figure 8 shape for direct pulling
but this method was presenting some difficulty with grip and introduction of
secondary stresses while being pulled.
Whatever may be the methods adopted for finding out the ultimate direct
tensile strength, it is almost impossible to apply truly axial load. There is always
some eccentricity present. The stresses are changed due to eccentricity of loading.
These may introduce major error on the stresses developed regardless of specimen
size and shape. The third problem is the stresses induced due to the grips.
There is a tendency for the specimen to break near the ends. This problem is
always overcome by reducing the section of the central portion of the test specimen.
The method in which steel plates are glued with the epoxies to the ends of test
specimen, eliminates stresses due to griping, but offers no solution for the
eccentricity problem.
All direct tension test methods require expensive universal testing machine.
This explains why these tests are not used on a routine basis and are not yet
standardised.
REPORTS
The Flexural strength of the concrete is reported to two significant figures.
Indirect Tensile Strength of Concrete
Objective:
This method covers the determination of the splitting tensile strength of
cylindrical concrete specimens.
Cylinder Splitting Tension Test:
This is also sometimes referred as, “Brazilian Test”. This test was developed
in Brazil in 1943. At about the same time this was also independently developed in
Japan.
The test is carried out by placing a cylindrical specimen horizontally between
the loading surfaces of a compression testing machine and the load is applied until
failure of the cylinder, along the vertical diameter. Figure 10.6 shows the test
specimen and the stress pattern in the cylinder respectively.

33
When the load is applied along the generatrix, an element on the vertical
diameter of the cylinder is subjected to a vertical compressive stress of
where, P is the compressive load on the cylinder
L is the length of cylinder
D is its diameter
and r and (D – r) are the distances of the elements from the two loads respectively.
The loading condition produces a high compressive stress immediately below
the two generators to which the load is applied. But the larger portion corresponding
to depth is subjected to a uniform tensile stress acting horizontally. It is estimated
that the compressive stress is acting for about 1/6 depth and the remaining 5/6
depth is subjected to tension.
In order to reduce the magnitude of the high compression stresses near the
points of application of the load, narrow packing strips of suitable material such as
plywood are placed between the specimen and loading platens of the testing
machine. The packing strips should be soft enough to allow distribution of load over
a reasonable area, yet narrow and thin enough to prevent large contact area.
Normally, a plywood strip of 25 mm wide, 3 mm thick and 30 cm long is used.
The main advantage of this method is that the same type of specimen and the
same testing machine as are used for the compression test can be employed for this
test. That is why this test is gaining popularity. The splitting test is simple to perform
and gives more uniform results than other tension tests. Strength determined in the
splitting test is believed to be closer to the true tensile strength of concrete, than the
modulus of rupture. Splitting strength gives about 5 to 12% higher value than the
direct tensile strength.
Apparatus:
Testing Machine - The testing machine may be of any reliable type, of sufficient
capacity for the tests and capable of applying the load at the rate specified in 5.5.
The permissible error shall be not greater than ± 2 percent of the maximum load.
Cylinders -The cylindrical mould shall be of 150 mm diameter and 300 mm height
conforming to IS: 10086-1982.

34
Weights and weighing device, Tools and containers for mixing, Tamper (square in
cross section) etc.
Procedure
1. Sampling of Materials
Samples of aggregates for each batch of concrete shall be of the desired grading and
shall be in an air-dried condition. The cement samples, on arrival at the laboratory,
shall be thoroughly mixed dry either by hand or in a suitable mixer in such a manner
as to ensure the greatest possible blending and uniformity in the material.
2. Proportioning
The proportions of the materials, including water, in concrete mixes used for
determining the suitability of the materials available, shall be similar in all respects
to those to be employed in the work.
3. Weighing
The quantities of cement, each size of aggregate, and water for each batch shall be
determined by weight, to an accuracy of 0.1 percent of the total weight of the batch.
4. Mixing Concrete
The concrete shall be mixed by hand, or preferably, in a laboratory batch mixer, in
such a manner as to avoid loss of water or other materials. Each batch of concrete
shall be of such a size as to leave about 10 percent excess after moulding the desired
number of test specimens.
5. Mould
The cylindrical mould shall be of 150 mm diameter and 300 mm height conforming
to IS: 10086-1982.
6. Compacting
The test specimens shall be made as soon as practicable after mixing, placed in 6
layers with 25 blows per layer is done in such a way as to produce full compaction
of the concrete with neither segregation nor excessive laitance.
7. Curing
The test specimens shall be stored in a place, free from vibration, in moist air of at
least 90 percent relative humidity and at a temperature of 27° ± 2°C for 24 hours ±
½ hour from the time of addition of water to the dry ingredients.
8. Placing the Specimen in the Testing Machine
Specimens when received dry shall be kept in water for 24 h before they are
taken for testing. Unless other conditions are required for specific laboratory
investigation specimen shall be tested immediately on removal from the water whilst
they are still wet. Surface water and grit shall be wiped off the specimens and any

