# Acoustic Emission for Structural Monitoring of Historical Masonry: An In-Field Application
## 6 Discussion and Conclusions  
The results of the experimental campaign confirm the applicability of Acoustic Emission (AE) monitoring for detecting and interpreting damage mechanisms in historic masonry structures, even under the constraints posed by conservation requirements and complex material heterogeneity. The integration of AE analysis with traditional displacement measurements enabled a comparative analysis of the structural response during three loading cycles performed under semi-static conditions. Displacement data revealed a predominantly elastic behaviour up to the peak load of 20.00 kN, with minor residual deformations and a tendency toward displacement recovery in the later stages of the test. These trends may be attributed to internal stress redistribution and micro-settling phenomena rather than significant structural degradation. From the AE perspective, activity was mostly concentrated in the second and third loading cycles. The absence of hits during the first cycle was attributed to an initially overestimated threshold value, later corrected to enable the detection of lower-intensity signals. Once adjusted, the AE system proved effective in capturing signals related to the mechanical response of the arch, including those associated with minor damage progression during loading and unloading phases. Source localisation via the Arrival Time Method revealed a concentration of events in the area between sensors AE3 and AE4, indicating a zone of relatively greater mechanical stress, in line with expectations based on the geometry and loading configuration of the arch. Isolated low-intensity sources were also recorded near the keystone and around sensor 2. These localisations, though simplified by the adoption of a linear model, offer a useful approximation for evaluating the spatial distribution of AE activity in structures where full three-dimensional sensor networks are unfeasible. The load test was carried out on a restored historical arch that had not previously experienced significant structural loading. The adopted loading protocol involved the gradual application of weights in increments of 0.5 kN, with the explicit aim of safeguarding the structure's integrity, given its cultural value and recent rehabilitation. The maximum applied load of 20 kN was deliberately selected to exceed typical service loads, allowing for the observation of structural behaviour under relatively high stress while still remaining within a conservative safety range. In this context, Acoustic Emission signals recorded during the test were mainly related to first-time loading effects rather than the reactivation of pre-existing damage. Due to the absence of a previous loading history and the irregular nature of the observed AE activity, it was not possible to carry out a meaningful quantitative evaluation of the Kaiser effect or to derive reliable Felicity ratios. The test was also interrupted at the end of the third cycle, as the increasing AE activity suggested that continuing the loading process might have compromised the condition of the structure. While some signals occurred at low load levels or during unloading in the later cycles, these can be reasonably attributed to stress redistributions within the masonry rather than to clear reopening of previously activated sources. Several limitations must be acknowledged. The heterogeneity of the masonry, the presence of a reinforcement system (FRCM), and the use of linear localisation in place of a 3D array all impose constraints on the precision and interpretability of the AE data. Additionally, the lack of established attenuation models specific to this type of historic material limits the possibility of fully quantitative assessments. Despite the methodological limitations discussed above, the integration of AE data with displacement measurements offered a more nuanced understanding of the structural behaviour during the load test.