## 3 Innovative SHM Techniques
### Distributed Sensors  
FBG sensors can provide a quasi-distributed measurement by multiplexing many FBG sensors at discrete points along the fibre. This technology, however, allows for a limited number of gratings (usually less than 100) and can be expensive. A truly distributed monitoring can be achieved by using distributed fibre optic sensors (DFOSs), enabling the mapping of parameters at any point along a fibre. Their operating principle is based on the scattering phenomena related to the interaction between light and an optical medium [116]. Three scattering processes may occur in a DFOS: Raman, Brillouin and Rayleigh. An exhaustive review of DFOS and their applications for SHM of civil engineering structures can be found in [116; 117; 118]. An essential benefit of DFOS technology is that it only requires a single connection cable to transfer the acquired data to the reading unit. In contrast, discrete sensors typically require many connecting cables. As a result, distributed sensing improves cost-effectiveness and, at the same time, opens a wide range of significant applications, such as the continuous (in space and time) monitoring of structures. However, this is still a recent and developing technology, and few applications in SHM have been found in the literature. DFOSs based on Brillouin optical time domain reflectometry (BOTDR) were applied by [PERSON] et al. [119] to study the behaviour of a 17 th century Italian palace severely damaged by the Umbria-Molise earthquake in 1997. The sensors were embedded into carbon fibre reinforced polymers (CFRP) adopted to strengthen and repair damaged walls and vaults. The effectiveness of Brillouin strain monitoring was verified, even for weak strain levels, while the effectiveness of the \"smart\" composite for crack opening detection was also successfully verified. This application introduced the integration of FOS to retrofitting systems to simultaneously combine strengthening and monitoring sytems for historic structures, which is extensively discussed in the next section. [PERSON] et al. [120] implemented a DFOS system based on the optical backscattered reflectometer (OBR) technique [115] to monitor the masonry vaulted floor of Sant Pau Hospital (a UNESCO World Heritage Site) in Barcelona during the replacement of two columns. This system allowed for the successful assessment of the structural stability and safety of the floor by analysing stress redistribution in a distributed and continuous way (in both time and space), without any service interruption. [PERSON] et al. [121] used three different monitoring systems, namely total stations, laser scanning, and distributed fibre optic cables based on BOTDR, to investigate the structural response of historic barrel vaults to piling-induced settlements during the London Bridge Station Redevelopment project. Compared with the total stations method, distributed sensors provided significantly higher spatial coverage. Moreover, the fibre optic systems were able to estimate the location and width of new intrados radial cracks that formed during piling. This unique capability is helpful for serviceability-based damage assessment. Integration of FOS in Textile-Based Composites  
The fibrous nature of the sensors proved to be suitable for integrating them into technical textiles frequently used as reinforcement for the structural retrofit and seismic upgrade of earthenwork and masonry structures. The integration of FOS for monitoring purposes in technical textiles with reinforcing capabilities has been actively explored in the last 15 years. In particular, their use of unreinforced masonry structures in seismic-prone areas fulfills the market need for research and development [122]. [PERSON] [123] and [PERSON] et al. [124] reported the activities carried out by the EU project POLYTECT [125], which focused on the development of novel geotextiles with embedded FOS for monitoring geotechnical and masonry structures.