35
projecting fins removed from the surfaces which are to be in contact with the packing
strips
The bearing surfaces of the supporting and loading rollers shall be wiped
clean, and any loose sand or other material removed from the surfaces of the
specimen where they are to make contact with the rollers.
Age at Test
Tests shall be made at recognized ages of the test specimens, the most usual
being 7 and 28 days. Where it may be necessary to obtain the early strengths, tests
may be made at the ages of 24 hours ± ½ hour and 72 hours ± 2 hours. The ages
shall be calculated from the time of the addition of water to the dry ingredients.
Number of Specimens
At least three specimens, preferably from different batches, shall be made for
testing at each selected age.
9. Two bearings strips of nominal (1/8 in i.e., 3.175mm) thick plywood, free of
imperfections, approximately (25mm) wide, and of length equal to or slightly longer
than that of the specimen should be provided for each specimen.
10. The bearing strips are placed between the specimen and both upper and lower
bearing blocks of the testing machine or between the specimen and the
supplemental bars or plates.

36
11. Draw diametric lines each end of the specimen using a suitable device that will
ensure that they are in the same axial plane. Centre one of the plywood strips along
the centre of the lower bearing block.
12. Place the specimen on the plywood strip and align so that the lines marked on
the ends of the specimen are vertical and centred over the plywood strip.
13. Place a second plywood strip lengthwise on the cylinder, centred on the lines
marked on the ends of the cylinder. Apply the load continuously and without shock,
at a constant rate within, the range of 689 to 1380 kPa/min splitting tensile stress
until failure of the specimen
14. Record the maximum applied load indicated by the testing machine at failure.
Note the type of failure and appearance of fracture.
Calculation:
Sr.
No.
Age of
Specimen
Identification
Mark
Dia of
Specimen
(mm)
Depth
(mm)
Maximum
Load (N)
Tensile
strength
(N/mm2)
Average
Tensile
strength
(MPa)
Calculate the splitting tensile strength of the specimen as follows:

37
The measured splitting tensile strength (fct), of the specimen shall be calculated to
the nearest 0.05 N/mm2,using the following formula
fct =
Where
d: diameter of specimen
L: Length, m
P: maximum applied load indicated by testing machine, kN.
Conclusion:
i) The average 7 Days Tensile Strength of concrete sample is found to be ….…...
ii) The average 28 Days Tensile Strength of concrete sample is found to be …...…...
Reference:
IS: 516 - 1959, IS: 1199-1959, SP: 23-1982, IS: 10086-1982, IS 5816: 1999.
Report:
The following information shall be included in
the report on each specimen:
a) Date of test,
b) Identification mark, shape and size of the specimen in mm,
c) Age of specimen at date of test when known,
d) Curing history,
e) Weight of specimen in Newtons,
f) Type of fracture and the appearance of concrete on the fractured face if these are
unusual,
g) Splitting tensile strength to the nearest 0.05 N/mm’ on the lower side, and
h) Estimate of the proportion of coarse aggregate fractured during test.
NON-DESTRUCTIVE TESTING OF CONCRETE (NDT ON CONCRETE)
Non-destructive test is a method of testing existing concrete structures to
assess the strength and durability of concrete structure. In the non-destructive
method of testing, without loading the specimen to failure (i.e., without destructing
the concrete) we can measure strength of concrete. Now days this method has
become a part of quality control process. This method of testing also helps us to
investigate crack depth, micro cracks and deterioration of concrete.
Non-destructive testing of concrete is a very simple method of testing but it
requires skilled and experienced persons having some special knowledge to interpret
and analyse test results.
What is the Importance of Non-Destructive Testing?
Testing hardened concrete in-place is often necessary to determine the
suitability of a structure for its intended use. Non-destructive testing methods are

