ULTRASONIC PULSE VELOCITY TEST (NON-DISTRUCTIVE TESTING)
INTRODUCTION
Ultrasonic Pulse Velocity test is a
recognized non-destructive evaluation test to qualitatively asses the homogeneity
and integrity of concrete.
PUNDIT (Portable Ultrasonic Non – Destructive Digital Indicating Tester):
UPV methods can play an important role in this area, since they allow us to monitor the density and homogeneity of the material, providing information about the strength evolution and about the internal flaws and defects. The UPV methods have been used in inspection operations and monitoring of concrete structures. This test allows to measure and to control a series of basic parameters to determine the concrete quality. However, interpreting the result of this type of test need to be made in a criteriously form and demand a specific knowledge of the influential factors. In order to collaborate with the development of the models that consider these factors, it was decided to carry a study aiming to analyze how the cure process influences the ultrasonic readings.
Using the UPV, it was possible to collect results of concrete specimens, leading to an opportunity to analyze how the cure process of concrete affects the readings. This analysis is important for concrete durability prognostics and can be useful considering the economic aspect.
PRINCIPLES
The principle that velocity of sound within
a solid mass in the test .
\[V = {f} {\sqrt {gE\over δ}} \]
\[Where,\]
\[f = ~is~a~function \]
\[E = ~is~ the ~ratio ~of ~Modulus ~of ~Elasticity \]
\[δ = ~is~ the~ Density \]
\[g = ~is~ the~ acceleration~ due~ to ~gravity. \]
For assessing the quality of materials from
ultrasonic pulse velocity measurement, it is necessary for this measurement to
be of a high order of accuracy. This is done using an apparatus that generates
suitable pulses and accurately measures the time of their transmission (i.e.
transit time) through the material tested. The distance which the pulses travel
in the material (i.e. the path length) must also be measured to enable the velocity
to be determined from:-
\[~ Pulse ~velocity = {~Path ~length \over ~Transit ~time }\] \[ V = {L \over t} \] \[~ expressed~ in~ { km \over s }\]
\[~ Pulse ~velocity = {~Path ~length \over ~Transit ~time }\] \[ V = {L \over t} \] \[~ expressed~ in~ { km \over s }\]
Path lengths and transit times should each
be measured to an accuracy of about ±1%. The instrument indicates the time
taken for the earliest part of the pulse to reach the receiving transducer
measured from the time it leaves the transmitting transducer when these transducers
are placed at suitable points on the surface of the material. Figure 1 shows
the transducers may be arranged on the surface of the specimen tested, the transmission
being either direct, indirect or semi-direct. The direct transmission arrangement
is the most satisfactory one since the longitudinal pulses leaving the transmitter
are propagated mainly in the direction normal to the transducer face.
The indirect arrangement is possible because the ultrasonic beam of energy is scattered by discontinuities within the material tested but the strength of the pulse detected in this case is only about 1 or 2% of that detected for the same path length when the direct transmission arrangement is used. Pulses are not transmitted through large air voids in a material and, if such a void lies directly in the pulse path, the instrument will indicate the time taken by the pulse that circumvents the void by the quickest route. It is thus possible to detect large voids when a grid of pulse velocity measurements is made over a region in which these voids are located.
The indirect arrangement is possible because the ultrasonic beam of energy is scattered by discontinuities within the material tested but the strength of the pulse detected in this case is only about 1 or 2% of that detected for the same path length when the direct transmission arrangement is used. Pulses are not transmitted through large air voids in a material and, if such a void lies directly in the pulse path, the instrument will indicate the time taken by the pulse that circumvents the void by the quickest route. It is thus possible to detect large voids when a grid of pulse velocity measurements is made over a region in which these voids are located.
There are 3 types of ways to arrange the
transducer.:-
1. Direct transmission
2. Semi-direct transmission
3. Indirection
transmission
OBJECTIVES
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.
APPARATUS
1.
Reference bar
2. Main control unit (42.5µs)
3. Transducer
4. Receiver
2. Main control unit (42.5µs)
3. Transducer
4. Receiver
PROCEDURE
1. The equipment is calibrated by placing
the transducer and receiver at either end of the reference bar.
2. Four suitable test locations is chosen
for each transducer arrangement. The surface of test location must be clean,
smooth and dry. Preferably mould or formed surface, but it trowelled surface
are unavoidable, rub smooth the surface using a suitable polish material.
3. The path length is measured using
measuring tape or calipers.
4. Grease is applied to the surface of test
location to ensure proper contact of the transducers with the concrete surface.
5. The transducers is positioned at the
chosen test location. They are ensured properly in contact with the concrete
surface.
