Infrared Thermography and its Applications in Civil Engineering

Chapter 1

INTRODUCTION
1.1 General:
Durability of reinforced concrete structures is a serious problem throughout the world,
and thus assessment of the condition concrete structures in order to determine the
remaining service life and the method of repair is becoming increasingly important. In
developed countries, the costs of repairs exceed the cost of building new structures.
The objective of condition assessment of existing reinforced concrete structures is to
provide insight into current condition of the observed structure, to predict development
of deterioration, and to assess the influence of defects found on the safety and life
expectancy of the structure.

During the condition assessment of reinforced concrete structures, it is necessary to

implement reliable and effective non-destructive testing methods which can detect
localize and characterize different types of defects. The advantage of non-destructive
methods is their simplicity and the fact that they do not damage the structure or cause
only minor surface damage. Interpretation of the results, however, is one of the most
challenging tasks in modern construction process. Although active Infrared
Thermography (IR) is being used in civil engineering for identification of heat losses
through the building’s thermal envelope, it has been realized its potential as one of the
non-destructive thermal methods for non-destructive testing of materials and structure.

The use of IRT as a means of structural health monitoring has significantly increased
in recent years, due in large part to the advancement of IR cameras and the
considerable reduction in their cost. Even though in the inspection of metals, IRT is an
accepted practice, in the field of reinforced and prestressed concrete, the use of active
IRT is relatively new. Non-destructive testing using active IR thermography generally
consists of external thermal stimulation of the object under examination and
monitoring of its surface temperature variation during the transient heating or cooling
phase.

The existence of in homogeneities in the core material affects the heat transfer and
thus causes local changes of temperature distribution on the surface of the observed

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 Infrared Thermography and its Applications in Civil Engineering
structure, which is then recorded using an IR camera. It can detect internal voids,
delamination’s, and cracks in concrete structures such as bridge decks, highway
pavements, garage floors, parking lot pavements, building walls etc.

The only precondition for the detection is that defects within the object under
examination lead to a sufficient variation of the temperature, compared to the bulk
material. It is well known that IRT has certain limitations when low thermal
conductivity materials comprising deep defects are being tested, but at the same time
IRT was proven useful when used in conjunction with NDT methods that enable
detection of deeper defects. Many safety-relevant cases of damage in concrete
structures originate from defects that are close to the surface, e.g., voids and
honeycombing in the top layer of reinforcement, delaminations of carbon-fiber-
reinforced plastic (CFRP) laminates used for strengthening of concrete structures,
delamination’s of protective coating systems as well as surface and subsurface cracks.
Therefore, active thermography is very well suited for condition assessment, damage
evaluation and quality assurance of the built infrastructure. Compared with IR
thermography, other NDT techniques are cost ineffective and require much time and
significant data interpretation.

One of the most significant advantages of the IR technique is the shortest detection
time among alternatives, as well as its capability for evaluating an area of subsurface
delamination around 3 inches (7.62 cm) in depth. However, quantifying the depth of
delamination appears to be a big challenge, and only a few researchers have attempted
to evaluate the depth of delamination using the IR technique.

Infrared Thermography

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 Infrared Thermography and its Applications in Civil Engineering
1.2PRACTICAL CONSIDERATIONS
I. THERMOGRAPHIC PROCESS
The data required to carry out an IR investigation include the emissivity factor, the
reflective temperature, and the atmospheric temperature and relative humidity.
Additionally the inspection is performed in specific environmental conditions. The
temperature difference between the interior and the exterior of the investigated
building is expected to be at least 10 C. The temperature difference is easily achieved
O

according to the time of the day of the investigation: night-time during the winter
period and day-time during the summer period.

The external environmental conditions settings are followed by the setting of the IR
camera towards the target object. Typically in building diagnostics the target is an
exposed structural element and the measurement is performed from the interior of the
building. The definition of the emissivity and the reflective temperature factors
facilitates the data correction of the measurements. To avoid any reflection of the
thermographer into the resulting images, the measurement is performed from a
minimum of 5 angle of the thermographer to the target object to a maximum of 5 .
O O

The temperature conditions, including the ambient temperature and the relative
humidity at the point of investigation are also recorded. The two available approaches
for thermography inspections are the passive and active. The passive approach
measures the temperature differences of a structure that occur under normal
conditions, while the active approach generates the temperature differences of the
structure using an external stimulus. External stimulus comprises any kind of external
heat source, such as lamps, ovens, and hot packs, while the nature of the stimulus
distinguishes active thermography into pulsed thermography (PT) and lock-in
thermography (LT).

