Artigo Nondestructive test methods for concrete bridges A review rehman 2016

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Construction and Building Materials 107 (2016) 58–86
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Construction and Building Materials
journal homepage: www.elsevier .com/locate /conbui ldmat
Review
Nondestructive test methods for concrete bridges: A review
http://dx.doi.org/10.1016/j.conbuildmat.2015.12.011
0950-0618/� 2015 Elsevier Ltd. All rights reserved.
Abbreviations: ACI, American Concrete Institute; AE, acoustic emission; ASTM, American Standards for Testing Materials; EM, electromagnetic; ER, electrical re
GPR, ground penetrating radar; IE, impact echo; IR, impulse response; NDE, Non-destructive evaluation; NDT, Non-destructive test; PCC, plain cement conc
reinforced concrete; RCP, Rapid Chloride Ion Penetration; SAFT, Synthetic Aperture Focusing Technique; SHM, structural health monitoring; UPV, ultrasonic pulse
VBM, vibration-based monitoring; VBDD, Vibration Based Damage Detection.
⇑ Corresponding authors.
E-mail addresses: kashif@engineersdaily.com (S. Kashif Ur Rehman), zainah@um.edu.my (Z. Ibrahim), shazimalimemon@gmail.com (S.A. Memon), jameel@um
(M. Jameel).
Sardar Kashif Ur Rehman a,⇑, Zainah Ibrahim a,⇑, Shazim Ali Memon b,⇑, Mohammed Jameel a
aDepartment of Civil Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
bDepartment of Civil Engineering, COMSATS Institute of Information Technology, Pakistan
h i g h l i g h t s
� NDT techniques applicable to concrete bridges are reviewed.
� Different damage levels, NDT methods along with remedial measures are proposed.
� NDT methods are suggested to address specific problems related to structures.
� Importance of structural health monitoring for concrete bridges is highlighted.
a r t i c l e i n f o
Article history:
Received 21 May 2015
Received in revised form 21 October 2015
Accepted 5 December 2015
Available online 6 January 2016
Keywords:
NDT methods
Structural health monitoring
Damage levels
Remedial measures
Concrete bridges
a b s t r a c t
NDT methods applicable to concrete bridges are reviewed. The methodology, advantages and disadvan-
tages along with up to date research on NDT methods are presented. Different damage levels, having less
dependence on inspector judgment, are suggested. Moreover, a flow chart based on damage level along
with NDT methods and potential remedial measures are proposed for periodic health monitoring of
structures. NDT methods are also suggested to address specific problems related to structures. Finally,
the relation between some of the well-known NDT methods and most common problems encountered
by the field engineers is proposed. Hence, the importance of structural health monitoring is highlighted.
� 2015 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
2. Nondestructive Test (NDT) methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
2.1. Audio–Visual methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
2.1.1. Visual inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
2.1.2. Chain drag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
2.1.3. Coin tap test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
2.2. Stress-wave methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
2.2.1. Acoustic emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
2.2.2. Impact echo testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
2.2.3. Sonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.2.4. Ultrasonic NDT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
2.2.5. Impulse response (IR). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
sistivity;
rete; RC,
velocity;
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S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 59
2.3. Electro-magnetic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
2.3.1. Ground penetrating radar (GPR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
2.3.2. Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
2.3.3. Half-cell potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
2.3.4. Electrical resistivity measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
2.4. Deterministic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
2.4.1. Proof load test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
2.4.2. Coring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
2.5. Miscellaneous tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
2.5.1. Dynamic/vibration testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
2.5.2. Infrared thermography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .et al. / Construction and Building Materials 107 (2016) 58–86
roughly costs around 6% of bridge replacement [106,110]. Applica-
tion of proof load test on a bridge in early 60s is represented in
Fig. 34.
Saraf and Nowak [109], carried out proof load test on three sim-
ply supported steel girder bridges. These selected bridges varied in
span length from 11.0 to 15.5 m and aged more than 60 years. Two
military tanks were used to load the bridges as shown in Fig. 35.
Weighed of each tank was 530 kN and these tanks were used to
increase the mid-span bending moment in different steps. Values
of Stress and deflection were measured on the steel girder at
selected points. The authors found that these measured stresses
and deflections were less as compared to the analytical calcula-
tions. The composite action between steel girders and concrete
slab was observed even at maximum load level. It was noted that
nonstructural members contributed to the flexural stiffness and
for all bridges bearings provided partial restraint to rotation at
supports.
2.4.2. Coring
Cores are considered as semi-destructive method and are used
to determine the internal defects in structures. ASTM C 42/C
42M-04 [112], provide the standard test method for obtaining
and testing the drilled cores. Cores are extracted to measure the
compressive, splitting tensile and flexural strength of in-situ con-
crete. These are used to check the quality of construction and mea-
sure the strength of old structures [10,25,112,113]. Strength of
concrete is affected by the location of the cores, amount and distri-
bution of moisture and orientation of cores [112]. Core drill and
saw is used for the drilling of cores from specimen. Usually, cores
are extracted from concrete structures particularly from highways
and bridges after performing preliminary NDT survey. Scott et al.
[10], used 10 cores samples from different locations marked by
preliminary survey for detail investigations. The cores extracted
from different locations were thoroughly inspected for the cracks,
delamination, deterioration, seepage and corrosion (Fig. 36). After
the experiment, the core places are filled up by grouting or any
other method. It is worth mentioning here that the data obtained
from cores is reliable and can be used for further investigation
and selection of repair and maintenance work. This method is
applied extensively in highways, bridge-deck and concrete struc-
tures. Core is a destructive and expensive method. However, its
results are used with NDT to achieve the high level of confidence.
Suzuki et al. [114] obtained the cylindrical core samples from
the walls of canals (more than 40 years old), in Hokkaido prefec-
ture, Japan so as to determine the mechanical properties of the
concrete. In cores freeze–thaw damage was heavily observed.
Compression test showed that samples effected by freeze–thaw
have 3.87 times lower strength as compared with non-cracked
samples. The cracked in cores were also inspected by X-ray com-
puted tomography method and AE. Moreover, the damage of con-
crete structures was evaluated quantitatively by AE and X-ray
method.
McIntyre and Scanlon [113], proposed the relationship between
core-cylinder specimens. Cross-section of core and cylindrical
specimens are given in Fig. 37. Due to the destructive nature of col-
lecting samples, core strength was found to be less than the cylin-
der strength. Some of the energy used to cut and remove the
sample invariably cause micro-cracks, bond failure or weakening
between cement matrix to aggregate surface and major cracks in
core. Authors proposed that overall average core-cylinder strength
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 77
ratio was 0.923. The major problemwith the core test is that it pro-
vide information about small region of total volume of concrete
from where it is obtained.
2.5. Miscellaneous tests
2.5.1. Dynamic/vibration testing
Vibration testing is most commonly used for measuring the
structural health of structures. In late 19th century, vibration/dy-
namic field tests were applied to different concrete structures
[115]. Various researches used this method on bridges to assess
their condition and structural health [115–124]. In dynamic test,
the structure is subjected to excitation forces and its response is
measured by means of transducers [125]. Natural frequency, mode
shapes and modal damping values are measured during testing
and are known as modal parameters [125]. The values of these
modal parameters change whenever a structure deteriorates. After
inspecting these parameters it is easier to find out the structural
health and condition of the structure [125]. It is worth mentioning
here that the changes in these parameter mainly depend upon the
location, type and severity of defects and flaws [125]. Two types of
methods (ambient and forced vibration) are used for vibration test-
ing. In ambient vibration testing method, there is no control on the
input or applied force [122] while in forced vibration testing
method, the input/applied forces is controlled.
Vibration-based monitoring (VBM) of civil infrastructure is
explored by Brownjohn et al. [126]. VBM is a subcategory of struc-
tural health monitoring plan and emphasis on the dynamic part of
performance [126]. It provides enough data for the structural
investigation and leads to the another study area which is a sub-
group of VBM and known as Vibration Based Damage Detection
(VBDD) [126]. VBDD methods are not only limited to determining
the natural frequencies of the structures, but can also be used for
diagnosing the change in support conditions/bearing, degradation
of the material properties and contribute in probabilistic post-
earthquake assessment using modal parameters [126]. This
method is relatively cheap and easy and the measurement points
can be selected in accordance to test situation.
A model calibration method was proposed by Robert-Nicoud
et al. [127]. This method is based on the structural behavior such
as material characteristics and support conditions. A methodology
for dynamic based assessment, using ambient vibration testing,
was applied to reinforced concrete arch bridge by Gentile [123].
Output-only modal identification and updating different uncertain
parameters of the computer models [123].
In [128], Brencich and Sabia used dynamic tests on a masonry
bridge to detect the natural frequencies and mode shapes. The
Fig. 38. Free vibration t
compressive tests were also conducted for the comparative charac-
terization of the brickwork. Dynamic characteristics were deter-
mined by using natural and forced vibration loading. These
loadings were used to find out stiffness and to some extent esti-
mate the structural damage and some types of global deterioration.
The effects of harmonically forced vibration tests on two-span
post-tensioned reinforced concrete bridge was investigated by
Bedon and Morassi [119]. This bridge was located in northern Italy
which is prone to high seismicity. The Frequency response function
(FRF) was determined by using low levels of excitation with series
of harmonic forced-vibration applied in the form of stepped-sine
technique. Thereafter, the acceleration response of the bridge
was measured and the acceleration-time histories were used to
determine the FRF of the bridge.
In recent past, Cunha, et al. reviewed the latest features of
vibration/dynamic testing [120]. According to the authors, forced
vibration has a disadvantage especially when it is applied to long
span bridges. The problem was related to exciting the important
and significant vibration modes in a controlled way specifically
in a low range of frequencies with adequate energy. However,
the latest advancement in technology make it possible to measure
precisely low levels of dynamic response produced by the ambient
excitations like traffic or wind. The ambient or free vibration tests
are the most real and effective method used for determining the
modal parameters[120]. Moreover, this method can be applied
during the construction and rehabilitation stage [120]. Fig. 38
shows the application of free vibration test on the bridge. The
non-contact measurement techniques such as radar or laser pro-
vided ease and efficiency in application of dynamic testing [120].
2.5.2. Infrared thermography
Infrared thermography, a global inspection method, allows
large surface area to be inspected in a short span of time. Concrete
surfaces absorb the solar energy in day and release it in night
[129,130]. Therefore, Infrared thermography is used for detecting
the delaminations and anomalies in concrete surface [6,7,25,131–
134]. The three main properties that influence the heat flow and
distribution within a material include the thermal conductivity,
specific heat capacity (Cp), and the density (q). The electromagnetic
spectrum range used to measure the Infrared thermography is
shown in Fig. 39. Infrared thermography for testing concrete is
based on two heat transfer mechanism, radiation and conduction
[7]. In the first mechanism, a surface emits energy (electromag-
netic radiation) and the rate of energy emitted by the surface per
unit area can easily be calculated by Stefan–Boltzmann law. Since
infrared radiations are not visible to human eye, therefore, special
sensors are required to detect these radiations. After calibration,
est at bridge [120].