6. Three (3) readings are taken per test
location. Four (4) readings will be adequate to plot the best fit straight line
for in-direct transmission.
7. The average reading for each test
location is calculated. Plot the best fit straight t line for the in-direct transmission.
The average velocity is given by slope of the best fit line.
8. The compressive strength of the test
specimen is estimated by using the chart provided
PRECAUTIONS TO BE TAKEN:
To carry out this test and obtain accurate
measurements, the following precautions should be taken:
1. The technique normally assumes that the
only volatile component is water and any significant contamination by other
volatile compounds would invalidate the test.
2. The moisture content of all specimens
must be identical and kept constant during testing.
3. Avoid obstacle in between the
transmitter and obstacle except the specimen.
4. Before the experiment is started, make
sure the apparatus set base in the standard.
5. There are no impurities on the surface
of the specimen.
6. Calibrate the instrument and make sure
the condition of instrument is good.
ADVANTAGES & DISADVANTAGES:
ADVANTAGES
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DISADVANTAGES
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1. Non-destructive strength testing determination and
monitoring of concrete strength.
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1. Requires skills to provide accurate reading and
measurements.
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2. Check the uniformity of concrete and deterioration.
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2.
Attenuation of signal in order or soft masonry restricts distance between
transducers for indirect and semi-direct use.
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3. Can detect flaws, crack, or voids.
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3. Coupling material needed between masonry and
transducer, which may alter the appearance of the masonry.
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4. Measurement of layer thickness and elastic
modulus.
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4.
Grinding may be required to prepare a rough surface.
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5. Equipment readily available and only moderately
expensive.
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5. No direct correlation with material properties.
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FACTORS AFFECTING MEASUREMENT:
Pulse velocity tests can be carried out on both laboratory-sized specimens and completed concrete structures, but some factors affect measurement.
1. Path-lengths desired to be at least 12 in. (30 cm) in order to avoid any errors introduced by heterogeneity.
2. There must be smooth contact with the surface under test; a coupling medium such as a thin film of oil is mandatory.
3. The presence of reinforcing steel in concrete has an appreciable effect on pulse velocity. It is therefore desirable and often mandatory to choose the paths that avoid the influence of reinforcing
steel or to make corrections if steel is in the pulse path.
Can we use UPV to monitor the quality of concrete during construction?
We can use UPV to monitor the quality of
the concrete during construction. Ultrasonic pulse velocity is an ideal tool
and suitable 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 assume that defective or deteriorated
concrete is present. High pulse velocity readings are generally is an indicative
of good quality concrete.
The general guidelines between concrete quality and pulse velocity based on UPV is given in Table below
The general guidelines between concrete quality and pulse velocity based on UPV is given in Table below
Longitudinal
pulse (Km/s)
|
Concrete
Quality
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> 4.5
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Excellent
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3.5 – 4.5
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Good
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3.0 – 3.5
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Doubtful
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2.0 – 3.0
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Poor
|
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 accuracy of
Ultrasonic Pulse Velocity Test does affected by various factors. Such as age of
concrete beam , surface area condition, skills of person who test it and the
inside situation of concrete. For example ,as concrete ages increases, 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 2000 to 3000 psi ( 13.6
to 20.4 MPa ) accuracy in determining strength is less than ±20 per cent.
Accuracy depends on careful calibration and use of the same concrete mix
proportions and aggregates 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
RESULT
Ultrasonic pulse velocity test results
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Reading
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Test location
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1
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2
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3
|
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Length of specimen (mm)
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300
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500
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470
|
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Speed in micro-seconds
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1
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100
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110
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100
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2
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98
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108
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96
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3
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110
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130
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105
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Average
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102.67
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116.00
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100.33
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Velocity in km/sec
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2.9
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4.3
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4.7
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Member
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Column
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Column
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Pile cap
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poor
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Good
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Excellent
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CONCLUSION
Ultrasonic Pulse Velocity (UPV) test is one
of the non-destructive tests. It detects cracks, strength of concrete and other
properties of concretes. It is portable and easy for testing. Since UPV use
velocity principle testing method, hence the frequency of the velocity should
be consider too when taking measurement on different material. Higher frequency
is used when the specimen is high density while for the less dense specimen,
the lower frequency is used. Besides that, except for mishandling of apparatus
that affects the actual reading and environment condition. Composites of
concrete also affect the concrete strength such as the water aggregate ratio,
size of aggregate and type of cement. To minimize the error of measuring,
transmitter and receiver must tightly in contact with surface of concrete when
taking the measurement. The surface area of concrete should be smooth and
grease should be applied on the surface of transmitter and receiver.
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