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In meteorology, weather forecasting uses thermal images from satellites.

Thermal imaging cameras are also often used by law enforcement helicopters at night during rescue operations or when tracking fugitives.

The potential use of thermal imaging in civil engineering is less well established or known. This paper presents a field study where thermal imaging technology is used as a tool for civil engineering application. Although the thermal imaging process is not new, in recent years the technology has greatly improved to the extent that temperature data can now be shown for each image pixel, and graphs can be created showing temperature change over an area.

Furthermore, thermal imaging has become more accessible and less costly compared to what was available a few years ago. Thermal imaging is now possible with small hand-held devices such as the Landguide M4 or Flir C2, as a smartphone attachment like Flir One, or as an in-built small phone such as Cat S60. The thermal images presented in this paper were obtained using a Landguide M4, which is one of the smallest and lightest devices of its kind in use today.

2. Background to thermal imaging

Thermal imagers detect radiation in the infrared range (Figure 1) of the electromagnetic spectrum – that is, waves of approximately 700 nm to 1 mm wavelengths.  The basis for infrared imaging technology is that any object, whose temperature is above 0 K,

1. Introduction

Thermal imaging originated for military use and remained relatively restricted for several decades.  It provided military personnel with the ability to view opposing forces during darkness or in smoke-covered battlegrounds. Over the last decade, the use of thermal imaging technology has propagated into several fields and its effectiveness has led to many successful new applications – including in civil engineering.

The properties that have made thermal imaging detection valuable to military forces around the world also make it valuable in many other disciplines, including medicine, emergency and rescue services, electrical and building engineering, meteorology and law enforcement.

In the medical field, thermal images are being used to assess inflammation in the arteries; to identify and screen travellers with high fever in airports; to diagnose a variety of disorders associated with the neck, back and limbs; and for early detection of breast cancer.

In emergency and rescue applications, firefighters use thermography to see through smoke, to locate injured persons and to locate the base of a fire, while rescue personnel use the technology to locate trapped persons following an earthquake or building collapse.

In electrical engineering, maintenance engineers use thermography to locate and help repair any overheating joints on power lines.

Building engineers use thermal imaging to identify any areas of faulty thermal insulation and use the results to improve the efficiency of heating and air-conditioning systems.

Innovative uses of thermal imaging in civil engineering1 Indrasenan Thusyanthan CEng, PhD, CMarEng, MICE, BA,

MEngGeotechnical Consultant, Thusyanthan Consultants Ltd., London, UK (corresponding author: it206@cantab.net)

2 Tim Blower CEng, FICE, BSc, MSc, DIC, CGeol, FGS, SiLCTechnical Director, Mott MacDonald, Cambridge, UK

3 William Cleverly CEng, MEng, MICE, MAGeotechnical Engineering Manager, Offshore Wind Consultants Ltd, London, UK

Thermal imaging captures information in the infrared spectrum of light invisible to people, thus it can

provide valuable extra information. Innovative use of thermal imaging technology can therefore play an

important role in many civil engineering applications. This paper provides insight into thermal imaging

technology and its uses in civil engineering. A field example of early leak detection in an earth embankment

using thermal imaging technology is presented. With ever-improving thermal imaging technology and

decreasing cost of thermal imaging cameras, many future uses of thermal imaging in civil engineering are

envisaged – from pipeline leak detection to structural integrity inspections, energy efficiency surveys and

pollution monitoring. The small size of the equipment means it can also be carried by drones, offering

access to remote or otherwise inaccessible areas.

Proceedings of the Institution of Civil Engineershttp://dx.doi.org/10.1680/jcien.16.00014Paper 1600014Received 22/04/2016 Accepted 26/09/2016Keywords: embankments/geotechnical engineering/pipes & pipelines

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Innovative uses of thermal imaging in civil engineeringThusyanthan, Blower and Cleverly

ICE Publishing: All rights reserved

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processing. The thermal imager measures the thermal radiation and converts it to corresponding temperature.  As with all measuring instruments, an initial calibration ensures that distance and other environment conditions are accounted for in the conversion process, thus leading to an accurate temperature measurement. There are several ways of post-analysing the thermal images to produce an effective presentation of the temperature measurements. Some of the key features of such post-analyses are illustrated here using the images taken.