Fig. 39. The electromagnetic spectrum [131].
Fig. 40. Infrared image looking down a span, camera is set to include all
temperatures in the frame [132]. Fig. 42. Schematic of radiographic method [7].
78 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
the output can be converted to temperature [7]. In the second
mechanism, the surface temperature is measured under condition
of heat flow. Since heat flow through concrete is affected by sub-
surface anomalies, therefore, it can be used to detect subsurface
anomalies [7]. It should be noted that delaminated areas heat up
faster and cool down more quickly when compared with concrete.
Moreover, they can develop surface temperatures from 1 �C to 3 �C
higher than the surrounding areas when ambient conditions are
favorable [25].
Typical Infrared thermography equipment consists of a scanner,
data acquisition system and a visual recorder. The scanner trans-
mits infrared radiation, the data acquisition system records and
analyzes the data while a visual recorder such as digital video cam-
era is used to record the scanned data.
This method has been used to locate delamination in reinforced
concrete pavements and bridges decks. The advantage of applying
Infrared thermography is that it is safe, efficient, cost effective in
terms of time, non-invasive and non-contact. The inspected mate-
rials can have distances ranging from a few millimeters to several
kilometers, if provided with suitable lens [130]. Some of the
limitations of this method include [7] (1) the equipment is very
Fig. 41. Schematic of direct transmission nuclear gage [7].
expensive, (2) it cannot give the depth of the defects because it
takes the image of a surface, (3) the testing is influenced by envi-
ronmental conditions such wind speed, solar radiation, moisture
content and surface emissivity, hence corrections are required for
these parameters [133], (4) trained operator and analyst is
required to capture data and interpret it correctly. Therefore, it is
suggested to use this method with ground penetrating radar
[133]. Moreover, combination of the two methods (Infrared ther-
mography and impulse radar) is widely used in civil engineering
industry and applied to highway bridges, asphalt pavement, sewer
system, canals, aqueducts and airport pavement for various find-
ings [132,133]. Infrared image of a span of a bridge is shown in
Fig. 40.
Infrared thermography in general can be divided into two major
categories: active and passive. It is based on excitation methods for
observing the temperature difference. Usually, external stimula-
tions are not required for passive thermography while different
types of stimulus are employed in active thermography. The funda-
mental concept of Infrared thermography for Non-destructive test-
ing is that abnormal temperature profiles indicate potential defect.
Difference between temperature at a suspected area and a refer-
ence area indicates abnormal behavior. In general, passive ther-
mography is rather qualitative because it is simply used to
pinpoint anomalies. On the other hand, active thermography
requires some energy to be brought to the target in order to obtain
significant temperature differences for indication of subsurface
anomalies [135].
According to Stanley and Balendran [134], thermography is the
best method used by many researchers for detecting the seepage
sources and heat loss calculation from the buildings. The scanning
system was much sensitive and can identify the temperature dif-
ferences of up to 0.1 �C. For normal scanning of buildings, the max-
imum recommended distance is 50–100 m. Authors enlightened
that infrared thermographic method not only identify the existing
defects and problems in the structure but also provide the advance
Table 1
Capabilities and limitations of Non-destructive test methods.
Test name Description Applications/capabilities Limitations
Impact echo Seismic or stress wave
based method
(1) Detect defects in concrete
(2) Detect delamination in PCC and RC element
(3) Detect wave reflectors
(4) Surface opening cracks Characterization
(5) Ducts detection
(6) Voids detection
(7) Concrete modulus of elasticity evaluation
(8) Concrete compressive strength estimation
(9) Grouting characterizations
(10) Detect overlay debonding
(1) Asphalt concrete overlay detectable only with low temperature of asphalt concrete
(2) Detection is possible for highly viscous material
(3) Loosely bonded overlay to the deck is detectable only
(4) Dense grids are required for marking the boundaries of delimited area
(5) Deck boundaries greatly affect the signals specially near the edges
(6) Interference of boundary on signal is more prominent on limited dimensional ele-
ments like girders and piers
Ultrasonic pulse
velocity
Ultrasonic (acoustic) stress
wave based method
(1) Measure depth of elements
(2) Assess faults in concrete
(3) Detect debonding of steel bars
(4) Detect shallow cracking
(5) Detect delamination
(6) Detect material interface, specifically interfaces between concrete
and air (e.g. grouting defects) and concrete and steel (reinforce-
ment) [60,62,66,140]
(7) Detect grouting defects
(1) Requires very close grid spacing
(2) Time consuming method
(3) Data quality strongly depends upon the coupling of the sensor unit
(4) Difficult on rough surface to couple the sensor
(5) Shallow defects may remain undetected
(6) Some defects might remain undetected due to use of lower frequencies
Impulse response Also known as transient
dynamic or mechanical
impedance method,
extension of vibration test
(1) Detect cracks
(2) Detect honeycombing or low density concrete
(3) Detect voids
(4) Detect delamination in concrete
(5) Determine loss of support below rigid pavement
(6) Determine load transfer at joints of concrete pavements
(7) Detect debonding of overlays
(8) Quality control and condition assessment of pavements and deep
foundations
(9) Determine subgrade modulus
(10) Used as screening tool for potential damage detection
(1) Reliable data interpretation is highly dependent on the selection of test points
(2) Smaller defects may remain undetected
(3) Automated apparatus is not available
Acoustic emission Transient elastic waves (1) Real-time damage detection
(2) Remote monitoring
(3) Applicable for both local and global monitoring
(4) Continuous monitoring without interrupting the traffic flow
(5) Reliable analysis
(1) Background noise problem
(2) Difficult to apply analysis to real structure
(3) No standard procedure for all types of bridges
Ground
penetrating
radar
Use electromagnetic waves (1) Ableto determine the buried objects
(2) Produce contour maps of subsurface features
(3) Used for condition assessment of civil structures specially related to
soil
(4) Detect voids and anomalies
(5) Evaluate thickness of members
(6) Measure thickness of concrete cover
(7) Determine rebar configuration
(8) Detect delamination and its potential
(9) Detect concrete deterioration
(10) Sensitive to corrosive environment
(11) Estimate concrete properties
(1) Inability to direct image
(2) Unable to detect delamination in bridge decks, unless they are epoxy-impregnated
or filled with water
(3) Extreme cold conditions effects the GPR data
(4) Completely frozen moisture will remain undetected
(5) Frozen moisture influence the acquired signal
(6) Deicing salts affect the dielectric constant
(7) Unable to estimate mechanical properties of concrete like strength, modulus
(8) Unable to provide absolute information about the presence of corrosion, corrosion
rate or rebar section loss
(9) Expensive method as compared to other ones
Half-cell potential Electrochemical method (1) Determine active corrosion
(2) Can be used at any time
(3) Can be used in any climatic condition
(1) Minimum temperature must be higher than 2 �C [96]
(2) Influence of concrete cover depth is still unknown
(3) Correcting data for depth is difficult
(4) It is not used for moisture or salt content calculation
(5) Cannot take reading on the isolating layer (asphalt, coating and paint)
(6) Measurements are often unreliable (concrete is wet, dense or polymer modified)
(continued on next page)
S.K
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80 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
warning of deterioration or pre-empt potential problem areas. It
also provide a historic map of varying trends in structure. Com-
puter based condition monitoring gives benefits to be readily
achieved. They recommended that thermography is the best
known for its application in detecting heat loss from buildings. Fur-
thermore, in future, this field has potential for scanning the exte-
rior cladding of buildings.
Clark et al. [132], used Infrared thermography to identify areas
delamination in a concrete bridge. The authors showed that even
with the low ambient temperatures experienced in Europe, it is
possible to use Infrared thermography to correctly identify known
areas of delamination in a concrete bridge structure. The Infrared
image of a span of a bridge investigated by the authors [132] is
shown in Fig. 40.
2.5.3. Radiography
Radiography is a nuclear NDT method in which high energy
electromagnetic radiation passing through the test object/member
is recorded on a photographic film placed on the other side of the
member [7]. This method can be effectively used to detect rein-
forcement location, voids in concrete, material heterogeneity and
layers of different materials, honeycombing and voids in grouting
of post-tensioning ducts [6,7,136–139]. However, the test person-
nel require specialized safety training and licensing. The radiation
can be produced by X-ray tube (X-radiography) [136] or radioac-
tive isotope [137] (gamma radiography). Schematic illustration is
given in Figs. 41 and 42. Radioactive isotope generate radiations
which pass through the concrete and sensed by the detector. The
detector converts these radiations into the electrical pulses, which
are further analyzed or counted by other techniques [136]. The
selection of radiation source and amount of energy absorbed by
the material depends on the thickness and the density of the mem-
ber as well as the exposure time that can be tolerated [7]. On the
radiation based photograph, high density materials (reinforce-
ment) appear as a light area while low density regions (voids)
appear as a dark area [7].
According to Song and Saraswathy [138], radiography is a reli-
able method for locating internal cracks, variations in density of
material and voids in material. Moreover, Ibrahim [139], evaluated
the application of different NDT methods on the composite struc-
tures. It was demonstrated that the number of voids having diam-
eter in between 20 and 50 mm can be detectable by X-ray imaging
while Neutron methods is suitable for inspection of moisture in
thick structures.
Suzuki et al. [114], used X-ray computed tomography method
for damage evaluation in a concrete canal. Concrete cores were
taken from the existing canal and the distributions of cracks in
these cores were inspected with helical computerized tomography
(CT) scans. The Output images were visualized in gray scale where
dark portion represents the air while white segment shows the
dense part. The samples were scanned constantly at 0.5 mm pitch
overlapping and depending on the length of each specimen a total
of 200–400 two dimensional images were obtained. The values of
the absorption coefficients obtained from the CT scanning system
were transformed into CT numbers using the international Houns-
field scale. The authors found that the CT numbers varied accord-
ing to the material properties. Variance of the CT number
increases with the increase in damage. Finally, crack properties
were quantified using X-ray CT data.
2.6. Capabilities and limitations
The capabilities and limitations of some of the well-known and
most important NDT methods are summarized in Table 1.
Fig. 43. Steps for NDT method selection.
Table 2
Identification of damage levels based onperiodic inspection [11].
Damage
level
Deficiency
level
Description
0 None Minor deterioration to any elements
1 Marginal Minor deterioration to primary supports
elements
2 Moderate Moderate deterioration to some of bridge
element
3 Severe Serious deterioration to some of structural
element
4 Severe Serious deterioration to primary structural
elements
5 Failed Some portion collapse/demolished
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 81
3. Planning and selection of NDT method
In the previous section, we highlighted the basic working prin-
ciple, advantages & disadvantages of various NDT methods used for
the evaluation of civil infrastructures. Depending on cost, available
Table 3
Detail description of damage levels.
