Internal Strain Measurements in Concrete Elements by Fiber Optic Sensors

The measurement of internal strain with a fully noninvasive method should allow us to obtain very useful information, both in laboratory testing and in structural monitoring. Fiber optic sensors are suitable for this purpose because of their very small dimensions. Various applications of fiber optic strain sensors to concrete are available in the literature. This paper reports two applications in particular. The first is aimed at measuring the transversal strain in the core of concrete cylinders subject to compressive tests, avoiding any disturbance to the stress state, and the second deals with reinforced concrete beams externally strengthened with sheets of fibrous composite material. In the latter case, the writers attempt to measure the concrete strain at the interface in order to compare it with the strain of the composite. In both cases, specific procedures were developed to properly place the sensors, avoiding any damage. The performance of fiber optic sensors embedded in concrete specimens was very good and allowed us to achieve a reliable measurement of the transversal strain and Poisson's ratio. The strain measurements at the concrete-FRP interface showed that this technique is promising and can improve the knowledge of the strain state at a debonded region.