38
used to evaluate concrete properties by assessing the strength and other properties
such as corrosion of reinforcement, permeability, cracking, and void structure. This
type of testing is important for the evaluation of both new and old structures. For
new structures, the principal applications are mainly used to determine the quality
of materials. Testing existing structures is usually related to an assessment of
structural integrity.
The Benefits of Non-Destructive Testing
 Non-destructive testing can also be used as an initial step to subsequent
coring and more invasive measures such as:
 Gauging characteristics of pre-cast, cast-in-place, or in-situ construction
 Determining the acceptability of supplied material and components
 Locating and categorizing cracks, voids, honeycombing, and other defects in
a concrete structure
 Determining the concrete uniformity prior to core cutting, load testing, or
other more expensive or disruptive tests
 Monitoring strength development related to formwork removal, cessation of
curing, and load application
 Determining the position, quantity, or condition of reinforcement
 Confirming or locating suspected deterioration of concrete resulting from such
factors as overloading, fatigue, external or internal chemical attack or change,
fire, explosion, environmental effects
 Assessing the potential durability of concrete while monitoring long-term
changes in properties
IS 13311(Part 2): 1992 CODAL Provisions for NDT Testing
This Indian Standard was adopted by the Bureau of Indian Standards, after
the draft finalized by the Cement and Concrete Sectional Committee bad been
approved by the Civil Engineering Division Council.
There are occasions when the various performance characteristics of concrete
in a structure are required to be assessed. In most of the cases, an estimate of
strength of concrete in the structure is needed, although parameters like overall
quality, uniformity, etc, also become important in others.
The various methods that can be adopted for in-situ assessment of strength
properties of concrete depend upon the particular aspect of strength in question.
For example, if the load-carrying capacity of structural ensemble is to be assessed,
carrying out a full-scale load test as per IS 456: 1978 ‘Code of practice for plain and
reinforced concrete (third revision)’ or IS 1343’: 1980 ‘Code of practice for
prestressed concrete (first revision)’ is the most direct way;
On the other hand, when the actual compressive strength of a concrete in the
structure is to be measured, core testing as per IS 516: 1959 Method of test for
strength of concrete’ is more reliable. However, both these methods are relatively
cumbersome and the latter method may leave the structure damaged locally in some
cases,
Use is, therefore, made of suitable non-destructive tests, which not only
provide an estimate of the relative strength and overall quality of concrete in the

39
structures, but also help in deciding whether more rigorous tests like load testing or
core drilling at selected locations are required.
There are various such non-destructive testing methods which can be broadly
classified as those which measure the overall quality of concrete, for example
dynamic or vibration methods like resonance frequency and ultrasonic pulse
velocity tests; and
those which involve measurement of parameters like
 surface hardness,
 rebound,
 penetration,
 pull-out strength, etc, and are believed to be indirectly related to the
compressive strength of concrete.
In addition, radiographic, radiometric, nuclear, magnetic and electrical
methods are also available. Since such non-destructive tests are at best indirect
methods of monitoring the particular characteristic of concrete and the
measurements are influenced by materials, mix and environmental factors, proper
interpretation of the results calls for certain degree of expertise.
It is more so, when the data on the materials and mix proportions used in the
construction are not available as is often the case. In view of the limitations of the
method for predicting the strength of concrete in the structure, it is preferable that
both ultrasonic pulse velocity given in Part 1 of the standard and rebound hammer
method is used in combination to alleviate the errors arising out of influence of
material, mix and environmental parameters on the respective measurements.
Relationships between pulse velocity, rebound number and compressive
strength of concrete are obtained by multiple regression of the measured values on
laboratory test specimens. However, this approach has the limitation that the
correlations are valid only for the materials and mix proportions used in the trials.
The intrinsic difference between the laboratory test specimens and in-situ
concrete,
for example
 surface texture,
 moisture condition,
 presence of reinforcement, etc, also affect the accuracy of results.
The correlation is valid only within the range of values of pulse velocity,
rebound number and compressive strength employed and any extrapolation beyond
these is open to question.
The rebound hammer test is not intended as a substitute for standard
compression test, but as a method for determining the uniformity of concrete
in the structure and comparing one concrete with another.
Because of the above limitations, the combined use of these two methods is
made in another way. In this, if the quality of concrete is assessed to be ‘excellent or
good’ by pulse velocity method, only then the compressive strength is assessed from

40
the rebound hammer indices, and this is taken as indicative of strength of concrete
in the entire cross-section of the concrete member.
When the quality assessed is ‘medium’, the estimation of compressive strength
by rebound indices is extended to the entire mass only on the basis of other collateral
measurements, for example, strength of site concrete cubes, cement content in the
concrete or core testing. When the quality of concrete is doubtful, no assessment of
concrete strength is made from rebound indices.
In most of the situations, the records of the original materials or mix
proportions used in the structure are not available. Therefore, considerable
improvisation has to be done in evolving the testing scheme and use are made of
comparative measurements made on adjoining portions of the structures or even
other structures in the vicinity of the one in question.
In doing so, an approach is taken the same materials and similar mix
proportions and level of workmanship were employed for the situations, any
significant difference in the ultrasonic pulse velocity or rebound indices between
them must be due to some inherent differences in the overall quality. If the nominal
grades of concrete or mix proportions are known to be different in either case,
suitable allowance is made for the same in interpretation of results.
The test results on ultrasonic pulse velocity and rebound indices are analysed
statistically and plotted as histograms and the lower fractiles of results are taken for
assessing the quality or ‘characteristic strength’ of concrete, in line with the current
limit state concepts of design.
The composition of the technical committee responsible for the formulation of
this standard is given at Annex A. For the purpose of deciding whether a particular
requirement of this standard is complied with, the final value, observed or
calculated, expressing the result of a test or analysis, shall be rounded off in
accordance with IS 2: 1960 ‘Rules for rounding off numerical values (revised) ‘. The
number of significant places retained in the rounded off value should be the same
as that of the specified value in this standard.
DIFFERENT METHODS OF NON-DESTRUCTIVE TESTING OF CONCRETE
Various non-destructive methods of testing concrete have been developed to
analyse properties of hardened concrete, which are given below.
1. SURFACE HARDNESS TEST
These are of indentation type, include the Williams testing pistol and impact
hammers, and are used only for estimation of concrete strength.
2. REBOUND HAMMER TEST
The rebound hammer test measures the elastic rebound of concrete and is primarily
used for estimation of concrete strength and for comparative investigation.
3. PENETRATION AND PULLOUT TECHNIQUES
These include the use of the simbi hammer, spit pins, the Windsor probe, and the
pull-out test. These measure the penetration and pull-out resistance of concrete and
are used for strength estimation, but they can also be used for comparative studies.