Level Inspection Crack lengths Spalling Support sett
0 Periodic None/1 lm None 100 lm
1 Periodic Partial/100 lm None 1 mm
2 Detailed Some/61 mm (NDT
limit) [143]
Partial bulge outward 50 mm
3 Special Fair amount/610 mm Partial spalling 95 mm (Rea
4 Special Large amount/
6100 mm
Fair amount of bulge
and spalling
200 mm (ma
allowable) [
5 Special Large/61 m Large amount of bulge
and spalling
P200 mm
time, reliability and simplicity, some methods are preferred over
others. Ideally, it is required to adopt a method which can detect
all important defects and anomalies present in the structure.
Therefore, the selection of NDT method must be based on the
yielding of the information.
In construction industry, most of the field engineers and spe-
cialists are dissatisfied by use of NDT methods due to using inap-
propriate method for a specific problem. Therefore, it is
recommended to do preliminary study before selecting any NDT
method. In literature, this study is normally referred as desk study
[6]. Different steps of desk study are given in Fig. 43.
According to the requirements of the Standards for Technical
Condition Evaluation of Highway Bridges [141], every bridge needs
to be inspected at least once every 24 months. The inspection is
done to monitor the evolution or possible deterioration of the
bridge’s working condition and to allow timely decision for repair,
rehabilitation, and/or emergency treatment such as strengthening
or closure [142]. Periodic inspection of the structure is the first
step for continuous health monitoring. Uemoto [11], identified
the damage levels based on periodic inspection (Table 2). The
author provided the scale of deterioration (0–5) and explained
the evaluation procedure by using visual aids. All the parameters
were elaborated in general terms and for simplicity the parame-
ters were limited to cracks, stains and spalling. Hence, the judg-
ment was based purely on the inspector understanding, which
may lead to wrong selection of method. Therefore, in this research,
the description of damages levels is provided in terms of number-
ing system based on severity of defects to the bridge members. The
measurement units are used which will provide comparatively
better information and less dependence on the inspector judg-
ment. Elaboration of each damage level based on crack lengths,
spalling of concrete cover, support settlement, titling of foundation
(due to settlement of subsoil or erosion of soil) and reinforcement
of corrosion is given in Table 3. This will provide an ease to the
field engineers for better understanding of the damage levels.
For example, crack lengths, support settlements and tilting of sup-
ports are measured and based on quantitative reading; they are
classified under certain damage level. However, further research
is required so as to correlate the source of error with the damage
level. For example, the source of crack propagation is important.
The behavior of cracks due to temperature and shrinkage would
be different when compared to shear and flexural cracks. Some
other important features such as the inspection type and potential
remedial action against each proposed damage level are included.
The inspection types include periodic, detailed and special inspec-
tion which is carried out to request for categorize the condition of
structure. Potential remedial measures such as routine preserva-
tion work to maintenance, repair and replacement of damaged
members and addition of new members are included to cater for
damage.
lement Tilting of
foundation
Reinforcement
corrosion
Potential remedial action
0� None Routine preservation
activities
0.05� Some stains on
surface
Minor maintenance
activities
0.1� (Realized)
[121]
Partially stains Major maintenance
activities
lized) 0.3� Fair amount of
stains
Priority to remediate and
repair
ximum
121]
0.5� (max
allowable)
Fair amount of
rusts
Rehabilitation and
replacement options
P0.5� Large amount of
rusts
Strengthen & upgrading
Fig. 44. Schematic layout of SHM procedure, NDT method selection and potential remedial measure.
Table 4
Selection of nondestructive method based upon damage criteria modified from Uemoto [11].
Measurement parameter Non-destructive test method Remarks
Delamination Impact echo, ultrasonic pulse echo, chain drag and hammer sounding, ultra sonic surface
waves, GPR, Infrared thermography, coin tap test, impulse response
In PCC, RCC and pre-stress concrete
Dimension measurement Laser transit measure Exposed dimension
Thickness/depth GPR, impact echo, ultra-sonic, radar Embedded dimension
Concrete cover GPR, electro-magnetic, radar, X-ray
Spacing between
reinforcement
Electromagnetic, GPR, radar, impact-echo, X-ray Top bars only/surface reinforcement
Diameter of bars Electro-magnetic method, GPR Top bars only
Stiffness Dynamic testing, oscillation test Frequency and amplitude
Deterioration Acoustic emission, photograph, GPR, Infrared thermography, visual inspection Stain, cracks
Surface defects Thermography, digital still camera, impulse response, Infrared thermography, Coin tap test Cold joints Including honeycomb
Inside defects X-ray, sonic, ultra-sonic, impact echo, thermograph, impulse response, GPR, radar Voids inside and at back of the
structure
Micro deformation Digital gauge, strain gauge
Large deformation Laser transit measure
Vibration Laser deformation measurement, LVDT, Acceleration sensor
Stress Optical sensor, mold gauge
Concrete compressive
strength
Rebound hammer test, Pull-out test, core sample test, impact echo General method, accuracy issues
Elastic modulus Impact echo, ultra sonic surface waves, core sample test, ultra-sonic velocity
Cracking Acoustic emission, impact echo, ultrasonic pulse echo, impulse response, electrical
resistivity, Infrared thermography, Coin tap test
Continuous measurement required
Crack depth Ultra-sonic pulse echo, ultrasonic surface waves Effects of bars
Crack width Thermography, Coin tap test, digital still camera Direct measurement
Crack distribution Thermography, digital still camera
Permeability Electrical resistivity, on-site permeability test
Corrosion location GPR, half-cell potential, Galvanostic Pulse measurement, electrical resistivity, natural
potential
In situ and real time location
Corrosion degree Electrical resistivity, electric current analysis, Galvanostic Pulse measurement, natural
potential
Periodic measurement required
Debonding detection Impact echo, ultrasonic pulse echo, impulse response, Infrared thermography, coin tap test
Grouting characteristics Impact echo, ultrasonic pulse echo
Material layers/interface Ultrasonic pulse echo, Infrared thermography
Chloride, carbonation, acids Core sample test, electrical resistivity Analysis by core sample
82 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
Fig. 45. Selection of NDT based on different defects and damages.
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 83
In this research, we have proposed a flow chart for periodic
inspection of concrete bridges (Fig. 44). The health of structures
is monitored and classified in terms of damage level. After classify-
ing the damage level, several suitable NDTmethods are suggested
for detailed investigation of bridge. Since maintenance activities
are the routine interventions which are applied to the structure
to preserve the structural performance and health. Therefore, sub-
sequently corrective measures are also proposed to recover the lost
strength. Repair activities involve restoration of the existing dam-
aged members to improve the condition, while rehabilitation and
replacement activities involve the replacement of damaged mem-
bers and focus on the root cause of the problem. Strengthening and
upgrading activities involve the modification in existing structural
members to improve and enhance the structural performance.
Most of the time field engineers are familiar with the problem
related to concrete structures specifically bridges. However, they
require precise NDT method which addresses their problem. In
Table 4, NDT methods are suggested according to different issues
related to structures. It will provide a guideline for field engineers
to select NDT method.
The selection of NDT method based on the defect type is also
proposed in this paper. The relation between some of the well-
known NDT methods and most common problems encountered
by the field engineers is illustrated in Fig. 45. Normally, periodical
inspection is done by using visual aids or by simple NDT method
(s). Currently, the most prominent on-site methods for detecting
disbonds, defects, impact damages, cracks, fiber breakage and
misalignment are visual inspection and tap testing [144]. Visual
inspection is the first step for deciding the health of a structure
and selecting NDT method for further examination. AE method is
also used for periodic inspection. In comparison to visual inspec-
tion, it is obviously advantageous as it can provide the global aswell
as local monitoring of the structures. Moreover, it provides the real
time monitoring of the structure. After the periodic inspection,
detailed investigation is performed based on the defect type. Each
defect typemention here can be further categorized and elaborated
into different groups. We would like to mention here that these are
the most common type of damages which are required to be inves-
tigated in almost all types of civil structures. This will provide an
ease to the field engineers in selection of an NDT method. Coring,
which is considered as semi-destructive method, can be used only
for validating the results of other methods. Once the detail study
is completed, the maintenance work can be started.
The selected method must be able to include all those problem-
atic areas which cannot be examined by using traditional methods.
The data recovered from the method must be reproducible and
should serve as reference for carrying out investigation in future.
In should also be kept in mind that the selected methods are con-
strained by allocated time and budget of the project. It is worth
mentioning here that the experience of the investigator is most
84 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
critical for executing any testing scheme in a successful manner.
The testing scheme should be designed such that it provides the
optimum level of knowledge about the health of structure. The
investigator must also be familiar with the construction technique,
damage mechanism and structural behavior besides knowing the
capabilities and limitation of selected NDT method. The knowledge
about the construction technique will help in anticipating about
the exact location of possible anomaly in concrete. Moreover,
information about damage mechanism will guide the investigator
to choose the parameters that are required to be measured. Finally,
the structural behavior will provide valuable information about
selecting most vulnerable member for the existence of defects.
4. Conclusions and recommendations
In this paper, various NDT methods which are applicable espe-
cially to concrete bridges are reviewed. The methodology, advan-
tages and disadvantages along with the up to date research on
NDT methods are presented. Following are some of the important
conclusions.
� For those structures which are inaccessible, dynamic testing,
infrared and radar technology are the better choice. Moreover,
global and local monitoring of structures in real time make AE
a preferred method with compared to other NDT methods.
� For long span bridges, it is very difficult to inspect visually all
the portions or elements of bridge, especially soffit of bridge.
Proof load test is especially good for stone and masonry struc-
tures, whose drawings and design parameters are unknown.
However, in reinforced concrete structures their structural
details can be found by using electromagnetic method and cor-
ing. The Coin tap, chain drag and hammer tests are inexpensive
in nature but these tests are depended on human factor, which
make these tests less reliable. The investigators can use these
methods for periodic inspection as they will give some better
understanding about structure.
� The sonic and ultra-sonic tests are more reliable and have more
capabilities to address maximum problems if coupling issue is
properly resolved. GPR, Impact echo and Infrared thermography
also provide reliable analysis of structure. If the shortcomings of
these methods (GRP, Impact echo and IR) are addressed than
these methods can provide the deterministic results. Electric
based tests can be performed rapidly at any time and in any cir-
cumstance, but these tests provide information related to rein-
forcement only. Moreover, electric based tests are not helpful
for brick masonry and stone structures. Radiation based tests
provide good information about voids and cracks but X-ray,
gamma rays and neutron rays are hazardous for the operator
and community. Therefore, great care is required during these
tests. Dynamic and vibration tests are reliable and extensively
used in civil industry. However, these tests are unable to pick
the problem related to elastomeric bearing pads. If NDT tests
are used in combinations then it provides detailed and reliable
information e.g. using chain drag in combination with GPR test
or with sonic/ultrasonic tests.