Most thermal image cameras capture both a digital (visual) and a thermal image and these are stored in the camera memory.

radiates infrared energy. Radiated energy is proportional to the body’s temperature raised to the fourth power, the Stefan–Boltzmann law. Therefore, the amount of radiated energy is a function of the object’s temperature and its relative efficiency of thermal radiation, known as its emissivity.

When pointed towards an object, a thermal imager captures this radiation energy and converts it into the corresponding temperature of the object. Hence the thermal imager allows identification of variations in surface temperature from a distance. Thermal imagers have a variety of controls, including relative humidity and emissivity settings that can be adjusted to obtain the optimal measurement accuracy.

The specifications for the Languide M4 thermal imager (Figure 2) used in the paper are as follows.

■ Temperature measurement range from –20°C to 250°C. ■ Sensitivity of 0·12°C. ■ Temperature differences are detected as high-resolution, 8-bit

thermal images. ■ Built-in laser locator to pinpoint hotspots. ■ 1GB memory (stores up to 600 images).

To demonstrate the accuracy of the thermal imager used in this paper, a thermal image of hot water in a cup was evaluated and compared with the temperature measurement obtained from a thermal probe submerged in the hot water. The thermal probe had an accuracy of ±0·2°C. Figure 3 shows both the thermal image and digital image of the cup containing hot water. The maximum temperature of the hot water measured by the thermal imager was 63·9°C, while that detected by the temperature probe was 64·2°C. Given that the thermal probe accuracy is ±0·2°C and that of thermal image is ±0·12°C, the measurement given by the thermal imager was concluded to be equivalent to that obtained from the probe. Comparisons were also made for various temperatures in the range from 0°C to 100°C and results showed that the thermal image temperature measurement was as accurate as the thermal probe measurement.

The simple demonstration provides confidence in the ability of the thermal imager to differentiate surface temperatures from a distance.  It is this unique ability which makes thermal imaging a useful tool in many civil engineering applications. It should be noted that the emissivity settings in the thermal imaging camera are critical for obtaining accurate thermal readings and this is discussed later on.

3. Features of thermal image and analysis

The thermal imager produces high-quality thermal images that can be analysed in real time on site or stored for post-

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Figure 2. A Languide M4 thermal imaging camera was used to take the thermal images

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Figure 3. Comparison of temperature measurement from thermal image with that from thermal probe

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and accuracy of the thermal image data, the emissivity setting in the thermal imaging camera needs to be fine-tuned to match that of the observed object.

Other factors that could affect the accuracy and reliability of thermal imaging are atmospheric temperature and atmospheric moisture content.  It should be noted that most thermal imaging cameras have very good accuracy, typically ±1°C, with their original preset values of emissivity. Furthermore, for many civil engineering applications of thermal imaging technology, it is the ability of the thermal imaging equipment to detect differences in temperature, rather than absolute temperature values that is the great strength of the method.  Therefore, the accuracy of the temperature measurement from the thermal image is less critical for the success of thermal imaging in civil engineering.

These images can be post-analysed using the thermal imaging software.  For example, Figures 4(a) and 4(b) show the digital image and thermal image of a tree and its surroundings under sunlight.  The thermal image clearly shows that the lowest temperature is in the shadow of the tree, while the highest temperature is at the soil next to the tree trunk, which is experiencing direct sunlight.

The post-analysis software allows a user to draw a line in the thermal image and obtain the temperature variation along this line as shown in Figure  4(c). This enables one to obtain the maximum temperature along a required area without any contact measurements.  As can be seen from the figure, the maximum temperature is about 40°C.  If a different temperature range is of importance, then the same image can be reanalysed by rescaling the temperature range in the thermal image as shown in Figure 5.

Another feature of the thermal imaging software is that it contains several coloured palettes, which assist in visualising thermal differences.  For example, Figure  6(a) shows a digital image, while Figures 6(b) and 6(c) show two different renditions of the same image. It is interesting to observe the portrayal of the thermal reflection of the footbridge in the water. Therefore, any interpretation from a thermal image needs to take into account the fact that thermal radiation can be reflected in the same way as light.

4. Thermal emissivity

The concept of emissivity is critical to the understanding of a thermal image of an object. Thermal emissivity is a measure of how the thermal emissions of an object deviate from those of an ideal black body. Therefore, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature. For example, two objects at the same temperature will not produce identical thermal images if they have different emissivities.

