What is Infrared Thermography?
The method of infrared thermography involves the data acquisition and its analysis with the help of thermal imaging devices that works without any contact with the material to be analyzed. With years, the method has been developed into more proficient manner as excellent infrared cameras were developed, that supports the method by converting the radiations into images (thermal images). The concept of working is simple. The method involves the conversion of infrared radiations that are emitted (by the body under observation) into thermal images or thermographs. Thermography means writing with heat.Thermography and Infrared Camera
The data is obtained from the material observed based on the thermal radiation emitted by them. The characteristics of the radiation emitted are based on the body temperature, their surface condition as well their thermal properties. An infrared camera here has the role in sensing the thermal energy from the body. The emitted energy may be due to reflection or transmission or directly radiated energies. This sensed data is converted into thermographs or thermal images. These only bring the results based on the radiant energy and no visible light is considered. Thermal images are entirely different from visual images. These images can be taken under total darkness.
Fig. An Infrared Camera
Construction Thermography
One of the main application of infrared thermography in the civil engineering is depended on the construction thermography. As the results are based on the emission properties of the materials, a good assessment of that area has to be performed. The emission properties of building materials are represented in terms of emission coefficients, which mainly range between 0.90 and 0.96. The emission coefficients of construction materials are given in table1.Table-1: Emission Coefficients of Construction Materials
| Construction Material | Emission Coefficient |
|
Concrete |
0.94 |
|
Sand |
0.93 |
|
Brick |
0.93-0.94 |
|
Limestone |
0.96 |
|
Plaster |
0.90-0.96 |
|
Glass |
0.93-0.96 |
|
Wood |
0.96 |
|
Felt(Roofing) |
0.93 |
|
Gypsum |
0.90 |
|
Paint |
0.90-0.95 |
|
Clay |
0.95 |
|
Brickearth |
0.93 |
Applications of Infrared Thermography in Construction
The Infrared thermography possesses a wide variety of application in the civil engineering field, which is explained in the following section.Determination of Moisture Penetration
The method shows the variation in moisture through photos, called the thermographs. These images show the different color of shades for respective temperature. Now a material under normal temperature if subjected to any kind of moisture penetration will be at a lower temperature. This differences can be evident in the thermographs. Figure-1 shows the thermograph of moisture penetration in a canopy.
Fig.1: Thermograph of moisture penetration
The temperature for respective band colors is shown. The image was obtained in the evening on a summer day. The masonry structure in the majority is subjected to a temperature of 47oC due to solar radiation. From a closer observation, it was found that the brick structure has three free sides that were exposed to rainwater. With this muck and dust got deposited along the lines that created a decrease in temperature, which is shown by the dark bands. Figure 2 represents the moisture penetration at the bottom of the window of a masonry structure at a temperature lower than 31 degree Celsius. Rest at a temperature of about 34 degree Celsius.
Fig.2: The moisture penetration under the windows of a masonry structure represented by dark bands
Plumbing Assessment
The thermographs help in assessing the performance of conduits and pipelines. Figure 3 represents the visual and the thermograph of a control valve. The images were taken on a hot day. The water flowing is hot, due to the water tank located on the terrace is exposed to the sun. From the figure, the wall is at 31 degree Celsius, the concealed pipe, and exposed pipe are at 32 and 33 degree Celsius respectively.
Fig.3: (a) Visual Image (b)Thermal Image
Determination of Fresh and hardened stages of Concrete
Figure 4 shows the thermal image of a ready mix concrete delivery truck at the site during a hot climate. It is observed that the truck tyre is at 46 degree Celsius, the pump at 50 degrees and drum of the truck is at 38 degrees. But the uniform color of the drum makes it clear that it has a good mixing and quality.
Fig.4: The thermal image of ready mix concrete truck drum
Figure 5 shows the concrete flow through the pipe. There are certain patches seen, that shows leakage of the slurry at the joints
Fig.5: The thermal image of concrete flowing through the pipe
It is clear that any obstruction in the flow can be seen at the spot and corrective measure can be taken accordingly. The system can be applied in the casting of the slab. During compaction, how the temperature is varying through the large area can be studied simultaneously, which help in knowing the faster evaporation problems resulting in inadequate hydration of concrete. The method is applied to know the effect of curing. Columns curing with gunny bags and by spraying can be differentiated by the different color shades shown for different temperature.Accurate Tension Test of Rebar
We know that the reinforcing bars fail for tension test by the formation of the neck. And the section fails at this neck formed. The normal tensile tests do not give us an idea about the yield point precisely.
Fig.6: The thermal image of a deformed bar under the tension test
It is observed that the tip of the failed section is at a higher temperature when compared with any point of the specimen. So, this method helps in giving more accurate strain measurements, compared to conventional methods.