41
4. DYNAMIC OR VIBRATION TESTS
These include resonant frequency and mechanical sonic and ultrasonic pulse
velocity methods. These are used to evaluate durability and uniformity of concrete
and to estimate its strength and elastic properties.
5. COMBINED METHODS
The combined methods involving ultrasonic pulse velocity and rebound hammer
have been used to estimate strength of concrete.
6. RADIOACTIVE AND NUCLEAR METHODS
These include the X-ray and Gamma ray penetration tests for measurement of
density and thickness of concrete. Also, the neutron scattering and neutron
activation methods are used for moisture and cement content determination.
7. MAGNETIC AND ELECTRICAL METHODS
The magnetic methods are primarily concerned with determining cover of
reinforcement in concrete, whereas the electrical methods, including microwave
absorption techniques, have been used to measure moisture content and thickness
of concrete.
8. ACOUSTIC EMISSION TECHNIQUES
These have been used to study the initiation and growth of cracks in concrete.
Methods of Non-Destructive Testing of Concrete
Following are different methods of NDT on concrete:
 Penetration method
 Rebound hammer method
 Pull out test method
 Ultrasonic pulse velocity method
 Radioactive methods
1. Penetration Tests on Concrete
The Windsor probe is generally considered to be the best means of testing
penetration. Equipment consists of a powder-actuated gun or driver, hardened alloy
probes, loaded cartridges, a depth gauge for measuring penetration of probes and
other related equipment.
A probe, diameter 0.25 in. (6.5 mm) and length 3.125 in. (8.0 cm), is driven
into the concrete by means of a precision powder charge. Depth of penetration
provides an indication of the compressive strength of the concrete.
Although calibration charts are provided by the manufacturer, the instrument
should be calibrated for type of concrete and type and size of aggregate used.

42
Benefits and Limitations
The probe test produces quite variable results and should not be expected to
give accurate values of concrete strength. It has, however, the potential for providing
a quick means of checking quality and maturity of in situ concrete.
It also provides a means of assessing strength development with curing. The
test is essentially non-destructive, since concrete and structural members can be
tested in situ, with only minor patching of holes on exposed faces.
2.What is Rebound Hammer Test?
Rebound Hammer test is a Non-destructive testing method of concrete which
provide a convenient and rapid indication of the compressive strength of the
concrete. The rebound hammer is also called as Schmidt hammer that consist of a
spring-controlled mass that slides on a plunger within a tubular housing.
The operation of rebound hammer is shown in the fig.1. When the plunger of
rebound hammer is pressed against the surface of concrete, a spring-controlled
mass with a constant energy is made to hit concrete surface to rebound back. The
extent of rebound, which is a measure of surface hardness, is measured on a
graduated scale. This measured value is designated as Rebound Number (rebound
index).
A concrete with low strength and low stiffness will absorb more energy to yield
in a lower rebound value.
Operation of the rebound hammer
Fig.1.Operation of the rebound hammer

43
Objective of Rebound Hammer Test
As per the Indian code IS: 13311(2)-1992, the rebound hammer test has the
following objectives:
 To determine the compressive strength of the concrete by relating the rebound
index and the compressive strength
 To assess the uniformity of the concrete
 To assess the quality of the concrete based on the standard specifications
 To relate one concrete element with other in terms of quality
 Rebound hammer test method can be used to differentiate the acceptable and
questionable parts of the structure or to compare two different structures
based on strength.
Principle of Rebound Hammer Test
Rebound hammer test method is based on the principle that the rebound of
an elastic mass depends on the hardness of the concrete surface against which the
mass strikes. The operation of the rebound hammer is shown in figure-1.
When the plunger of rebound hammer is pressed against the concrete surface,
the spring-controlled mass in the hammer rebounds. The amount of rebound of the
mass depends on the hardness of concrete surface. Thus, the hardness of concrete
and rebound hammer reading can be correlated with compressive strength of
concrete.
The rebound value is read off along a graduated scale and is designated as the
rebound number or rebound index. The compressive strength can be read directly
from the graph provided on the body of the hammer.
Procedure for Rebound Hammer Test
Procedure for rebound hammer test on concrete structure starts with
calibration of the rebound hammer. For this, the rebound hammer is tested against
the test anvil made of steel having Brinell hardness number of about 5000 N/mm2.
After the rebound hammer is tested for accuracy on the test anvil, the rebound
hammer is held at right angles to the surface of the concrete structure for taking the
readings. The test thus can be conducted horizontally on vertical surface and
vertically upwards or downwards on horizontal surfaces as shown in figure below If
the rebound hammer is held at intermediate angle, the rebound number will be
different for the same concrete.

