1. UNIT IV NON DESTRUCTIVE
TESTING METHODS
Load testing on structures, buildings, bridges and towers – Rebound Hammer –
acoustic emission – ultrasonic testing principles and application – Holography – use of
laser for structural testing – Brittle coating, Advanced NDT methods – Ultrasonic pulse
echo, Impact echo, impulse radar techniques, GECOR , Ground penetrating radar
(GPR).
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5. SCHMIDT REBOUND HAMMER TEST
The Schmidt rebound hammer is principally a surface hardness
tester. It works on the principle that the rebound of an elastic
mass depends on the hardness of the surface against which the
mass impinges. There is little apparent theoretical relationship
between the strength of concrete and the rebound number of the
hammer .
Rebound hammer test is done to find out the compressive strength
of concrete by using rebound hammer as per IS: 13311 (Part 2)
– 1992. The underlying principle of the rebound hammer test is
9. SCHMIDT REBOUND HAMMER TEST
The main components include the outer body, the plunger, the hammer mass, and
the main spring. Other features include a latching mechanism that locks the hammer
mass to the plunger rod and a sliding rider to measure the rebound of the hammer
mass. The rebound distance is measured on an arbitrary scale marked from 10 to
100. The rebound distance is recorded as a “rebound number” corresponding to the
position of the rider on the scale.
The plunger is then held perpendicular to the concrete surface and the body pushed
towards the concrete, Fig. 4.2b. This movement extends the spring holding the mass
to the body. When the maximum extension of the spring is reached, the latch
releases and the mass is pulled towards the surface by the spring, Fig. 4.2c.The
mass hits the shoulder of the plunger rod and rebounds because the rod is pushed
hard against the concrete, Fig. 4.2d. During rebound the slide indicator travels with
the hammer mass and stops at the maximum distance the mass reaches after
rebounding. A button on the side of the body is pushed to lock the plunger into the
retracted position and the rebound number is read from a scale on the body
10.
11. SCHMIDT REBOUND HAMMER TEST
RANGE AND LIMITATIONS OF SCHMIDT REBOUND HAMMER TEST
1. Smoothness of the test surface
2. Size, shape and rigidity of the specimen
3. Age of the specimen
4. Surface and internal moisture conditions of concrete
5. Type of coarse aggregate
6. Type of cement
7. Carbonation of the concrete surface
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14.
15. ULTRASONIC TESTING
PULSE VELOCITY TEST
A pulse of longitudinal vibrations is produced by an electro-acoustical
transducer, which is held in contact with one surface of the concrete under
test. When the pulse generated is transmitted into the concrete from the
transducer using a liquid coupling material such as grease or cellulose
paste, it undergoes multiple reflections at the boundaries of the different
material phases within the concrete. A complex system of stress waves
develops, which include both longitudinal and shear waves, and
propagates through the concrete. The first waves to reach the receiving
transducer are the longitudinal waves, which are converted into an
electrical signal by a second transducer. Electronic timing circuits enable the
transit time T of the pulse to be measured.
16. ULTRASONIC TESTING
PULSE VELOCITY TEST
Longitudinal pulse velocity (in km/s or m/s) is given by:
v =L/T
• where
• v is the longitudinal pulse velocity,
• L is the path length,
• T is the time taken by the pulse to traverse that length.
• The receiving transducer detects the arrival of that component of the pulse,
which arrives earliest. This is generally the leading edge of the longitudinal
vibration. Although the direction in which the maximum energy is propagated
is at right angles to the face of the transmitting transducer, it is possible to
detect pulses, which have travelled through the concrete in some other
direction. It is possible, therefore, to make measurements of pulse velocity by
placing the two transducers on either:
• opposite faces (direct transmission) , adjacent faces (semi-direct transmission): or the same face (indirect or
25. ULTRASONIC TESTING
Factors influencing pulse velocity measurements
1. Moisture content
2. Temperature of the concrete - 10oC and 30oC
3. Path length - minimum path length should be 100 mm nominal
maximum size of aggregate is between 20 mm and 40 mm
4. Shape and size of specimen
5. Effect of reinforcing bars
6. Determination of concrete uniformity
26. Ultrasonic pulse echo
High Frequency sound waves from a transmitting transducer are
transmitted into a part to interrogate the material. The sound waves travel
through the material and return to either the same transducers or a
different transducer.
The input and return signals are displayed on an ultrasonic instrument.
Differences between the input signals and return signals are analyzed to
determine the flaws, defects, changes of thickness, and other material
characteristics. The received signals are compared to signals of a
reference standard.
