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INTRODUCTION
Over the past few years, many techniques have been developed in order to evaluate the state of reinforced concrete structures, to detect the location and the extent of construction defects or damage zones in a non-destructive way.
Many NDT methods are presently available, such as impact-echo method, ultrasonic pulse echo, ground penetrating radar and infrared thermography. Of these, the first three techniques work based on wave propagation. The characteristics of the reflected wave are used for the evaluation of the concrete properties or the location of the defects. These methods are useful in the identification of internal voids, honeycombs, delamination, cracks and other subsurface defects. Although Infrared (IR) thermography has been widely used in civil engineering for the identification of heat losses in building envelopes, it has been realized its potential as one of the non-destructive thermal methods for NDT of materials and structures.
Infrared thermography technique relies on the variation in the temperature caused by the presence of air packets in concrete. This information is used for the identification of defects. It uses the infrared rays emitted by the investigated object to assess its temperature gradient on the surface of the test object /specimen.
Deterioration in concrete increases with time due to induced stresses. The induced stresses can result from applied loads, especially fatigue loadings, and expansive reactions in concrete. The expansive reactions can result from sulfate attack, alkali-silica reaction, carbonation, freezing and thawing actions, rust forming reactions, etc. However, in cold climates, occurrence of expansions due to freezing and thawing actions, and rust forming reactions is more common. When water penetrates into concrete and freezes, cracking and spalling of concrete occurs due to the resulting expansion. The spalling can lead to formation of "pot holes". Additionally, when deicing chemicals, especially chloride-based chemicals are used, chloride ions penetrate into the concrete and favor rust forming expansive reactions around the top of the reinforcement mat. The resulting expansion causes cracking and eventually spalling of concrete cover. This type of cracking is referred to as delamination. Damage to concrete due to the above expansive reactions can range from micro-cracks to macro-cracks that can lead to sudden failure of structures. Initial stages of cracking in concrete is not detectable by visual inspections. With a view to avoid catastrophic failure and massive repair of structures, it is essential to determine damage at low levels. To accomplish this, a number of nondestructive test methods have been developed and used. Nondestructive methods for concrete can be categorized into two major types. The first type of nondestructive testing (NDT) methods are primarily concerned with determination of in-place strength properties. Whereas, the second type of NDT methods are employed to locate hidden flaws in existing structures. The flaws include cracks, voids, honeycombing, delaminations, and debonding of reinforcement. The second type of NDT methods include infrared thermography, ground penetrating radar, electrical/magnetic, radioactive/nuclear and impact-echo. Of these, infrared thermographic testing is an area testing method, while other techniques are either point or line testing methods. Additionally, it is fast and accurate in subsurface flaws detection of existing structures. The present investigation is primarily concerned with infrared thermography for NDT structures.
Infrared thermography (IRT) is a science dedicated to the acquisition and processing of thermal information from non-contact measurement device. It is based on infrared radiation (below red), a form of electromagnetic radiation with longer wavelengths than those of visible light. Any object at a temperature above absolute zero (i.e. T > 0oK) emits infrared radiation. The human eye cannot see this type of radiation. Thus, infrared measuring devices are required to acquire and process this information. Infrared camera is used as infrared measuring device and is used for capturing the infrared rays from the object.
1.1.History
Thermography has a long history, although its use has increased dramatically with the commercial and industrial applications of the past fifty years. Sir William Herschel, an astronomer, discovered infrared in 1800. He built his own telescopes and was therefore very familiar with lenses and mirrors. Knowing that sunlight was made up of all the colours of the spectrum, and that it was also a source of heat, He wanted to find out which colour(s) were responsible for heating objects. He devised an experiment using a prism, paperboard, and thermometers with blackened bulbs where he measured the temperatures of the different colours. He observed an increase in temperature as he moved the Infrared Thermography thermometer from violet to red in the rainbow created by sunlight passing through the prism. He found that the hottest temperature was actually beyond red light. The radiation causing this heating was not visible; Herschel termed this invisible radiation "calorific rays". Today, we know it as infrared.
LITERATURE REVIEW
Sakagami and kubo (2002), developed a new processing technique to evaluate delamination defects in concrete structures based on phase delay measurement using lock-in thermography. Experimental program was carried out on a concrete block specimen of width 1m, height 1m and depth 0.3m. Square shaped polystyrene foam sheets were embedded in the specimen to simulate delamination. The heating was done through quartz lamp heater and an infrared camera was used to capture images of the specimen. It was found that the location and size of defect can be estimated from the contrast change and that the depth can be estimated from the relationship between phase delay and heating period.