44
Fig.2.Rebound Hammer Positions for Testing Concrete Structure
The impact energy required for the rebound hammer is different for different
applications. Approximate Impact energy levels are mentioned in the table-1 below
for different applications.
Table-1: Impact Energy for Rebound Hammers for Different Applications As
per IS: 13311(2)-1992
Sl. No Applications
Approximate Impact Energy
for Rebound Hammer in Nm
1 For Normal Weight Concrete 2.25
2
For light weight concrete / For
small and impact resistive
concrete parts
0.75
3
For mass concrete testing Eg:
In roads, hydraulic structures
and pavements
30.00
Points to Remember in Rebound Hammer Test
 The concrete surface should be smooth, clean and dry.
 Ant loose particles should be rubbed off from the concrete surface with a
grinding wheel or stone, before hammer testing.
 Rebound hammer test should not be conducted on rough surfaces as a result
of incomplete compaction, loss of grout, spalled or tooled concrete surface.
 The point of impact of rebound hammer on concrete surface should be at least
20mm away from edge or shape discontinuity.
 Six readings of rebound number are taken at each point of testing and an
average of value of the readings is taken as rebound index for the
corresponding point of observation on concrete surface.

45
Correlation between compressive strength of concrete and rebound number
The most suitable method of obtaining the correlation between compressive
strength of concrete and rebound number is to test the concrete cubes using
compression testing machine as well as using rebound hammer simultaneously.
First the rebound number of concrete cubes is taken and then the
compressive strength is tested on compression testing machine. The fixed load
required is of the order of 7 N/ mm2 when the impact energy of the hammer is about
2.2 Nm.
The load should be increased for calibrating rebound hammers of greater
impact energy and decreased for calibrating rebound hammers of lesser impact
energy. The test specimens should be as large a mass as possible in order to
minimize the size effect on the test result of a full-scale structure. 150mm cube
specimens are preferred for calibrating rebound hammers of lower impact energy
(2.2Nm), whereas for rebound hammers of higher impact energy, for example 30 Nm,
the test cubes should not be smaller than 300mm.
The concrete cube specimens should be kept at room temperature for about
24 hours after taking it out from the curing pond, before testing it with the rebound
hammer. To obtain a correlation between rebound numbers and strength of wet
cured and wet tested cubes, it is necessary to establish a correlation between the
strength of wet tested cubes and the strength of dry tested cubes on which rebound
readings are taken.
A direct correlation between rebound numbers on wet cubes and the strength
of wet cubes is not recommended. Only the vertical faces of the cubes as cast should
be tested. At least nine readings should be taken on each of the two vertical faces
accessible in the compression testing machine when using the rebound hammers.
The points of impact on the specimen must not be nearer an edge than 20mm and
should be not less than 20mm from each other. The same points must not be
impacted more than once.
Interpretation of Rebound Hammer Test Results
After obtaining the correlation between compressive strength and rebound
number, the strength of structure can be assessed. In general, the rebound number
increases as the strength increases and is also affected by a number of parameters
i.e., type of cement, type of aggregate, surface condition and moisture content of the
concrete, curing and age of concrete, carbonation of concrete surface etc.

46
Relationship Between Cube Strength and the Rebound Number
Fig.3.Relationship Between Cube Strength and the Rebound Number
Moreover, the rebound index is indicative of compressive strength of concrete
up to a limited depth from the surface. The internal cracks, flaws etc. or
heterogeneity across the cross section will not be indicated by rebound numbers.
Table-2 below shows the quality of concrete for respective average rebound number.
Table.2. Quality of Concrete for different values of rebound number
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 found 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 gets greatly
increased.
Advantages and Disadvantages of Rebound Hammer Test
The advantages of Rebound hammer tests are:
 Apparatus is easy to use
 Determines uniformity properties of the surface
 The equipment used is inexpensive
 Used for the rehabilitation of old monuments
 The disadvantages of Rebound Hammer Test