II. INFRARED CAMERA

An IR camera is an advanced device that produces a sequence of images of the thermal

distribution and is calibrated to measure the emissive power of surfaces in an area at
various temperature ranges. Typically IR cameras also contain cooled detectors that
are necessary for the operation of the semiconductors that detect the IR energy coming
from the target object. The emitted radiation from the target is focused by the optics

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 Infrared Thermography and its Applications in Civil Engineering
onto the IR sensor and the electrical response signal is converted into a digital image
(Fig.).

Infrared camera

The different colours of the image correspond to the temperature distribution of the
targeted surface. The different thermal abnormalities indicated by the different colours,
such as increase and decrease of surface temperatures or retained heat arise when a
crack, moisture accumulation, heat leakage, or any other problem area is encountered.
Further analysis of the thermo graphic images enables the quantification of the
recorded temperatures and subsequently the quantification of the severity of the
problem.

The Swedish AGA/Bofors companies, now known as FLIR Systems, developed the
first radiometric thermal imagers in the 1970s. These cameras used a single detector.
The infrared detector employed in them was the photon type, its main features being a
quite short response time, a limited spectral response, but required cooling. Succeeding
radiometers used several types of cooled photon detectors, with lower time constants,
allowing frame rates of 15/30 Hz and improved sensitivity. In the 1980s, the
commercial cameras used single cooled photon detectors with opt mechanical
scanning, of which the Focal Plane Array (FPA) detector enabled 30/60 Hz frame
rates. Infrared cameras based on no cooled FPA thermal detectors, such as micro
bolometers, emerged in the mid-1990s and led to the development of thermal imagers
with no cooling. After 1991, thermal imaging employment expanded into several
industries due to the decreased costs, while thermal imaging based on Barium
Strontium Titanate (BST) and micro bolometer has now made the application widely
available across all sectors. Technological breakthroughs have occurred at a constant
pace, resulting in improvements in resolution, sensitivity and detector packaging,

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 Infrared Thermography and its Applications in Civil Engineering
while pixel uniformity and operability are also improving due to production changes.
Thus, it can be deduced that both technological advancements and economies of scale
are the contributors to the steady price decline of IR cameras.

The current market is driving the reduction of the manufacturing cost through the
development of new low-cost detectors and optics, pushing also the manufacturers to
transformations of the IR factories to high volume manufacturing for the maximization
of the benefits of increased economies of scale. Also the next generation arrays are
projected to have a 12 µm pixel pitch geometry and increased sensitivity to reach the
50 mK target, further reducing the size of detectors and optics, the size of infrared
cameras, as well as the manufacturing costs. Additional cost reductions of uncooled IR
imaging are also driven by the demand of the market for low-cost and low-resolution
sensors. Table 1 shows the infrared camera wavelength. Table 2 shows technical
specifications of FLIR camera.

Table 1: Choice of infrared camera wavelength

Camera
Test environment Example
wavelength
High voltage electrical environment,
High temperature
concrete pavement in hot desert climate. Short
difference
e.g. Arizona/Nevada
Low temperature Concrete or masonry bridge in
Long
difference the UK

Table 2: Technical specifications of FLIR camera

IR Camera characteristics Specifications

Spectral range 7.5 - 13µm
IR resolution 640 x 480 pixel
Field of view 24o x 180
Minimum focus distance 0.3m
Spatial resolution 0.65mrad for 24o lens
Thermal sensitivity 60 mK @ 30oc
Image frequency 30 HZ
- 40 C to + 120oC
o

Temperature range 0OC to + 500oC

300OC to 2000OC

1.3Advantages of IR Thermography
 Remote sensing -1:
No direct contact is required between the camera and the object under
investigation. Camera and object separation can range from a few millimeters

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 Infrared Thermography and its Applications in Civil Engineering
to several kilometers thus allowing measurements to be made identifying
potentially hazardous areas. As no external source of illumination is necessary,
both day and night operation is possible.
 Remote sensing-2:
Due to the separation between camera and object, measurement by
thermography should not cause interference with the object and, hence,
acquired data. In reality, some interference will be caused by the camera
shielding the object from some radiation, which would otherwise be incident
upon it and by the radiation reflected and emitted from the camera itself. These
effects can normally be assumed to be negligible.
 Large monitoring capacity.
Thermal imaging cameras are capable of monitoring temperature at many
different points within a scene simultaneously.
 Visibility.
Since thermal radiation can penetrate smoke and mist more readily than visible
radiation, visually obscured objects can be detected readily.
 Range of measurement.
By altering the camera lens aperture and by introducing various filters, the
sensitivity of the system and its response to thermal radiation can be altered to
suit. Typical temperature ranges are of the order of -20 to 1600oC.
 Fast response rate.
Thermal imaging equipment is capable of detecting and monitoring rapid
temperature fluctuations to an accuracy of 0.08oC.

