Components, application, the procedure for using, interpretation of results, advantages & limitations of Ultrasonic pulse velocity method of Non-Destructive Testing is briefly described in this slide.

1. EXPERIMENTAL TECHNIQUES
NON- DESTRUCTIVE TESTING-
ULTRASONIC PULSE VELOCITY
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2. Ultrasonic Pulse Velocity Technique
 This method consists of measuring the time of travel of an
ultrasonic pulse passing through the concrete being tested.
 The equipment consists of transmitter and receiver probe and
a time measuring device
 Ultrasonic pulses are generated by electro-acoustical
transducer and transmitted through the concrete

3.  A complex system of stress waves is developed which includes
longitudinal (Compressional), shear (Transverse) and surface
(Rayleigh) waves.
 This transducers convert electrical signals into mechanical
vibrations (transmit mode) and mechanical vibration into
electrical signals (receive mode).
 The receiving transducer detects the onset of the longitudinal
waves which is the fastest wave.
 The travel time is measured with an accuracy of +/- 0.1
microseconds.

5. Transducers
 Piezo- electric Transducers with natural frequencies between
20 kHz and 200 kHz are available.
 Generally, high frequency transducers are preferable for short
path length and low frequency transducers for long path lengths.
 Transducers with a frequency of 50 to 60 kHz are useful for
most all-round applications.

6. Calibration of V meter

7. Schematic Diagram of Ultrasonic Pulse Velocity Method

8. Methods of receiving the ultrasound waveform
Reflection (or pulse-echo) mode:
The transducer performs both the sending and the
receiving of the pulsed waves as the "sound" is reflected back to the
device.
Reflected ultrasound comes from an interface, such as
the back wall of the object or from an imperfection within the object.
The diagnostic machine displays these results in the
form of a signal with an amplitude representing the intensity of the
reflection and the distance, representing the arrival time of the
reflection.

9. Reflection mode
A probe sends a sound wave into a test material. There are two indications, one from the initial pulse of the
probe, and the second due to the back wall echo. A defect creates a third indication and simultaneously
reduces the amplitude of the back wall indication. The depth of the defect is determined by the ratio D/Ep

10. Attenuation mode (or through-transmission) mode:
 A transmitter sends ultrasound through one surface, and a
separate receiver detects the amount that has reached it on
another surface after traveling through the medium.
 Imperfections or other conditions in the space between the
transmitter and receiver reduce the amount of sound transmitted,
thus revealing their presence.
 The pulse strength is attenuated and it passes around the
discontinuity, thereby making path length longer and hence the
transit time.

11. Procedure
 Clean the surface of concrete to remove grits and scales
 Smoothen with grinding wheel
 Mark grids and measure the distance (L) between them where the
transmitter and receiver are to be placed
 Apply grease or petroleum jelly on the grid points
 Press the transmitter and receiver on pre-determined grit points

12.  The time (T) required to traverse the ultrasonic pulse from
transmitter to receiver is recorded
 The apparent velocity (V) is computed V= (L / T)
 Apparent velocity is more for direct concrete and low for
damaged or cracked concrete
 Comparative higher velocities are obtained when the quality of
concrete in terms of density, homogeneity and uniformity is good.
 In case poorer quality of concrete, lower velocities are obtained.

14. Measurement of pulse velocity through concrete
 Direct Transmission (Cross Probing) through Concrete :
In this method transducers are held on opposite face of
the concrete specimen under test.
The method is most commonly used and is to be
preferred to the other two methods because this results in maximum
sensitivity and provides a well defined path length.

15. Direct method

16.  Semi-direct Transmission through Concrete :
Sometimes one of the face of the concrete specimen
under test is not accessible, in that case we have to apply semi-
direct method.
In this method, the sensitivity will be smaller than cross
probing and the path length is not clearly defined.

17. Semi- Direct method

18. Indirect Transmission (Surface Probing):
This method of pulse transmission is used when only one
face of concrete is accessible.
Surface probing is the least satisfactory of the three
methods because the pulse velocity measurements indicate the
quality of concrete only near the surface and do not give information
about deeper layers of concrete.
The weaker concrete that may be below a strong surface
can not be detected. Also in this method path length is less well
defined.

19. Indirect method

20. Applications of UPV results
Monitoring strength development or deterioration in
laboratory specimens subjected to varying curing
conditions or to aggressive environment
Measurement of in-situ concrete uniformity
Detection of cracking and honeycombing in in-situ
concrete
Measurement of crack depth
Strength estimation of in-situ concrete
Assessment of in-situ concrete deterioration

21. Measurement of layer thickness in in-situ concrete
Measurement of elastic modulus of in-situ concrete
UPV test may be used for monitoring strength
development or deterioration in laboratory specimens
subjected to varying curing conditions or to aggressive
environment
To ascertain the quality of the concrete in relation to the
specified standard requirements
Quality assurance prior to purchase & Dispute settlement
Forensic investigation -post damage investigation of
structure (Fire damage)

22. Measurement of in-situ concrete uniformity
Measurement of in-situ concrete uniformity is probably the
most valuable and reliable application of UPV test in the
field
The uniformity of concrete may be obtained by conducting
the UPV test on a regular grid over the member (with a
spacing of 1 m or less)