47
 The results obtained is based on a local point
 The test results are not directly related to the strength and the deformation
property of the surface
 The probe and spring arrangement will require regular cleaning and
maintenance
 Flaws cannot be detected with accuracy
Factors Influencing Rebound Hammer Test
Below mentioned are the important factors that influence rebound hammer test:
Type of Aggregate
Type of Cement
Surface and moisture condition of the concrete
Curing and Age of concrete
Carbonation of concrete surface
Type of Aggregate
The correlation between compressive strength of concrete and the rebound number
will vary with the use of different aggregates. Normal correlations in the results are
obtained by the use of normal aggregates like gravels and crushed aggregates. The
use of lightweight aggregates in concrete will require special calibration to undergo
the test.
Type of Cement
The concrete made of high alumina cement ought to have higher compressive
strength compared to Ordinary Portland cement. The use of super sulphated cement
in concrete decreases the compressive strength by 50% compared to that of OPC.
Type of Surface and Moisture Condition
The rebound hammer test work best for close texture concrete compared with open
texture concrete. Concrete with high honeycombs and no-fines concrete is not
suitable to be tested by rebound hammer. The strength is overestimated by the test
when testing floated or trowelled surfaces when compared with moulded surfaces.
Wet concrete surface if tested will give a lower strength value. This underestimation
of strength can go lower to 20% that of dry concrete.
Type of curing and age of concrete
As time passes, the relation between the strength and hardness of concrete will
change. Curing conditions of concrete and their moisture exposure conditions also
affects this relationship. For concrete with an age between 3days to 90 days is
exempted from the effect of age. For greater aged concrete special calibrated curves
is necessary.
Carbonation on Concrete Surface
A higher strength is estimated by the rebound hammer on a concrete that is
subjected to carbonation. It is estimated to be 50% higher. So, the test has to be
conducted by removing the carbonated layer and testing by rebound hammer over
non-carbonated layer of concrete.

48
3. Pull-Out Tests on Concrete
A pull-out test measures, with a special ram, the force required to pull from
the concrete a specially shaped steel rod whose enlarged end has been cast into the
concrete to a depth of 3 in. (7.6 cm).
The concrete is simultaneously in tension and in shear, but the force required
to pull the concrete out can be related to its compressive strength.
The pull-out technique can thus measure quantitatively the in-situ strength
of concrete when proper correlations have been made. It has been found, over a wide
range of strengths, that pull-out strengths have a coefficient of variation comparable
to that of compressive strength.
Limitations and Advantages
Although pull-out tests do not measure the interior strength of mass concrete,
they do give information on the maturity and development of strength of a
representative part of it. Such tests have the advantage of measuring quantitatively
the strength of concrete in place.
Their main disadvantage is that they have to be planned in advance and pull-
out assemblies set into the formwork before the concrete is placed. The pull-out, of
course, creates some minor damage.
The test can be non-destructive, however, if a minimum pull-out force is
applied that stops short of failure but makes certain that a minimum strength has
been reached. This is information of distinct value in determining when forms can
be removed safely.
4. Dynamic Non-Destructive Test
At present the ultrasonic pulse velocity method is the only one of this type
that shows potential for testing concrete strength in situ. It measures the time of
travel of an ultrasonic pulse passing through the concrete.
The fundamental design features of all commercially available units are very
similar, consisting of a pulse generator and a pulse receiver.
Pulses are generated by shock-exciting piezoelectric crystals, with similar
crystals used in the receiver. The time taken for the pulse to pass through the
concrete is measured by electronic measuring circuits.
Pulse velocity tests can be carried out on both laboratory-sized specimens and
completed concrete structures, but some factors affect measurement:
There must be smooth contact with the surface under test; a coupling medium
such as a thin film of oil is mandatory.
It is desirable for path-lengths to be at least 12 in. (30 cm) in order to avoid
any errors introduced by heterogeneity.

49
It must be recognized that there is an increase in pulse velocity at below-
freezing temperature owing to freezing of water; from 5 to 30°C (41 - 86°F) pulse
velocities are not temperature dependent.
The presence of reinforcing steel in concrete has an appreciable effect on pulse
velocity. It is therefore desirable and often mandatory to choose pulse paths that
avoid the influence of reinforcing steel or to make corrections if steel is in the pulse
path.
Applications and Limitations
The pulse velocity method is an ideal tool for establishing whether concrete is
uniform. It can be used on both existing structures and those under construction.
Usually, if large differences in pulse velocity are found within a structure for no
apparent reason, there is strong reason to presume that defective or deteriorated
concrete is present.
High pulse velocity readings are generally indicative of good quality concrete. A
general relation between concrete quality and pulse velocity is given in
Table: Quality of Concrete and Pulse Velocity
General Conditions Pulse Velocity ft/sec
Excellent Above 15,000
Good 12,000-15,000
Questionable 10,000-12,000
Poor 7,000-10,000
Very Poor below 7,000
Fairly good correlation can be obtained between cube compressive strength
and pulse velocity. These relations enable the strength of structural concrete to be
predicted within ±20 per cent, provided the types of aggregate and mix proportions
are constant.
The pulse velocity method has been used to study the effects on concrete of
freeze-thaw action, sulphate attack, and acidic waters. Generally, the degree of
damage is related to a reduction in pulse velocity. Cracks can also be detected.
Great care should be exercised, however, in using pulse velocity
measurements for these purposes since it is often difficult to interpret results.
Sometimes the pulse does not travel through the damaged portion of the concrete.
The pulse velocity method can also be used to estimate the rate of hardening
and strength development of concrete in the early stages to determine when to

