Non Destructive Testing of Bridges
Non Destructive Testing of Bridges
Non Destructive Testing of Bridges
MINISTRY OF RAILWAYS
GUIDELINES
ON
NON-DESTRUCTIVE TESTING
OF
BRIDGES
BS - 103
August, 2009
B&S DIRECTORATE
RESEARCH DESIGNS AND STANDARDS ORGANISATION
LUCKNOW-226011
PREFACE
Non-destructive testing of bridges has assumed a greater
significance in the present scenario because our existing inspection system
is not adequate to identify the internal defects in the structures. With the
recent collapses of bridges in India and other countries the objective
inspection of bridges has become the need of the hour.
Indian Railway Bridge Manual (IRBM) prescribes periodical Health
Monitoring of Very Important Bridges by an independent agency which
includes corrosion monitoring, deterioration of material, system damage,
retrofitting etc. All these can be done by using suitable NDT methods.
Since the work will be executed by the independent agency, it is important
to understand the details of the testing procedures as the engineer may
have to supervise the work at the site. Since non-destructive testing
methods do not form part of IRBM, the various methods used for testing
different types of bridges have been included in this publication.
I hope these Guidelines will be found very useful by field engineers,
who are entrusted with the work of nondestructive testing of bridges, and
will help as a guide for implementation of testing methods for inspection
and testing of bridges.
CONTENTS
CHAPTER-1 NON - DESTRUCTIVE
GENERAL
TESTING
OF
BRIDGES
1.1
Introduction
1.2
Concrete Bridges
1.3
Steel Bridges
1.4
Masonry Bridges
FOR
STRENGTH
2.1
2.2
10
2.3
19
2.4
19
2.5
22
2.6
25
2.7
30
2.8
32
2.9
Permeability Test
33
2.10
Bond Test
37
2.11
Maturity Method
38
2.12
38
Introduction
40
3.2
40
3.3
Resistivity Test
44
i
3.4
47
3.5
47
3.6
Endoscopy Technique
47
3.7
Profometer
48
3.8
Micro Covermeter
50
Introduction
52
4.2
52
4.3
55
4.4
58
4.5
61
4.6
72
4.7
Boroscope
73
4.8
Nuclear Method
76
4.9
76
4.10
84
TESTING
OF
STEEL
5.1
Introduction
88
5.2
88
5.3
99
5.4
107
5.5
Radiographic Testing
114
5.6
Ultrasonic Inspection
115
5.7
122
5.8
122
ii
Introduction
123
6.2
123
6.3
124
6.4
124
6.5
Infrared Thermography
124
6.6
Boroscope
124
iii
125
CHAPTER 1
NON-DESTRUCTIVE TESTING OF BRIDGES - GENERAL
1.1
Introduction:
There are about 1, 27,000 bridges of different types with varying spans on Indian
Railways. About 40% of these bridges are over 100 years old and have
completed their codal life. The present method of bridge inspection is mostly
visual and gives only subjective assessment of the condition of bridge. Moreover
present inspection system is not capable of assessing hidden defects, if any.
We may group the various bridges mainly in three types, based upon material of
construction.
(i)
Concrete bridges
(ii)
Steel bridges
(iii)
Masonry bridges
Various types of bridges are having their own strengths, weaknesses and
maintenance related problems. Each type of bridge is having different properties
i.e. concrete is a heterogeneous material but the steel is a denser and
homogeneous material. Similarly in masonry structures, the condition of joints
and material of construction is of utmost importance. Considering the uniqueness
of each type of bridge, there are different methods adopted for inspection and
maintenance based on material of construction. In this book, various NonDestructive Testing (NDT) methods for testing concrete, steel and masonry
bridges have been discussed separately.
1.2
Concrete Bridges:
The quality of new concrete structures is dependent on many factors such as
type of cement, type of aggregates, water cement ratio, curing, environmental
conditions etc. Besides this, the control exercised during construction also
contributes a lot to achieve the desired quality. The present system of checking
slump and testing cubes, to assess the strength of concrete, in structure under
construction, are not sufficient as the actual strength of the structure depend on
many other factors such as proper compaction, effective curing etc.
Considering the above requirements, need of testing of hardened concrete in
new structures as well as old structures, is there to asses the actual condition of
structures. Non-Destructive Testing (NDT) techniques can be used effectively for
investigation and evaluating the actual condition of the structures. These
techniques are relatively quick, easy to use, and cheap and give a general
indication of the required property of the concrete without damaging the structure
or any hindrance in the service of structure. This approach will enable us to find
1
suspected zones, thereby reducing the time and cost of examining a large mass
of concrete. The choice of a particular NDT method depends upon the property
of concrete to be observed such as strength, corrosion, crack monitoring etc.
The subsequent testing of structure will largely depend upon the result of
preliminary testing done with the appropriate NDT technique.
Purpose of Non-destructive Tests: The non-destructive evaluation techniques
are being increasingly adopted in concrete structures for the following purposes:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Monitoring changes in the structure of the concrete which may occur with
time.
(vii)
(viii)
(ix)
(x)
(xi)
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 result of measurements. Moreover the test
measure some other property of concrete (e.g. hardness) yet the results are
interpreted to assess the different property of the concrete e.g. (strength). Thus,
interpretation of the result is very important and a difficult job where
generalization is not possible. Even though operators can carry out the test but
interpretation of results must be left to experts having experience and knowledge
of application of such non-destructive tests.
Variety of NDT methods have been developed and are available for investigation
and evaluation of different parameters related to strength, durability and overall
quality of concrete. Each method has some strength and some weakness.
Therefore prudent approach would be to use more than one method in
combination so that the strength of one compensates the weakness of the other.
The various NDT methods for testing concrete bridges are listed below
A.
(ii)
(iii)
Bond Test
(ii)
(iii)
Endoscopy Technique
(vi) Profometer
(vii) Micro covermeter
C.
(ii)
(iii)
(vi) Boroscope
(vii) Nuclear Method
(viii) Structural scanning equipment
(ix) Spectral Analysis of surface waves for unknown foundation
3
All the above said methods have been discussed in detail in this book.
1.3
Steel Bridges:
On Indian Railways, the superstructure of the large number of major bridges is of
steel, and substructure is generally of concrete/masonry. These steel bridges are
fabricated using structural steel section i.e. channels, angles ,plates and Isections etc. The bridges are subjected to severe dynamic stresses under
passage of traffic and because of these stresses, the deterioration of the
materials takes place.
In our system of inspection, we are mainly carrying out the visual inspection of
the various parts of bridges, rivet testing and inspection of bearings etc. But all
these methods do not given any indication about the microcracking, presence of
flaws/ internal blow holes/lamination etc. in the bridge members. Moreover some
of the members of the bridge girders are difficult to inspect because of
inaccessibility and in those cases, the NDT technique can be used effectively for
inspection and evaluation of structures.
The various NDT methods for testing steel bridges are listed below:
(i)
(ii)
(iii)
(iv)
Radiography
(v)
Ultrasonic testing
(vi)
(vii)
All the above said methods have been discussed in detail in this book.
1.4
Masonry Bridges:
A large no. of bridges on Indian Railways are masonry bridges in which
foundation or substructure is of either stone or brick masonry. In addition, a large
no. of bridges are masonry arch bridges which have become quite old and
already outlived their design life. The weakest location in a masonry bridge is the
joint, as the deterioration gets initiated from the joints. With the passage of loads
and over a period of time the deterioration of the material itself take place due to
which the strength of the masonry structures gets affected. At the time of
inspection, normally the condition of joints or the material on the outer surface is
noted but it does not give any indication about the inherent defects within the
structures. Moreover the present system of inspection is not about detecting the
deterioration in strength of the stone/brick masonry because of the weathering
action and other factors. In India, the NDT of masonry structures is still in
4
necessant stage. There are lot of methods available for NDT of masonry
structure, as indicated below:
(a)
(b)
(c)
(d)
Infrared thermography
(e)
Boroscope
As the application of the above said NDT methods for masonry inspection is not
very common in India, details given in this book are just for general guidance.
**********
CHAPTER-2
NON-DESTRUCTIVE TESTS FOR STRENGTH ESTIMATION
OF CONCRETE
(b)
(c)
(d)
This method can be used with greater confidence for differentiating between the
questionable and acceptable parts of a structure or for relative comparison
between two different structures.
6
2.1.2 Principle:
The method is based on the principle that the rebound of an elastic mass
depends on the hardness of the surface against which mass strikes. When the
plunger of rebound hammer is pressed against the surface of the concrete, the
spring controlled mass rebounds and the extent of such rebound depends upon
the surface hardness of concrete. The surface hardness and therefore the
rebound is taken to be related to the compressive strength of the 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.
The impact energy required for rebound hammer for different applications is
given below
Sr.
No.
Approximate impact
energy required for the
rebound hammers (N-m)
Application
1.
2.25
2.
0.75
3.
30.00
Depending upon the impact energy, the hammers are classified into four types
i.e. N, L, M & P. Type N hammer having an impact energy of 2.2 N-m and is
suitable for grades of concrete from M-15 to M-45. Type L hammer is suitable for
lightweight concrete or small and impact sensitive part of the structure. Type M
hammer is generally recommended for heavy structures and mass concrete.
Type P is suitable for concrete below M15 grade.
2.1.3 Methodology:
Before commencement of a test, the rebound hammer should be tested against
the test anvil, to get reliable results. The testing anvil should be of steel having
Brinell hardness number of about 5000 N/mm2. The supplier/manufacturer of the
rebound hammer should indicate the range of readings on the anvil suitable for
different types of rebound hammer.
For taking a measurement, the hammer should be held at right angles to the
surface of the structure. The test thus can be conducted horizontally on vertical
surface and vertically upwards or downwards on horizontal surfaces (Fig.2.1.2).
(b)
The loosely adhering scale should be rubbed off with a grinding wheel or
stone, before testing
(c)
(d)
The point of impact should be at least 20mm away from edge or shape
discontinuity.
Around each point of observation, six readings of rebound indices are taken and
average of these readings after deleting outliers as per IS 8900:1978 is taken as
the rebound index for the point of observation.
8
materials and mix proportion, then the accuracy of results and confidence
thereon gets greatly increased.
2.1.6 Standards:
The rebound hammer testing can be carried out as per IS-13311 (Pt.2).
2.1.7 Other types of Rebound Hammer are:
Concrete Test Hammer (Pendulum Type):
This is a new type of test hammer. In addition to testing of concrete, this
measures the strength of masonry structures as well, although
approximately. The equipment is very handy and to the fair extant reliable
also. Further more, this is only equipment which is most pre-dominantly used
in the field. Its new addition is having so many additional features.
Digital Concrete Test Hammer
The digital concrete test hammer is a microprocessor operated standard unit
equipped with electronic transducer which converts the rebound of the
hammer into electric signal and displays it in the selected stress unit. It has
capability of setting of test of the testing angle, selection of units in use
(Kg/cm2 , Mpa or Psi ). It is battery operated instrument and can be easily
connected to a PC and has large memory to store up-to 5000 results.
2.2
(b)
(c)
Amplifier
(d)
2.2.1 Object:
The ultrasonic pulse velocity method could be used to establish:
(a)
(b)
(c)
change in the structure of the concrete which may occur with time
(d)
(e)
(f)
2.2.2 Principle:
The method is based on the principle that the velocity of an ultrasonic pulse
through any material depends upon the density, modulus of elasticity and
Poissons ratio of the material. Comparatively higher velocity is obtained when
concrete quality is good in terms of density, uniformity, homogeneity etc. The
ultrasonic pulse is generated by an electro acoustical 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
(compression), shear (transverse) and surface (Reyleigh) waves. The receiving
transducer detects the onset of longitudinal waves which is the fastest.