For any pre-set emissivity value, objects with higher emissivity will appear hotter, and those with a lower emissivity will appear cooler.  The typical emissivity value of pure water is 0·96 and concrete is 0·91, but silver has a much lower range of 0·03–0·04. Therefore, if very accurate temperature measurements by thermal imaging are required, then an accurate estimate of the thermal emissivity of the object is critical. In order to increase the reliability

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Figure 5. Rescaling the temperature range in a thermal image

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is close to the ground surface, giving civil engineers the chance to carry out remediation before the leakage initiates.

Using thermal imaging technology to identify potential future leakage areas in an embankment was piloted in an aqueduct in Hertfordshire, UK. This embankment had experienced several leaks in the past and the aim was to identify any potential future leak areas along a section of 1 km length. Thermal images were taken along the embankment, section by section, as shown in Figure 8(b). It should be noted that there was a practical difficulty in trying to identify temperature differences at the embankment surface since the thermal images showed that areas in direct sunlight were much hotter compared to the regions in shadow. This is clearly evident in Figures 9(a) and 9(b). Therefore, identifying a temperature difference due to the presence of the phreatic line near the surface was not possible.

After several attempts to capture the thermal image of the embankment surface without the direct sunlight effect, it was

5. Civil engineering applications of thermal imaging

5.1 Embankment leakageEarth bunds and dams play a vital role in storing and conveying

water for irrigation or consumption. Water leakage from aqueducts and embankment dams is a significant issue faced by civil authorities as it wastes large quantities of water that could be utilised for public use. Worldwide, millions of dollars are spent annually around the world on earth embankment, aqueduct and river channel embankment leakage repairs.  The complexity, duration and cost of these repairs can be reduced if the repairs are carried out prior to a full leak being developed. However, there is no established method in which areas of potential leakage can be identified in an earth embankment prior to the leak occurring. Thermal imaging could be an effective tool to address this issue.

An embankment can start to leak when there is a localised path for the water to pass through the embankment (i.e. cracks) or when the phreatic line reaches the surface of the embankment.  One of the factors affecting the surface temperature of an embankment is the distance to the phreatic line or water source. In a location where a leak is imminent, the water is closer to the surface and therefore the surface temperature at this location is lower than at locations where the phreatic line is relatively far away from the surface. This fact can be used in thermal imaging to identify the locations in an embankment where leakage is just starting to develop and therefore a bigger leak is highly likely to occur. This would enable early detection of such locations and allow for preventative repair and maintenance activities. The concept is illustrated in Figures 7(a) and 7(b).

If the phreatic surface were to exit on the downstream slope of the embankment, the surface would gradually erode away, the water flowing out of the face carrying soil particles with it. This process can eventually cause the entire structure to fail. The thermal imager can locate these areas where the phreatic surface

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concluded that the best time for thermal imaging was after sunset when the influence of direct sunlight on surface temperature would be largely dissipated. Thus, 1 h after sunset, a series of thermal images was captured along the embankment sections as shown in Figure 8(b). To show the location clearly, the digital image shown was captured during daytime but the thermal images were captured after sunset. The thermal images were post-processed to evaluate the thermal characteristics of the surface.

One of the thermal images highlighted an area of concern which indicated a linear path with lower temperature. This area was cooler by almost 2°C compared to the rest of the region which was at approximately 17°C. This evidence is presented in Figures 10(b) and 10(c). Figure  10(d) shows a thermal image which is typical from other sections and which does not show lower temperature paths. The small patches of hotter surface in the thermal images in Figures 10(b) and 10(d) are where there was no grass and the bare ground had been warmed directly by the sunlight. The section shown in Figure  10(b) was the only region of concern identified by the thermal imaging assessment, thus further investigation near the channel was undertaken at the location of this region of lower temperature.

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Figure 8. River embankment digital image (a); thermal image (b) was captured in series of sections as shown

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Visual detection is difficult and often impractical when the pipelines are tens of kilometres long; however, thermal imaging can be an efficient tool for identifying the location of a leak and facilitating quicker repair. The following example demonstrates the effectiveness of thermal imaging in identifying such leaks, since a leak in a buried pipeline would lead to soil temperature variation and this could easily be identified in a thermal image.

As a demonstration of the idea, a small quantity of hot water was poured onto a lawn area and visual and thermal images were obtained as shown in Figure 11. It is evident that the thermal image can clearly identify the exact location of the hot spot, while such identification is not possible by visual inspection. While there are many factors – such as ground temperature and the depth of the buried pipeline – that would affect effectiveness of any pipeline leak detection, use of thermal images, especially from aerial drones, can be an effective way to monitor the integrity of long pipelines.