� Different damage levels based on crack lengths, spalling of con-
crete cover, support settlement, titling of foundation (due to
settlement of subsoil or erosion of soil) and corrosion of rein-
forcement are suggested, which will limit dependence on the
inspector judgement. A flow chart based on damage level along
with NDT methods and potential remedial measures are pro-
posed for periodic health monitoring of structures. Most of
the time field engineers are familiar with the problem related
to concrete structures specifically bridges. However, they
require precise NDT method which addresses their problem.
Therefore, NDT methods are also suggested according to
different issues related to structures. Finally, the relation
between some of the well-known NDT methods and most com-
mon problems encountered by the field engineers is proposed.
This will provide an ease to the field engineers to select the
NDT method. Hence, this research tries to overcome the misper-
ception about the NDT methods and encourage the field engi-
neers an ease to use NDT as integral part of testing.
� Some of the NDT methods are developed for some specific
industries, like radiography for the medical industry, GPR for
the geologist and geo-physics, Electrical based methods for
solving the electrical problems. Hence, there is a strong need
to introduce a new discipline which bridges the gap between
civil engineering and these technologies.
Acknowledgements
This research was supported by University Malaya Research
Grant (UMRG – Project No. RP004A/13AET), University Malaya
Postgraduate Research Fund (PPP – Project No. PG217–2014B)
and Fundamental Research Grant Scheme, Ministry of Education,
Malaysia (FRGS – Project No. FP028/2013A). Authors would like
to thanks to all those who gave written permission to use the
figures.
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2.5.3. Radiography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
2.6. Capabilities and limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
3. Planning and selection of NDT method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4. Conclusions and recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
1. Introduction
Maintaining safe and reliable civil infrastructures for daily use
is important for the well-being of mankind. Operation and mainte-
nance have become more complex with the increased age of the
structures. The process of determining and tracking structural
integrity and assessing the nature of damage in a structure is often
referred as health monitoring [1].
In history a lot of work has been done regarding infrastructure
monitoring, inspection, repair and design code specifications. For
example, in United States of America in every two years almost
600,000 bridges are inspected and depending upon their condition
they are scaled [2]. Federal Highway Administration defined the
scaling system (0–9) for the rating of the structures, in which 9
stands for newly constructed bridge while 0 for failed bridge.
According to this rating, more than 40% of the national bridges
are structurally deficient or not functioning [2]. Moreover, the
average life of a bridge in USA is 42 years while bridges are
designed and constructed for at least 50 years [3]. Therefore, there
is a great need for the health monitoring and maintenance of the
concrete bridges [2].
Concrete degradation, steel corrosion, change in boundary con-
ditions, and weakening of connections in structures over time are
major concerns in highway bridges. If a damaged bridge remains
unattended, the structural integrity and service capability of the
bridge would deteriorate over time [4]. Therefore, frequent condi-
tion assessment and health monitoring of the highway bridges is
required. Health monitoring of structures specially bridges can be
classified into two broad categories i.e. global health monitoring
and local health monitoring. Global health monitoring is the tech-
nique in which only the occurrence of damage is detected, while in
local health monitoring, the extend, location and severity of dam-
age is identified [1]. Both global and local health monitoring tech-
niques are necessary and important for safe and sound operation of
structures.
Non-destructive test (NDT) and nondestructive evaluation
(NDE) offer skills to engineers and owners to speedily and effec-
tively examine and monitor aging structures. These methods are
used to detect the damage and used for local health monitoring
of structures [1]. Moreover, NDT can prevent the unpredictable
and premature collapse of structures. Various researchers have
provided the guidelines and application of these methods for the
evaluation of structures [5–13]. In this paper, we have reviewed
various NDT methods which are applicable especially to concrete
bridges. The methodology, advantages and disadvantages along
with the up to date research on NDT methods are presented in
Section 2. For better understanding, capabilities and limitation of
the well-known NDT methods is presented in tabular form. The
planning and selection of NDT methods is discussed in Section 3.
Here, we have proposed different damage levels based on crack
lengths, spalling of concrete cover, support settlement, titling of
foundation (due to settlement of subsoil or erosion of soil) and cor-
rosion of reinforcement along with inspection type are suggested.
Thesemeasurement units will provide relatively better information
and less dependence on the inspector judgement. As field engineers
and specialist are dissatisfied by NDT methods, a flow chart based
on damage level along with NDT methods and potential remedial
measures are proposed for periodic healthmonitoring of structures.
In this section, NDT methods are also suggested to address specific
problems related to structures. Moreover, the relation between
some of thewell-knownNDTmethods andmost common problems
encountered by the field engineers is proposed. Finally, in Section 4,
the conclusions and recommendations are covered.
2. Nondestructive Test (NDT) methods
In civil engineering field, engineers and specialists mainly used
NDT methods for providing the counter check information related
to structures rather than as an integral part of testing procedures.
The main reason for this is the lack of awareness about testing pro-
cedures, equipment handling and data collection using NDT [6].
McCann and Forde [6], presented variety of NDT methods which
are appropriate for civil structures. According to ACI 228.2R-98
[7], NDT methods are used in construction industry mainly due
to following reasons.
� Quality control in new construction.
� Troubleshooting of problems.
� Condition assessment of existing structures.
� Quality assurance of repair works.
Furthermore, NDT methods are selected (a) If direct physical
measurement is not possible or too expensive like monitoring of
soffit of bridge on the river or sea. (b) When it is required to spread
a limited physical analysis [6].
Non-destructive tests are classified on the basis of test sources,
nature or methodology of tests and purpose of test. However, in
this review paper, NDT methods are classified on the basis of test
sources. The techniques which are based on the Audio–Visual char-
acteristics of the operator are grouped in Audio–Visual methods
while those techniques which use the stress waves for detection
of damages and material properties are placed in stress-wave
methods. The techniques which use electrical or electro-magnetic
signals for material analysis are placed in the category of
Fig. 1. Concrete deterioration between spans (a) seepage, (b) efflorescence, (c) concrete cover delamination, (d) widen cracks [18].
Fig. 2. Description of the bridge structure in the inventory module, by Gattulli and Chiaramonte [16].
60 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
electro-magnetic methods while the techniques which give defi-
nite results are placed under heading of deterministic methods.
Finally, in order to make this review paper more comprehensive,
some other test methods like vibration method, Infrared method
and Radiographic methods are also discussed.
2.1. Audio–Visual methods
2.1.1. Visual inspection
Visual inspection is one of the versatile and powerful NDT
methods for inspecting visible surfaces. However, its effectiveness
depends on the experience and knowledge of the investigator
especially with structural behavior, materials and construction
methods [7]. Visual inspection is most extensively used for moni-
toring the damages in concrete structures [1,7,14–18]. According
to Perenchio [19], visual inspection is one of the first steps in eval-
uating concrete structures. It is a quick method for identifying the
apparent and superficial problems. This method is rapid & inex-
pensive but it never offers detailed and quantitative information
about the interior defects. Ithttp://refhub.elsevier.com/S0950-0618(15)30690-5/h0710
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	Nondestructive test methods for concrete bridges: A review
	1 Introduction
	2 Nondestructive Test (NDT) methods
	2.1 Audio–Visual methods
	2.1.1 Visual inspection
	2.1.2 Chain drag
	2.1.3 Coin tap test
	2.2 Stress-wave methods
	2.2.1 Acoustic emission
	2.2.2 Impact echo testing
	2.2.3 Sonics
	2.2.4 Ultrasonic NDT
	2.2.5 Impulse response (IR)
	2.3 Electro-magnetic methods
	2.3.1 Ground penetrating radar (GPR)
	2.3.2 Conductivity
	2.3.3 Half-cell potential
	2.3.4 Electrical resistivity measurement
	2.4 Deterministic methods
	2.4.1 Proof␣load test
	2.4.2 Coring
	2.5 Miscellaneous tests
	2.5.1 Dynamic/vibration testing
	2.5.2 Infrared thermography
	2.5.3 Radiography
	2.6 Capabilities and limitations
	3 Planning and selection of NDT method
	4 Conclusions and recommendations
	Acknowledgements
	Referencesis normally carried out to determine
cracking, seepage, spalling, exposed reinforcement, staining, mois-
ture ingress, beam delamination, concrete deterioration and rein-
forcement corrosion [17]. Hand held magnifier, stereo
microscope, fiberscopes and borescopes are some of the tools that
Fig. 3. Chain drag test [25].
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 61
can be used for visual inspection [7]. Some of the defects observed
by Gokhan [18] using this technique during the inspection of con-
crete bridge are illustrated in Fig. 1.
The main disadvantage of visual inspection is that it detects the
cracks, deterioration and damage only when it begins to affect the
life of the structure or in some cases it has badly affected the inter-
nal layers of the structure while only minor cracks appear on the
surface [20]. This is especially true in case of long span bridges
where it is very difficult and almost impossible to visually inspect
the complete bridge. It was reported by Dubin and Yanev [21] that
the biennial visual inspection of Brooklyn Bridge in New York cost
around 1 Million USD [2]. According to one study conducted by
Federal Highway Administration (USA), at least 56% of the average
condition ratings of concrete bridges were found incorrect and
most probably 95% information was taken from the visual inspec-
tion [2]. It is due to the reason that inspection of all the bridge ele-
ments particularly soffit of the bridge on the stream or river is very
difficult. Its effectiveness, however, to a large extent is governed by
the investigator’s experience and knowledge. We would like to
mention here that visual inspection is typically one aspect of the
total evaluation plan. Therefore, it is suggested that this method
should be combined with other NDT methods so as to obtain real
condition of the structure.
Gattulli and Chiaramonte [16], used visual inspection for condi-
tion assessment of a bridge management system for Italian rail-
way. Their approach for condition assessment was applicable to
concrete, steel and masonry bridges. Deficiency levels and associ-
ated maintenance and repair priority was also provided in the
study. They proposed four different simulation models (bridge
inventory, computer aided visual inspection, automated defect cat-
alog and priority-ranking procedure) to predict the rating index. An
automated bridge inventory was proposed (Fig. 2) which offered
complete data necessary for the bridge characterization and
assessment.
Fig. 4. Coin tap test for checking the debonding and delamination in concrete and
steel.
