1. Objective and Principle
Objective
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.
Principle
The ultrasonic pulse is generated by an
electroacoustical 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.
The actual pulse velocity obtained depends
primarily upon the materials and mix proportions
of concrete. Density and modulus of
elasticity of aggregate also significantly affect
the puise velocity.
2. Apparatus required
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Fig. 1: Ultrasonic Pulse Velocity Meter
The apparatus for ultrasonic pulse velocity measurement shall consist of the following:
a) Electrical pulse generator,
b) Transducer - one pair,
c) Amplifier, and
d) Electronic timing device.
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3. Reference
IS-516(Part 5/Sec 1):2018 “Part 5 Non-Destructive Testing of Concrete- Section 1 Ultrasonic Pulse Velocity Testing"
4. Procedure
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In this test method, the ultrasonic pulse is
produced by the transducer which is held in
contact with one surface of the concrete member
under test. After traversing a known path length
L in the concrete, the pulse of vibrations is
converted into an electrical signal by the second
transducer held in contact with the other surface
of the concrete member and an electronic timing circuit enables the transit time (T) of
the pulse to be measured. The pulse velocity (V) is given by:
V = L/T
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Once the ultrasonic pulse impinges on the
surface of the material, the maximum energy is
propagated at right angles to the face of the
transmitting transducer and best results are,
therefore, obtained when the receiving transducer
is placed on the opposite face of the
concrete member (direct transmission or cross
probing). However, in many situations two
opposite faces of the structural member may
not be accessible for measurements. In such
cases, the receiving transducer is also placed on
the same face of the concrete members (surface
probing). Surface probing is not so efficient as
cross probing, because the signal produced at
the receiving transducer has an amplitude of only
2 to 3 percent of that produced by cross probing
and the test results are greatly influenced by the
surface layers of concrete which may have
different properties from that of concrete inside
the structural member. The indirect velocity is
invariably lower than the direct velocity on the
same concrete element. This difference may vary
from 5 to 20 percent depending largely on the
quality of the concrete under test. For good
quality concrete, a difference of about 0.5 km/
sec may generally be encountered.
-
To ensure that the ultrasonic pulses generated
at the transmitting transducer pass into the
concrete and are then detected by the receiving
transducer, it is essential that there be adequate
acoustical coupling between the concrete and
the face of each transducer. Typical couplants
are petroleum jelly, grease, liquid soap and
kaolin glycerol paste. If there is very rough
concrete surface, it is required to smoothen and
level an area of the surface where the transducer
is to be placed. If it is necessary to work on
concrete surfaces formed by other means, -for
example trowelling, it is desirable to measure
pulse velocity over a longer path length than
would normally be used. A minimum path length
of 150 mm is recommended for the direct transmission
method involving one unmoulded surface
and a minimum of 400 mm for the surface
probing method along an unmoulded surface.
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The natural frequency of transducers should
preferably be within the range of 20 to 150 kHz. Generally, high frequency transducers
are preferable for short path lengths and
low frequency transducers for long path lengths.
Transducers with a frequency of 50 to 60 kHz
are useful for most all-round applications.
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Since size of aggregates influences the pulse
velocity measurement, it is recommended that
the minimum path length should be 100 mm for
concrete in which the nominal maximum size of
aggregate is 20 mm or less and 150 mm for
concrete in which the nominal maximum size of
aggregate is between 20 to 40 mm.
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In view of the inherent variability in the test
results, sufficient number of readings are taken
by dividing the entire structure in suitable grid
markings of 30 x 30 cm or even smaller. Each
junction point of the grid becomes a point of
observation.
-
Transducers are held on corresponding points
of observation on opposite faces of a structural
element to measure the ultrasonic pulse velocity
by direct transmission, i.e., cross probing. If one
of the faces is not- accessible, ultrasonic pulse
velocity is measured on one face of the structural
member by surface probing.
-
Surface, probing in general gives lower
pulse velocity than in case of cross probing and
depending on number of parameters, the difference
could be of the order of about 1 km/sec.
5. Influence of Test Conditions
5.1 Influence of Surface Conditions and Moisture Content of Concrete
Smoothness of contact surface under test affects
the measurement of ultrasonic pulse velocity.
For most concrete surfaces, the finish is usually
sufficiently smooth to ensure good acoustical
contact by the use of a coupling medium and
by pressing the transducer against the concrete
surface. When the concrete surface is rough and
uneven, it is necessary to smoothen the surface
to make the pulse velocity measurement
possible.
In general, pulse velocity through concrete
increases with increased moisture content of
concrete. This influence is more for low strength
concrete than high strength concrete. The pulse
velocity of saturated concrete may be up to 2
percent higher than that of similar dry concrete.
In general, drying of concrete may result in
somewhat lower pulse velocity.