5.3 Building heat lossEnergy conservation is now a global goal of all organisations

around the world.  Use of thermal imaging for thermal audits of buildings is not new, but the advancement in thermal imaging is enabling the assessment to be carried out very quickly and effectively. As an example, the thermal imager used in this study was used for heat loss inspection for a building and the results are provided in Figures 12(a) and 12(b).

It is to be noted that the digital image shown was captured during daytime, but the thermal image was captured during night-time when the internal heating was constant. The building had the same internal temperature in all the rooms.  The thermal image is effective in clearly identifying the windows that are less effective in conserving the building’s internal heat. Thus thermal imaging provides a quick and inexpensive means of identifying defective thermal insulation of windows or doors in a building and hence enables the owners to take cost-effective actions to repair the defective areas.

5.4 Other usesThermal imaging is also an effective tool in research, where

temperature variation within soils or fluids needs to be investigated (Kodikara et  al., 2011; Liang et  al., 2012; Thusyanthan et  al., 2011).

Divers identified cracks in the base of the channel lining near the identified location. Therefore, the embankment could have started to leak in the very near future. However, since the location of the potential leak was identified before any significant leakage from the embankment, repair work was planned and executed efficiently without any loss of water from the channel or disruption to traffic flow next to the embankment. The study demonstrates that thermal imaging can be successfully used to identify potential leakage locations quickly and thereby enable engineers to plan and carry out remedial works and possibly even prevent a major leakage from earth embankments.

The thermal imaging leak detection method should be considered in the context of other available systems.  In the dams industry, there are other methods of leakage detection, but they are few in number. Some methods use a series of thermal probes driven into the embankment at close centres, which can be used to measure soil/groundwater temperature differences. Other methods use a low-voltage electrical current that follows water-bearing features, including leakage paths, to generate a magnetic field which can then be measured at the ground surface. These methods are claimed to give a two- or three-dimensional picture of leakage pathways through a water-retaining embankment, and they have the advantage that they can detect leakages in the core of a dam or embankment, which may be the critical zone. However, they presuppose that one already knows roughly where any leakage is or might be.

Thermal imaging in the context of dam engineering and maintenance may be of limited use, in the sense that it does not allow a picture to be built up of conditions within the core of the embankment, where critical leakage may be initiated. However, as a rapid assessment tool for long embankments or over large areas or to identify surface cracks or defects in concrete, it has considerable advantages over other methods.  It would be cheap to carry out surveys over large distances, and comparisons could be made between repeated surveys on different dates.  It would therefore appear to have its best application in the rapid preliminary assessment, for maintenance purposes, of flood embankments, river levees, earth aqueducts and canals.

5.2 Pipeline leakageLocating leaks in a water, gas or oil pipeline at an early stage is

critical to ensure repair work is undertaken quickly and efficiently.

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References

Abdullah QA and McClellan R (2011) Airborne infrared thermography for environmental and facility management of the army national guard training facilities. Proceedings of Pecora 18: the 18th William T. Pecora Memorial Remote Sensing Symposium – Forty Years of Earth Observations: Understanding a Changing World, Herndon, VA, USA.

Clark MR, McCann DM and Forde MC (2003) Application of infrared thermography to the non-destructive testing of concrete and masonry bridges. NDT & E International 36(4): 265–275.

Dumoulin J, Ibos L, Ibarra-Castanedo C et al. (2010) Active infrared thermography applied to defect detection and characterization on asphalt pavement samples: comparison between experiments and numerical simulations. Journal of Modern Optics 57(18): 1759–1769.

Gunn DA, Marsh SH, Gibson A et al. (2008) Remote thermal IR surveying to detect abandoned mineshafts in former mining areas. Quarterly Journal of Engineering Geology and Hydrogeology 41(3): 357–370.

Hopkins P (2011) Chasing water with thermal imaging. In InfraMation 2011 Proceedings. Infrared Training Center, Nashua, NH, USA, paper 2011-078 (CD-ROM).

Ishimwe R, Abutaleb K and Ahmed F (2014) Applications of thermal imaging in agriculture – a review. Advances in Remote Sensing 3(3): 128–140.