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.
For good quality concrete pulse velocity will be higher and for poor quality it will
be less. 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 passed
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 affects the pulse velocity.
Any suitable type of transducer operating within the frequency range of 20 KHz
to 150KHz may be used. Piezoelectric and magneto-strictive types of
transducers may be used and the latter being more suitable for the lower part of
the frequency range. Following table indicates the natural frequency of
transducers for different path lengths
11
Path length
(mm)
Natural Frequency of
Transducer(KHz)
Minimum transverse
dimensions of members
(mm)
Upto 500
150
25
500 700
>60
70
700 1500
>40
150
above 1500
>20
300
The electronic timing device should be capable of measuring the time interval
elapsing between the onset of a pulse generated at the transmitting transducer
and onset of its arrival at receiving transducer. Two forms of the electronic timing
apparatus are possible, one of which use a cathode ray tube on which the
leading edge of the pulse is displayed in relation to the suitable time scale, the
other uses an interval timer with a direct reading digital display. If both the forms
of timing apparatus are available, the interpretation of results becomes more
reliable.
2.2.3 Methodology:
The equipment should be calibrated before starting the observation and at the
end of test to ensure accuracy of the measurement and performance of the
equipment. It is done by measuring transit time on a standard calibration rod
supplied along with the equipment.
A platform/staging of suitable height should be erected to have an access to the
measuring locations. The location of measurement should be marked and
numbered with chalk or similar thing prior to actual measurement (pre decided
locations).
Mounting of Transducers:
The direction in which the maximum energy is propagated is normally at right
angles to the face of the transmitting transducer, it is also possible to detect
pulses which have travelled through the concrete in some other direction. The
receiving transducer detects the arrival of component of the pulse which arrives
earliest. This is generally the leading edge of the longitudinal vibration. It is
possible, therefore, to make measurements of pulse velocity by placing the two
transducers in the following manners (Fig.2.2.2)
12
(b)
(c)
There should be adequate acoustical coupling between concrete and the face of
each transducer to ensure that the ultrasonic pulses generated at the
transmitting transducer should be able to pass into the concrete and detected by
the receiving transducer with minimum losses. It is important to ensure that the
layer of smoothing medium should be as thin as possible. Couplant like
petroleum jelly, grease, soft soap and kaolin/glycerol paste are used as a
coupling medium between transducer and concrete.
Special transducers have been developed which impart or pick up the pulse
through integral probes having 6mm diameter tips. A receiving transducer with a
hemispherical tip has been found to be very successful. Other transducer
configurations have also been developed to deal with special circumstances. It
should be noted that a zero adjustment will almost certainly be required when
special transducers are used.
Most of the concrete surfaces are sufficiently smooth. Uneven or rough surfaces,
should be smoothened using carborundum stone before placing of transducers.
Alternatively, a smoothing medium such as quick setting epoxy resin or plaster
can also be used, but good adhesion between concrete surface and smoothing
medium has to be ensured so that the pulse is propagated with minimum losses
into the concrete.
Transducers are then pressed against the concrete surface and held manually. It
is important that only a very thin layer of coupling medium separates the surface
of the concrete from its contacting transducer. The distance between the
measuring points should be accurately measured.
Repeated readings of the transit time should be observed until a minimum value
is obtained.
Once the ultrasonic pulse impinges on the surface of the material, the maximum
energy is propagated at right angle to the face of the transmitting transducers
and best results are, therefore, obtained when the receiving transducer is placed
on the opposite face of the concrete member known as Direct Transmission.
The pulse velocity can be measured by Direct Transmission, Semi-direct
Transmission and Indirect or Surface Transmission. Normally, Direct
Transmission is preferred being more reliable and standardized. (various codes
gives correlation between concrete quality and pulse velocity for Direct
Transmission only). The size of aggregates influences the pulse velocity
measurement. The minimum path length should be 100mm for concrete in which
the nominal maximum size of aggregate is 20mm or less and 150mm for
aggregate size between 20mm and 40mm.
Reinforcement, if present, should be avoided during pulse velocity
measurements, because the pulse velocity in the reinforcing bars is usually
higher than in plain concrete. This is because the pulse velocity in steel is 1.9
times of that in concrete. In certain conditions, the first pulse to arrive at the
receiving transducer travels partly in concrete and partly in steel. The apparent
14
Above 4.5
Excellent
3.5 to 4.5
Good
3.0 to 3.5
Medium
Below 3.0
Doubtful
Note: in case of doubtful quality, it will be desirable to carry out further tests.
15
16
60
+5
+4
40
+2
+1.7
20
-0.5
-1
-4
-1.5
-7.5
Path Length: The path length (the distance between two transducers) should be
long enough not to be significantly influenced by the heterogeneous nature of the
concrete. It is recommended that the minimum path length should be 100mm for
concrete with 20mm or less nominal maximum size of aggregate and 150mm for
concrete with 20mm and 40mm nominal maximum size of aggregate. The pulse
velocity is not generally influenced by changes in path length, although the
electronic timing apparatus may indicate a tendency for slight reduction in
velocity with increased path length. This is because the higher frequency
components of the pulse are attenuated more than the lower frequency
components and the shapes of the onset of the pulses becomes more rounded
with increased distance travelled. This apparent reduction in velocity is usually
small and well within the tolerance of time measurement accuracy.
With indirect transmission, there is some uncertainty regarding the exact length
of the transmission path. It is, therefore, preferable to make a series of
measurements with placing transducers at varying distances to eliminate this
uncertainty. To do this, the transmitting transducer should be placed in contact
with the concrete surface at a fixed point x and the receiving transducer should
be moved at fixed increments xn along a chosen line on the surface. The
transmission times recorded should be plotted as points on a graph showing
their relation to the distance separating the transducers.( Fig.2.2.3).
17
Vc = 4.0
Vc = 4.5
24
146
167
188
54
65
74
83
82
43
49
55
150
23
27
30
18
2.3
2.4
(b)
(b)
(c)
(d)
2.4.2 Principle
The pull off test is based on the concept that the tensile force required to pull a
metal disk, together with a layer of concrete, from the surface to which it is
attached, is related to compressive strength of concrete There are two basic
approaches that can be used. One is where the metal disk is glued directly to the
concrete surface and the stressed volume of the concrete lies close to the face
of the disk, and the other is where surface carbonation or skin effect are present
and these can be avoided by use of partial coring to an appropriate depth. Both
the approaches are illustrated in Fig. 2.4.2.
20
2.5
(b)
(c)
Loading ram seated on a bearing ring for applying pull out force
2.5.1 Object:
The pull out test can be used to determine the following properties.
(a)
(b)
(c)
2.5.2 Principle:
The pullout test measures the force required to pull an embedded metal insert
with an enlarged head from a concrete specimen or a structure. The Fig 2.5.1
illustrates the configuration of a pull out test. The test is considered superior to
the rebound hammer and the penetration resistance test, because large volume
and greater depth of concrete are involved in the test. The pull out strength is
proportional to the compressive strength of concrete. The pull out strength is of
the same order of magnitude as the direct shear strength of concrete, and is 10
to 30% of the compressive strength. The pull out test subjects the concrete to
slowly applied load and measures actual strength property of the concrete. The
concrete is subjected, however, to a complex three dimensional state of stress,
and the pull out strength is not likely to be related simply to uniaxial strength
properties. Nevertheless, by use of a previously established correlation, the pull
out test can be used to make reliable estimates of in situ strength.
2.5.3 Methodology:
The pull out tests falls into two basic categories
(i)
those in which an insert is cast along with concrete i.e. the test is
preplanned for new structures and
23
(ii)
This method is generally known as drilled hole method. There are various
Dd
2h
The ultimate pullout load measured during the in place test is converted to an
equivalent compressive strength by means of a previously established
relationship.
As per ASTM C-900-82, following are the requirements for metal insert
(a)
(b)
(c)
Apex angle 53 to 70
2.5.6 Standards:
The pull out test is conducted as per ASTM C 900-01 & BS-1881 Part 207.
2.6
2.6.1 Object:
This test can be used both for quality control and quality assurance. The most
practical use of the BO test method is for determining the time for safe form
removal and the release time for transferring the force in prestressed posttensioned members. This test can be planned for new structures as well as for
existing structures.
2.6.2 Principle:
The method is based upon breaking off a cylindrical specimen of in place
concrete. The test specimen has a 55mm diameter and a 70mm height. The test
specimen is created in the concrete by means of a disposable tubular plastic
sleeve, which is cast into the fresh concrete and then removed at the planned
time of testing, or by drilling the hardened concrete at the time of the break off
(BO) test. Fig 2.6.1 and 2.6.2 show tubular plastic sleeves and a drill bit,
respectively.
25
Both the sleeve and the drill bit are capable of producing a 9.5mm wide groove
(counter bore) at the top of the test specimen (see Fig 2.6.3).A force is applied
through the load cell by means of a manual hydraulic pump. Fig 2.6.3 is a
schematic of a BO concrete cylindrical specimen obtained by inserting a sleeve
or drilling a core. The figure also shows location of the applied load at the top of
the BO test specimen. In principal, the load configuration is the same as a
cantilever beam with circular cross section, subjected to a concentrated load at
its free end. The force required to break off a test specimen is measured by
mechanical manometer. The BO stress can then be calculated as:
26
fBO
M
S
where M = PBOh,
PBO = BO force at the top
h = 65.3 mm
S=
d
32
d = 55mm
In this case the cracks are initiated at the point 55mm away from the concrete
surface.
2.6.3 Methodology:
The load cell has two measuring ranges: low range setting for low strength
concrete up to approx. 20 MPa and high range setting for higher strength
concrete up to abut 60Mpa. The equipment used for this test is shown in Fig
2.6.4.
27
A tubular plastic sleeve of diameter 55mm and geometry shown in Fig. 2.6.1, is
used for forming cylindrical specimen in fresh concrete. A sleeve remover as
shown in Fig. 2.6.5 is used for removing the plastic sleeve from hardened
concrete.
A diamond tipped drilling bit is used for drilling cores for the BO test in hardened
concrete (Fig. 2.6.2). The bit is capable of producing a cylindrical core, along
with a reamed ring (counter bore) in the hardened concrete at the top with
dimensions similar to that produced by using a plastic sleeve.
The sleeves should be at center to center and edge distance of minimum
150mm. Concrete inside the sleeve and the top of plastic sleeve itself should
then be tapped by fingers to ensure good compaction for the BO specimen.
Sleeves should then be moved gently up and down in place and brought to the
same level as the concrete surface at its final position. For stiff mixes (i.e. low
slump concrete) a depression may occur within the confines of the sleeve during
the insertion process. In such cases the sleeve should be filled with additional
concrete, tapped with fingers. For high slump concrete, the sleeve may move up
28
ward due to bleeding. For such cases, sleeves should be gently pushed back in
place, as necessary, to the level of finished concrete surface. Grease or other
similar material, should be used to lubricate the plastic sleeves for its easier
removal after the concrete hardens.
For core drilling from hardened concrete the concrete surface should be smooth
in order to fix the vaccum plate of the core drilling machine. The core barrel
should be perpendicular to the concrete surface. The length of the drilled core
should be 70mm and in no case shorter than 70mm.