2.1.2. Chain drag
Chain drag test is performed according to the guidelines given
in ASTM D 4580-86 [22]. This test is used to mark the delaminated
areas on the surface of the deck [10,22–25]. Initially a grid system
is laid on the bridge deck. Thereafter, chains are dragged on the
surface of the deck and all those areas where a dull or hollow
sound is heard by the operator are marked. Finally, with the help
of grid lines, a map is prepared for locating these delaminated
areas. Frequently, one operator is enough for chain dragging and
defining the boundaries of delaminated areas. In Fig. 3, it can be
seen that one person is performing the chain drag test on the deck
where grid lines are also visible. However, efficiency and reliability
of this method is increased by using two persons [10]. One person
initially drags the chain and hear the dull sound over the deck
while the second person further clarify the boundaries of the
defected areas with the help of rock hammer and record the find-
ings. A clear ringing sound represents a sound deck while a muted
or hollow sound represents a delaminated deck. The hollow sound,
which is a result of flexural oscillations in the delaminated section
of the deck, creates a drum like effect. Its frequency is typically in
between 1–3 kHz range and is within the audible range of a human
ear. The presence of any delamination changes the frequency of
oscillation and, therefore, the audible response of the deck [25].
From above discussion, it is clear that this method depends upon
the human factor which makes it less reliable. However, investiga-
tors can use this technique for periodic inspection in conjunction
with other methods. Moreover, this method can be used for com-
parison of data with other NDT methods.
Barnes et al. [23], used the chain drag method in combination
with half-cell method and compared the results with GPR method.
They investigated the defects by applying chain drag and half-cell
method on bridge deck. The bridge deck consisted of two-lanes and
was constructed by using reinforced concrete on steel girders.
Fig. 5. (a) Force–time histories of impacts on good and disbonded areas of CFRP
skinned honeycomb structure; (b) spectra of time histories [27].
62 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
Delaminations were marked by using chain drag method in geo-
metric shapes/figures which provided the estimate regarding the
actual delaminated areas and were often used as the boundaries
for repair and maintenance area. The authors suggested removing
the cover material during repair and maintenance work, especially
for the exposed corroded steel bars, which lie outside of the bound-
aries marked by chain drag survey. The difference between chain
drag based delaminated area and GPR based delaminated area
was 2.5 m2 or 0.4% with a standard deviation of 1.3% of the deck
surface area. Chain drag results and GPR results were much similar
and very close to each other using thresholds developed by the
authors.
2.1.3. Coin tap test
Coin tap test is the simplest form of Impulse echo method [6]. It
is amongst the oldest methods of NDT and frequently used for
crack and damage detection [6,26–29]. Specifically, it is used for
laminated structures and bonded joints [27]. In the past, this
method was widely used in UK for detecting defects/discontinu-
ities during tunnel linings [6]. In this method, wall or lining or
bridge portion is strike with coin or lightweight hammer and echo
or ringing is heard. Whenever, the impulse hammer hits the cavity
or defaulted portion of structure, a significant change in the fre-
quency is noticed. It is a rapid method and can be used anywhere
easily, because human ears are very sensitive to detect change in
the frequency of sound. Moreover, different types of equipment
can also be used to detect the difference in frequency and make
this technique convenient to use anywhere. Fig. 4 shows the appli-
cation of coin tap test in steel and concrete structures.
The physical basis of coin-tap test was investigated by Cawley
and Adams [27]. It was observed that during the tap the presence
of defect such as delamination beneath the surface of the structure
changes the force characteristics. As defect becomes larger, the
duration of impact increases and the peak force reduces. It was also
observed from the force–time history spectra that as the defect
become larger, the rate of reduction in force amplitude with fre-
quency increases. Fig. 5(a) shows measured force–time histories
from taps on good and disbonded regions of a honeycomb struc-
ture. The impact over the good area is more intense and of shorter
duration than that on the damaged area. In Fig. 5(b), the amplitude
of the force input to the damaged area falls off rapidly with
increasing frequency, while the impact on the sound area has a
much lower rate of decrease of force with frequency. The authors
showed that the defects like delaminations and disbonds can be
modeled as spring where the stiffness of spring would be same
as of layer(s) above the defect. The authors concluded that this
method is more reliable and provide satisfactory results on thin
structures. In this method no coupling fluid is required whichmake
this technique more rapid then ultrasonic testing techniques.
In Cawley [26] research, a high frequency version of coin tap
test by tapping the structure(aluminum plates with flat-bottom
holes) with a lighter coin (below 1 g) was investigated. The lighter
Fig. 6. Principle of acoustic emission [36].
coin was used to enhance the frequency range of excitation. Test
results showed that high frequency coin tap is more capable to
observe the cracks and voids in greater depth as compared to con-
ventional coin. This method when tested on aluminum plates with
flat-bottom holes showed that high frequency coin tap can detect
50 mm diameter defect located at 7 mm depth, while the same
defect with 5 mm depth was not detectable by conventional coin
tap test. However, the results of this low mass striker were not
good on honeycomb specimen with fiber reinforced plastic skin.
According to the authors, it may be due to reduction in excitation
frequency range by the low out of plane stiffness of the fiber rein-
force plastic. It was recommended that the method may be effi-
cient on structures made with metals or unreinforced plastic.
Wu and Siegel [28], used hammer to apply force on structure
and measured its value by accelerometer and the resulting sound
with the help of microphone. From the experimental results, it
was found that both accelerometer and microphones have valid
role as instruments for automating defect and damage detection.
However, according to the authors, ‘‘it is hard to say in any univer-
sal sense whether forces-only or sound-only methods are more
useful coin-tap methods”.
The traditional coin tap test is low cost but it is operator depen-
dent. Therefore, in Georgeson et al. [29] research work, an illustra-
tion of low cost tap hammer developed by Boeing was given. This
low cost tap hammer provides quantitative measure about impulse
time of hammer/composite which can further be linked with
delaminations in structure. The instrumented tap hammer pro-
vides numeric value that can easily be linked with the quality of
local parts and thus removes discrimination of workers. Moreover,
the effect of operator differences and background noise on investi-
gated data can be eliminated.
2.2. Stress-wave methods
2.2.1. Acoustic emission
Acoustic emission (AE) test is most extensively used for damage
detection in several fields in general and transportation sector in
particular [9,30–36]. In 1939, Hopwood [30] for the first time
applied the concept of AE testing to a bridge for structural health
monitoring [9]. Carter and Holford [31] and Holford and Lark
[33] reviewed the application of AE in bridge monitoring. AE test-
ing is used for three major areas of damage i.e. source location,
source identification and severity assessment [32]. The automated
source location capability of AE make this technique more distin-
guished in NDT [32,34]. This test is based on transient elastic
waves. These elastic waves are produced from the structure under
observation by rapid release of energy and the transducers are
used to collect these waves. Thus, two integral components are
necessary for AE test. The first is the material deformation which
produces the elastic waves and becomes the source while the sec-
ond is transducers that receive these elastic stress waves.
The schematic layout of the AE testing which represents the
overall operational principle of an AE monitoring system is shown
in Fig. 6. Whenever a crack, damage or flaw develop within the
interior layers of structure, it release burst of energy [36]. This
energy (primary and secondary emissions) is mostly in the form
of high frequency sound waves [36]. The primary emissions are
those which are generated from the under observation structure
while the secondary emissions are those which are produced from
some external material or source. These waves travel within the
object and are received by transducers and sensors [36]. The travel
time of these waves is very important. If both transmitter and
transducer are placed on the opposite sides than the shortest travel
time would indicate the sound concrete while the longer travel
time is an indication of inferior quality of the concrete. The longer
travel time may be due to the reason that the signal got diffracted
Fig. 7. Wireless sensing of large structures using radio frequency transmission scheme [39].
Fig. 8. Different types of AE Sensors, Pacndt.com [36].
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 63
along the edge of the large void. Moreover, no arrival at transducer
may indicate that signal got reflected through the air interference
in void [7].
There are numerous qualitative as well as quantitative ways to
interpret these signal parameters or waveforms. For example,
parametric analysis of the AE signal resulted in evaluation criteria
such as: (i) A concrete beam integrity (CBI) ratio, defined as the
ratio of the load at the onset of new AE in a subsequent load cycle
to the maximum prior load [37] and (ii) calm and load ratios of
reinforced concrete beams; where the calm ratio is the cumulative
AE activities ratio during the unloading process to the maximum of
the last loading cycle and the load ratio is the ratio between the
load at onset of AE to the prior load [38], etc.
AE testing provides the real time examination of the material
characteristics when subjected to under load/stress [36]. The major
advantage of AE is that it can be used for both global and localized
monitoring of damage portion. This method can be used for contin-
uously monitoring and can detect the activity from considerable
distance [35]. The wireless sensing of large structures using radio
frequency transmission scheme given by Grosse et al. [39] is
Fig. 9. Schematic of impact-echo method [7].
shown in Fig. 7. Traffic flow remains undisturbed during perfor-
mance of this test and it also provide a reliable analyzes [36].
The major disadvantage of AE is that no standard procedure is
available which can be applicable to all types of the structures
and specifically bridges [36]. This method requires experienced
personnel and several trial sessions so as to eliminate the real-
time noise from the gathered data. It is also problematic to apply
the quantitative AE analyzes on actual structures [36]. Another dis-
advantage of local AE technique is that it is limited to 8 AE chan-
nels [35].
Acoustic emission monitoring plan is mainly categorized into
two types i.e. global and local monitoring [31]. In global monitor-
ing, the entire integrity of complete structure is assessed while in
local monitoring, a particular zone of damage is examined. The glo-
bal AE monitoring involves connection of sensors in such a way
that the major portion of the structure or entire structure can
easily be investigated. Fig. 8 shows different types of sensors used
in AE. Load can be applied from regular traffic or by heavy truck to
stimulate the flaws. The global AE monitoring can be used for peri-
odically evaluation of the structures. The main benefit of global AE
monitoring is that it examines the complete structure and if inves-
tigations are required for some specific area than it can be further
done by using local AE monitoring [35]. The continuous monitoring
will enable the investigators to constantly assess the health of any
old or newly constructed structure. Moreover, there is a strong
possibility that, in future, visual inspection can be replaced by glo-
bal AE monitoring [35]. In local AE monitoring, small or local areas
of the structure are concentrated. The dimension of local area may
vary from few centimeters to meters [35]. Loading is normally
applied by normal traffic for release of energy.
The application of AE for locating damages in steel–concrete
composite bridges was investigated by Holford et al. [32]. The
source location of damage was based on measurement of time dif-
ference between arrivals of individual AE events at different sen-
sors in an array. The source location in global monitoring
detected certain suspected regions in the complete structure.
Thereafter, local monitoring was used to locate the defects and
supply informationabout the emitted waveforms. The authors
concluded that the method gives realistic information about the
concerned region.
2.2.2. Impact echo testing
Impact echo (IE), is a stress-wave method that can be used for
flaw detection and determining the thickness of structural member
such as bridge decks and slabs. In early 1970s, impact methods
were used for evaluation of concrete piles and drilled shaft founda-
tions [40] while, in 1980s, it was developed for testing of concrete
members such as assessing the quality of bond in overlays and
Fig. 10. Devices used for the IE method (a) stepper, (b) bridge deck scanner [25].