Jadin MS and Ghazali KH (2014) Gas leakage detection using thermal imaging technique. In UKSIM 2014, UKSim-AMSS 16th International Conference on Computer Modelling and Simulation Cambridge, United Kingdom 26-28 March 2014 (Al-Dabass D, Orsoni A, Cant R et al. (eds)). IEEE Computer Society, Los Alamitos, CA, USA, pp. 302–306.

Kodikara J, Rajeev P and Rhodenb NJ (2011) Determination of thermal diffusivity of soil using infrared thermal imaging. Canadian Geotechnical Journal 48(8): 1295–1302.

Liang DF, Chong KJY, Thusyanthan NI and Tang HW (2012) Thermal imaging study of scalar transport in shallow wakes. Journal of Hydrodynamics 24(1): 17–24.

Lo TY and Choi KTW (2004) Building defects diagnosis by infrared thermography. Structural Survey 22(5): 259–263.

Moropoulou A, Palyvos J, Karoglou M and Panagopoulos V (2007) Using IR thermography for photovoltaic array performance assessment. In 4th International Conference on Non-Destructive Testing, Chania, Crete, Greece. Hellenic Society of NDT, Athens, Greece (CD-ROM).

Sham J (2009) Infrared Flash Thermography (FT) for Building Diagnosis: Detection of Surface Cracks, Subsurface Defects and Water-Paths in Building Concrete Structures. VDM Verlag, Saarbrücken, Germany.

Thusyanthan NI, Cleverly W, Haigh SK and Ratnam S (2011) Thermal imaging, thermal conductivity of soil and heat loss from buried pipelines. Proceedings of the 34th Annual Offshore Pipeline Technology Conference, Amsterdam, the Netherlands.

Titman DJ (1990) Infra-red thermal imaging construction fault location. In Infrared Technology and Applications (Lettington AH (ed.)). Sira, Ltd, Chislehurst, UK, SPIE Proceedings vol. 1320.

There are numerous further instances in civil engineering applications where thermal imaging can be an effective diagnostic tool. These include concrete integrity inspection (Lo and Choi, 2004; Sham, 2009; Titman, 1990), bridge inspections (Clark et al., 2003), asphalt pavement inspections (Dumoulin et  al., 2010), subsurface cavity detection (Gunn et  al., 2008), environmental inspections such as pollution dispersion (Abdullah and McClellan, 2011), solar panel performance assessments (Moropoulou et  al., 2007), insulation loss or leak detection in facilities (Hopkins, 2011) and even agriculture (Ishimwe et  al., 2014). Clark et  al. (2003) have demonstrated that areas of delamination in a concrete bridge structure can be correctly identified using infrared thermography.

The use of thermal imaging as an inspection tool in petrochemical plants is already well established and it greatly enhances the efficiency of operation and maintenance activities while increasing equipment and worker safety. Thermal imaging can quickly identify any leaks or defects (Jadin and Ghazali, 2014) without the need for any shutdown in operations. Furthermore, the thermal survey can be carried out remotely and without the need for any contact with plant items. The use of thermal imaging allows the identification of problems at an early stage and potentially avoids them leading to major issues later. Thermal imaging surveys can reduce inspection costs and increase equipment and plant reliability.

6. Conclusion

This paper presents the potential of thermal imaging to provide innovative solutions for several civil engineering applications.  A site example has been presented of the use of thermal imaging in identifying areas of future leakage in earth embankments.

The ability of thermal imaging to identify potential leakage areas in aqueducts, canals or earth bunds is valuable information that engineers can use for effective maintenance and repair. Pipeline and plant equipment leakage detection through remote survey are some other applications where thermal imaging offers an effective solution.

With the cost of thermal imaging technology becoming more economical all the time, this method could become an important tool in many civil engineering applications. Thus, thermal imaging could become an integral part of civil engineering solutions in future.

How can you contribute?If you would like to comment on this paper, please email up to 200 words to the editor at journals@ice.org.uk.

If you would like to write a paper of 2000 to 3500 words about your own experience in this or any related area of civil engineering, the editor will be happy to provide any help or advice you need.

Acknowledgement

The authors would like to thank The Royal Society, University of Cambridge and St Catharine’s College for the funding and assistance which enabled this study.

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Figure 12. Visual image of a building (a) and thermal image of the same building (b) obtained at night where internal temperature was same – windows with poor thermal insulation can easily be identified