In the single transducer method, one transducer is used to both send and
receive signals. One advantage of this method is that only one side of the
test part needs to be accessible to the inspector.
27. Ultrasonic pulse echo
Types of Inspections
Pulsed Echo Inspection
- Uses high frequency (MHz) sound to determine distance of a feature
through measurement of time of flight or amplitude response
- Employs a single transducer for transmitter/receiver
Through Transmission
- Similar to pulsed echo, but uses two transducers
- Requires access to both sides of the test specimen
Pitch Catch
- Also uses two transducers along with an angle beam when access to
only one side of the part is available
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33.
34. Ultrasonic pulse echo
In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over
the object being inspected. The transducer is typically separated from the test object by a couplant
(such as oil) or by water, as in immersion testing. However, when ultrasonic testing is conducted
with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required.
There are two methods of receiving the ultrasound waveform: reflection and attenuation. In
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 anamplitude representing the
intensity of the reflection and the distance, representing thearrival time of the reflection. In
attenuation (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. Using the
couplant increases the efficiency of the process by reducing the losses in the ultrasonic wave
energy due to separation between the surfaces.
35. Ultrasonic pulse echo
Advantages
High penetrating power, which allows the detection of flaws deep in the part.
High sensitivity, permitting the detection of extremely small flaws.
Only one surface needs to be accessible.
Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the
thickness of parts with parallel surfaces.
Some capability of estimating the size, orientation, shape and nature of defects.
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.
36. Ultrasonic pulse echo
Disadvantages
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, after follow up with other nondestructive testing methods.[1]
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.
Surface must be prepared by cleaning and removing loose scale, paint, etc., although paint that is
properly bonded to a surface need not be removed.
Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and
parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser
and Electro Magnetic Acoustic Transducers (EMAT).
Inspected items must be water resistant, when using water based couplants that do not contain rust
inhibitors.
37.
38. The Impact-Echo Method
Impact-echo is based on the use of transient stress waves generated by elastic
impact. A diagram of the method is shown in Figure. A short-duration
mechanical impact, produced by tapping a small steel sphere against a
concrete or masonry surface, is used to generate low-frequency stress waves
that propagate into the structure and are reflected by flaws and/or external
surfaces. Surface displacements caused by reflections of these waves are
recorded by a transducer, located adjacent to the impact. The resulting
displacement versus time signals are transformed into the frequency domain,
and plots of amplitude versus frequency (spectra) are obtained. Multiple
reflections of stress waves between the impact surface, flaws, and/or other
external surfaces give rise to transient resonances, which can be identified in
the spectrum, and used to evaluate the integrity of the structure or to
determine the location of flaws.
39. The Impact-Echo Method
It is the patterns present in the waveforms and spectra (especially the latter)
that provide information about the existence and locations of flaws, or the
dimensions of the cross-section of the structure where a test is performed,
such as the thickness of a pavement. For each of the common geometrical
forms encountered in concrete structures (plates; circular and rectangular
columns; rectangular, I-, and T-beams; hollow cylinders; etc.), impact-echo
tests on a solid structure produce distinctive waveforms and spectra, in which
the dominant patterns-especially the number and distribution of peaks in the
spectra-are easily recognized. If flaws are present (cracks, voids,
delaminations, etc.) these patterns are disrupted and changed, in ways that
provide qualitative and quantitative information about the existence and
location of the flaws
44. The Impact-Echo Method
A short-duration mechanical impact, produced by tapping a small steel sphere against a concrete
or masonry surface, produces low-frequency stress waves (up to about 80 kHz) that
propagate into the structure and are reflected by flaws and/or external surfaces. The
wavelengths of these stress waves are typically between 50mm and 2000mm -- longer than
the scale of natural inhomogeneous regions in concrete (aggregate, air bubbles, micro-
cracks, etc.). As a result they are only weakly attenuated, and propagate through concrete
almost as though it were a homogeneous elastic medium. Multiple reflections of these waves
within the structure excite local modes of vibration, and the resulting surface displacements
are recorded by a transducer located adjacent to the impact. The piezoelectric crystal in the
transducer produces a voltage proportional to displacement, and the resulting voltage-time
signal (called a waveform) is digitized and transferred to the memory of a computer, where it
is transformed mathematically into a spectrum of amplitude vs. frequency. Both the waveform
and spectrum are plotted on the computer screen. The dominant frequencies, which appear
as peaks in the spectrum, are associated with multiple reflections of stress waves within the
structure, or with flexural vibrations in thin or delaminated layers.
45. The Impact-Echo Method
The fundamental equation of impact-echo is d = C/(2f), where d is the depth
from which the stress waves are reflected (the depth of a flaw or the
thickness of a solid structure), C is the wave speed, and f is the dominant
frequency of the signal. The frequency f is obtained from the results of a test.