Fig. 11. Void detection using DAI-1 impact-echo system at a concrete bridge [53].
Fig. 12. Results of void detection along a scanning line in a concrete bridge [53].
64 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
determining the depth of crack [41–52]. Schematic working princi-
pal of impact-echo method is shown in Fig. 9. Initially a two
dimension grid system is drawn on structure and IE is carried over
the specified location. Scott et al. [10], used a spacing of 2 ft
between the grid lines. An impactor is used to produce the tran-
sient pulse in the structure by applying impact loading (Fig. 10).
Depending on the materials characteristics, the stress pulse infil-
trate into the structure at a specific speed and produces waves
(P, S and surface waves) that are reflected by internal interfaces/
defects or external boundaries. Transducers are used to receive
these reflected waves from any discontinuity present in concrete.
Fast Fourier Transform (FFT) analysis is then used to transform
time domain signal into frequency domain signal. The location of
the damage portion, crack or discontinuity and depth of member
can be found by using simple formula [10].
T ¼ Cp=2f
where T is location of discontinuity or point from where waves
reflect, f is the wave frequency and Cp is the velocity of the compres-
sional propagating wave.
Different authors used various ways for interpreting the sever-
ity of delamination in bridge deck with IE method [25]. For exam-
ple, a delaminated point in the deck will theoretically demonstrate
a shift in the return frequency toward higher values because the
Fig. 13. Experimental and testing schemes [54].
Fig. 14. Transmission modes for sonic wave tests: (a) direct; (b) semi-direct; (c)
indirect [6].
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 65
wave reflections occur at shallower depths. Moreover, depending
on the extent and continuity of the delamination, the partitioning
of the wave energy reflected from the bottom of the deck and the
delamination may vary. Hence, the initial or incipient delamina-
tion, described as occasional separation within the depth of the
slab, can be identified through the presence of return frequencies
associated with the reflections from both the bottom of the deck
and the delamination. It is worthy to mention here that progressed
delamination is characterized by a single peak at a frequency cor-
responding to the depth of the delamination. Finally, in cases of
wide or shallow delaminations, the dominant response of the deck
to an impact is characterized by a low frequency response of
flexural-mode oscillation. This response is almost always in the
audible frequency range, unlike responses from the deck with
incipient delamination that may exist only in the higher frequency
ranges [25,51]. Impact echo method can be applied to plate like
structure. Plate like structures are those in which transverse direc-
tion of structure is at least five times the thickness of structure.
Bridge decks, slabs and walls are the examples of plate-like struc-
tures [10].
A new concept for impact echo testing was presented by Grosse
et al. [53]. They showed that IE has great ability to detect more
accurately large voids, inhomogeneity and the thickness of con-
crete structures (Figs. 11 and 12). According to the authors, this
method effectively detects cracks and reliability of this method is
increasing with the passage of time.
Azari et al. [54], investigated the sensitivity of Impact Echo
method to different parameters like slab thickness and defects in
slab and found that it is very sensitive to slab dimensions
(Fig. 13). The size of slab panel was found to greatly affect the
energy reflected from lateral boundaries. They suggested that these
reflected waves may interfere with outgoing signal and can impact
IE frequency response and measurement of thickness. For a fixed
size of slab, as slab thickness increases less energy was reflected
back from bottom boundary as compared to lateral boundary.
The authors concluded that the combination of IE with ultra-
sonic wave method is more effective in detecting and locating
the defects within concrete slab.
Fig. 15. Initial assumed density of sonic ray paths [57].
66 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
2.2.3. Sonics
Non-destructive sonic test was abundantly used in past for
assessing and determining the civil structures and materials
[6,7,55–58]. This method is based on the stress waves. Mechani-
cally, stress waves are transmitted into the structure and when
intervene by any discontinuity; these waves reflect and collected
by the receivers. Transmission modes for sonic wave tests include
(a) direct, (b) semi-direct, (c) indirect [6]. The different modes of
transmission are illustrated in Fig. 14.
In direct transmission method compressional waves of frequen-
cies ranging from 500 Hz to 10 kHz penetrate through the struc-
ture [6]. Waves are transmitted from one side of structure and
received at opposite site. Upgraded form of direct transmission is
semi-indirect transmission mode [6]. Semi-indirect transmission
method, also known as sonic tomography [6], is used on the paths
which are not transverse to wall surfaces of structure. The effi-
ciency of results is increased by adopting a dense grid system for
data recording (Fig. 15) [57]. Colla et al. [57], used the recorded tra-
vel time data of sonic signal waves to draw 3-D velocity distribu-
tion across the entire structure. McCann and Forde [6] plotted
velocity magnitude into the format of contour map [6]. Grid points
were drawn on x–y axes and velocity coordinates on z-axis. This
format provided information about the relative conditions of inter-
nal fabrics of structure [6]. Moreover, 3-D velocity distribution
map helped in diagnosing the local alterations in velocity which
can be further linked with the different areas of weakness and
flaws [6]. In indirect method, also known as sonic reflection
method or sonic-echo method, the signal generator and receiver
are placed on the same face of structure. Recorded stress waves
(direct stress waves) are echoed from the inner discontinuities or
from rear face. This method is not recommended due to poor res-
olutions attained from the low frequency energy [6]. Moreover, it is
difficult to discriminate between reflected waves and surface
waves [6]. Sonic methods can be utilized for evaluation of material
uniformity, void detection, depth of surface crack, internal proper-
ties of fill material or structure, average compressive strength,
internal dimensions and shape of the structure [6]. As an example,
if the length of the pile shaft is known and the transmission time
for the stress wave to return to the transducer is measured, then
its velocity can be calculated. Conversely, if the velocity is known,
then the length can be calculated. Since the velocity of the stress
wave is primarily a function of the dynamic elastic modulus and
density of the concrete. Therefore, the calculated velocity can pro-
vide information regarding the concrete quality [7].
When the stress wave has traveled the full length of the shaft,
these calculations are based on the following formula
Cb ¼ 2L=Dt
where Cb = bar wave speed in concrete; L = shaft length; Dt = transit
time of stress wave.
Empiricaldata show that a typical range of values for Cb can be
assumed, where 3800–4000 m/s would indicate good-quality con-
crete, having a compressive strength on the order of 30–35 MPa
[59]. For the case where the length of the shaft is known, an early
arrival of the reflected wave would mean that it has encountered a
reflector (change in stiffness or density) other than the toe of the
shaft.
Colla et al. [57], evaluated and compared the performance of
sonic and ultrasonic tests on rubble masonry walls. The authors
found that ultrasonic frequencies have high depletion rate due to
presence of voids, joints and homogeneities. Therefore, they sug-
gested that sonic test should be preferred for rubble masonry
walls. Berra et al. [58], also concluded that for masonry walls and
in comparison to ultrasonic test, sonic test is more suitable and
appropriate in detecting the cracks, delaminated joints and large
voids.
The effect of sonic test on existing partially damaged and
repaired structures was investigated by Binda et al. [55]. Sonic
velocities were used to analyze the structural health of masonry
walls of Cathedral of Noto. According to the authors, this test can
be used to identify different levels of damage in structural ele-
ments and can also examine the efficiency of grout injection.
In 2003, Binda et al. [56], investigated the effect of sonic and
radar tests on masonry structures. Both tests gave reliable informa-
tion about the damage portions. However, the results of sonic tests
were found to be better than radar test. The authors suggested that
these test can be used to recognize different materials on the sur-
face of large areas and can identify variation in material character-
istics. Furthermore, this test might be used for evaluating the
effectiveness and distribution of grout in the structure. Results of
sonic tests were found better then radar test.
Fig. 16. Bridge deck survey using ultrasonic equipment (top) test results, (bottom) equipment and data collection [25].
Fig. 17. Validation test specimen: (a) validation test site; (b) sketch depicting locations of built-in defects as well as marked test lines and test points [65].
Fig. 18. Point-by-point condition rating of validation test specimen based on: (a) longitudinal measurements; (b) transverse measurements [65].
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 67
Fig. 19. Example of modern IR test equipment [68].
68 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
2.2.4. Ultrasonic NDT
Ultrasonic pulse velocity (UPV) method is one of the oldest NDT
methods for determining the relative condition of concrete by
measuring the time of pulse of ultrasonic waves over a known path
length [7]. It can be used to locate abnormal regions in the member
[7,54,60–67]. The ultrasonic device consists of a transmitting and a
receiving transducer. The transmitting transducer produces stress
pulse that propagates in the member. The signal is then received
by the receiving transducer and the results are displayed in the
form of travel time. As the wave reach out to a defect, a small part
of the emitted energy is reflected back to the surface. Defects, in
this case, are identified as any anomaly of acoustical impedance
different from the concrete element tested. However, in regions
where there is significant deterioration or micro cracking, concrete
will have a noticeably lower velocity when compared with con-
crete in intact regions. ASTM C597-09 [67], covers the require-
ments of suitable ultrasonic pulse velocity device. The main
difficulty which needs to be addressed is good coupling of the
transducer with the surface. This issue can be addressed by using
viscous material such as petroleum jelly or grease. However, on
uneven surfaces it is very difficult to achieve adequate coupling.
It needs to be pointed out here that for most applications, 50 kHz
frequency transducers are suitable [7]. Fig. 16 illustrate the work-
ing of ultrasonic NDT equipment on a bridge deck.
The development work for the ultrasonic imaging of concrete
was described by Schickert [63]. Ultrasonic method has a strong
potential for detail and in-depth assessment of concrete structures
by using short wavelengths as compared to other nondestructive
method. SAFT (Synthetic Aperture Focusing Technique) and tomo-
graphic reconstruction, which are imaging methods were used to
provide good quality and high resolution images. These algorithms
utilized the data and measurement taken from pulse-echo method.
Amongst all measurement system, transducers were found most
vital for transmitted signals since ultrasonic waves passed twice
from transducers to the concrete. Due to highly different acoustic
impedances, air gap between transducer and concrete surface
greatly deteriorated the quality of pulse waves. Therefore, the cou-
pling of transducers directly affected the image quality (signal to
noise ratio, resolution and uniform amplitude distribution) of con-
crete. Schickert mentioned that conventional coupling techniques
were not adequate and difficult to apply. Therefore, air coupled
transducer may be useful. Moreover, water or dry coupling was
recommended for future work.
The accuracy and precision of ultrasonic testing with low fre-
quency of approximately 55 kHz for characterization of delamina-
tion in deck of concrete bridge was evaluated by Shokouhi et al.