At the time of conducting BO test, remove the inserted plastic sleeve by means
of key supplied with the tester (Fig 2.6.5). Leave the plastic ring in place.
Remove loose debris from around the cylindrical slit and the top groove. Select
the desired range setting and place the load cell in the groove on the top of the
concrete surface so that load is applied properly. The load should be applied to
the test specimen at a rate of approx. one stroke of hand pump per second. After
breaking off the test specimen, record the BO manometer reading. The BO
meter reading can then be translated to the concrete strength using curves
relating the BO reading to the desired concrete strength.
Before conducting the test, the BO tester should be calibrated as per the
procedure given by the manufacture. The BO manufacturer provides correlation
curves relating the BO reading and the compressive strength of the standard
150mm cubes. However, it is desirable that the user should develop his own
correlation curves for a particular concrete batch. Developing correlation curves
for different types of concrete would increase the accuracy and dependability of
method in predicting the in-place strength. The following precaution should be
taken when developing data for correlations:
(a)
(b)
(c)
An average of the five BO readings and the average of the three standard
cube test results represent one point on the graph relating the BO reading
to the desired standard strength of the concrete.
(d)
(ii)
the minimum member thickness for which it can be used. The max.
aggregate size is 19mm and min. member thick ness is 100mm.
However, the principle for the method can be applied to accommodate
large aggregate sizes or smaller members. The test cause the damage to
the con crete which needs repair after conducting the test.
2.6.5 Standards:
The break off test is conducted as per ASTM C 1150.
2.7
The probes have a tip dia of 6.3 mm, a length of 79.5mm, and a conical point.
Probes of 7.9mm dia are also available for the testing of concrete made with light
weight aggregates.
2.7.1 Object:
The Windsor probe test is used to determine
(a)
(b)
(c)
(d)
2.7.2 Principle:
The Windsor probe, like the rebound hammer, is a hardness tester, and the
penetration of probe can be related to the compressive strength of concrete
below the surface, using previously developed correlations between strength
properties and penetration of the probe. The underlying principal of this
penetration resistance technique is that for standard test conditions, the
penetration of probe in to the concrete is inversely proportional to the
compressive strength of the concrete. In other words larger the exposed length
of the probe, greater the compressive strength of concrete.
2.7.3 Methodology:
The method of testing is simple and is given in the manual supplied by the
manufacturer. The area to be tested must have a smooth surface. To test
structure with coarser finish, the surface must be first ground smooth in the area
of the test. The powder actuated driver is used to drive a probe into concrete. If
flat surfaces are to be tested, a suitable locating template to provide 158mm
equilateral triangular pattern is used and three probes are driven into the
concrete at each corner. The exposed length of individual probes are measured
by a depth gauge. For testing structures with curved surfaces, three probes are
driven individually using the single probe locating template. In either case, the
measured average value of exposed probe length may then be used to estimate
the compressive strength of concrete by means of appropriate correlation data.
The manufacturer of the Windsor probe test system supply tables relating
exposed length of the probe with compressive strength of concrete. For each
exposed length value, different values of compressive strength are given,
depending upon the hardness of aggregate. However the manufacturers table
do not always give satisfactory results. Sometimes they considerably over
estimate the actual strength and in some cases they underestimate the strength.
It is, therefore, imperative to correlate probe test result with the type of concrete
being used. In addition to hardness of the coarse aggregate, the type and size of
coarse aggregate also have a significant effect on probe penetration. The degree
of carbonation and the age of concrete may also affect the probe penetration
strength relationship.
2.7.4 Advantages & Limitations:
Windsor probe testing method is basically hardness method, and like other
hardness methods, should not be expected to yield absolute values of strength
of concrete in a structure. However, like surface hardness tests, penetration tests
provide an excellent means of determining the relative strength of concrete in the
same structure, or relative strength in different structures.
31
One of the limitation of this test is minimum size requirements for the concrete
member to be tested. The minimum distance from a test location to any edges of
the concrete member or between two given test locations is of the order of
150mm to 200mm, while the minimum thickness of the member is about three
times the expected depth of penetration. The test also causes some minor
damage to the surface, which generally needs to be repaired.
The main advantages of this test are the speed and simplicity and only one
surface is required for testing.
2.7.5. Standards:
The penetration resistance test is coducted as per ASTM C 803 / C 803-03 and
BS 1881 Part 207.
2.8
2.9
Permeability Test:
2.9.1 Introduction
32
33
RS 232 C interface
Integrated software for printout of measured objects and transmission to
pC
The volume of inner chamber and hose and the cross sectional area of the
inner chamber are terms in the formula for calculating kT and L. They
must therefore not be changed.
Electrode spacing 50 mm
(4) Vacuum pump:
1.5 m3/h
approx 10 bar
In case of dry concrete, the results are in good agreement with laboratory
methods, such as oxygen permeability, capillary suction, chloride penetration
and others. The quality class of the cover concrete is determined from kT using a
table as shown below.
Index
5
4
3
2
1
kT (10-16 m2 )
> 10
1.0 - 10
0.1 1.0
0.01 0.1
< 0.01
The most accurate values are obtained for dry concrete ( p measurement is
superfluous).
The quality classification of cover concrete from table and the monogram
from figure related to young concrete i.e. concrete age about 1-3 months.
Some measurements on concrete a few years old have shown that the
classification in Table and the monogram cannot be directly applied.
The moisture content of the concrete has a major effect on the gas
permeability. The correction of this effect by the measurement of the
electrical resistance generally leads to satisfactory results in the case of
young concrete. For old concrete, further investigations must be carried out.
There may be three further reasons why the desired vacuum (10-50 mbar ) is
not reached.
The concrete cover is too permeable (normal function of the unit).
The concrete surface is too uneven: the rubber seals can compensate
only a certain degree of unevenness (abnormal function).
The unit has a leak (abnormal function).
37
Compressive strength of well cured concrete increases with time. But this
increase is dependent on the temperature of curing also. The combined
influence of time and temperature is considered as the maturity. It is thus defined
as the integral of time multiplied by temperature with a datum temperature of
10 C, since below this temperature cement in concrete ceases to hydrate.
The maturity of in-place can be monitored by thermocouples or by instruments
called maturity maters The strength of in-place concrete is then estimate using
the established correlation graph between maturity and compressive strength of
concrete. The advantage of maturity concept is that by prior placing of maturity
meters in the formwork at the time of the construction, the strength of early age
concrete can be monitored and accordingly formwork can be removed
confidently.
The subsequent analytical modeling techniques range from a simple planner grid
model to a three dimensional finite element representation. The response of the
model in systematically compared with the field test results using multiple gauge
location and load configurations. Structural parameters such as lateral deck
stiffness, rotational restraints are then modified through an interactive process
until the analytical responses closely match the field measurement. This
calibrated model can be used to predict stress levels at critical locations due to
rating and overloaded vehicle. Rating factors can then be developed using
either allowable stress method (ASD) or the load factor method (LFD).
This approach is suitable to use on highway bridges, rail road bridges and other
structures.
Where the live load can be easily applied and load stresses
significant, by approaching to this method steel, pre-stressed concrete, reinforce
concrete and timber structures can be tested successfully.
2.12.2 Indian Railways has planned to procure the complete structural testing
equipment for performing live load tests on short to medium span Railway Bridge
38
for static load, low speed, full sectional speed i.e. upto 200 kmph, braking and
acceleration of trains so as to complete the full test for load rating of bridge.
39
CHAPTER-3
NONDESTRUCTIVE TESTS FOR CORROSION
ASSESSMENT, LOCATION AND DIAMETER OF
REINFORCEMENT AND COVER THICKNESS OF CONCRETE
BRIDGES
3.1
Introduction:
For effective inspection and monitoring of concrete bridges, the condition
assessment of reinforcement is an important step. Even for deciding appropriate
repair strategy for a distressed concrete bridge, the determination of corrosion
status of reinforcing bars is a must. Most of NDT methods used for corrosion
assessment are based on electrochemical process. But apart from the process, it
is advisable that the persons involved for conducting the tests should have
enough experience in this particular field. The following methods are normally
used for the condition assessment of reinforcement in concrete structures:
(a)
(b)
Resistivity test
(c)
(d)
(e)
Endoscopy Technique
The following methods are generally used for determining the location / diameter
of reinforcement bars and cover thickness in concrete bridges
(a)
Profometer
(b)
Micro covermeter
Each of the above said methods has been discussed in detail in this chapter.
3.2
40
3.2.1 Object:
This test is used to assess the corrosion conditions in a reinforced concrete
structure. The method detects the likelihood of corrosion of steel but can not
indicate the rate of corrosion. By making measurements over the whole surface,
a distinction can be made between corroded and non-corroded locations.
3.2.2 Principle:
CANIN corrosion analyzer is based on electro-chemical process to detect
corrosion in the reinforcement bars of structure. It represents a galvanic element
similar to a battery ,producing an electrical current, measurable as an electric
field on the surface of concrete. The potential field can be measured with an
electrode known as half cell. The electrical activity of the steel reinforcement and
concrete leads them to be considered as one half of battery cell with the steel
acting as one electrode and concrete as electrolyte. The name half cell surveying
derives from the fact that the one half of the battery cell is considered to be the
steel reinforcing bars and surrounding concrete. The electrical potential of a point
on the surface of steel reinforcing bar can be measured comparing its potential
that of copper copper sulphate reference electrode/silver- silver nitrate
reference electrode on the surface.
The positive terminal of the voltmeter is attached to the reinforcement and the
negative terminal is attached to the copper-copper sulphate half cell. If there is
any corrosion in the bars, the excess electrons in the bar would tend to flow from
the bar to the half cell. Because of the way the terminals of the voltmeter are
connected in the electrical circuit (Fig. 3.2.1), the voltmeter indicates a negative
voltage. The measured half cell potential is the open circuit potential, because it
is measured under the condition of no current in the measuring circuit. A more
negative voltage reading at the surface is to interpreted to mean that the
embedded bar has more excess electrons, and there is, therefore, a higher
likelihood that the bar is corroding.
The half cell potential readings are indicative of the probability of corrosion
activity of the reinforcing bars located beneath the copper-copper sulphate
reference cell. However, this is true only if the reinforcing steel is electrically
connected to the bar attached to the voltmeter.
3.2.3 Methodology:
The corrosion analyzing instrument CANIN operates as digital voltmeter. Voltage
of + 999 mV DC can be measured using this instrument. The potential in
millivolts is measured with rod electrodes at different locations on the structure.
The measured voltage depends upon the type of the half-cell, and conversion
factors are available to convert readings obtained with other half cells to coppersulphate half cell.
Testing is usually performed at points arranged in a grid. The required spacing
between test points depends on the particular structure. Excessive spacing can
42
miss points of activity or provide insufficient data for proper evaluation, while
closer spacing increase the cost of survey. In surveying bridge decks, ASTM C
876 recommends a spacing of 1.2 meter. If the difference in voltage between
adjacent points exceed 150 mV, a closer spacing is suggested. A key aspect of
this test is to ensure that the concrete is sufficiently moist to complete the circuit
necessary for a valid measurement. If the measured value of the half cell
potential varies with time, pre wetting of the concrete is required. Although pre
wetting is necessary, there should be no free surface water between test points
at the time of potential measurement. The concrete is sufficiently moist if the
measured potential at a test point does not change by more than + 20 mV within
a 5 min. period. If stability cannot be achieved by pre-wetting, it may be because
of stray electrical currents or excessive electrical resistance in the circuit. In
either case, the half cell potential method should not be used. Testing should be
performed between temperature range of 17 to 280C.