To determine thickness or depth of a flaw, the wave speed C must be known.
It can be measured by observing the travel time of a stress wave between
two transducers held a fixed distance apart on the concrete surface or by
performing a test on a solid slab of known thickness and observing the
dominant frequency. In the latter case the equation is rearranged to give C =
2df (where d is the known thickness).
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48.
49. Impulse radar techniques
Basically, the proposed radar demining system on moving platform (Fig. 2)
with radar mounted on it can operate sequentially in two operation stages:
1) at far distance in the stand-off mode to detect harmful objects located
beneath ground surface and covered by vegetation; 2) at short distance
resulted from radar-target approach in the stand-over mode used mostly
for radar demining at present. In order to examine this radar concept,
laboratory and field tests have been conducted. The developed
experimental set-up that consists of modified impulse radar (Fig. 3a) of the
1GHz middle frequency band, monostatic Tx-Rx antennas arrangement,
control and processing electronics, computer with control/processing real-
time software. The total 500 MHz – 1.5 GHz frequency band used is
optimal and compromises to achieve maximum spatial resolution and
minimum signal attenuation under real operation conditions in vegetation
and subsurface soil regions.
50. Impulse radar techniques
Transmitting Tx antenna is driving by pulse with front duration £ 0.2
nanoseconds, amplitude of about 35 V, pulse repetition rate of 10
microseconds. Stroboscopic receiver unit is employed in radar with 0.0125
nanoseconds time resolution. The bow-tie antennas with backed reflector
and the TEM horn antennas have been used. The 12 bit 500 kHz sampling
frequency ADC and Analog Device ADSP-2105 digital signal processor
(DSP) have been employed that installed both on computer card boarded
in computer via the standard PCI bus. Radar returns data stored by
computer has 512 samples per each radar’s sounding period, i.e. trace.
Radar is able to give out 20-50 traces per second that is preferable value
for movable radar carrier. Total performance factor of radar is 140 dB and
its total dynamic range is 120 dB including internal analog summation
implemented in the receiver unit.
51. Impulse radar techniques
The next task to be solved is improving SBR (signal-to-background ratio )
and signal-to-clutter ratio (SCR) to ensure required radar performances
that can be summarized in receiver operation characteristic (ROC) . Let
note that in contrast to classical theory of optimal detector where detection
rate and false alarm rate are determined by signal-to-noise ratio (SNR) in
the presented stand-off system both SBR and SCR are determining factor
mostly. Next operating block in Fig. 3b after subtraction shows a linear
filtration to minimize non-compensate residual interface signals to improve
finally SBR.
56. Ground penetrating radar (GPR).
GPR uses high-frequency (usually polarized) radio waves and transmits into the ground. When the
wave hits a buried object or a boundary with differentdielectric constants, the receiving antenna
records variations in the reflected return signal. The principles involved are similar to reflection
seismology, except that electromagnetic energy is used instead of acoustic energy, and reflections
appear at boundaries with different dielectric constants instead ofacoustic impedances.
The depth range of GPR is limited by the electrical conductivity of the ground, the transmitted
center frequency and the radiated power. As conductivity increases, the penetration depth decreases.
This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in
signal strength at depth. Higher frequencies do not penetrate as far as lower frequencies, but give
better resolution. Optimal depth penetration is achieved in ice where the depth of penetration can
achieve several hundred metres. Good penetration is also achieved in dry sandy soils or massive dry
materials such as granite, limestone, and concrete where the depth of penetration could be up to 15-
metre (49 ft). In moist and/or clay-laden soils and soils with high electrical conductivity, penetration is
sometimes only a few centimetres.
Ground-penetrating radar antennas are generally in contact with the ground for the strongest signal
strength; however, GPR air-launched antennas can be used above the ground.
64. GECOR
GECOR 8- World's most advanced system for
analyzing corrosion of concrete steel reinforcement
bars.
Gecor 8 represents the latest technology in steel
reinforcing bar corrosion rate determination. It combines
state of the art embedded microprocessor systems and
computerized flash technology with the world's leading
research in reinforcing bar corrosion rate analysis.
65. GECOR has the following advantages:
• It´s possible to measure in submerged structures (should be proved).
• It´s possible to measure in structures with cathodic protection.
• It has noise level indicator.
• It has a user-friendly operator interface.
• Advanced software with the possibility to update it.
And the following disadvantages:
The measurement takes around 5 minutes per location.
The reference electrodes are Copper / Copper Sulphate (Cu/CuSO4)
with CuSO4 solution reservoirs. The maintenance of these electrodes it
takes time because they need to be refilled.
