Application—Temperature Detection of C80 High Performance Concrete By Infrared Thermal Imaging

Temperature Detection of C80 High Performance Concrete By Infrared Thermal Imaging

   High-strength and high-performance concrete is more and more widely used in large-span buildings, port buildings and high-rise buildings because of its high strength, high durability, high workability, and high volume stability. However, high-strength and high-performance concrete is more likely to burst when fire is exposed to high temperature. This is because high-strength and high-performance concrete is dense, and the steam pressure generated inside it cannot be released when subjected to the high temperature of the fire, and the steam pressure increases with the fire temperature. When the steam pressure exceeds the tensile strength of high-strength and high-performance concrete, the concrete bursts. Through experiments, it is found that the compressive strength and elastic modulus of high-strength and high-performance concrete decrease after high temperature, and the higher the maximum temperature, the greater the decrease in strength. Moreover, studies have found that the flexural strength of high-performance concrete decreases with increasing temperature.

   However, there are few reports on the axial compressive strength and damage detection of C80 high performance concrete mixed with polypropylene fiber (PP fiber for short) after high temperature, and further research is needed. For this reason, C80 high performance concrete (HPC and PPHPC) with PP fiber volume content of 0% and 0.2% was simulated fire high temperature test, and the burst after high temperature was observed, and the fire temperature and axial compression of C80 high performance concrete were studied. The relationship between intensity and infrared temperature rise.

Figure 1. Infrared thermal image of HPC under different fire temperatures.

   The principle of infrared thermal imaging detection is: infrared thermal imaging detection is a kind of non-destructive testing that uses the difference between the surface temperature and radiation emissivity of the object to form a visible infrared thermal image to detect the surface structure and defects of the object, and to judge the nature of the material. method. After high temperature, the concrete will be damaged by cracking and loosening. The higher the fire temperature, the more serious the concrete damage. Use an external heating source to irradiate the concrete after high temperature. The concrete has different infrared radiation due to different damage conditions. The infrared thermal image is collected by an infrared thermal imager, and the temperature change data is analyzed to establish the relationship between the temperature rise and the fire temperature, and then infer the concrete high temperature. Damage situation.

   The size of the concrete specimen is 150 mm × 150 mm × 300 mm. After the specimen is formed and demolded, the specimen is cured for 28 days, and then it is allowed to stand indoors and dry naturally for three weeks. For HPC and PPHPC simulating fire high temperature, the heating rate of this test is 5 ℃/min, and the fire temperature level is set to 100 ℃, 200 ℃, 300 ℃, 400 ℃, 500 ℃, 600 ℃, and HPC and A set of PPHPC is left as a normal temperature (20 ℃) comparison group. In order to ensure that the internal temperature of the concrete specimen is consistent with the surface temperature, another 6 150 mm cube specimens with a central embedded thermocouple are prepared. When the temperature reaches the set temperature, the temperature is kept constant for 20 minutes to keep the internal and external temperature of the concrete consistent. That is, the test piece is considered to be burnt through, the power is turned off, and the furnace door is opened. There are seven groups in this test, each with three pieces. In order to prevent the test piece from bursting and affecting the accuracy of the test data, both HPC and PPHPC prepared 27 pieces.

Figure 2. Infrared thermal image of PPHPC under different fire temperatures

   In the experiment, an infrared thermal imager was used to take an infrared thermal image of concrete after high temperature. The external heating source is an infrared light bulb. When testing, the distance between the test piece and the external heating source (distance measurement) is 0.8 m, 1.0 m, 1.2 m, and 1.5 m, respectively. The C80 high-performance concrete specimen is 10,000 square meters When the data starts to be heated and the heating time is 3 minutes, one infrared thermal image is taken, and the infrared thermal image is analyzed and processed by MikroSpec to obtain the infrared temperature rise value of the specimen after different fire temperatures.

   After high temperature, the HPC and PPHPC ranging from 1.0 m and the infrared thermal images are shown in the figure when irradiated for 3 min. With the increase of the fire temperature, the color of infrared thermal imaging changes significantly. After MikroSpec analyzes and processes the infrared thermal image, it is found that the temperature of the infrared thermal imaging increases with the increase of the fire temperature. The reason may be high temperature. The concrete damage is more serious, and the concrete surface has more pores and cracks. When the infrared heat source is irradiated, the concrete surface after high temperature has more heat accumulation than the concrete surface at normal temperature, so the infrared thermal image temperature of concrete after high temperature is larger. The further research results are:

(1) When the fire temperature is less than 200 ℃, both HPC and PPHPC have no cracks. As the fire temperature rises, the concrete cracks and gradually increase; at 600 ℃, the edges of the HPC prism will peel off, and the number of PPHPC cracks will increase significantly, but No peeling occurred.

(2) As the fire temperature increases, the axial compressive strength of HPC and PPHPC both show a downward trend; when the fire temperature is less than 300 ℃, the axial compressive strength decreases slowly, and when the temperature is greater than 300 ℃, it decreases rapidly. At 600 ℃, HPC and PPHPC The remaining axial compressive strengths are 12. 67 MPa and 11.87 MPa, respectively; the axial compressive strength of PPHPC is slightly higher than that of HPC, indicating that the addition of PP fiber reduces the damage to the concrete axial compressive strength at high temperature.

(3) As the fire temperature rises, the infrared temperature rise of HPC and PPHPC both show an increasing trend, and the temperature rises sharply at 400 ℃. At the same fire temperature, the infrared temperature rise of HPC and PPHPC increases with the increase in distance measurement. Decrease, the infrared temperature rise of PPHPC is slightly larger than that of HPC.

(4) The relationship between infrared temperature rise, fire temperature and axial compressive strength of C80 high performance concrete when the volume of PP fiber is 0% and 0.2% is established, which can be the axial compressive strength of C80 high performance concrete after fire. Provide reference for the damage estimation.