3.2.4 Interpretation of test results:
As per ASTM C 876, two techniques can be used to evaluate the results (i) the
numeric technique (ii) the potential difference technique.
In the numeric technique, the value of the potential is used as an indicator of the
likelihood of the corrosion activity. The potential measured at the surface of
concrete can be interpreted as per table given below:Phase of Corrosion Activity
(b)
(c)
(d)
In the potential difference technique, the areas of active corrosion are identified
on the basis of the potential gradients. In the equipotential contour plot, the
closer spacing of the voltage contour indicates regions of high gradients. The
higher gradient indicates, higher risk of corrosion. The potential difference
technique is considered more reliable for identifying regions of active corrosion
than is the use of numerical limits.
3.2.5 Limitations:
For conducting this test access to the reinforcement is must. The method cannot
be applied to epoxy coated reinforcement or concrete with coated surfaces. The
concrete should be sufficiently moist for conducting this test.
This test only indicates the liklyhood of corrosion activity at the time of
measurement. It does not furnish direct information on the rate of corrosion of
the reinforcement.
3.2.6 Standards:
This method is covered under ASTMC 876-91 (Reapproved 1999).
3.3
Resistivity Test:
This test is used to measure the electrical resistance of the cover concrete. Once
the reinforcement bar loses its passivity, the corrosion rate depends on the
availability of oxygen for the cathodic reaction. It also depends on the concrete,
which controls the ease with which ion migrate through the concrete between
anodic and cathodic site. Electrical resistance, in turn, depends on the
microstructure of the paste and the moisture content of the concrete.
The combination of resistance measurement by resistivity meter and potential
measurement by corrosion analyzing instrument give very reliable information
about the corrosion condition of the rebar.
The equipment used for this test is a portable, battery operated, four probe
device which measures concrete resistivity. ( Fig. 3.3.1)
44
L
A
45
With =12 K W cm
Corrosion is improbable
2.
With = 8 to 12 K W cm
Corrosion is Possible
3.
With = 8 K W cm
46
3.3.4 Limitations:
The method is slow because it covers small area at a time. The system should
not be used in isolation because it gives better indication of corrosion in
reinforced concrete if used in combination with half - cell potentiometer.
3.4
3.5
3.6
Endoscopy Technique:
Endoscopy consists of inserting a rigid or flexible viewing tube into holes drilled
into concrete bridge components or cable ducts and view them with light
provided by optical glass fibers from an external source. This is a most useful
method for inspecting or detecting voids in the grout and corrosion in steel in the
cable ducts. It is also useful for detail examination of other part of the bridge
structure, which could not other wise be assessed. Endoscopes are available
as attachments for a camera or a TV monitor. It, however, needs an
experienced engineer to make assessment of most likely locations of voids in the
grout and probable points of entry of chlorides into the ducts.
47
3.7
Profometer:
In any RCC/PSC structure, adequate cover thickness is essential to prevent
corrosion of the reinforcement. In old structures, sometimes the detailed
drawings are not traceable due to which it becomes very difficult to calculate the
strength of the structure which is essentially required for finalizing the
strengthening scheme. Sometimes, the bridges are to be checked from strength
point of view to permit higher axle load and in absence of reinforcement details it
becomes very difficult to take a decision.
To overcome all these problems, the methods have been developed for
investigation and evaluation of concrete structures. Profometer is a small
versatile instrument for detecting location, size of reinforcement and concrete
cover. This instrument is also known as rebar locator. This is a portable and
handy instrument which is normally used to locate the reinforcement on LCD
display. This instrument is available with sufficient memory to store measured
data. Integrated software is loaded in the equipment for carrying out and printing
statistical values. One of the equipment which is commercially available in the
market is shown in Fig. 3.7.1.
3.7.2. Principle:
The instrument is based upon measurement of change of an electromagnetic
field caused by steel embedded in the concrete.
3.7.3 Methodology:
To ensure satisfactory working of profometer and to get accurate results, it
should be calibrated before starting the operations and at the end of the test. For
this purpose, test block provided with the instrument should be used. To check
the calibration accuracy, the size and cover of the reinforcement of the test block
is measured at different locations on test block and the recorded data should
match with the standard values prescribed on the test block.
Path measuring device and spot probes are together used for path
measurements and scanning of rebars. These are connected with profometer
with cables and are moved on the concrete surface for scanning the rebars and
measuring the spacing. As soon as the bar is located, it is displayed on the
screen. Once the bar is located, it is marked on the concrete surface.
Diameter probe is used for measuring the dia of bars. It is also connected with
profometer by one cable. After finding out the location of rebar, the dia probe is
placed on the bar parallel to bar axis. Four readings are displayed and mean
value of these readings is taken as diameter of bar.
Depth probe of the profometer is used to measure the cover. It is also connected
with profometer by cable and is placed exactly on the bar As soon as, the depth
probe is above a rebar or nearest to it, it gives an audio signal through a short
beep and visual display. Simultaneously, the measured concrete cover is stored
in memory.
For carrying out this test, the proper assess is essential. For this purpose, proper
staging, ladder or a suspended platforms may be provided. Before actual
scanning, marking is done with chalk on the concrete surface by dividing it into
panels of equal areas.
3.7.4 Advantages and Limitations:
This is a purely non-destructive test for evaluation of concrete structures
particularly old structures. The methods is very fast and gives quite accurate
results if the reinforcement is not heavily congested. The equipment is very light
and even one person can perform the test without any assistance.
The equipment is not being manufactured in India and needs to be imported.
Some of the Indian Firms are marketing the instrument and this is a costly
equipment.
49
3.8
Micro covermeter:
This is a portable and handy instrument weighing about 0.5 kg. This is normally
provided with two types of search heads one for parallel bars having range
approx. 360mm and other for mesh and close spaced bars having range of
approx. 120mm. It can function over the temp. range of 0C to 45C. One of the
micro cover meter commercially available in the market is shown in Fig. 3.8.1.
the dial. The probe should be kept parallel to the length of rebar. Depending
upon the diameter of the bar, the dial readings gives directly the cover to the
reinforcement.
3.8.3 Methodology:
The equipment should be calibrated before starting and at the end of the test to
get accurate results. For this purpose, one spacer is provided with each
equipment. For calibration, the cover should be measured at one location and
then it is remeasured after placing the spacer between the concrete surface and
probe. The difference between two readings should not vary more than +/- 5% of
the thickness of the spacer.
For locating the reinforcement bar, the search head should be placed on the
surface of concrete in such a way so that the length of the search head should
be parallel to the reinforcement provided in the structure. The location of the
main reinforcement should be decided based upon the geometry of the structure.
The search head should be moved from one end to other end in a direction
perpendicular to the main reinforcement. The sound of the buzzer /beep will be
strongest when the bar will come just above or below the probe.
For measurement of cover, the search head is moved on the surface. While
moving, the cover displayed on the screen reduces and sound of the
buzzer/beep increases when probe comes near reinforcement bars. The
minimum reading displayed will be the cover and the sound of the buzzer is
strongest when the reinforcement bar is just below the search head.
3.8.4 Advantages and Limitations:
The method is very fast and large area can be covered within short time. The
instrument work on batteries and does not require any electric supply. Since the
equipment is very small and portable, the test can be conducted by single person
without any assistance.
**********
51
CHAPTER 4
NON-DESTRUCTIVE TESTS FOR DETECTION OF
CRACKS/VOIDS/ DELAMINATIONS ETC. IN CONCRETE
BRIDGES
4.1
Introduction:
For the assessment of the actual condition of a concrete bridge, the detection of
internal crack, void, lamination etc. is very much necessary. The NDT methods
for testing surface hardness, strength and for checking the condition of
reinforcement do not indicate the internal condition of concrete. Sometimes
these internal cracks, voids etc. may lead to the corrosion of reinforcement,
cracking of section which may ultimately result into reduced life of bridge. The
following methods are normally used for detection of cracks/voids/laminations
etc. in concrete bridges
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Boroscope
(vii)
Nuclear Method
(viii)
(ix)
Some of these methods are used extensively all over the world for condition
assessment of various components of concrete bridges while some methods are
still in the laboratory trial stage. All the above methods are discussed in detail in
this chapter.
4.2
(b)
(c)
The environment
Normally the testing should be conducted during times of the day or night when
the solar radiation or lack of solar radiation would produce the most rapid heating
or cooling of the concrete surface. The test should not be conducted when sky is
cloudy. The measurements should be taken when wind speed is lower than
25kmph. The test should not be conducted when the temperature is below 0C. If
the concrete surface is covered with standing water, the test should not be
conducted.
4.2.3 Methodology:
For performing this test efficiently, a movement of heat must be established in
the structure. Normally the inspection should be conducted during the sunny day,
i.e. the testing should be avoided during monsoon. The inspection may begin
either 2 to 3 hours after sunrise or 2 to 3 hours after sunset, both are times of
rapid heat transfer. The surface to be tested should be cleaned thoroughly.
The next step is to locate a section of sound concrete. This work can be done by
chain dragging (sounding), coring, or ground penetrating radar or by using some
other suitable method. Image the reference area and set the equipment controls
so that an adequate temperature image is viewed and recorded.
The next step is to image the area, which is known to have defects i.e.
voids/delamination/cracks etc. and images of this defective area are viewed and
recorded. Now the setting of the equipment should be done in such a way to
allow viewing of both the sound and defective reference areas in the same image
with the widest contrast possible.
If a black and white monitor is used, better contrast images will normally be
produced when the following convention is used, black is defective concrete and
white is sound material. If a colour monitor or computer enhanced screen is
used, three colours are normally used to designate definite sound areas, definite
defective areas and indeterminate areas. When tests are performed during day
light hours, the defective concrete areas will appear warmer, whereas during test
performed after dark, defective areas will appear cooler.Once the control are set,
the recording of images can be done and stored.
54
4.3
(b)
Preamplifier Because of the low voltage output, the leads from the
transducer to the preamplifier must be as short as possible. Sometimes
the preamplifier is integrated within the transducer itself. This amplifies
the output signals.
(c)
55
(d)
Main amplifier - This further amplifies the signals, typically within a gain of
20 to 60 dB.
(e)
4.3.1 Object:
This method is used mainly to detect the cracking in concrete, whether due to
externally applied loads, drying shrinkage or thermal stresses. This method can
56
be helpful in determining the internal structure of the material and to know the
structural changes during the process of loading.
The method can also be used to establish whether the material or the structure
meet certain design or fabrication criteria. In this case, the load is increased only
to some predetermined level. The amount and nature of acoustic emissions may
be used to establish the integrity of the specimen or structure and may also be
used to predict the service life.
4.3.2 Principle:
When an acoustic emission event occurs at a source with in the material, due to
inelastic deformation or cracking, the stress waves travel directly from the source
to the receiver as body waves. Surface waves may then arise from mode
conversion. When the stress waves arrive at the receiver, the transducer
responds to the surface motion that occurs. A typical acoustic emission signal
from concrete is shown in Fig. 4.3.2.
By using a number of transducers to monitor acoustic emission events, and
determining the time differences between the detection of each event at different
transducer positions, the location of acoustic emission event may be determined
by using triangulation techniques.
57
4.3.3 Methodology:
Acoustic emission test may be carried out in the laboratory or in the field.