Three different sensors are needed.
It needs a battery pack, it´s not possible to use common batteries. The
device should not be operated in temperatures below 0ºC.
66.
67. GECOR
Features and Benefits
• Rapid mapping capabilities for analysis of large structures.
• Advanced method for more accurate corrosion rate determination.
• New sensor design for analysis of wet or submerged structures.
• New method for analysis of cathodic protection systems while the
system is running.
• Personal computer software for data analysis and report
generation.
• Graphical user friendly interface to facilitate measurements
68. GECOR
Corrosion of steel reinforced concrete affects the safety and
durability of concrete structures in the following ways:
A. The steel cross section is reduced, weakening the
concrete strength.
B. The concrete is cracked due to the increased volume in
the rust.
C. The steel to concrete bond is reduced when cracking and
spalling are initiated
A true measure of the corrosion rate is possible by the polarization
resistance technique. It has been well established by Stern and
Geary that corrosion current is linearly related to polarization
69. GECOR
A true measure of the corrosion rate is possible by the polarization
resistance technique. It has been well established by Stern and
Geary that corrosion current is linearly related to polarization
resistance. This gives a direct quantitative measurement of the
amount of steel turning into oxide at the time of measurement. By
Faraday’s equation, this can be extrapolated to direct metal
sectional loss
73. GECOR
The Gecor 8 features:
1. A rapid mapping technique that allows the engineer to quickly classify areas of a structure.
Both the classical corrosion potential as well as the resistivity of the concrete can be
measured. Each individual parameter can be mapped in a multi-color contour graph.
2. Our advanced modulation confinement technique precisely measures the true polarization
resistance of the steel reinforcing bar. This allows the Gecor 8™ to reach a quasi steady-state
condition for the 30 to 100 seconds required for determining the polarization resistance
through a galvanostatic pulse. This advanced technology provides the most accurate field test
currently available for the determination of corrosion rate.
3. The Gecor 8™ also has the ability to measure corrosion rate in submerged or very wet
structures. An optional sensor has been designed to measure corrosion rate in extremely wet
environments, eliminating the need for an external guard ring.
4. PC software assists the user to graphically interpret, collate, organize and generate reports
with the data generated from the device. It allows the user to set up the Gecor 8™ for more
rapid testing later in the field, and will automatically link to the PC for data collection.
74. Acoustic emission
Acoustic emission is a technique to monitor
defect formation and failures in structural
materials used in services or laboratories.
Moreover, the method has been developed and
applied in numerous structural components,
such as steam pipes and pressure vessels, and
in the research areas of rocks, composite
materials, and metals
75. Acoustic emission
Acoustic emission (AE) is the phenomenon of radiation of acoustic
(elastic) waves in solids that occurs when a material undergoes
irreversible changes in its internal structure, for example as a result of
crack formation or plastic deformation due to aging, temperature
gradients or external mechanical forces
Acoustic emissions (AEs) are the stress waves produced by the sudden
internal stress redistribution of the materials caused by the changes in the
internal structure. Possible causes of the internal-structure changes are
crack initiation and growth, crack opening and closure, dislocation
movement, twinning, and phase transformation in monolithic materials and
fiber breakage and fiber-matrix debonding in composites. Most of the
sources of AEs are damage-related; thus, the detection and monitoring of
these emissions are commonly used to predict material failure.
76. Acoustic emission
The research on AE can be generally divided into two categories
– Traditional acoustic emission
– Source-function and waveform analysis
TRADITIONAL ACOUSTIC-EMISSION TECHNIQUE
The traditional AE method only captures certain parameters (sometimes
called AE features), including AE counts, peak levels, and energies. Then,
the AE features are correlated with the defect formation and failures. These
AE characteristics are only related to the captured signals and do not
account for the source of the signal
77. Acoustic emission
Figure 1 shows a burst AE signal and the commonly used parameters of AE techniques. When the
AE transducer senses a signal over a certain level (i.e., the threshold), an AE event is captured. The
amplitude of the event is defined at the peak of the signal. The number of times the signal rises and
crosses the threshold is the count of the AE event. The time period between the rising edge of the
first count and the falling edge of the last count is the duration of the AE event. The time period
between the rising edge of the first count and the peak of the AE event is called the rise time. The
area under the envelope of the AE event is the energy.
