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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. An initial stage 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 types 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 includes 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.