23. Detection of cracking and honeycombing
 UPV test may directly detect cracking and honeycombing in
concrete, without need for detailed correlation of V with any other
property of concrete
 Since presence of cracking or honeycombing (i.e., void) along the
pulse path decreases the velocity due to increase in attenuation,
the cracking and honeycombing may be detected by observing at
a location where measured value of V is found to be less than that
found at a sound location
 Water-filled cracks can not be detected using UPV test
 A given void is more difficult to detect as the path length increases

24. Measurement of crack depth
An estimate of crack depths may be obtained by the use
of indirect surface readings
Ts = transit time observed when the transducers are
placed at sound concrete
Tc = transit time observed when the transducers are
placed each side of the cracked concrete

25. n = depth of crack
2x = Path length without crack
2 𝑥2 + 𝑛2 = Path length with crack
V = velocity observed
n = x (
𝑇𝑐
𝑇𝑠
2 − 1)

26. Strength estimation
With the help of suitable calibration chart or model, UPV
test may be used to estimate the strength of concrete.
However, the strength estimate by UPV method is not
found to be accurate. The error ranges from ±10 to
±20%.
Error in the strength estimation increases at higher
strength levels, and estimates above 40 MPa should be
treated with great caution

27. Assessment of concrete deterioration
 UPV test may be used to assess the deterioration of concrete by
following:
Fire, Mechanical effects, Frost attack, Chemical attack
 The depth of fire or surface chemical attack may be determined
using the following:
t = (TVc ) – L
where t = depth of deterioration
T = transit time for a path length L including one damaged surface zone
Vc = pulse velocity measured at a sound location

28. Assessment of fire damage
When a concrete member is subjected to fire, the exterior
of the member is heated up drastically, while the interior
remains at a relatively low temperature
Therefore, only a thin surface layer of the concrete is
subjected to severe damage
Knowledge of the thickness of damaged layer is very
useful to the engineer in estimating the repair work
Thickness of the fire damaged layer can be assessed by
measuring UPV along the surface

29. Vs = Velocity in sound concrete
Vd = Velocity in damaged concrete

30. Measurement of elastic modulus
 Elastic modulus is the property of concrete which can be
determined with the greatest accuracy using UPV test
 Elastic modulus may be calculated by substituting the measured
value of pulse velocity and suitably assumed values of Poisson’s
ratio and density in the equation relating pulse velocity with the
elastic modulus
 Dynamic modulus of elasticity of concrete, Ed = Kn2l2ρ (GPa)
where L = Length of specimen n = resonant frequency
ρ = density of concrete
K= 4x 10 -15 when L is in mm and ρ is in kg/m3

31. Velocity Criteria For Concrete Quality Grading
As per Table 2 of IS 13311 ( Part 1 ) : 1992
Sr.
No.
Pulse Velocity by Cross
Probing( km/sec )
Concrete Quality
Grading
1. Above 4.5 Excellent
2. 3.5 to 4.5 Good
3. 3.0 to 3.5 Medium
4. Below 3.0 Doubtful
Interpretation of Results

32. Factors affecting the measurements of pulse velocity
 Smoothness of Concrete Surface under Test
 Moisture Condition of Concrete : Velocity is increased with
increased moisture content , the influence being more in lower
quality concrete
 Lateral Dimensions: If least lateral dimension is less than the
wavelength of the pulse vibrations, the pulse velocity will be
influenced. 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

33. Temperature of Concrete

34.  Effect of Reinforcing Bars : velocity of pulses is higher in steel and
so the velocity measurements made in the vicinity of reinforcing
steel is high and not representative of concrete
 Influence of Path Length on Pulse Velocity: 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.
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

35. Advantages
 High penetrating power, which allows the detection of flaws deep
in the part.
 High sensitivity, permitting the detection of extremely small flaws.
 Greater accuracy than other nondestructive methods in
determining the depth of internal flaws and the thickness of parts
with parallel surfaces.
 Non hazardous to operations or to nearby personnel and has no
effect on equipment and materials in the vicinity.
 Capable of portable or highly automated operation.
 Results are immediate. Hence on the spot decisions can be
made.

36. Limitations
 Manual operation requires careful attention by experienced technicians. The
transducers alert to both normal structure of some materials, tolerable
anomalies of other specimens (both termed “noise”) and to faults therein
severe enough to compromise specimen integrity. These signals must be
distinguished by a skilled technician, possibly requiring follow up with other
nondestructive testing methods.
 Extensive technical knowledge is required for the development of inspection
procedures.
 Parts that are rough, irregular in shape, very small or thin, or not
homogeneous are difficult to inspect.

37.  In view of the limitation of the methods for the predicting the
strength of concrete in the structure, IS 13311 ( Part 1 ) : 1992
Code has suggested to use combined method of ultrasonic pulse
velocity and rebound hammer methods to alleviate the errors
arising out of influence of materials, concrete mix proportions and
environmental parameters on the respective measurement.
 The use of more than one methods are capable of providing
useful information and statically improved accuracy for estimation
of in situ strength of concrete.