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 Infrared Thermography and its Applications in Civil Engineering

 Portability.
Thermal imaging equipment is lightweight and can be easily transported. It is
also possible to use the equipment whilst mobile.
 Data manipulation.
The recorded data can be monitored and processed on standard PC running
dedicated imaging software

1.4Problems in Applying IR Thermography

 From the above, the radiation reaching a thermal imaging system is not only a
function of the temperature of the object but also of its emissivity. Since
emissivity varies from material to material, the brightness of different objects
within a scene do not necessarily give a clear indication of their relative
temperatures.
 Any material with emissivity less than one will reflect radiation from
surrounding objects as well as radiating its own radiation. Thus, the
temperature obtained for an object may be influenced by other objects in the
surrounding area.
 Attenuation of radiation in the atmosphere caused by the absorption of energy
by suspended particles and subsequent re-radiation in random directions can
affect the obtained results. These effects can be assumed negligible for cases
where camera – object separation is small.

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 Infrared Thermography and its Applications in Civil Engineering
1.5Methods of Thermography
I. Passive Thermography
The passive approach measures the temperature differences of a structure that occur
under normal conditions. Typically passive thermography is rather a qualitative test
since the objective is simply the identification of the location of the thermal anomalies.
IRT under natural excitation sources has been used in civil engineering for the last 40
years. The method has been applied to the identification of internal voids,
delamination’s, and cracks in concrete structures such as bridge decks, highway
pavements, parking garages, pipelines and other applications.

The ASTM standard for detecting delamination’s in concrete bridge decks using IRT
was developed. This standard defines restrictions and limitations of the method,
together with required proper environmental conditions present for testing, varying
environmental conditions during testing (temperature gradient, shaded or direct
sunlight, cloud cover and surface water). Bridge decks receive significant attention
because the decks typically require repair sometime during the service life of a bridge,
frequently due to corrosion-induced delamination’s that develop.

The main advantage of natural excitation is that it is both cheap and an environment
friendly technique which provides a perfectly even heating. Disadvantages include
relatively low available heating power and the dependence to weather conditions, to
surface orientation, and to the color of the concrete, as the sun mainly heats in visible
light. Figure shows thermos grams of the defected bridge deck of Krapina Bridge from
the passive thermography method.

Fig : Thermo grams of the defected bridge deck of Krapina Bridge

II. Active Thermography

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 Infrared Thermography and its Applications in Civil Engineering
If a thermal gradient between the scene and the object of interest exist, the target can
be inspected using the passive approach. However, when the object or feature of
interest is in equilibrium with the rest of the scene, it is possible to create a thermal
contrast on the surface using a thermal source; this is known as the active approach in
infrared thermography. Energy brought to the object of interest will cause the change
of thermal gradient compared to the bulk material thus witnessing the presence of
subsurface anomalies.
i. Pulsed Thermography
Pulsed thermography (PT) is one of the most common thermal stimulation methods
used in thermography for nondestructive testing. One reason for this is the
quickness of the inspection, in which a short thermal stimulation pulse lasting from
a few milliseconds for highconductivity material, such as metal, to a few seconds
for low conductivity specimens, such as plastics, is used. Basically, PT consists of
heating the specimen briefly and then recording the temperature decay curve, Figure
5 shows schematics of the pulse thermography test procedure. Qualitatively, the
phenomenon is as follows, the temperature of the material changes rapidly after the
initial thermal pulse because the thermal front propagates by diffusion under the
surface and also because of radiation and convection losses.
The presence of a subsurface defect modifies the diffusion rate so that when
observing the surface temperature, a different temperature with respect to the
surrounding sound area appears over a subsurface defect once the thermal front has
reached it. As for the detection depth, it is limited since thermography for
nondestructive testing is a “border technique”, but often, anomalies such as cracks
start close to the surface.

Schematics of pulsed thermography test procedure

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 Infrared Thermography and its Applications in Civil Engineering
ii. LOCK-IN THERMOGRAPHY

Schematics of lock-in thermography test procedure

Lock-in thermography (LT) is based on thermal waves generated inside a specimen

and detected remotely. Wave generation, for example is performed by periodic
deposition of heat on a specimen´s surface while the resulting oscillating
temperature field in the stationary regime is recorded remotely through thermal
infrared emission, Figure shows the schematics of the Lock-in thermography test
procedure. Lock-in refers to the necessity to monitor the exact time dependence
between the output signal and the reference input signal, the modulated heating.
This is done with a locking amplifier in point-by-point laser heating or by computer
in full-field (lamp) deployment so that both phase and magnitude images become
available.
Phase images are related to the propagation time, and since they are relatively
insensitive to local optical surface features such as no uniform heating. The depth
range of images is inversely proportional to the modulation frequency, so that
higher modulation frequencies restrict the analysis in a near surface region.