Figure 2 presents a typical AE system setup. The AE transducers are generally very sensitive
piezoelectric sensors. Because the traditional AE technique only uses AE features, the actual
waveforms are not critical to this method. The AE sensors (transducers) used are usually resonance
sensors, which are only very sensitive to a certain frequency. Since the AE signals are very weak, a
preamplifier is connected right after the AE transducer to minimize the noise interference and
prevent the signal loss. Sometimes, the transducer and the preamplifier are built as a unit. Then, the
signals pass through a filter to remove the noise. The signals are amplified by the main amplifier
before being sent to the signal conditioner. After that, the AE features are subtracted and stored in a
computer for further analysis. During investigations, other parameters, such as load, deformation,
pressure, and temperature, can also be recorded as parametric inputs.
79. Holography
Holography is a technique which enables three-
dimensional images to be made. It involves the use of
a laser, interference, diffraction,
light intensity recording and suitable illumination of the
recording. The image changes as the position and
orientation of the viewing system changes in exactly
the same way as if the object were still present, thus
making the image appear three-dimensional.
86. Holography
It all starts with the properties of laser light.
Of course you know how a laser is created. Ordinary light is beamed
through a crystal. After bouncing around the internal structure of the
crystal, the light comes out in a highly organized beam. That’s not the
only way to create laser light. LASER actually stands for, “Light
Amplification by Stimulated Emission of Radiation”. Besides crystals
laser can be produced by stimulating some molecules until they emit
laser light.
The properties of laser light are still being explored, but those already
discovered have revolutionized modern technology. One of the
concepts illuminated by laser light is that of holography.
87. Holography
What makes a hologram work is that the two beams of laser light that
illuminate the object are not only the same type, but in perfect
synchronization. This is accomplished by passing the light through glass
that reflects half of the light, thus splitting the beam in two. The main
beam is then directed at the object, while the reflected one is directed at
the photographic filmfrom an angle. Where the reflected and direct laser
light intersect, interference patterns are created that are recorded on the
surface of the film.
The resulting pattern is meaningless when seen in ordinary light, but
when illuminated by the original laser, it produces a 3D image of the
original object.
89. Laser for structural testing
Laser ultrasonic testing (LUT) is a remote, noncontact extension of
conventional, contact or near-contact ultrasonic testing (UT). A
schematic layout of a laser ultrasonic system is shown in the figure.
A laser pulse is directed to the surface of a sample through a fiber or
through free space. The laser pulse interacts at the surface to induce
an ultrasonic pulse that propagates into the sample. This ultrasonic
pulse interrogates a feature of interest and then returns to the surface.
A separate laser receiver detects the small displacement that is
generated when the pulse reaches the surface. The electronic signal
from the receiver is then processed to provide the measurement of
interest.
90. Laser for structural testing
Laser ultrasonic testing offers many advantages when compared with
traditional contact inspection techniques:
Remote, non-contact, reconfigurable
Can scan measurement head or sample
Proven at speeds ≥ 5 m/sec
Proven at temperatures ≥ 2000°F
High bandwidth operation
High spatial resolution
Micrometer thickness accuracy
Small contact area on sample
91.
92.
93. Brittle Coating
CoatingSelection:
Brittle coating are designed to fracture at a specified strain,
usually 500 microstrain. Since fracture strain depends on
application conditions, the particular brittle coating to be used
should be chosen for the test conditions to be used. The most
important factors that determine the coating fracture strain for
well-applied coatings are temperature and relative humidity.
Coating manufacturers produce coatings for application and
use at avariety of temperatures and humidities. The coating
chosen should be appropriate for the test conditions
95. Brittle Coating
Coating Application:
The brittle coating must be built up slowly by applying several light coats
The final coating thickness should be 0.06 mm - 0.11 mm (0.0025 in -
0.0045 in). Coating thickness can be measured from before and after
measurements of the calibration specimen thickness, and sometimes on
the actual test surface depending on the actual test part. For the brittle
coating used in the lab a coating of about 0.09 mm (0.0035 in)thickness
has a pale green color. Each coat should be applied in one spray pass.
Spray passes should be quick and steady from a distance of about 15 cm
(6 in). Coats should not be applied so wet/thick that theyrun nor so dry
that they appear dusty. Excessive coating thickness causes sagging,
running and trapping of large air bubbles. The first coat may not cover the
surface evenly,but subsequent coats should even out the coating
99. Part-A
1. Which NDT methods used to asses the surface and
core strength of concrete
2. Define the term Holography and it uses
3. Define GECOR
4. List the various types of NDT
100. Part – B
1. i) Explain the uses of NDT testing techniques
ii) Explain how Holography is used in structural application purpose
iii) Uses of ground penetrating radar
2. Write short notes on
i) Rembound Hammer
ii) Ultrasonic testing and principles
3. Write short notes on
i) Brittle coating
ii) Impact eco