Basically one or more acoustic emission transducers are attached to the
specimen. The specimen is then loaded slowly, and the resulting acoustic
emissions are recorded. The test is generally conducted in two ways.
(a)
When the specimens are loaded till failure. (to know about internal
structure/to study about structural changes during loading).
(b)
4.3.4 Limitations:
The acoustic emission techniques may be very useful in the laboratory to
supplement other measurement of concrete properties. However, their use in the
field is still very limited. Another draw back is that acoustic emissions are only
generated when the loads on a structure are increased and this create
considerable practical problems.
4.4
58
AR 2 1
.......... ..(1)
1,2
AI
2
1
Where 1, 2 is the reflection coefficient at the interface, & are the wave
impedances of the material 1,2 respectively, in ohms. For non-metallic material,
such as concrete or soil the wave impedance is given by
59
where is the magnetic permeability of air, which is 4 x10 -7 henry /meter and is
the dielectric constant of material in farad / meter
Since the wave impedance of air, o is equal to
o
o
1,2
r
1
r
1
r
2
.....................(3)
r
2
4.5
(b)
(c)
63
Fig.4.5.2.2
4.5.2.3 Instrumentation:
An impact-echo test system is comprised of three components:
An impact source.
A displacement transducer.
64
65
Fig.4.5.2.4
The P-wave generated by the impact propagates back and forth between the
top and bottom surfaces of the plate. Each time the P-wave arrives at the top
surface it produces a characteristic displacement. Thus the waveform is
periodic, and the period, t, is equal to the travel path, 2T, divided by the P-wave
speed. Since frequency is the inverse of the period, the frequency, fp, of the
characteristic displacement pattern is:
fp = Cp/2T Eq. (1)
Thus, if the frequency of an experimental waveform can be determined, the
thickness of the plate (or distance to a reflecting interface) can be calculated:
T = Cp/2fp Eq. (2)
Note that Eq. (2) is an approximation that is suitable for most applications in
plate-like structures. When using the method to measure plate thickness, a
correction factor is needed. For prismatic members, the value of the correction
factor depends on the geometry of the member. In practice, the frequency
content of the recorded waveforms is obtained using the fast Fourier transform
(FFT) technique to obtain the amplitude spectrum. Appendix A gives additional
background information on digital frequency analysis.
66
Fig.4.5.2.5
Appendix A explains that the resolution in the amplitude spectrum, that is, the
frequency difference between adjacent points, is equal to the sampling
frequency divided by the number of points in the waveform record. This
imposes a limit on the resolution of the depth calculated according to Eq. (2).
Because depth and frequency are inversely related, it can be shown that for a
fixed resolution in the frequency domain, the resolution of the calculated depth
improves as the frequency increases, that is, as depth decreases.
4.5.2.6 ASTM Standard
In 1998, ASTM adopted a standard test method on using the impact-echo
method to measure the thickness of concrete members (ASTM C 1383,
Standard Test Method for Measuring the P-wave Speed and Thickness of
Concrete Plates Using the Impact-Echo Method). The method involves two
procedures. Procedure A is to determine the P-wave speed in the concrete by
measuring the travel time between two surface receivers separated by a known
67
68
The
objective
of
frequency
analysis is to determine the
dominant frequency components in
the digital waveform. This is most
easily accomplished by using the
fast Fourier transform (FFT)
technique. The FFT results can be
used to construct the amplitude
spectrum,
which
gives
the
amplitudes
of
the
various
frequency components in the
waveform.
The
amplitude
spectrum obtained by the FFT
contains half as many points as
the time domain waveform, and
the maximum frequency in the
spectrum is one-half the sampling
rate, which for this example is 500
Hz. Figure 4.5.2.7(b) shows the
initial portion of the computed
amplitude spectrum; the peaks
occur at 20, 40, and 60 Hz. Each
of the peaks corresponds to one of
the component sine curves in Eq.
(A.1).
Fig.4.5.2.7
In the FFT technique, the frequency interval, deltaf, in the spectrum is equal to
the sampling frequency divided by the number of points in the waveform. For
this example, there are 256 points in the complete time domain waveform, and
the frequency interval is equal to 1000 Hz divided by 256, or 3.9 Hz. Since the
frequency interval is proportional to the sampling frequency, a slower sampling
rate enhances resolution in the frequency domain. However, slower sampling
rates leads to longer record lengths which can result in complex spectra due to
reflections from side boundaries of the test object. Experience has shown that a
sampling frequency of 500 kHz with 1024 points per record is desirable in most
applications.
4.5.3 Impulse Response method:
Impulse response equipment is used to produce a stress wave in the considered
component. The stress wave may e.g. be produced by an impact with an
instrumented rubber tipped hammer. The impact causes the component to act in
bending mode. A velocity transducer placed adjacent to the impact point
measures the response of the component.
In contrast to the Impact-Echo method the impulse response equipment does not
measure the reflection of the impact. Furthermore, the impact used to produce
the response is considerably larger than the impulse used for the Impact-Echo
method.
69
The hammer used to produce the impact and the transducer used to measure
the response of the component are both connected to a laptop PC. The laptop
performs a spectral analysis of the impact as well as the response. Dividing the
resultant velocity spectrum by the force spectrum then derives the mobility. An
example of a mobility graph is shown below.
For each measurement the resulting mobility graph is shown. On the basis of the
mobility graph the following parameters are determined:
Average mobility: The average mobility is shown as the green line in the
figure above. The average mobility depends on the thickness of the material.
If the thickness is reduced the average mobility is increased. This implies that
laminated concrete has a higher average mobility than non-laminated
concrete.
Mobility slope: The presence of honeycombs in the concrete will reduce the
damping of the signal. This implies that the mobility graph will be increasing
within the considered frequency range, see figure below.
70
Voids index: The voids index is defined as the ratio between the initial maximum
of the mobility and the average mobility. If the component is laminated the initial
maximum of the mobility will be con siderably higher than the average mobility. If
the voids index is higher than 2 4 it indicates a potentially weak area, see
figure below.
The impulse response method is a fast method which may be used to screen a
relatively large area within a short period of time. The equipment delivers surface
graphs of the measured parameters. In the figure below a surface graph of the
average mobility of a bridge deck is shown.
71
The results of the impulse response testing shall always be calibrated on the
basis of e.g. cores, break-ups or a visual inspection using a boroscope. The
locations of these tests are selected on the basis of the surface graphs of the
measured parameters.
The method is used mainly for testing of piles. Testing of piles by this method is
covered in ASTM test method D 5882 and is known as transient response
method.
4.6
72
4.7
Boroscope:
This method can be used for concrete, steel and masonry structures. The
method is most commonly used on concrete and masonry structures.
A boroscope is used to look inside inaccessible or small voids. For example, if
cable ducts are not injected, it is possible to inspect the strands by means of an
endoscope through a contact drilling (here a drilled hole from the surface to the
cable duct).
For steel structures the method is usually used for investigation of closed profiles
to gain information regarding the condition of the interior surfaces of the closed
profiles.
For masonry structures the boroscope can be used to gain information of the
depth of the outer layer of bricks or natural stones and it can provide information
of the filling material in between the arches. It may also be used to examine the
mortar between bricks or natural stone.
73
The boroscope equipment includes a lighting source and a fibre optic cable to
transfer the light to the boroscope.
A system of lenses enables the boroscope to be used as a monocular. A camera
or video camera can also be mounted on the boroscope for photo
documentation.
Generally speaking, the method is appropriate and may also be used for
inspections of structural components such as expansion joints, honeycombs and
cracks/slots.
The many variations and features which can be obtained for boroscopes make
them an almost universal tool for internal inspections. These include a wide
range of lengths and diameters, solid tubular or flexible bodies, lenses for
forward, sideways or retro viewing, still and video camera attachments, and
mains or battery power supplies.
4.7.1 Specification
Video Boroscope Remote High Resolution CCD camera is comprising of three
items as below.
(i) Videoscope 3M 6MM dia
Industrial videoscope with diameter 6 mm, length 3m, tungsten sheet, deflection
150 degree (up/down) and 120 degree (right/ left), direction of view 0 degree,
field of view 50 degree, consisting of VP06030AE Videoscope 3m x 6 mm dia,
V0625WAE Interchangeable lens 0 degree, 80135A Carrying case
(ii) Interchangeable lens 90 degree
Interchangeable lens 90 degree, interchangeable lens for videoscope with 6 mm
diameter. Direction of view 100 degree, depth of field 1 to 15 mm.
(iii)Portable Pack , PAL
consisting of 81040020 Portable pack base unit, PAL 81040030 Portable pack
battery 81040031, Portable pack battery charger 81040040 Portable pack
carrying strap 81040045 Support cover type 1 81040050 Support cover type 2
81040055 Portable pack soft carrying bag 88003001 C Compact flash card 128
MB, 88002013 Card reader.
Portable Pack
24W Xenon light source, 6.4 TFT monitor, integrated battery with life of 2.5 hrs,
1GB CF card as standard for image recording
4.7.2 Operational Procedure :- Each company specifies the operational procedure.
The operational procedure of KARL STORZ Borescope is given for information
as below.
(i) System startup
74
Insert video connector into the camera port of the camera control unit / the KARL
STORZ TECHNO PACK.
Insert light connector into the light tapping point of the cold light projector / the
KARL STORZ TECHNO PACK.
Turn on camera control unit / KARL STORZ TECHNO PACK, light source and
monitor (if a separate monitor is used).
(ii) System Shutoff
Turn off monitor (if a separate monitor is used), light source and camera control
unit / KARL STORZ TECHNO PACK.
Disconnect light connector from the light tapping point of the cold light projector /
the KARL STORZ TECHNO PACK.
Disconnect video connector from the camera port of the camera control unit / the
KARL STORZ TECHNO PACK.
(iii) White Balance
The white balance adjusts the colour reproduction of the system to match the
colour temperature of cold light projector used. The white balance remains in
memory after the base unit is turned off. When the system is started again , a
new white balance must be set whenever a different light source or a different
lamp or a different cable is used.
Point VIDEOSCOPE towards a white surface while the camera control unit is
turned on. Ensure that no objects with other colour can be seen in the image
section.
(iv) Press the white balance button ( blue function button on handpiece ) for a long
time. If the white balance is correct, the screen display is briefly inverted. If this
does not happen, the white balance was not completed correctly. Either too
much or too little light was received. In this case, increase or, as the case may
be, decrease the distance between the VIDEOSCOPE tip and the white surface.
(v)
(vi) The viewing of the VIDEOSCOPE may be adjusted by deflecting the movable tip
in four directions.
(vii)
By rotating the larger of the two hand wheels on the handpiece, the movable tip
of the VIDEOSCOPE is deflected downward or upward. The viewing direction
that was set either downward or upward can be locked in with a locking lever
between the large hand wheel and the handpiece.
(viii) By rotating the smaller of the left or right wheels on the handpiece, the movable
tip of the VIDEOSCOPE is deflected to the left or right. The viewing direction
75
that was set either to the left or right can be locked in with the rotary dial on the
small hand wheel.
4.7.3 List of suppliers :- The list is not exhaustive. Some of the suppliers are as below :1) M/s S.G. Marketing Pvt. Ltd., 15, Birbal Road, Jangpura xtn., New Delhi
110014
2) M/s NDT Technologies Pvt. Ltd., Plot No. 11, Sector 23, Turbhe, New Bombay
400705
3) M/s J.Mitra & Co. Pvt. Ltd., A 180-181, Okhla Industrial Area I, New Delhi
110020
4) M/s KARL STORZ Endoscopy India Pvt. Ltd., C-126, Okhla Industrial Area
Phase I, New Delhi 110020.
