The application of infrared thermal imaging technology in the detection of building exterior walls
Release time：2023-12-20Visitors：
Infrared thermal imaging technology, as a new non-destructive testing technology, is still in its early stages of application in the field of engineering construction testing and monitoring. Taking the construction quality inspection of exterior wall decoration in a high-rise building as an example, this paper introduces the application of infrared thermal imaging technology in exterior wall inspection of buildings; And combined with other cases, the reliability, sensitivity, and detection conditions of this technology were analyzed. The results indicate that if the external detection conditions are good, the selection of detection time is reasonable, and infrared thermal imaging technology is used to reliably and accurately detect the quality of building exterior walls, it can provide a reliable basis for the acceptance of building construction or the survey of decorative quality.
The exterior walls of buildings in our country generally require decorative engineering, but in recent years, accidents of collapse of exterior wall decorative materials such as decorative bricks and mortar have occurred frequently, and even multiple incidents of casualties have occurred. In order to improve the quality of exterior wall decoration engineering in China, the Ministry of Construction has successively issued the "Inspection Standards for Bond Strength of Decorative Bricks in Building Engineering" and the "Technical Regulations for External Wall Insulation Engineering". Although the release of these standards has played a significant role in controlling the quality of decorative construction, the inspection methods specified in the standards belong to local damage tests, and there are also certain limitations on the number of samples during testing, and scaffolding needs to be installed during implementation. Therefore, this method poses great difficulties for the quality inspection of newly constructed large-scale decorative projects and building exterior walls that have been in use for many years. Even if inspected, the sampling quantity is very small and lacks comprehensiveness.
In this context, the engineering inspection community hopes to find a method that can conduct a large-scale survey of the quality of building exterior wall finishes without damaging the finishing materials. In recent years, research has found that using infrared thermal imaging technology can detect the quality of building exterior wall finishes by examining the temperature distribution on the surface of the exterior wall; And this method does not require scaffolding, and can also conduct fast, non-contact, and large-scale surveys. Given the advantages of infrared thermal imaging technology, researchers have conducted extensive experimental research on its feasibility in building decoration engineering.
The author takes the construction quality inspection of exterior wall decoration in a high-rise building as an example to introduce the application of infrared thermal imaging technology in exterior wall inspection of buildings; And combined with other cases, the reliability, sensitivity, and detection conditions of this technology were analyzed.
The principle of infrared thermal imaging technology
All objects in nature with temperatures above absolute zero are constantly emitting radiation energy (infrared) into the surrounding space, and the same applies to the exterior walls of buildings. The infrared radiation characteristics of an object are closely related to its surface temperature; Therefore, by measuring the infrared energy radiated by the object itself, the surface temperature of the object can be accurately measured, which is the theoretical basis for infrared radiation temperature measurement. In fact, infrared radiation is an electromagnetic band with a wavelength between microwave and visible light, commonly referred to as infrared or infrared light, with a wavelength range of [0.75] μ M~1 mm æ -5].
Infrared imaging detection is a diagnostic method that applies the principle of infrared thermal imaging to detect the magnitude of radiation energy on the surface of building exterior walls. By measuring the temperature distribution on the surface of building exterior walls, it can determine whether there are defects in the exterior wall insulation layer.
In 1964, the world's first industrial infrared thermal imager was successfully developed in Sweden. Two years later, Swedes used infrared thermal imaging technology to test the energy-saving and insulation performance of buildings. Researchers from multiple countries such as the United States and Germany also conducted research in this area. China's infrared detection technology began in 1979. In recent years, multiple departments in China have successively released national or industry standards for infrared detection, established infrared qualification certification training centers, and developed corresponding infrared detection instruments, providing strong support for the promotion of infrared detection in China.
Due to construction quality reasons, there are two types of quality defects in the external wall insulation layer of buildings. One is that the quality of the materials used in the insulation layer does not meet the standard or the amount used is insufficient, which cannot achieve the expected insulation effect; Another type is that the external wall surface of some parts (mainly the reserved holes for the manhole frame, passenger and freight elevators, and tower crane connections) has not been insulated during later repair. When pasting decorative bricks on exterior walls of buildings, two types of quality defects are also prone to occur: one is that the construction unit cut corners, resulting in insufficient mortar for pasting decorative bricks, and artificially causing hollowing between the decorative bricks and the wall; Another type is due to the improper mix ratio of cement mortar used for pasting exterior tiles, which leads to unstable pasting and the formation of voids between the exterior tiles and the wall. Some old buildings have long-term exposure to sunlight and rain on their exterior, which can easily create hollowing between the exterior tiles and the walls.