50
remove formwork. Holes have to be cut in the formwork so that transducers can be
in direct contact with the concrete surface.
As concrete ages, the rate of increase of pulse velocity slows down much more
rapidly than the rate of development of strength, so that beyond a strength of 2,000
to 3,000 psi (13.6 to 20.4 MPa) accuracy in determining strength is less than ±20%.
Accuracy depends on careful calibration and use of the same concrete mix
proportions and aggregate in the test samples used for calibration as in the
structure.
In summary, ultrasonic pulse velocity tests have a great potential for concrete
control, particularly for establishing uniformity and detecting cracks or defects. Its
use for predicting strength is much more limited, owing to the large number of
variables affecting the relation between strength and pulse velocity.
Ultrasonic Pulse Velocity Test
Ultrasonic testing of concrete or ultrasonic pulse velocity test on concrete is a
non-destructive test to assess the homogeneity and integrity of concrete. With this
ultrasonic test on concrete, following can be assessed:
 Qualitative assessment of strength of concrete, its gradation in different
locations of structural members and plotting the same.
 Any discontinuity in cross section like cracks, cover concrete delamination
etc.
 Depth of surface cracks.
Ultrasonic Testing of Concrete
Ultrasonic pulse velocity test consists of measuring travel time, T of ultrasonic
pulse of 50 to 54 kHz, produced by an electro-acoustical transducer, held in contact
with one surface of the concrete member under test and receiving the same by a
similar transducer in contact with the surface at the other end.
With the path length L, (i.e., the distance between the two probes) and time of
travel T, the pulse velocity (V=L/T) is calculated. Higher the elastic modulus, density
and integrity of the concrete, higher is the pulse velocity.
The ultrasonic pulse velocity depends on the density and elastic properties of
the material being tested. Ultrasonic Pulse Velocity Testing Instrument for Concrete

51
Fig.1: Ultrasonic Pulse Velocity Testing Instrument
Though pulse velocity is related with crushing strength of concrete, yet no
statistical correlation can be applied. The pulse velocity in concrete may be
influenced by:
a. Path length
b. Lateral dimension of the specimen tested
c. Presence of reinforcement steel
d. Moisture content of the concrete
The influence of path length will be negligible provided it is not less than 100mm
when 20mm size aggregate is used or less than 150mm for 40mm size aggregate.
Pulse velocity will not be influenced by the shape of the specimen, provided its least
lateral dimension (i.e., its dimension measured at right angles to the pulse path) is
not less than the wavelength of the pulse vibrations.
For pulse of 50Hz frequency, this corresponds to a least lateral dimension of
about 80mm. the velocity of pulses in steel bar is generally higher than they are in
concrete. For this reason, pulse velocity measurements made in the vicinity of
reinforcing steel may be high and not representative of the concrete.
The influence of the reinforcement is generally small if the bars run in a direction
at right angles to the pulse path and the quantity of steel is small in relation to the
path length. The moisture content of the concrete can have a small but significant
influence on the pulse velocity.
In general, the velocity is increased with increased moisture content, the
influence being more marked for lower quality concrete. Ultrasonic Pulse Velocity
Method - Method of propagating and receiving pulses

52
Fig.2: Method of propagating and receiving pulses
Measurement of pulse velocities at points on a regular grid on the surface of
a concrete structure provides a reliable method of assessing the homogeneity of the
concrete. The size of the grid chosen will depend on the size of the structure and the
amount of variability encountered.
Table: 1 – Concrete Quality based on Ultrasonic Pulse Velocity Test
Sl.
No.
PULSE VELOCITY CONCRETE QUALITY
1 >4.0 km/s Very good to excellent
2 3.5 – 4.0 km/s Good to very good, slight porosity may exist
3 3.0 – 3.5 km/s Satisfactory but loss of integrity is suspected
4 <3.0 km/s Poor and los of integrity exist.

53
Table 1 shows the guidelines for qualitative assessment of concrete based on
UPV test results. To make a more realistic assessment of the condition of surface of
a structural member, the pulse velocity can be combined with rebound number.
Table 2 shows the guidelines for identification of corrosion prone locations by
combining the results of pulse velocity and rebound number.
Table:2 – Identification of Corrosion Prone Location based on Pulse
Velocity and Hammer Readings
Sl.
No.
Test Results Interpretations
1
High UPV values, high
rebound number
Not corrosion prone
2
Medium range UPV
values, low rebound
numbers
Surface delamination, low quality of
surface concrete, corrosion prone
3
Low UPV, high rebound
numbers
Not corrosion prone, however to be
confirmed by chemical tests,
carbonation, pH
4
Low UPV, low rebound
numbers
Corrosion prone, requires chemical and
electrochemical tests.
Detection of Defects with Ultrasonic Test on Concrete
When ultrasonic pulse travelling through concrete meets a concrete-air
interface, there is a negligible transmission of energy across this interface so that
any air-filled crack or void lying directly between the transducers will obstruct the
direct beam of ultrasonic when the void has a projected area larger than the area of
transducer faces. The first pulse to arrive at the receiving transducer will have been
directed around the periphery of the defect and the time will be longer than in similar
concrete with no defect.
Estimating the depth of cracks
An estimate of the depth of a crack visible at the surface can be obtained by
the transit times across the crack for two different arrangements of the transducers
placed on the surface. One suitable arrangement is one in which the transmitting
and receiving transducers are placed on opposite sides of the crack and distant from
it.
Two values of X are chosen, one being twice that of the other, and the transmit
times corresponding to these are measured. An equation may be derived by
assuming that the plane of the crack is perpendicular to the concrete surface and
that the concrete in the vicinity of the crack is of reasonably uniform quality. It is
important that the distance X be measured accurately and that very good coupling
is developed between the transducers and the concrete surface. The method is valid