4.8
Nuclear Methods:
Neutron Moisture Gauges- These are used to measure moisture content in
concrete. They are based on the principle that hydrogen containing materials
(water) act as excellent moderators for fast neutrons, i.e. such materials produce
a rapid decrease in neutron energy, depending on amount of hydrogen. Thus,
counting of the slowed down neutrons gives a measure of the hydrogen content
of the concrete. Isotopic neutron sources (such as radium with beryllium) are
generally used in moisture gauges.
4.9
4.9.1
medium are important to convert a time domain radargram model into a distance
domain radargram model.
4.9.2.1 Operation Principles:
There are several antenna manufacturers, antenna types, signal pre- and postsetting options; operating frequencies software packages, etc. to consider for a
specific application within the engineering and construction industry, geological,
environmental and/or archaeological fields. Each radar system must be designed
to meet the objective(s) of a given project. For concrete evaluation studies, there
are several options available all of which have certain advantages and
disadvantages. For the evaluation of various concrete structures, which include
streets/highways, parking lots, bridge decks, pools, tilt wall panels, sidewalks,
various foundation systems and retaining walls, a versatile and highly portable
radar system with a ground coupled, monostatic antenna is suitable. However,
for specialized projects, such as road condition evaluation, an air launched
(horn) antenna is commonly used due to the efficient data collection,
characteristic of this antenna. Currently, GPR data can be collected with this airlaunched antenna at highway speeds.
A typical radar system for concrete evaluation studies generally consists of a
control unit (computer), pulse generator, transmitting and receiving antennae and
video monitor. A bistatic antenna describes a radar system with two antennae,
one to transmit and the other to receive. An antenna that both transmits and
receives is defined as a monostatic antenna. There are advantages and
disadvantages of each antenna type for a given application; however, for
concrete evaluation studies, monostatic antennae are typically more
advantageous due to higher data collection and processing efficiency.
GPR is a non-destructive technique that uses electromagnetic (EM) waves to
look into a material. GPR systems operate in a similar manner to sonar i.e. by
emitting a series of brief pulses and estimating distance to objects from the time
it takes to detect reflections. Figure 4.9.2.1 shows a schematic of a GPR system
in operation. Transmitting and receiving antennas are used to emit the EM pulse
and detect the reflections.
Reflections occur when the EM wave passes from one material into another
material with contrasting electrical properties. The strength of the reflection
depends on the electrical contrast between materials (i.e. a strong contrast
produces a strong reflection).
The GPR records the strength of reflections detected for a set duration after
each pulse. A plot of this data is called a trace. Figure 4.9.2.2 shows a typical
trace and illustrates how the EM reflections correspond to the material
boundaries.
79
Fig.4.9.2.3-Unprocessed radargran
In figure 4.9.2.3 the black line near the top is the near side of the girder and the
far side of the girder is around 9ns (somewhat difficult to see). The location of an
internal feature has been highlighted. The white squares near the top are
markers that were entered at metre intervals.
The depth to the feature can be estimated as follows:
D=Vr tr /2
Eqn.1(2)
80
Fig.4.9.2.4-Processed radargran
After signal processing it is now easier to see the far side of the girder, which
was previously obscured because of signal artifacts.
4.9.3 Performance and Other Considerations:
GPR surveys, signal processing and interpretation of the results should be
undertaken by personnel experience in using these techniques. Poorly
configured systems will lead to poor results.
Interpretation of the results is often an involved process. Successful
interpretation requires a good understanding of the underlying principles and
limitations of these techniques.
GPR is suitable for investigating relatively non-conductive materials such as
concrete, timber and road pavements. It is not suitable for investigating materials
with high conductivities such as metals, or materials with high moisture contents.
As GPR uses very weak non-ionising radiation there are no particular safety
issues for site personnel (i.e. similar but much weaker than a mobile phone)
4.9.4
Bridge Decks:
On bridge decks, various issues can be addressed with the help of GPR. Some
examples are asphalt pavement thickness, concrete cover of re-bar, position of
pre-stressing-tendons or tendon ducts and bridge deck deterioration.
Locate rebar, tension cables, conduits, voids, PVC pipes, and measure
slab thickness.
Locate a target depth of 18 inches and more in concrete.
Detect and map the relative concrete condition for rehab planning.
(B) SIR SYSTEM ANTENNAS
The structure Scan Standard system comes with a very high resolution
antenna model 5100 of Central frequency 1500 MHz that is specially
configured for access to small areas up to a depth range of 0 18.
Built for durability and reliability
Rugged, military-style connectors.
Coated, sealed electronics.
Shielded to eliminate above-ground interference.
All temperature conditions, 200C to 500C.
Low resistance, long-life replaceable wear skids.
Rugged, high-density molded cases.
Heavy duty cable.
Physical Properties:
Depth Range 0-0.5m (0-18in),
Dimensions 3.8 x 10 x 16.5 cm,
Weight
1.8 kg (4 lbs),
(C) SOFTWARE
(i) RADAN 6.0 Processing & Interpretation Software
It is Advanced Post Processing Software that works on Windows-XP and
Windows 2000 Professional Program, projects processing which includes
16 million colors, horizontal scaling, distance normalization, surface
normalization, static corrections, zero position adjustments, arithmetic
functions, range gain, gain restoration, vertical and spatial filters,
predictive deconvolution, 2D constant and variable velocity migration,
interactive interpretation and structure identification.
Now RADAN for Windows has applications specific add-on modules with
more new features and capabilities. Some of them are as below:
(ii) Software 3D QuickDraw mapping module
This add-on module features 3D presentations of data with simple
manipulations of the entire data cube so that it can be sliced and diced
along various x-y, y-z, or x-z planes.
83
This module also uses some of the variable velocity migration capabilities
that are featured in the RADAN main program to appropriately size point
targets that appear like hyperbolic shapes in the raw data.
By first performing a migration operation, the final 3-D data set is now
ready for linear feature recognition-a capability that enables the module to
assist in identification and display of linear features such as walls or
utilities that may be embedded in the earth.
New Features:
4.10
4.10.1
profile versus depth from ground surface without drilling a boring. Subsequently,
SASW tests are useful in determining depth of unknown foundations of more
massive abutments, piers and footing provided the substructure geometry allows
for proper access. Access for SASW test in term of unknown bridge foundations
means that the foundation is more massive and has an exposed fairly flat ledge
or top surface on which impacts are applied and a pair of receivers placed.
Lately, SASW testing has been adopted for offshore/underwater use.
4.10.2 Principle: This method is based on the principle that foundation sub-structure
materials have different stress wave velocities (stiffness) than the underline
supporting soil and bed blocks, which typically have slower velocities, i.e. they
are less stiff than foundation material. SASW method measures variation in
surface wave velocity with depth in layered material. The bottom depths of
exposed substructures or footings are indicated by slower velocities of surface
wave travel in underlying soils. The brief description of the method is given in the
figure below:
Applications:
The SASW tests has applications for unknown foundation depth determination
where flat, wide structure access is available for geometry determination of
abutment
wall thickness and exposed footings/pilecaps, for determining
substructure material properties v.s. depth, and for measurement of the variation
of stiffness (velocity) of soil and bedrock with depth. The equipment for SASW
method includes hammers from 1-lb to a 4-lb hand sledge to a 12-lb sledge
hammer (vibrators can also be used), a dynamic signals analyzer, and two
seismic accelerometers (or suitable geophones for greater depths and for testing
of soils).
4.10.5
**********
87
CHAPTER-5
NON-DESTRUCTIVE TESTING OF STEEL BRIDGES
5.1
Introduction:
Flaws and cracks can play havoc with the performance of structures and for
improving the performance, the timely detection of these defects is very much
necessary. Our present system of inspection of bridges mainly emphasize on the
visual inspection which does not give correct picture of internal structural defects.
By using non-destructive testing methods, the structures can be evaluated to a
greater degree of accuracy, without damaging them. These methods can be
used as quality control measures at the time of construction of structures as well
as tool for detection of defect during the service. The biggest advantage of NDT
methods is that these are quick and large no. of structure can be covered to
evaluate their service performance without causing any damage to the structure.
In this chapter, only the following methods which are commonly used for
evaluating the steel structures, are discussed
5.2
(a)
(b)
(c)
(d)
Radiography testing
(e)
Ultrasonic testing
(f)
(g)
basically two ways that a penetrant inspection process makes flaws more easily
seen. First, LPI produces a flaw indication that is much larger and easier for the
eye to detect than the flaw itself. Many flaws are so small or narrow that they are
undetectable by the unaided eye. Due to the physical features of the eye, there is
a threshold below which objects cannot be resolved. This threshold of visual
acuity is around 0.003 inch for a person with 20/20 vision.
The second way that LPI improves the detect ability of a flaw is that it produces a
flaw indication with a high level of contrast between the indication and the
background which also helps to make the indication more easily seen. When a
visible dye penetrant inspection is performed, the penetrant materials are
formulated using a bright red dye that provides for a high level of contrast
between the white developer that serves as a background as well as to pull the
trapped penetrant from the flaw. When a fluorescent penetrant inspection is
performed, the penetrant materials are formulated to glow brightly and to give off
light at a wavelength that the eye is most sensitive to under dim lighting
conditions.
89
Penetrant Dwell: The penetrant is left on the surface for sufficient time to
allow as much penetrant as possible to be drawn from or to seep into a
defect. Penetrant dwell time is the total time that the penetrant is in
contact with the part surface. Dwell times are usually recommended by the
penetrant producers or required by the specification being followed. The
times vary depending on the application, penetrant materials used, the
material, the form of the material being inspected, and the type of defect
being inspected. Minimum dwell times typically range from 5 to 60
minutes. Generally, there is no harm in using a longer penetrant dwell time
as long as the penetrant is not allowed to dry. The ideal dwell time is often
90
7.
91
8.
Clean Surface: The final step in the process is to thoroughly clean the
part surface to remove the developer from the parts that were found to be
acceptable.
Glass
Rubber
Plastics
Dye penetrant inspection is used to inspect for flaws that breaks the surface of
the sample. Some of these flaws are listed below:
Cracks
Porosity
Laps Seams
The method has few material limitations, i.e. metallic and nonmetallic,
magnetic and nonmagnetic, and conductive and nonconductive materials may
be inspected.
Indications are produced directly on the surface of the part and constitute a
visual representation of the flaw.
Primary Disadvantages:
Metal smearing from machining, grinding, and grit or vapor blasting must be
removed prior to LPI.
The inspector must have direct access to the surface being inspected.
spread easily over the surface of the material being inspected to provide
complete and even coverage.
93
remain in the defect but remove easily from the surface of the part.
remain fluid so it can be drawn back to the surface of the part through the
drying and developing steps.
Penetrant materials come in two basic types. These types are listed below:
94
Water washable (Method A) penetrants can be removed from the part by rinsing
with water alone. These penetrants contain some emulsifying agent (detergent)
that makes it possible to wash the penetrant from the part surface with water
alone. Water washable penetrants are sometimes referred to as self-emulsifying
systems. Post emulsifiable penetrants come in two varieties, lipophilic and
hydrophilic. In post emulsifiers, lipophilic systems (Method B), the penetrant is oil
soluble and interacts with the oil-based emulsifier to make removal possible.