When the exterior walls of a building are insulated, if the quality of the insulation layer is reliable, it is equivalent to having an air filled isolation panel between the exterior tiles and the wall, which has good insulation properties. Therefore, under sunlight, the heat transfer between the exterior wall surface and the wall is less; If there are certain defects or no insulation treatment in the insulation layer of the local area of the exterior wall, under the irradiation of sunlight, when the temperature of the exterior wall surface is higher than the wall temperature, heat will quickly transfer from the exterior wall surface to the interior. Therefore, the temperature of the exterior wall surface in the area with insulation layer defects will be lower than that of the surrounding normal parts. Similarly, if there are quality defects in the pasting of local decorative bricks on the exterior wall, or if there is hollowing between the decorative bricks on the exterior wall of an old building and the wall, the surface temperature of the exterior wall under sunlight is higher than that of the surrounding normal parts. The schematic diagram of the infrared imaging detection principle for building exterior walls is shown in Figure 1.
2 Engineering application examples
A newly built hotel in a city in southern China is a high-rise building with a full frame structure. Its exterior walls are insulated with polystyrene particles, and the outermost layer is decorated with tiles. According to the construction materials submitted by the construction unit, the insulation layer construction of this project fully complies with the relevant requirements of the national industry standard JGJ144-2004 "Technical Specification for External Wall Insulation Engineering". During the quality acceptance of building exterior walls, at the request of the owner, the infrared thermal imaging method was used to comprehensively inspect the bonding quality of building exterior wall decorative materials and the construction status of the external insulation layer. Here is a brief introduction to the issues discovered during the testing process.
The on-site inspection was conducted before the hotel was officially put into use, with all exterior wall finishes completed, scaffolding removed, and interior decoration in progress. The inspection was conducted in March of spring, with daily average temperatures exceeding 10 ℃. The local weather remained sunny the week before the inspection, and on the day of the inspection, the exterior wall had sufficient sunlight, low air humidity, and a gentle breeze. According to the orientation characteristics of the building, the east and west exterior walls are inspected in the morning and afternoon respectively, while the south and north exterior walls are inspected at noon. The detection scope covers all exterior wall finishes of the entire building.
During on-site inspection, multiple inspection shooting points are set up on each side of the exterior wall according to the different detection areas, and the shooting distance is required to be controlled within 50m as much as possible. The angle between the observation direction of the infrared thermal imager and the normal direction of the emitting surface of the exterior wall decorative layer is generally less than 35 °; A few detection shooting points in certain areas, when the shooting distance is greater than 50m, are equipped with long focal length lenses on the infrared thermal imager. For each detection point, high-resolution cameras were used to take visible light photos based on the infrared shooting range, and the photos taken by the infrared thermal imager and camera were uniformly numbered to avoid errors. Before the formal inspection, the infrared thermal imager was debugged based on the emissivity of the tested exterior wall surface to ensure the normal operation of the inspection work.
The equipment used for on-site testing is the NEC infrared thermal imager produced in Japan, which has a temperature resolution of 0.03 ℃ and can fully meet the accuracy requirements of exterior wall inspection.
The test results indicate that the insulation and decorative engineering quality of the majority of the exterior walls of the building is good, with only local abnormalities and quality issues. Examples of its detection photos and visible light photos are shown in Figure 2.
The exterior insulation layer of this building is made of polystyrene particles, and the decorative layer is made of tiles. From Figure 2 (a), it can be seen that most areas of the exterior wall have similar temperatures. Due to different structures, the window positions display certain high temperature phenomena, and the contours are very clear, making it easy to see the specific situation of each window. Moreover, the glass status of each window can be accurately distinguished, indicating a significant infrared imaging effect. There is a clear irregular low-temperature area in the lower middle of Figure 2 (a), which is 2-3 ℃ lower than the temperature of the surrounding exterior wall. According to infrared imaging theory, it can be determined that there are certain defects in the quality of its exterior wall. From the visible light photo in Figure 2 (b), it can be seen that the exterior wall is clean and beautiful, and the ceramic tiles used as the exterior decoration have a good bonding effect, without any quality issues. Based on the display results of the two photos and the characteristics of the project, it can be basically determined that there are quality defects or deficiencies in the external insulation layer (if there is a quality problem with the pasting of the external decorative surface layer tiles, the infrared image should show local high temperature anomalies). However, according to the construction records submitted by the construction party, the cause could not be identified. In order to take responsibility for the quality of the project, on-site sampling and verification of the external wall insulation layer and decoration in the abnormal area were carried out under the witness of the owner, supervisor, and construction unit. At the same time, several decorative bricks were also taken from the normal external wall for comparison. Knocking open the infrared low-temperature abnormal area, it can be seen that the external facing tile (Figure 3 (a)) has not undergone insulation treatment in the abnormal area, only ordinary cement mortar; Take out a portion of the cement mortar here (see Figure 3 (b)), and it can be seen that the cement mortar in this area only contains a very small amount of polystyrene particles, which is far from the insulation material required by the specifications. Figure 3 (c) shows the removed infrared detection of the exterior wall insulation material in the normal area, indicating that its polystyrene particles are normal and meet the mandatory national standards, making it a qualified insulation material.
