Monitoring with FBG to control cracking of r.c. structures before retrofitting

The structural reliability of concrete beams can be strongly reduced by several cracking patterns, because of the rebar corrosion due to moisture. External bonding of fiber reinforced polymer (FRP) sheets is a technique currently used as structural strengthening system, but it can not protect the structure from bar corrosion, if the crack opening is not reduced before applying the sheets. The use of shape memory alloys (SMA) is evaluated in this paper as an alternative to traditional systems, as the application of upwards loads, in order to reduce both the midspan deflection and the crack width. This particular application requires an accurate control of the strain during the SMA actuation. Fiber Bragg grating (FBG) sensors can be successfully used with this aim. Therefore, the combined use of FBG as sensing and SMA as actuation systems is evaluated. Small scale smart concrete beams with FBG sensors and SMA actuators were designed and tested. FBG were used to monitor the deformation in the SMA wires and to determine the beam critical conditions. The effectiveness of both sensing and actuation methods was experimentally demonstrated, while a number of issues still need to be investigated regarding the different factors influencing the amount of repair provided by the SMA actuators.