Issue 53

D. Wang et alii, Frattura ed Integrità Strutturale, 53 (2020) 236-251; DOI: 10.3221/IGF-ESIS.53.20 249 As shown in Fig. 14, the event rates at all measuring points declined sharply to the valleys at around zero events/min. This shows that no new cracks or concrete spalling oc curred in this “quiet” stage. As shown in Fig. 15, the energy rates at all measuring points dropped deep to the valleys at around zero mv ∙ μs/min. As shown in Fig. 16, the b-values at all measuring points remained at a high value (2), exhibiting as serrated curves. This means, when the plate was under the same load, the crack width changed in a certain degree in the later period of heating, but the crack development was rather limited. (5) Cooling stage After flameout, the data collection was stopped at 550 min. As shown in Figs. 14 and 15, the event rates and energy rates at all measuring points surged up to the peak values between 326 and 329 min. The 3 min difference is due to multiple factors, such as the attenuation of acoustic waves in the propagation path. The peak event rate (137 events/min) was measured by 8#, while the peak energy rate (50,098 mv ∙ μs/min) was captured by 12#. These results indicate that, after flameout, the furnace temperature dropped sharply, and the heat was transferred from the bottom to the top of the specimen. The moisture in the plate continued to evaporate, creating temperature stress and internal stress, releasing some energy, and thus AE sources. After that, no new crack emerged, and no original crack widened. As a result, the event rates and energy rates at all measuring points dropped deeply to a low level, kicking off another “quiet” period. As shown in Fig. 16, after flameout, the b-values of all measuring points started to decrease and reached the valleys (1.55- 1.60) at around 326-329 min. It shows that in this stage, with the redistribution of internal forces, the internal cracks of concrete develop further. Then, the b-values gradually picked up, and the number of AE sources decreased slowly in the specimen. C ONCLUSIONS his paper mainly carried out two works: one is to record the development law of slab cracks under normal temperature and fire; the other is to use acoustic emission technology to monitor the acoustic emission signals at different positions of the specimen, and to collect the accumulated events, event rate, energy rate, b value and other parameters analysis. The main conclusions are as follows: (1) Before punch failure, the cracks mainly radiated from the center of the plate. Among them, the diagonal cracks developed quickly under fire. The cracks under high temperature could be monitored in real-time by infrared detection technology. (2) The cumulative number of AE events can reflect the activity of the specimen. According to the test results, the cumulative number of events grew linearly in room temperature loading stages, high temperature stages and cooling stage. This means the specimen has been active in the entire process, and each stage has a “quiet” period. (3) Throughout the test, the variation in event rate, energy rate and b-value can be split into several stages. Upon load increase or ignition, the event rate and energy rate exhibited as serrated curves, while the b-value hit the valley. The specimen suffered the most severe damage when event rate and energy rate suddenly spiked and b-value suddenly dropped. In this case, the structure could have failed. By monitoring these three characteristic parameters, it is possible to provide early warning of fire to reinforced concrete plate-column structure. (4) The crack density and change in internal forces could be derived from the trends of event rate and energy rate. The local energy changes of the specimen could be deciphered from the curves of energy rate and b-value, making it possible to judge if a component has reached the failure state. This provides an important reference for preventing structural instability and internal damage. However, there are several limitations of this research. Firstly, the AE testing for engineering structures under fire was only applied to reinforced concrete plate-column structure, due to the difficulty of full-scale testing. Secondly, the authors only analyzed the features and trends of some typical AE parameters due to the limited amount of test data. The analysis cannot cover all possible problems. Especially in the data collected from the acoustic emission test in this paper, there is almost no case that the number of events n is very small and the amplitude A is very large. Therefore, the calculation formula of b value proposed in this paper is suitable for the test in this paper. However, in the process of unstable crack growth, there are not many possible crack events, and the energy (amplitude) is large. At this time, the b value calculated by the formula in this paper is likely to be too large, resulting in misjudgment. Our attempt to apply AE techniques in fire safety of engineering structures has high universality and can be applied to various fields of civil engineering, such as health monitoring during the whole life of bridge engineering. To establish an AE-based early warning system for collapse of engineering structures under fire, more tests are needed to determine the failure thresholds of key AE parameters and realize the real-time online monitoring and damage assessment of engineering structures. T