Milovanovic and Pecur (2011), presented Infrared thermography as a method for detecting defects in reinforced concrete samples. They also included a short description of post processing methods used for detection of defects. The test specimen was cast with voids of various sizes. FLIR thermacam P640 was used in this study. Pulse Phase thermography method was used to transform the collected data from time domain to frequency domain and the results were obtained as phase grams and ampligrams. From the results, it was concluded that the method of pulse phase thermography (PPT) for post-processing of thermogram sequences can be used to detect defects in reinforced concrete samples.
Simoes and Tadeu (2012) reported the experimental applicability of Infrared thermography in detection of defects embedded in mortar specimens. The temperature patterns were compared with numerical results. Two specimens were cast by coating a steel cylinder with mortar. Internal defects were created between the mortar and steel core by inserting a rubber thread when the specimen was cast. Tests were performed using IR video camera. Numerical simulation was also employed to evaluate the heat diffusion in the systems. The comparison between numerical and experimental results showed that the numerical model can adequately simulate the heat diffusion phenomenon.
Gailius and Zukauskas (2003) performed investigations to find the relationship between percentage of damage and temperature variation caused by it. Three specimens for every percentage of damage were used for the determination of the temperature. Heating was done in the same direction as loading. Experimental investigations showed that different percentage of damage in concrete produced different thermal properties. Results of the experiments showed that infrared spectrum analysis can be used in the assessment of early stages of concrete damage. Experimental data showed that relationship between damages and temperature distribution in concrete specimen exist, and further investigations on this property of concrete can be useful for non-destructive technique evaluation and in future can be a precise method for non-destructive testing in concrete.
INFRARED THERMOGRAPHY
3.1. Definition
Infrared thermography can be defined as the science of acquisition and analysis of data from non-contact thermal imaging device. IRT technique is capable of monitoring the temperature variations on an area at every time instant. Infrared cameras convert the radiation sensed from the surfaces into thermal images. Thermography literally means “writing with heat”. Thus IRT method has the characteristics of an efficient non-destructive testing method and offers a rapid method for assessing large surface without the need of a scaffold to reach the area under investigation.
3.2. Principle of Infrared Thermography
Infrared radiation is the energy radiated by the surface of an object whose temperature is above absolute zero. The emitted radiation is a function of the temperature of the material; the higher the temperature, the greater the intensity of the infrared energy emitted. There are three ways by which the radiant energy striking an object may be dissipated: absorption, transmission and reflection. The fractions of the total radiant energy that are associated with each of these modes of dissipation are referred to as the absorptivity, transmissivity and reflectivity of the body. Three parameters are used to describe these phenomena: the spectral absorptance αλ, which is the ratio of the spectral radiant power absorbed by the object, the spectral reflectance ρλ, which is the ratio of the spectral radiant power reflected by the object, and the spectral transmittance τλ, which is the ratio of the spectral radiant power transmitted by the object. These three parameters are wavelength dependent.
3.3. Advantages
1. IRT is a non-contact technology: the devices used are not in contact with the source of heat, i.e., they are non-contact thermometers. In this way, the temperature of extremely hot objects or dangerous products, such as acids, can be measured safely, keeping the user out of danger.
2. IRT provides two-dimensional thermal images, which make a comparison between areas of the target possible.
3. IRT is in real time, which enables not only high-speed scanning of stationary targets, but also acquisition from fast-moving targets and from fast-changing thermal patterns.
4. IRT has none of the harmful radiation effects of technologies, such as X-ray imaging. Thus, it is suitable for prolonged and repeated use.
5. IRT is a non-invasive technique. Thus, it does not intrude upon or affect the target in any way.
3.4. Disadvantages
1. Interpretation of results and obtaining high accuracy is difficult.
2. High cost of the equipment.
3.5. Applications
1. Moisture damage and cracks in exterior walls occur due to poor design, deterioration of surface materials, and leakage from internal water pipes, etc.
2. Used to detect moisture damage to roofs.
3. Rapid and accurate evaluations of subsurface defects of structures such as bridge decks, highways, and airport pavements to avoid costly repairs.
4. Infrared thermography can also detect debonding between an asphalt overlay and the underlying concrete.
5. Infrared thermography was capable to detect voids around sewer system due to temperature differentials that existed between various types of materials, effluent, and cavities.
3.6. Types of Infrared Thermography
The IRT method is divided in to two types based on the source of heating the test object. They are:
1. Passive IRT method: The infrared rays emitted from the inherent heat of the test object are used for the assessment.
2. Active IRT method: It uses an external heat source on the test object for the evaluation process.
For the non-destructive evaluation in concrete components, the Active IRT method being used.
The two types of Active IRT methods are used. They are:
i. Pulsed Thermography (PT) and
ii. Lock-In Thermography (LT)
3.6.1. Pulsed Thermography (PT)
Pulsed thermography (PT) is one of the most common thermal stimulation method 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 high-conductivity 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. 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.
3.6.2. Lock-In Thermography (LT)
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. 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 non 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.