[65]. A reinforced concrete slab (20 ft � 8 ft � 8.5 in.) was used to
simulate a concrete bridge deck (Fig. 17). It was found that defects
as small as 30 cm2 could be reliably detected using multiprobe
ultrasonic array. Delaminations deeper than 6 in. were directly
detected and characterized while delaminations shallower than
2.5 in. were detected indirectly. Point-by-point condition rating
of validation test specimen was also given (Fig. 18). The authors
concluded that multi-sensor low frequency ultrasonic testing is
useful for delaminations detection and characterization and has
great potential for in-depth testing of selected areas of bridge
decks [65]. However, high initial cost and the need for lane closure
during testing are the main drawbacks of this technique.
The micro and macro scale defects in concrete were investi-
gated by Shah et al. [64] by measuring linear and non-linear ultra-
sonic parameters. It was found that wave depletion was much
susceptible to different levels of power and damage. With the
increase in damage level, high voltage pulses diminished more as
compared to low voltage pulses. Both linear and nonlinear meth-
ods were found to be sensitive to the w/c ratio. Moreover, for all
levels of damage, a high power resulted in higher wave velocities.
The authors suggested that more research should be conducted to
determine the influence of power levels on pulse velocity with dif-
ferent damage levels [64].
In [61], Bogas et al. experimentally investigated the influence of
several parameters (cement type and content, amount of water,
type of admixture, initial wetting conditions, type and volume of
aggregate and partial replacement of normal weight coarse and
fine aggregates by lightweight aggregates) on the relationship
between ultrasonic pulse velocity and compressive strength of
concrete. Eighty-four different compositions having compressive
strength ranging from 30 to 80 MPa were prepared. Test results
showed that the relationship between UPV and compressive
strength of concrete was less affected by the aggregate volume in
light weight concrete than in normal weight concrete. Moreover,
it was found that different types of cement and different initial
wetting conditions of aggregate have little effect on the relation
between UPV and compressive strength.
2.2.5. Impulse response (IR)
Impulse response (sonic mobility) is a stress wave method
[7,68–70] that is especially used for deep foundations [68]. This
method uses low-strain impact to produce the stress waves. When
the hammer strikes the concrete, compressive stress wavesare
generated whose frequency can vary from 0 to 3000 Hz depending
on the material used in the hammer (soft rubber, metal) [7]. The
force (measured by load cell) and the velocity (measured by the
receiver) time based signals are recorded by the data acquisition
system, which is then transformed by computer into frequency
domain using fast Fourier transform algorithm [7]. The ratio of
the velocity response spectra and impact spectra is termed as
mobility spectrum [25]. However, if measured response is dis-
placement, the ratio of the displacement and impact spectra repre-
sents a flexibility spectrum while the inverse ratio is termed
mechanical impedance (dynamic stiffness spectrum). In some
analyses, the flexibility spectrum is matched by a flexibility spec-
trum (response spectrum) for an assumed single-degree-of-
freedom (SDOF) system. Once the two spectra are matched, the
modal properties of the SDOF system provide information about
the stiffness and damping properties of the system. The underlying
assumption of this process is that a structure’s response can be
approximated by the response of SDOF system. The modern IR test
equipment is shown in Fig. 19 while the schematic working princi-
ple and field application is illustrated in Fig. 20. This method can be
Fig. 20. Impulse response test method: (a) the concept, (b) example of using in practice [69].
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 69
used to determine the length of piles and piers, cross-sectional area
of pier, provide information about dynamic stiffness of the shaft/-
soil composite [7]. Theoretical interpretation of impulse response
application regarding pile length and dynamic stiffness are given
by Turner [70]. Even though the concept behind impulse response
and impact echo is similar, there are significant differences when it
comes to bridge deck testing. Impact echo is based on the excita-
tion of particular wave propagation modes above the probable
anomalies within the deck or between the top and bottom of the
deck. The frequency range is typically in between 3 and 40 kHz.
On the other hand, impulse response relies more on the structural
response in the vicinity of the impact and, therefore, the frequency
range of interest is much lower i.e. 0–1 kHz for plate structures
[25].
Davis [68] applied this test method on different concrete struc-
tures which include cooling towers, arch bridge, bridge deck, pre-
stress box beam bridge, freeways and terra coda clad steel column.
Fig. 21 shows the exposed wall of cooling tower after the IR test
and stiffness contour map of pre-stressed box beam bridge.
According to Davis [68], this method play an important role in
evaluation of floor slabs, pavements, concrete bridges, decks, piers
fluid retaining structures and silos. Moreover, this test can be used
for rapid evaluation of large structures and identification of
Fig. 21. (Left) Exposed cooling tower wall after IR testing, (Right)
problematic zones for detailed investigations. The author con-
cluded that IR test plays an important role in rapid assessment of
large concrete structures during on site testing and immediately
allows identifying the problematic areas within structure for
detailed investigation. This results in financial benefit and
increased confidence on this technique.
The Impulse Response s’Mash test method was used by Gorze-
lanczyk et al. [69] to measure the length of piles. The authors used
this method to determine the length and continuity of concrete
prefabricated piles in a quick manner. The pile length measured
by IR method was found similar to the designed length. However,
professional person is required for analyzing stress waves signals
which is a major drawback of this method.
2.3. Electro-magnetic methods
2.3.1. Ground penetrating radar (GPR)
GPR is the most successful and well-known Non-destructive
test for investigation of bridge decks and pavements [7,23,71–
79]. It is used for structural health monitoring of concrete struc-
tures like buildings, bridges and tunnels [73,78,80,81] and provides
an electromagnetic (EM) wave-reflection survey. GPR is rapid and
quick method for assessing in-depth characteristics of subsurface
stiffness contour plot of pre-stressed box beam bridge [68].
Fig. 22. Reflections of electromagnetic radiation pulse at interfaces between materials with different relative dielectric constants [7,88].
Fig. 23. GPR testing [25].
70 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
layers and can be used for detecting damages, delaminations, voids
(more than 1 in. deep and 1 in. diameter) [82], cracks and their
lengths, cavities, recognizing steel reinforcement and its diameter
[72], thickness of member, identifying regions of high moisture
content, settlement of layers and monitoring of deformation
induced by strain [71,76,77,83–86]. Initially grid lines are marked
on structure at different intervals (usually 2.5 ft) [17]. Then on
these grid points GPR equipment moves for data recording of com-
plete structure. Typical GPR include an antenna, a control unit, a
display unit and a data recording device [7]. The antenna transmits
short pulses of electromagnetic energy that penetrate through the
material and a portion of this energy is received by the antenna
when it is encountered by an interface between materials of dis-
similar dielectric properties [7]. For continuous acquisition of data
along the bridge deck, the antenna is either mounted (150–
500 mm above the pavement surface) on the vehicle or it can be
dragged across the pavement surface [7]. The control unit controls
the pulse, amplify the received signal and provide output to the
display unit (oscillo-graphs, graphic facsimile recorders) [7]. The
data obtained (arrival time and energy level of reflected electro-
magnetic pulse) can be analyzed by various techniques such as
cluster analysis, quantitative peak analysis, and topographic plot-
ting [7]. The velocity and travel time of the electromagnetic pulse
signals provide information about the location and depth of the
discontinuity [87]. Moreover, since the travel time of signals and
depth of the discontinuity are directly proportional to each other,
therefore, it is also possible to detect interface surfaces between
air and concrete, steel and concrete, steel and water [87]. Reflec-
tions of electromagnetic radiation pulse at interfaces between
materials with different relative dielectric constants is shown in
Fig. 22. We would like to mention here that large amount of data
is obtained from this test; therefore, an experienced interpreter/-
analyst is required.
Hugenschmidt and Mastrangelo [75], quantified the accuracy
and reliability of GPR on bridge structure under real circumstances.
They applied the GPR method on a bridge which was marked for
demolition. For concrete cover over the top layer of re-bar, the dif-
ference in results between radar and reality was 10 mm. The main
reason for this difference in results was related to the resolution
problem in small cover areas and uncertainties in interpretation.
Moreover, the pavement thickness was also measured and the dif-
ference in the results between GPR and reality was found to be
9 mm. The main reason for the gap in the results was small con-
crete cover which resulted in overlapping reflections from re-bar
and bottom of pavement. According to the authors, the quality of
results depends on two parameters (1) the object under testing
and (2) qualification and experience of persons conducting test.
Fig. 23 shows the working of GPR test equipment.
Hugenschmidt [74], used mobile GPR system for concrete
bridge inspection (Fig. 24). The results of mobile and manual data
Fig. 24. EMPA’s mobile GPR system [74].
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 71
GPR were compared for accuracy. The accuracy of the mobile radar
for layer thickness was in between 5 and 15 mm if signal velocity is
assumed to behorizontally invariant. However, the accuracy can
be improved if velocity variations are taken into account. The
authors concluded that mobile GPR data acquisition system is cost
effective method as compared to manual data GPR. Moreover,
interruption in traffic flow can be reduced to minimum by using
this method.
An experimental investigation was carried out by Yehia et al.
[79] to evaluate the effect of different parameters such as mix vari-
ations, changes in temperature and concrete maturity on defect
detection ability using GPR method in bridge decks. Overall they
prepared sixteen concrete samples and eight were simulated with
defects. Fig. 25 shows the specimen made for experimental testing.
The authors found that GPR can detect defects in bridge deck with
radius to depth ratio (r/d) as small as 0.15. The existence of defects
under steel reinforced was found to limit the defect detection abil-
ity of GPR. Porous lightweight aggregate can absorb and retain
Fig. 25. Specimens used in the investigation (a) singly reinforced specimen
water which limit GPR to identify the damage at early stage. Fur-
thermore, defects were not detectable in lightweight concrete
mixes within 3 months of casting concrete.
2.3.2. Conductivity
Electromagnetic conductivity is a technique which provides
geometrical, electrical information and degree of saturation about
the materials [6]. Alteration in the conductivity characteristics of
concrete indicates the damage in concrete [6,57,89–91]. This
method uses the response of the ground against the propagation
of electromagnetic fields. The electromagnetic fields, which are
generated through the transmitting coils, passes through the object
and their response is monitored. Since the receiving and the trans-
mitted fields are different both in amplitude and phase, hence, this
difference provides certain information on materials geometry and
electrical properties [57]. In this method, normally a grid system is
laid on the surface of the material and the measurements are taken
on specific grid points. For better results and maximum accuracy
level, it is suggested to have overlapped reading for every half
meter [57]. Data is then recorded by the digital data recorder and
later on, it is transferred to the computer for plotting contour maps
of the conductivity distribution in both horizontal and vertical
plane.
The relationship between concrete mix proportions and electri-
cal characteristics of concrete was investigated by Whittington
et al. [91]. The determination of electrical conductivity of concrete
mix was based on the hydration of cement paste, rate of hydration
of cement paste within concrete, degree of hydration of concrete
and water cement ratio of mix. Indirectly, this method was used
to determine the quality of concrete and its strength at particular
time. A simplified model proposed for conduction of current
through concrete is shown in Fig. 26. Whittington concluded that
it can be used for real time structural health monitoring of struc-
tures. For example, the growth of a crack can be identified as an
alteration in the conductivity characteristics of concrete.