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 Infrared Thermography and its Applications in Civil Engineering
1.6Applications of infrared thermography
I. Bridge deck assessment
Bridge deck deterioration is an issue to be addressed with seriousness. Delamination
and disintegration of concrete lead to this. Inadequacy of Traditional methods like
sounding, chloride, corrosion potential gives way to IRT to be considered as the better
alternative.

a) Visible Spectrum and b) Infrared Spectrum of a concrete bridge deck.

II. Testing for FRP wrapped columns

(a) Digital picture of first pile after wrapping, and (b) infrared image

 Subsurface debonds form between the fabric and the underlying member
 This affects the strength and ductility of the member
 IRT in rehabilitation work and periodic monitoring
 External Heat source is used
 Detection of  subsurface debonds
 Repair using resin or replacement

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 Infrared Thermography and its Applications in Civil Engineering
III. Thermal Measurement and Control of HMA Pavement
Construction
The figure below shows the continuous thermal measurement system.
Continuous Thermal Measurement System: (a) Sensing Bar mounted to Paver (b)
Display Screen

 IRT can be used for real-time measurements of the surface temperature of the
installed asphalt mat
 Map thermal contour on the surface of a material
 Identify temperature anomalies in cold areas

IV. Energy Efficiency Assessment in Buildings

 It is used to identify and minimize the source of unnecessary heat flows.

 It makes use of the actual and expected 3D spatio-thermal models using EPAR
 The technique optimizes R-values using retrofit
 It helps to achieve optimal thermal comfort for occupants
 It also improves energy efficiency in buildings

V. Building Moisture Inspection

 In this application, IRT is utilized as a diagnostic tool to evaluate moisture

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 Infrared Thermography and its Applications in Civil Engineering
 It uses Moisture detector as a supporting device
 IRT identifies critical areas that were not detected visually
 Structural plans of the building should be checked

Thermal Imaging Detects Moisture

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 Infrared Thermography and its Applications in Civil Engineering

CASE STUDY
IRT is being explored by American Electric Power (AEP) as an experimental
tool for detecting defects.

I. Cooling tower at Amos plant located in central West Virginia

Figure presents a thermal image from one of the cooling towers. New post processing
techniques are currently being developed to detect defects.

a) The visual image, b)Thermal image of a cooling tower at Amos plant located
in central West Virginia;

III. Smith Mountain Dam

The research is also being extended to two of AEP’s hydroelectric facilities: Claytor
and Smith Mountain. A sample thermal image from Smith Mountain dam is presented
in Figure.

SUMMARY

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 Infrared Thermography and its Applications in Civil Engineering
1) Infrared thermography is a fast, clean and safe technology
2) IRT is dependent on the sensor and the surrounding environment
3) The defect can only be detected if it possesses enough thermal resistance
4) The two available approaches for thermography inspections are the passive and
active. The passive approach measures the temperature differences of a
structure that occur under normal conditions, while the active approach
generates the temperature differences of the structure using an external
stimulus.
5) The success of IRT to detect defects or damage in large concrete structures
would be a major breakthrough in NDE inspections of large structures such as
dams and cooling towers as traditional methods are very costly and time
consuming

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REFERENCES

1) Snehal R. Khedkar, Prof V.R. Dhawale, “Infrared thermography and its

application in building construction” volume: 06 issue: 05 may 2009
2) Gene F. Sirca Jr., Hojjat Adeli, “Infrared thermography for detecting defects in
concrete structures”, volume 24 ,issue 7: 508–515:(2008)
3) jungwon Huh, Quang Huy Tran, Jong-Han Lee Dongyeob Han, Jin-Hee ahn,
and Solomon yim, “Experimental study on detection of deterioration in
concrete using infrared thermography technique” volume 2016, issue 23 jun
(2016)
4) Bojan Milovanovic and Ivana Banjad Pecur, “Review of active ir
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5) V.K.Divya,K.Sivasubramanian, D.Suji,”Defect identification in concrete using
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6) Tarek Omar, Moncef L. Nehdi, “Application of passive infrared thermography
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7) Shahid Kabir, “Image processing in concrete pplications : review and
prospective” volume 6 (2008)
8) G. Wedler, A. Brink, Ch. Maierhofer, M. Röllig, F. Weritz and h.
Wiggenhauser,(2003),” Active infrared thermography in civil engineering –
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CE 2003)
9) J. K. C. Shih, Ming Hsin, R. Delpak, C, (1999), “Application of Infrared
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Publishers ISSUE 7,PAGE 3-319,1999

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