Thermal strain sensing of concrete piles using brillouin optical time domain reflectometry

Abstract

Recent advancement in distributed fiber-optic sensing offers new possibilities for performance monitoring in the field of geotechnical and civil engineering. Brillouin optical time-domain reflectometry (BOTDR) is a commercially available technology that allows distributed strain measurements in the microstrain range along the full length of an optical fiber. By integrating a single fiber-optic cable into soil or a structure, an unprecedented amount of reasonably accurate (±30με), spatially resolved data could be obtained. Since the BOTDR data is influenced by both strain and temperature, it is important that methods to separate the two effects are fully understood. This paper describes the BOTDR temperature compensation method by implementing appropriate thermal expansion coefficients of optical cables and structures to the raw data. In the laboratory study, validation of the instrumentation technique was conducted in a concrete beam by embedding two types of optical cables consisting of tight-buffered and loose-tubed coatings to measure thermal strains response during concrete curing. Temperature readings inferred from optical fibers were found to be in accordance to the thermocouples. A field study of axially loaded concrete pile subjected to cooling and heating cycle is presented. Measurements in the test pile and adjacent borehole indicate similar strain profiles and temperature changes between BOTDR and conventional instrumentation such as vibrating wire strain gauges and thermistors. General steps to derive the temperature compensated strain profiles observed in the thermal pile as a result of cooling and heating is presented. The data enables load-transfer profiles to be interpreted and used as framework to understand pile response to temperature changes.