Colla et al. [57], used this rapid, low cost and not-contacting
technique for the assessment (inhomogeneity identification, mois-
ture movement detection over time) of masonry arch bridge
(Fig. 27). The main parameters which controlled the conductivity
procedures were clay content, moisture level (humidity) and salin-
ity [57]. However, the conductivity method was unable to collect
data at deeper locations.
It was shown by Smith-Rose [92] that conductivity increased
approximately by the square of the moisture content in soil sam-
ple. In a typical case, the conductivity for dry soil at a frequency
cross-section and (b) doubly reinforced specimen cross-section [79].
Fig. 26. Conduction model for concrete [91].
72 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
of 1200 kilocycles per sec was of the order of 105 electrostatic units
(corresponding to a resistivity of 9 mega ohms per cm cube), while
at a moisture content in between 12 and 26 percent a limiting
value in between 108 and 2 � 108 electrostatic units (resistivity
9000–4500 ohms per cm cube) was obtained [92]. Burial-type
four-electrode units used for monitoring applications are shown
in Fig. 28.
2.3.3. Half-cell potential
Half-cell potential is an electrical technique used for evaluation
of corrosion activity of steel reinforcement [7,93–98]. The standard
equipment (Fig. 29) consists of a copper–copper sulfate half-cell,
connecting wires and a high voltmeter so that very little current
flows through the circuit [7]. When a metal is submerged into an
electrolyte, positive metal ions will resolve (oxidation) and will
accumulate at the metal–liquid interface. As a consequence, the
metal–liquid interface becomes positively charged, and a double
layer is formed. Anions, from the electrolytic solution (in concrete
–Cl� and –SO4
2�), are attracted to the positively charged side of this
double layer and accumulate there, forming the so-called half-cell.
A potential difference between the metal and the net charge of the
anions in the electrolyte builds up, which depends on the solubility
Fig. 27. Downstream elevation of
of the metal and the anions present in the solution. This potential
difference will give us information about the corroded reinforce-
ment [25]. As per ASTM C876-91 [93], if the reinforcement is cor-
roding, the electrons would tend to flow from the reinforcement to
the half-cell, where copper ions would be transformed into copper
atoms deposited on the reinforcement. A display of more negative
reading on the voltmeter would be an indication of high probabil-
ity that the reinforcement is corroding. During experiments, it was
observed that the negative potential will be more when some
defects like cavities are present in the surface [98]. For bridge
decks, initially a grid system is laid on the deck. ASTM C876-91
[93] suggests a spacing of 1.2 m, however, a closer spacing is rec-
ommended if the voltage difference between the adjacent points
exceeds 150 mV. For reliable assessment of corrosion activity,
some researchers suggested a spacing of about 0.6 m [94].
The results of the survey can be prepared in the form of equipo-
tential contour maps to assess corrosion activity on concrete bridge
and can then be used for the maintenance and repair work [6]. The
results of survey can also be formulated in the form of cumulative
frequency diagram [93]. It has been pointed out by Gu and Beau-
doin [97], that when the portable half-cell is moved on the marked
grid points, the longitudinal distribution of corrosion potential is
mapped. Finally, the results can be evaluated by either numeric or
potential difference technique [93]. Half-cell potential method
can be applied at concrete structures irrespective of thickness of
concrete cover and size and detailing of reinforcement [96]. It can
indicate the corroding reinforcement not only near the external
surface but also in greater depth [96]. It needs to be pointed out
here that the testing and interpretation of results must be done
by experienced personnel. Some limitations of this method include
(1) the wire has to be connected to the reinforcement, (2) it is dif-
ficult to take correct reading if any coating is present on the surface
like epoxy, paint or asphalt, (3) it does not provide information
about the rate of corrosion, (4) correction factor has to be applied
if the test is performed outside 17–28 �C range [7,93].
In [95], Elsener observed that half-cell potential method can be
easily used for assessing the corrosion of rebars after repair and
thus scrutinize the effectiveness and durability of repair work. It
was found that for accurate results and effectiveness of repair
work, two parameters are crucial, one mechanism of repair
principle and second measurement time after repair work. The
North Middleton Bridge [57].
Fig. 28. Commercial burial-typefour-electrode conductivity probe used for mon-
itoring changes in soil electrical conductivity [90].
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 73
interpretation of measurements was found to depend on the
resulting pH and on the conductivity of pore solution in concrete.
Elsener concluded that more negative potentials do not indicate
corroded bars but alkaline environment of bars. The schematic
illustration of half-cell measurements is given in Fig. 30.
According to Gu and Beaudoin [97], several factors must be con-
sidered for accurate interpretation of half-cell measurements such
as oxygen, electrical resistivity of concrete, concentration of chlo-
rides and chemical admixtures. It is generally believed that more
negative reading in potential is an indicative of high probability
of corrosion; however, this is not always true. The authors found
that many factors can shift the measurements toward positive
and negative values. They suggested that half-cell data provide
the probability of corrosion only at a given location on specific time
of test. However, long-term monitoring of this data is more
valuable.
2.3.4. Electrical resistivity measurement
Electrical Resistivity (ER) (the inverse of the conductivity) of
concrete is the ratio between applied voltage and resulting current
in a unit cell [99]. It is mainly influence by the moisture content
Fig. 29. Apparatus of half-cell
and varies from 101 to 106Xm [100,101]. By comparing different
reading points, valuable information regarding the concrete inner
surface characteristics can be extracted [6,99–104]. In this method,
minimum two electrodes are used out of which one is the reinforc-
ing steel bar. A voltage/current is applied between the electrodes
and resulting current/voltage is measured. The ratio of voltage
and current gives the resistance while the resistivity is calculated
by multiplying resistance with a factor known as cell constant.
The distribution of concrete resistivity in reinforced concrete is
depended upon several factors like location of steel and depth to
surface value [99]. The top surface layer is another important
parameter. If the top surface is too much dried out, the resistivity
will be higher than the normal concrete while if it is wet then
the resistivity will be less as compared with normal concrete. This
problem can be minimized by using the four point method. For
example, the Wenner setup uses four probes that are equally
spaced [25]. A current is applied between the outer electrodes
while the potential of the generated electrical field is measured
between the two inner ones. The resistivity is calculated according
to the following formula:
q ¼ 2pa=I
where q = resistivity (in Xm); a = electrode separation (in m);
V = voltage (in V); and I = current (in A).
Polder et al. [99], concluded that concrete resistivity method is
very important and matter of concern due to its linkage with cor-
rosion of steel. High corrosion rate was observed in areas of low
resistivity as compared with high resistivity.
This test can be used to determine moisture content, homo-
geneity and corrosion rate of reinforcing steel bars [99]. Different
reading points can be correlated with the ability of corrosion cur-
rents to flow through the concrete, which is a based upon three
parameter i.e. moisture content, salt content and water/cement
ratio [6]. Temperature related corrections must be done to the data
because resistivity value will got twice if temperature decrease
about 20 �C or a change of 3–5% per degree occurs [99].
potential method [7,93].
Fig. 30. Schematic illustrating the half-cell measurement [97].
Fig. 31. Schematic representation of electrical resistivity measurement set up [103].
Fig. 32. Four-probe square array [102].
74 S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86
Surface resistivity method can be used to evaluate the risk of
chloride ion penetration in the concrete. Hence, this method is
an alternative method of Rapid Chloride Ion Penetration (RCP) test.
Ryan et al. [104], used surface resistivity method to determine
resistance of concrete to chloride ion penetration and concluded
that this method is sound, easier, faster and more reliable method
as compared with RCP.
Saleem et al. [103], evaluated the effect of chloride and sulfate
ions on ER of concrete. Test results showed that the moisture con-
tent, chloride and sulfate contamination control the ER of concrete.
The ER of concrete decreased due to sulfate contamination. In this
situation corrosion of steel bars will increase due to carbonation. It
was observed that impact of moisture content on the electrical
resistivity of concrete is very small when the salt concentration
is high. Moreover, in nearly dry concrete, high salt concentration
could greatly increase the steel corrosion. The schematic of electri-
cal resistivity measurement set up is shown in Fig. 31.
Lataste et al. [102], used ER measurements to categorize the
damage in concrete specifically crack. Four-probe square array is
presented in Fig. 32. The measurements were taken on upper part
of a slab (1 � 0.5 m2 area) which was 40 years old. This area was
selected due to presence of delaminated region and large crack
as shown in Fig. 33. This area was also inspected by visual and ham-
mer test. ER was found to assess the crack width (crack opening)
Fig. 33. On site sounding of an altered slab [102].
Fig. 34. Proof load test on new Elizabeth Bridge in 1964 [111].
Fig. 35. Two tanks in maximum load position on bridge [109].
S. Kashif Ur Rehman et al. / Construction and Building Materials 107 (2016) 58–86 75
and depth, which are important for determining serviceability of
structures. The authors suggested that the coupling of ER with
acoustic method will produce the accurate diagnosis.
2.4. Deterministic methods
2.4.1. Proof load test
Proof load test provides the actual load carrying capacity of
structures [105–110]. It is also used as diagnostic test to verify ana-
lytical or predictive structural models. This test is appropriate only
when either unsatisfactory load rating is indicated by the analyti-
cal procedures or it is not possible to perform analytical test on
structure due to lack of structural documentation or deterioration.
Therefore, it is suggested to perform proof load test on all those
structures on which assessment is not possible due to insufficient
structural drawings or it does not meet with requirements of the
most advanced assessment theories.
Earlier proof load test was used as check to design assumptions
and quality of construction work. But, later on it was used for
assessing the load capacity of existing structures (especially
bridges) with the ultimate goal to determine directly the maxi-
mum allowable load for a structure with essential safety level.
Acoustic emission is the useful method to follow up the loading
process to control the load increment before propagation of any
damage to the structure [108]. According to Casas and Gomez
[105], proof load test is very efficient in assessing the capacity of
existing bridges. The targeted proof load can be calculated on the
basis of reliability theory as many variables involved are of random
nature. The value of the targeted proof load can also be obtained as
nominal traffic load multiplied by the target-proof load factor.
For bridges, if the proof load test is successful then it shows that
the bridge is capable of resisting more load than the applied proof
load. This reduces uncertainty and increase reliability of the struc-
ture. However, the main risk involved in performing this test is
that if unsuccessful, the structure may damage or collapse [106].
Moreover, the testing procedures for proof load test are not well
documented [107]. The proof load test is very expensive and
Fig. 36. Delamination observed in extracted cores [25].
Fig. 37. Cross-section of specimens (a) core and (b) cylindrical specimen [113].
76 S. Kashif Ur Rehman