54
provided the crack is not filled with water. This ultrasonic test is done as per IS:
13311 (Part 1) – 1992.
Procedure for Ultrasonic Pulse Velocity
i) Preparing for use:
Before switching on the ‘V’ meter, the transducers should be connected to the
sockets marked “TRAN” and” REC”. The ‘V’ meter may be operated with either:
 The internal battery,
 An external battery or
 The A.C line.
ii) Set reference:
A reference bar is provided to check the instrument zero. The pulse time for
the bar is engraved on it. Apply a smear of grease to the transducer faces before
placing it on the opposite ends of the bar. Adjust the ‘SET REF’ control until the
reference bar transit time is obtained on the instrument read-out.
iii) Range selection:
For maximum accuracy, it is recommended that the 0.1 microsecond range
be selected for path length up to 400mm.
iv) Pulse velocity:
Having determined the most suitable test points on the material to be tested,
make careful measurement of the path length ‘L’. Apply couplant to the surfaces of
the transducers and press it hard onto the surface of the material.
Do not move the transducers while a reading is being taken, as this can generate
noise signals and errors in measurements. Continue holding the transducers onto
the surface of the material until a consistent reading appears on the display, which
is the time in microsecond for the ultrasonic pulse to travel the distance ‘L’. The
mean value of the display readings should be taken when the units digit hunts
between two values.
Pulse velocity= (Path length/Travel time)
v) Separation of transducer leads:
It is advisable to prevent the two transducer leads from coming into close
contact with each other when the transit time measurements are being taken. If this
is not done, the receiver lead might pick-up unwanted signals from the transmitter
lead and this would result in an incorrect display of the transit time.
5. Radioactive Methods of NDT
Radioactive methods of testing concrete can be used to detect the location of
reinforcement, measure density and perhaps establish whether honeycombing has
occurred in structural concrete units. Gamma radiography is increasingly accepted
in England and Europe.

55
The equipment is quite simple and running costs are small, although the
initial price can be high. Concrete up to 18 in. (45 cm) thick can be examined without
difficulty.
Purpose of Non-Destructive Tests on Concrete
A variety of Non-Destructive Testing (NDT) methods have been developed or
are under development for investigating and evaluating concrete structures.
These methods are aimed at estimation of strength and other properties;
monitoring and assessing corrosion; measuring crack size and cover; assessing
grout quality; detecting defects and identifying relatively more vulnerable areas in
concrete structures.
Many of NDT methods used for concrete testing have their origin to the testing
of more homogeneous, metallic system. These methods have a sound scientific basis,
but heterogeneity of concrete makes interpretation of results somewhat difficult.
There could be many parameters such as materials, mix, workmanship and
environment, which influence the results of measurements.
Moreover, these tests measure some other property of concrete (e.g., hardness)
and the results are interpreted to assess a different property of concrete e.g.,
strength which is of primary interest.
Thus, interpretation of results is very important and difficult job where
generalization is not possible. As such, operators can carry out tests but
interpretation of results must be left to experts having experience and knowledge of
application of such non-destructive tests.
Purposes of Non-destructive Tests
 Estimating the in-situ compressive strength
 Estimating the uniformity and homogeneity
 Estimating the quality in relation to standard requirement
 Identifying areas of lower integrity in comparison to other parts
 Detection of presence of cracks, voids and other imperfections
 Monitoring changes in the structure of the concrete which may occur with
time
 Identification of reinforcement profile and measurement of cover, bar
diameter, etc.
 Condition of prestressing/reinforcement steel with respect to corrosion
 Chloride, sulphate, alkali contents or degree of carbonation
 Measurement of Elastic Modulus
 Condition of grouting in prestressing cable ducts
 Purposes of Non-destructive Tests
 Purposes of Non-destructive Tests
Equipment’s for Non-Destructive Testing
According to their use, non-destructive equipment can be grouped as under:
 Strength estimation of concrete
 Corrosion assessment and monitoring
 Detecting defects in concrete structure
 Laboratory tests

Building Materials and Concrete Technology Unit 5

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