PSI - Issue 64

Hernán Xargay et al. / Procedia Structural Integrity 64 (2024) 1790–1797 Hernán Xargay / Structural Integrity Procedia 00 (2019) 000 – 000

1793

4

25 mm away from each side of the beam's midpoint on the frontal surface, as illustrated in Fig. 1. Acoustic insulation tapes were placed at the supports in order to reduce friction noise. The system was configured with a detection threshold of 40 dB and a frequency filter range set from 20 kHz to 400 kHz, to ensure optimal signal capture and processing. 3. Results and Discussion 3.1. Flexural behavior Flexural strength ( R f ) results of each mortar temperature case are presented in Fig. 2 (a), with the vertical segments representing the range between the minimum and maximum values obtained from four tests. In Fig. 2 (b) the flexural strength loss is illustrated, normalized with reference to the ambient temperature of 20 °C, denoted as R f , 20 . From these graphs, it can be observed a consistent trend: as the temperature levels rise, there is a noticeable decline in flexural strength. Particularly noteworthy is the pronounced degradation in flexural behaviour observed in specimens subjected to temperatures exceeding 200 °C, primarily due to evaporation of both free and adsorbed water. Further deterioration is evident in those exposed to temperatures above 400 °C, mostly attributable to dehydration of the calcium silicate hydrates and aggregates expansions. Notably, R f , 500 equated to 37% of R f , 20 and R f , 600 was 14% of R f , 20 . As temperature increases, fractures typically occurred near the mid-span, along pre-existing weaker paths induced by thermal effects. This leads to a significant reduction in energy absorption capacity.

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1

(a)

(b)

R f / R f,20

R f [MPa]

20 °C 100 °C 200 °C 300 °C 400 °C 500 °C 600 °C

0

100 200 300 400 500 600 700

Mortar 8,4

8,0

7,1

5,0

4,9

3,1

1,1

Temperature [ ° C]

Fig. 2. (a) Flexural strengths ( R f ) at various thermal treatment levels and (b) R f / R f , 20 plots, being R f , 20 the reference flexural strength at 20 °C.

3.2. Temporal evolution of AE parameters Fig. 3 represents the applied load and AE parameters versus elapsed test time for each mortar temperature group. Each graph depicts the load-time curve in red, while the blue dots represent the AE signal amplitude parameter, and the black line shows the cumulative energy of AE events. As a general rule, it can be seen that the amplitudes distribution tends to increase as the fracture process progresses. The maximum amplitude events are typically observed approaching and immediately after the peak load. With increasing temperature damage, the cracks generated in the fracture zone become more tortuous, leading to more clusters of AE events in the post-peak curve. Specimens exposed to 500 °C and 600 °C exhibit comparatively lower AE amplitudes, especially at peak load and during the post-peak stage. The observed lower amplitudes can be attributed to the degradation of the inner structure and the presence of multiple micro-cracks resulting from thermal treatment. On one hand, thermal damage results in a significant irreversible release of internal energy during thermal process. On the other hand, the skeleton cohesive forces were weakened and then less energy is needed during load-induced fracture and less energy is emitted as elastic waves. Furthermore, preexisting thermal cracks dampen the propagation of AE waves and attenuate AE signals.