Post emulsifiable, hydrophilic systems (Method D), use an emulsifier that is a
water soluble detergent which lifts the excess penetrant from the surface of the
part with a water wash. Solvent removable penetrants (Method C) require the
use of a solvent to remove the penetrant from the part.
Penetrants are then classified based on the strength or detectability of the
indication that is produced for a number of very small and tight fatigue cracks.
The five sensitivity levels are shown below:
5.2.6 Penetrants:
The industry and military specification that control the penetrant materials and
their use all stipulate certain physical properties of the penetrant materials that
must be met. Some of these requirements address the safe use of the materials,
such as toxicity, flash point, and corrosiveness, and other requirements address
storage and contamination issues. Still others delineate properties that are
thought to be primarily responsible for the performance or sensitivity of the
penetrants. The properties of penetrant materials that are controlled by AMS
2644 and MIL-I-25135E include flash point, surface wetting capability, viscosity,
contact angle, color, brightness, ultraviolet stability, thermal stability, water
tolerance, and removability.
How some of these properties can affect penetrant testing are described next.
Some properties of a penetrant Capillary Action:
Capillary action is the tendency of certain liquids to travel or climb when exposed
to small openings. In nature there are many examples of capillary action. Plants
and trees have a network similar to capillary tubes that draw water upward
supplying nourishment. The earth brings water to the surface through the
capillary action of the earth's exterior.
95
Surface Tension:
There are many factors in capillary action; among these are surface tension,
cohesion, wetting ability, adhesion and contact angle. Each of these factors has
a strong influence in the performance of capillary action. Of these, surface
tension is one of the two most important factors. Water in a pond exhibits surface
tension when it supports the weight of an insect - a spider or mosquito for
example. The insect is supported by a molecular membrane created by the
attraction (cohesiveness) of one water molecule to another. Each water molecule
is attracted laterally and vertically (above and below) to adjacent molecules. The
molecules on the surface are attracted only laterally and below because of the
absence of molecules above them. This change in attraction between surface
molecules creates the effect of a stretched membrane on the surface of the
water strong enough to support small objects. Water has high surface tension
because of the strong cohesive attraction between the molecules of water. The
amount of surface tension will vary between different liquids depending upon
how cohesive the molecules are.
Wetting Ability and Contact Angle:
The second most important factor in capillary action is wetting ability. How well a
liquid wets the surface of a specimen is referred to as its wetting ability. The
wetting ability of a liquid is determined by the contact angle produced when a
liquid meets a surface. The cohesive force that determines surface tension
competes with the adhesive properties of a liquid producing a specific degree of
contact angle.
Adhesion:
Adhesion is how strongly the molecules of a liquid are attracted to a particular
surface. If a capillary tube is placed in a beaker of water, the water will rise in the
tube to a level higher than the water surrounding the tube. The water climbs in
the tube because the molecules of water are more strongly attracted to the inside
surface of the tube than they are to each other. The stronger the attraction
between the molecules of a liquid and a surface, the smaller will be the contact
angle and the higher a liquid will rise in a capillary tube.
5.2.7 Developers:
The role of the developer is to pull the trapped penetrant material out of defects
and to spread the developer out on the surface of the part so it can be seen by
an inspector. The fine developer particles both reflect and refract the incident
ultraviolet light, allowing more of it to interact with the penetrant, causing more
efficient fluorescence. The developer also allows more light to be emitted
through the same mechanism. This is why indications are brighter than the
penetrant itself under UV light. Another function that some developers performs
is to create a white background so there is a greater degree of contrast between
the indication and the surrounding background.
97
98
Ranking
Developer Form
Method of Application
Spray
Plastic Film
Spray
water-soluble
Spray
Water Suspendible
Spray
water-soluble
Immersion
Water Suspendible
Immersion
Dry
Dry
Fluidized Bed
Dry
10
Dry
Immersion (Dip)
5.3
5.3.1 Principle:
When ferromagnetic material or component (weld) is magnetized, magnetic
discontinuities that lie in direction approx. perpendicular to the field direction, will
result in formation of a strong leakage field. This leakage field is present at and
above the surface of magnetized component and its presence can be visibly
detected by the cluster of finely divided magnetic particle i.e. when crack is met
to magnetic field direction it will form local magnet and will attract fine particles
along the crack when sprayed. Magnetization may be induced in the component
by using permanent magnet or electromagnet. For simple illustration, consider a
bar magnet. It has a magnetic field in and around the magnet. Any place, that a
magnetic line of force exits or enters the magnet, it called a pole. A pole where
magnetic line of force exits is called a north pole and where a line of force enters
the magnet is called a south pole (Fig 5.3.1)
99
(b)
Electromagnets These type of magnets are most widely used in the MPI
equipments. In this equipment, electrical current is used to produce the
magnetic field. An electromagnetic yoke is a very common piece of
equipment which is made by wrapping an electrical coil around a piece of
soft ferromagnetic steel. A switch is included in the electrical circuit so
that current as well as magnetic field can be turn on and off. This type of
magnet generates a very strong magnetic field in a local area where the
poles of magnet touch the part to be inspected.
(c)
Prods These are hand held electrodes that are pressed against the
surface of the component being inspected to make contact for passing
electrical current through steel. The current passing between the prods
creates a circular magnetic field around the prods that can be used for
magnetic particle inspection. Prods are made from copper and have an
insulated handle. One of the prods have a trigger switch so that the
current can be quickly and easily turned on and off. Sometimes the two
prods are connected by any insulator to facilitate one hand operation.
This is known as dual prod also and generally used for weld inspection.
(d)
All the above said equipments are portable equipments and can be used in the
field without any handling problems.
Following methodology to be followed while performing the test in the field
(1)
(2)
(3)
Dust on the light layer of magnetic particles with the magnetizing force still
applied, remove the excess powder from the surface with few gentle puf
of dry air.
If wet suspension is used, then the suspension is gently sprayed or flowed
over the surface to be tested. Immediately after the application of wet
suspension the magnetizing force should be applied.
(4)
After this the area should be inspected carefully for finding out the cluster
of particles. Surface discontinuities will produce a sharp indication.
finer particles will be more easily blown away by the wind and, therefore, windy
conditions can reduce the sensitivity of an inspection. Also, reclaiming the dry
particles is not recommended because the small particles are less likely to be
recaptured and the "once used" mix will result in less sensitive inspections.
The particle shape is also important. Long, slender particles tend to align
themselves along the lines of magnetic force. However, research has shown that
if dry powder consists only of long, slender particles, the application process
would be less than desirable. Elongated particles come from the dispenser in
clumps and lack the ability to flow freely and form the desired "cloud" of particles
floating on the component. Therefore, globular particles are added that are
shorter. The mix of globular and elongated particles results in a dry powder that
flows well and maintains good sensitivity. Most dry particle mixes have particle
with L/D ratios between one and two.
Wet Magnetic Particles:
Magnetic particles are also supplied in a
wet suspension such as water or oil. The
wet magnetic particle testing method is
generally more sensitive than the dry
because the suspension provides the
particles with more mobility and makes it
possible for smaller particles to be used
since dust and adherence to surface
contamination is reduced or eliminated. The
wet method also makes it easy to apply the
particles uniformly to a relatively large area.
Wet method magnetic particles products differ from dry powder products in a
number of ways. One way is that both visible and fluorescent particles are
available. Most nonfluorescent particles are ferromagnetic iron oxides, which are
either black or brown in colour. Fluorescent particles are coated with pigments
that fluoresce when exposed to ultraviolet light. Particles that fluoresce greenyellow are most common to take advantage of the peak colour sensitivity of the
eye but other fluorescent colours are also available. (For more information on the
colour sensitivity of the eye, see the penetrant inspection material.)
103
The particles used with the wet method are smaller in size than those used in the
dry method for the reasons mentioned above. The particles are typically 10 m
(0.0004 inch) and smaller and the synthetic iron oxides have particle diameters
around 0.1 m (0.000004 inch). This very small size is a result of the process
used to form the particles and is not particularly desirable, as the particles are
almost too fine to settle out of suspension. However, due to their slight residual
magnetism, the oxide particles are present mostly in clusters that settle out of
suspension much faster than the individual particles. This makes it possible to
see and measure the concentration of the particles for process control purposes.
Wet particles are also a mix of long slender and globular particles.
The carrier solutions can be water- or oil-based. Water-based carriers form
quicker indications, are generally less expensive, present little or no fire hazard,
give off no petrochemical fumes, and are easier to clean from the part. Waterbased solutions are usually formulated with a corrosion inhibitor to offer some
corrosion protection. However, oil-based carrier solutions offer superior corrosion
and hydrogen embrittlement protection to those materials that are prone to attack
by these mechanisms.
5.3.4 Suspension Liquids:
Suspension liquids used in the wet
magnetic particle inspection method
can be either a well refined light
petroleum distillate or water containing
additives. Petroleumbased liquids are
the most desirable carriers because
they provided good wetting of the
surface of metallic parts. However,
water-based carriers are used more
because of low cost, low fire hazard,
and the ability to form indications
quicker than solvent-based carriers.
Water-based carriers must contain
wetting agents to disrupt surface films of oil that may exist on the part and to aid
in the dispersion of magnetic particles in the carrier. The wetting agents create
foaming as the solution is moved about, so anti-foaming agents must be added.
Also, since water promotes corrosion in ferrous materials, corrosion inhibitors are
usually added as well.
Petroleum based carriers are primarily used in systems where maintaining the
proper particle concentration is a concern. The petroleum based carriers require
less maintenance because they evaporate at a slower rate than the water-based
carriers. Therefore, petroleum based carriers might be a better choice for a
system that only gets occasional use and adjusting the carrier volume with each
use is undesirable. Modern solvent carriers are specifically designed with
properties that have flash points above 200 degrees F and keep nocuous
vapours low. Petroleum carriers are required to meet certain specifications such
as AMS 2641.
104
105
Inspect for indications: Look for areas where the magnetic particles are
clustered.
Wet Suspension Inspection:
Wet suspension magnetic particle
inspection, or more commonly wet
magnetic
particle
inspection,
involves applying the particles
while they are suspended in a
liquid carrier. Wet magnetic
particle
inspection
is
most
commonly performed using a
stationary,
wet,
horizontal
inspection unit but suspensions
are also available in spray cans for
use with an electromagnetic yoke.
A wet inspection has several
advantages over a dry inspection.
First, all the surfaces of the
component can be quickly and
easily covered with a relatively uniform layer of particles. Second, the liquid
carrier provides mobility to the particles for an extended period of time, which
allows enough particles to float to small leakage fields to form a visible indication.
Therefore, wet inspection is considered best for detecting very small
discontinuities on smooth surfaces. On rough surfaces, however, the particles
(which are much smaller in wet suspensions) can settle in the surface valleys
and loose mobility rendering them less effective than dry powders under these
conditions.
Steps in performing an inspection using wet suspensions:
Prepare the part surface: Just as is required with dry particle inspections, the
surface should be relatively clean. The surface must be free of grease, oil and
other moisture that could prevent the suspension from wetting the surface and
preventing the particles from moving freely. A thin layer of paint, rust or scale will
reduce test sensitivity, but can sometimes be left in place with adequate results.
Specifications often allow up to 0.003 inch (0.076 mm) of a nonconductive
coating (such as paint) and 0.001 inch max (0.025 mm) of a ferromagnetic
coating (such as nickel) to be left on the surface. Any loose dirt, paint, rust or
scale must be removed.