3.7. Infrared Camera
A camera consists of three basic components the lens, the photographic film and the mechanical device. In digital cameras the film is replaced by a series of image sensors. Each image sensor is a charged-coupled device (CCD) which converts light into electric charges. Infrared camera works on the same basic principle of a camera except that it detects infrared radiation. The electric charges created by the incident infrared light are converted into a color image where a particular temperature is associated for each color in the image. There are wide varieties of infrared cameras available in the market for various applications.
Post-Processing Techniques
3.8.1. Pulsed Phase Thermography (PPT)
Pulse phase thermography (PPT) is the active IR technique which enables the collected data to be transformed from the time domain to the frequency domain using one-dimensional discrete Fourier transform (DFT) [11, 12]. PPT is well known for NDT of materials and structures. It combines features of pulse thermography (PT) and lock-in thermography (LT) techniques and thus enables an easy data acquisition and fast data analysis.
3.8.2. Principle Component Thermography (PCT)
Principal Component Analysis (PCA) is a statistical analysis tool used for identifying patterns in data and expressing the data in way to highlight the similarities and differences in the patterns. The pattern matching of the data becomes very difficult when the data dimension is very high. In such situations PCA comes in handy for analyzing and graphically representing of such data. Once the patterns in the data are found, the data is compressed by reducing the data dimensions without much loss of information. PCA applied to the data in the form of thermogram sequence is called PCT. The image data captured by the IR cameras consist of undesirable signals and noise along with the IR image data. These image sequences are to be processed to eliminate the undesirable signals and enhance the useful IR information. The PCT used for processing IR sequences is based on thermal contrast evaluation in time. The PCT analysis is based on the second order statistics of IR image data.
3.8.3. Correlation Operators Technique
In probability theory and statistics, the correlation coefficient indicates the strength and direction of a linear relationship between two variables. In the IR context, the correlation coefficient refers to the strength and direction of the linear relationship between a given temperature evolution reference and all the temperature evolution of the pixels over the specimen under inspection. The correlation coefficient image is mostly sensitive to material changes, being much less sensitive to temperature non-uniformities or initial heat absorption as these are temperature offsets in terms of temperature evolution.
CHAPTER 4
CASE STUDY
A literature review was used to analyze approach of individual researchers to using IR thermography for defects determination in concrete structures. Specimen size was defined by the need of transferring specimens (limited by the dimensions and weight) and conditioning of the specimens, while on the other hand, specimens needed to be large enough to be able to simulate real defects, without the influence of edges or the influence of defects between themselves on the temperature field. The effect of reinforcement spacing on the possibility of detection of defects was also investigated. Influences of concrete properties (density and moisture content), on the possibility to carry out measurements were also investigated.
The research was conducted by using reflecting method, where surface temperature was monitored during 60 minute thermal excitation, together with the cooling period lasting also 60 minutes. Thermal excitation was performed using 1000 W halogen lamp, where heating distances were 1.5; 2.0 and 3.0 m from the surface of concrete specimens.
A schematic drawing of the concrete test specimen is shown in Figure 3. For simulating compaction defects and voids, defects made of polystyrene foam have been incorporated. These defects vary in size (discs of 15 cm in diameter and rhombus 17×12.6 cm both with thickness of 4 cm). Within the sample, defects are positioned underneath the reinforcement with different rebar spacing. The samples were tested from both sides, while the reinforcement was placed only from one side.
An IrNDT system, manufactured by Automation Technology GmbH was used, where halogen lamp was used as the thermal excitation source. IR camera FLIR ThermaCAM P640 was used with spectral range from 7.5 - 13 μm, thermal sensitivity (NETD) 60 mK @ 30°C.
The IR camera is connected to the computer and the thermal excitation using the active thermography electronic interface.
In the post-processing of thermogram sequences after step heating (SH) thermography was used to collect sequences of thermograms, pulsed phase (PPT) thermography, principle component thermography (PCT) and correlation operators’ techniques were used in order to determine the existence of defects in concrete samples.
CONCLUSION
1. Infrared thermography is a fast, clean and safe technology that is used in a wide variety of applications. The use of infrared thermography in two very important fields: temperature measurement and non-destructive testing. The principles and essential theoretical background in these two fields have been reviewed.
2. Infrared thermography is a mature technique for non-destructive testing. Recent advances in this field allow this technology to detect many types of defects. However, a defect can only be detected using infrared thermography if it opposes enough thermal resistance to create visible thermal contrast.
3. Infrared thermography has experienced a great evolution in a relatively short time. Important improvements were achieved in different fields. However, there is a variety of limitations that need to be taken into account. Infrared thermography is highly dependent on the sensor selection and the experimental setup.
4. It may be affected by the instrument and by the environment. These problems can be minimized, but only with adequate setup and testing procedures, which mostly depend on the operator’s skill.