Moisture Measurements In Masonry Materials Using Active Distributed Optical Fiber Sensors

A distributed measurement system for moisture in masonry materials is introduced. The system is based on active thermometry and Brillouin-based distributed temperature sensing (DTS). A fiber optic cable with an electrically conductive armoring is heated via electrical resistance. The thermal response of the cable to an electrical current pulse is monitored by measuring the Brillouin frequency shift variations through a Brillouin Optical time-domain analysis (BOTDA) reading unit operated at 50 cm spatial resolution. The feasibility of the BOTDA technology to provide distributed measurements of moisture in masonry materials is verified through field tests. In particular, the relationship between thermal conductivity, measured by the BOTDA sensor, and volumetric water content, measured gravimetrically, has been experimentally determined over samples of cement mortars and a clay brick. The obtained results show that thermal conductivity is sensitive to moisture variations, indicating DTS technology as a promising tool for moisture measurement of building materials. Owing to the feasibility of the proposed experimental procedure for moisture measurements of real masonry walls, the technique appears to be suitable for field monitoring applications. For the experimental tests, a fiber optic heatable cable comprising a central loose tube, 0.83 mm2 copper conductor for active sensing, stainless steel strength members and double layer polyamide (PA) outer sheath was employed. As an example, we show in figure 1 the time evolution of the Brillouin frequency shift distribution along the sensing fiber, upon application of a 10 A heat pulse and with the cable exposed to air (ambient temperature: 21.6 °C). Note that the heated fiber is comprised between z= 9.4 m and z = 11.2 m. In figure 2 we show the average temperature along the heated fiber, acquired during the heating and cooling processes at an acquisition rate of  50 seconds and with the fiber exposed either to air (blue line) or immersed into water (magenta line). It is seen that the BOTDA sensor is capable of tracking accurately the thermal response of the fiber, clearly distinguishing the different boundary conditions.