5.2 Influence of Path Length, Shape and Size of the Concrete Member
As concrete is inherently heterogeneous, it is
essential that path lengths be sufficiently long
so as to avoid any error introduced due to its
heterogeneity. In field work, this does not pose
any difficulty as the pulse velocity measurements
are carried out on thick structural concrete
members. However, in the laboratory where
generally small specimens are used, the path
length can affect the pulse velocity readings.
The shape and size of the concrete member do
not influence the pulse velocity unless the least
lateral dimension is less than a certain minimum
value, for example the minimum lateral dimension
of about 80 mm for 50 kHz natural
frequency of the transducer. Table 1 gives the
guidance on the choice of the transducer natural
frequency for different path lengths and minimum
transverse dimensions of the concrete
members.
5.3 Influence of Temperature of Concrete
Variations of the concrete temperature between
5 and 30°C do not significantly affect the pulse
velocity measurements in concrete. At temperatures
between 30 to 60°C there can be reduction
in pulse velocity up to 5 percent. Below
freezing temperature, the free water freezes within
concrete, resulting in an increase .in pulse
velocity up to 7.5 percent.
5.4 Influence of Stress
When concrete is subjected to a stress which is
abnormally high for the quality of the concrete,
the pulse velocity may be reduced due to the
development of micro-cracks. This influence is
likely to be the greatest when the pulse path is
normal to the predominant direction of the
planes of such micro-cracks. This occurs when
the pulse path is perpendicular to the direction
of a uniaxial compressive stress in a member.
This influence is generally insignificant unless
the stress is greater than about 60 percent of the
ultimate strength of the concrete.
5.5 Effect of Reinforcing Bars
The pulse velocity measured in reinforced concrete
in the vicinity of reinforcing bars is
usually higher than in plain concrete of the
same composition. This is because, the pulse
velocity in steel is 1.2 to 1.9 times the velocity
in plain concrete and, under certain conditions,
the first pulse to arrive at the receiving transducer
travels partly in concrete and partly in
steel.
The apparent increase in pulse velocity depends
upon the proximity of the measurements to the
reinforcing bar, the diameter and number of the
bars and their orientation with respect to the
path of propagation.
6. Interpretation of Result
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The ultrasonic pulse velocity of concrete is
mainly related to its density and modulus of
elasticity. This in turn, depends upon the
materials and mix proportions used in making concrete as well as the method of placing,
compaction and curing of concrete.
For example, if the concrete is not compacted
as thoroughly as possible, or if there is segregation
of concrete during placing or there are
internal cracks or flaws, the pulse velocity will
be lower, although the same materials and mix
proportions are used.
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The quality of concrete in terms of uniformity,
incidence or absence of internal flaws,
cracks and segregation, etc, indicative of the
level of workmanship employed; can thus be
assessed using the guidelines given in Table 2,
which have been evolved for characterising the
quality of concrete in structures in terms of the
ultrasonic pulse velocity.
S. No. |
Average velocity of Pulse Velocity by Cross Probing (km/sec) |
Concrete Quality Grading |
1 |
Below 3.5 |
Doubtful" |
2 |
3.50 to 4.50 |
Good |
3 |
Above 4.50 |
Excellent |
Table 1 : For concrete (Less than or Equal to M 25).
S. No. |
Pulse velocity by Cross Probing (km/sec) |
Concrete Quality Grading |
1 |
Above 3.75 |
Doubtful" |
2 |
3.75 to 4.50 |
Good |
3 |
Above 4.50 |
Excellent |
Table 2 : For concrete (Greater than M 25).
" In case of 'Doubtful quality', it shall be necessary to carry out additional tests.
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Since actual values of the pulse velocity
obtained, depend on a number of parameters,
any criterion for assessing the quality of concrete
on the basis of pulse velocity as given in Table 2
can be held as satisfactory only to a general
extent. However, when the comparison is made
amongst different parts of a structure, which
have been built at the same time with supposedly
similar materials, construction practices and
supervision, the assessment of quality becomes
more meaningful and reliable.
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The assessment of compressive strength of
concrete from ultrasonic pulse velocity values
is not adequate because the statistical confidence
of the correlation between ultrasonic pulse
velocity and the compressive strength of concrete
is not very high. The reason is that a large
number of parameters are involved, which
influence the pulse velocity and compressive
strength of concrete to different extents. However, if actual concrete materials and mix
proportions adopted in a particular structure are
available, then estimate of concrete strength can
be made by establishing suitable correlation
between the pulse velocity and the compressive
strength of concrete specimens made with such
materials and mix proportions, under environmental
conditions similar to that in the structure.
The estimated strength may vary from the actual
strength by 20 percent. The correlation so
obtained may not be applicable for concrete
of another grade or made with different types of
materials.