Apply the suspension: The suspension is gently sprayed or flowed over the
surface of the part. Usually, the stream of suspension is diverted from the part
just before the magnetizing field is applied.
Apply the magnetizing force: The magnetizing force should be applied
immediately after applying the suspension of magnetic particles. When using a
106
wet horizontal inspection unit, the current is applied in two or three short busts
(1/2 second) which helps to improve particle mobility.
Inspect for indications: Look for areas where the magnetic particles are
clustered. Surface discontinuities will produce a sharp indication. The indications
from subsurface flaws will be less defined and loose definition as depth
increases.
5.4
5.4.1 Introduction:
The most basic eddy current testing instrument consists of an alternating current
source, a coil of wire connected to this source, and a voltmoter to measure the
voltage change across the coil. An ammeter could also be used to measure the
current change in the circuit instead of using the voltmeter.
Eddy current equipment can be used for a variety of applications such as
detection of cracks (discontinuity), measurement of metal thickness, detection of
metal thining due to corrosion and erosion, determination of coating thickness
and the measurement of electrical conductivity and magnetic permeability.
For inspection of bridge girder, this technique can be used for detection of
surface breaking cracks. This is an excellent method for detecting surface and
near surface defects when the probable defect location and orientation is well
known. Defects such as cracks are detected when they disrupt the path of eddy
current and weaken their strength.
5.4.2 Principal:
This is one of the several NDT methods that use the principle of
electromagnetism as the basis for conducting the test. Eddy currents are created
through a process called electromagnetic induction. When alternating current is
applied to the conductor, such as copper wire, a magnetic field develops in and
around the conductor. This magnetic field expands as the alternating current
rises to maximum and collapses as the current is reduced to zero. If another
electrical conductor is brought into the close proximity to this changing magnetic
field, current will be induced in this second conductor. Eddy current are induced
electrical currents that flow in a circular path. They get their names from eddies
that are formed when a liquid or gas flows in a circular path around obstacles.
For generating eddy current, a probe is used which consists of electrical
conductor formed into a coil and housed inside the probe. These probes are
available in a large variety shapes and sizes. In fact, one of the major
advantages of eddy current inspection is that probes can be custom designed for
a wide variety of applications. Eddy current probes are classified by the
configurations and mode of operation of the test coils. The configuration of
probes generally refers to the way the coil or coils are placed with reference to
test area. An example of different configuration of probes would be bobbin
107
probes, which are inserted into a piece of pipe to inspect from inside out. While
in encircling probes, the coil or coils encircle the pipe to inspect from outside in.
The mode of operation refers to the way the coil or coils are wired and interface
with the test equipment. The mode of operation of a probe generally falls into
one of four categories, absolute, differential, reflection and hybrid. Normally
differential probes are used for flaw detection in steel members.
Basic Performance.
(ii)
Inputs/ Outputs
(iii)
Additional Features
(iv)
General.
(i)
Basic Performance.
Frequency Range
100 Hz 12 MHz
Gain
0 90.0 dB
Sensitivity
Flaw Response
0 2000 Hz nominal
Digitizing Rate
Rotation
Sweep
Prove Drive
Null
Variable persistence
Probe Types
Scanner Drive
Alarms
Alarms Mode
Waterfall Display
Conductivity Accuracy
Program Storage
Serial Interface
Printout
Supported Printers
110
Inputs/ Outputs
Power
RS232C
Outputs
Probe Connectors
Additional Features
Report Fields
Conductivity
Frequency:
Probe Type:
Conductivity Probe.
Dual Frequency
General
Dimensions
Weight
(241mm
Display (LCD)
Power
Operating Time
5.4.4 Methodology:
While performing the inspection with surface probe following methodology should
be adopted:
(i)
Connect the probe with the system. Power Link screen will appear. Rotate
the smart knobs to confirm and press enter. The screen will display the
information about the probe. There are three different types of probes.
These are denoted as LOW, MID, HI. The approximate peak to peak
voltages for each are 2, 6 and 12 volts respectively. Mid probe drive is
normally sufficient for most eddy current testing. Now press the MAIN key
to proceed with test setup.
(ii)
Adjust the frequency as required. Press the FREQ menu soft key. Rotate
the smart knobs for the required frequency until it appears in the
frequency box.
(iii)
Adjust the phase angle as required. Press the ANGLE menu soft key on
the instrument. Rotate the smart knobs until the same appears in the
angle box.
(iv)
Adjust the horizontal gain and vertical gain as required. If horizontal gain
and vertical gain to be kept same than press GAIN from the menu. Rotate
the smart knobs key until both reaches to the desired value. If horizontal
gain and vertical gain to be kept different than press HGAIN menu soft
key and rotate the smart knobs to the required value. Similarly select the
VGAIN and adjust the required value.
(v)
Place the probe on the specimen to be tested away from cracks. Press
NULL and ERASE keys for positioning of the probe and to clean the
screen.
(vi)
113
(a)
(b)
Waterfall data is data captured from a waterfall (multiple sweep) display using a
PS-5AL scanner. It is the only data that can be modified after it is captured. All
methods of saving will record the current data and time of storage.
Scan the probe over part of the surface in a pattern that will provide complete
coverage of the area being inspected. Care must be taken to maintain the same
probe to surface orientation as probe wobble can affect interpretation of the
signal.
5.5
Radiographic Testing:
This is the technique of obtaining a shadow image of a solid using penetrating
radiation such as X-rays or gamma rays. These rays are used to produce a
shadow image of an object on film. Thus if X-ray or gamma ray source is placed
on one side of a specimen and a photographic film on the other side, an image is
obtained on the film which is in projection, with no details of depth within the
solid. Images recorded on the films are also known as radiographs.
The contrast in a radiograph is due to different degrees of absorption of X-rays in
the specimen and depends on variations in specimen thickness, different
chemical constituents, non-uniform densities, flaws, discontinuities, or to
scattering processes within the specimen.
Some of the other closely related methods are Tomography, Radioscopy,
Xerography etc.
114
5.5.1 Methodology:
First step of the method is to examine carefully the specimen and to decide on
the direction to examine the object considering the probable orientation of
defects and the thickness of the specimen in relation to the diverging beam of Xrays.
Considering the thickness of object, density of the material etc., the wavelength
of X-ray to be used should be decided.
The images can be observed on an image intensifying tube with remote viewing
or recorded on film with or without intensifying screens. Grid or blocking
materials should be used to reduce scattering effects. The optimum time of
exposure need to be determined by experimental trials.
The last but the most important step is the interpretation of radiograph.
Radiographs are projections, providing no information about depth within the
specimen. While interpreting following factors normally should be considered
(a)
(b)
(c)
(d)
(e)
(f)
The X-ray target to film distance should not be less than 10 x the
thickness of specimen.
(g)
5.6
Ultrasonic Inspection:
This method can be used for steel structures.
115
After that the bright spot continues to the right with a speed that can vary from
about 1/200 to 5 times the velocity of sound in steel. See Figure 5.6.3.1.
first to die out before a new pulse is send out. In most ultrasonic equipment it
means that a pulse can move backwards and forwards in a 10 m long steel bar,
before a new pulse is send out.
Figure 5.6.3.2 and Figure 5.6.3.3 show some common ultrasonic equipment.
Figure 5.6.3.3
119
5.6.4 Probes:
Normal probes:
A normal probe generates longitudinal waves, which leaves the probe at a right
angle to its contact surface. If the probe is in contact with a specimen, the sound
wave penetrate into it. It travels in straight lines, with a certain beam spread. See
Figure 5.6.4.1.
The near resolution can be increased considerably by using a probe with two
separate crystals one for transmission and one for receiving. Figure 5.6.4.3
shows the inside of a TR- probe.
Testing a plate can be done manually with a plate tester as shown on Figure
5.6.4.5 or in an automatic ultrasonic testing installation, where the plate is moved
past a row of probes for example 40-80 probes according to the width of the
plate. Each probe scans the plate along a line and the results are registered on a
paper slip.
5.7
5.8
CHAPTER-6
NON-DESTRUCTIVE TESTING OF MASONRY BRIDGES
6.1
Introduction:
As already explained in chapter 1, the use of NDT methods for testing of
masonry structures is not very common in India. However there is a necessity of
adopting suitable NDT methods for evaluation of masonry structures as existing
system of inspection is not sufficient to cover all the aspects of inspection.
Sometimes very critical defects like, deterioration of masonry materials, internal
cavities formed due to rat holes etc and cracking of structures due to
overstressing go unnoticed which may proved to be very fatal for the safety of
structure. The NDT methods have a large potential to be part of system for
inspection and monitoring of structures. This includes quality assurance during
and after construction, identification of damages in an early stage and to decide
the repair strategy for rehabilitation of the structures. Some of the NDT methods
which are used for evaluation and inspection of masonry structures are listed
below
6.2
(a)
(b)
(c)
(d)
Infrared thermography
(e)
Boroscope
the flat jack calibration constant, providing a measure of the in situ masonry
compressive stress.
The other test is, in situ deformability test which is used for direct measurement
of masonry deformability properties and to estimate the masonry compressive
strength. For conducting this test, two parallel flat jacks are used which subject
the masonry between them to compressive stress. The stress strain curve which
is obtained during the test, is used for obtaining both compressive modulus and
an estimate of compressive strength.
6.3
6.4
6.5
Infrared Thermography:
Details are given at S. No. 4.2.
6.6
Boroscope:
Details are given at S. No. 4.7.
**********
124
CHAPTER 7
BRIEF OF NDT EQUIPMENTS AVAILABLE AND THEIR USE
S. N.
Measurement
Application
Equipment
2.0
2.1
Surface strength
(rebound number)
2.2
Homogeneity
Concrete
2.3
Combined
ultrasonic and
rebound number
determination
Uniformity/homogeneity,
Location of internal defects
Ultrasonic Pulse
velocity tester
2.4
Pull-off strength
(bond strength
2.5
of Quality of Concrete
Rebound Hammer
Ultrasonic
velocity meter
pulse
2.7
Penetration
resistance
Windsor Probe
2.8
Core strength
(Micro core)
2.9
Permeability test
2.10
Bond test
2.11
Maturity method
2.12
Complete
Structural
technique
125
Maturity meters.
3.0
3.2
3.3
Resistivity
Resistivity Meter
3.4
Carbonation depth
3.5
Chloride content
3.6
Voids
Corrosion
3.7
3.8
4.0
4.1
Length changes
Strain measurement
Measurement and
digital strain gauges
4.2
Cracks, delamination
4.3
4.4
4.5
Stress
wave Based
on
stress
wave Stress
propagation
propagation, used for non propagation
method
destructive testing of conc.
equipment.
(a) Pulse
method
(b) Impact
method
Echo -do-
(c) Impact
method
Endoscopy
126
&
radar
wave
4.6
Crack width
measurement
Microscopes, crack
width gauges for walls
and angles
4.7
Endoscope
Examination
4.8
Nuclear method
moisture
4.9
Structural
Scanning
equipment
It
is
complete
concrete Ground
inspection system. It is also radar
used
for
inspection
of
foundation
penetrating
4.10
5.0
5.2
Dye-penetrating
method
5.3
5.4
Eddy Current
5.5
Radiographic
testing
5.6
5.7
Complete
Structural
technique
5.8
6.0
6.2
Stress modulus of
deformation
of SASW equipment
in
the Magnetic
particle
inspection equipments
&
6.3
Impact
method
6.4
6.5
Cracks, delamination
6.6
Endoscope
Examination
Borescope
**********
128
wave
radar