On site verification shows that the exterior walls of the low-temperature abnormal area displayed by infrared detection have not undergone insulation treatment (or there are serious quality problems with the insulation materials), and the building cannot achieve insulation effect, resulting in serious quality defects on the exterior walls; In the normal area displayed by infrared detection, the external wall insulation material is suitable and the insulation effect is good. At the same time, it also proves the accuracy and reliability of infrared thermal imaging technology in the detection of exterior wall finishes.
3 Comprehensive analysis of infrared thermal imaging detection technology
3.1 Reliability analysis
In order to further verify the reliability of infrared detection technology, actual cases of infrared detection in recent years were collected and analyzed, and the results are shown in Table 1.
From the several cases listed in Table 1, it can be seen that for some areas with obvious defects in the external protective layer or quality problems in the exterior wall decorative layer, infrared thermal imaging technology can be easily distinguished, with accurate discrimination and good reliability.
3.2 Sensitivity analysis
The sensitivity of infrared exterior wall detection is mainly related to the sunlight intensity of the exterior wall. Due to the detector receiving infrared radiation from the exterior wall, it is greatly affected by changes in solar radiation. In order to understand the degree to which infrared detection is affected by solar radiation and analyze the sensitivity of infrared detection, a series of observations were conducted over time on a hollow outer wall of a certain building's exterior facing tiles. The observation results are shown in Table 2 (with the building facade facing west).
From Table 2, it can be seen that for the same exterior wall with quality issues, there is a significant difference in temperature difference values observed by infrared at different time periods. Among them, the temperature difference value observed at 15:30 in the afternoon is the highest, with the highest detection sensitivity and obvious detection effect; The temperature difference between morning and evening periods is relatively small, which may not be able to effectively distinguish. Therefore, when using infrared thermal imaging method for exterior wall detection, an appropriate detection time should also be selected based on the orientation of the building facade. The sensitivity of external wall infrared detection is not only related to the intensity of external wall sunlight (the amount of infrared radiation energy on the surface of the external wall), but also to environmental humidity, shooting angle, shooting distance, external wall materials, and instrument accuracy.
3.3 Testing condition requirements
Due to the fact that external wall infrared detection mainly receives infrared radiation emitted by the external wall, the detection effect is not only related to the detection time period, environmental humidity, shooting angle and distance analyzed above, but also to weather, external wall condition and other conditions. For example, testing cannot be conducted on rainy days; In cloudy weather, the temperature difference between normal parts and quality defect parts generally does not exceed 0.5 ℃. At this time, infrared detection is prone to misjudgment and omission; After rain, the surface of the exterior wall gets wet, and the exterior decoration surface is uneven in moisture content. Rainwater may seep into the hollow part of the wall through cracks. At this time, when testing, the infrared characteristics of the exterior wall are not significantly different, making it difficult to accurately distinguish whether there are defects. Testing during the process of rainwater evaporation may also cause misjudgment. Therefore, after rain, testing should be carried out after the walls are completely dry. In summer, it is best to start testing after three consecutive days of sunny weather, and in other seasons, it is best to start testing after one week of continuous sunny weather; For areas involving contamination, shadows, and areas affected by heat sources (such as hanging air conditioning units), special areas (balcony sides), etc., due to their significant external influence, they are prone to misjudgment. Therefore, special attention should be paid to the detection conditions when conducting infrared detection, and if necessary, other methods can be used for verification.
In summary, when using infrared thermal imaging technology for exterior wall detection, if the external detection conditions are good and the detection time period is selected reasonably, reliable and accurate detection results can be obtained.
The use of infrared imaging detection methods has gradually made the inspection and acceptance of building exterior walls more objective and fair, solving the problems of limited detection quantity and insufficient representativeness that used to rely solely on visual observation and manual tapping for judgment. Technically speaking, infrared thermal imaging detection technology has advantages such as high detection sensitivity and good reliability of detection results; In terms of cost, it has the advantages of wide detection coverage, non-destructive testing, and high detection efficiency; At the same time, it also has non-contact characteristics, which can effectively avoid safety accidents in testing and is a leapfrog progress in the field of engineering testing technology. This technology can be applied not only to the detection of exterior wall quality, but also to the damage detection of high-voltage transmission cables, on-site real-time monitoring of asphalt pavement paving quality, and water seepage inspection in multiple fields.
