Observation of prestress loss in post-tensioned concrete with FBG and LVDT sensors

We report on long-term measurement results on post-tensioned concrete specimens. A total number of 8 specimens, each of dimensions 700×150×150mm³ and four each of two different concrete mix designs, are subjected to prestress by post-tensioning iron bars conducted in a central longitudinal hole. During their drying phase the specimens undergo a prestress loss from creep and shrinkage, resulting in a reduction of their longitudinal size. This change of dimension is observed by a number of sensors applied to the arrangement. As the main sensors for strain, FBG sensing fibers spot bonded to the specimen and FBG sensing fibers bonded on their entire length are used. FBG sensing fibers are applied to both sides of the specimen to allow observation of bending. The experimental arrangement further includes a conventional length measurement apparatus consisting of LVDT sensors (also applied to opposite sides of the specimens) so comparisons can be made between the novel technique based on FBGs and this conventional measurement approach. As each of the 3 types is applied to both opposing sides of the specimens bending can also be monitored. In addition, our data logger will also monitor both room and specimen temperatures. We are especially interested in the performance of the FBG sensors bonded on their entire length that are measuring the local strain in the concrete through direct elastomechanical interaction as we want to clarify the appropriateness of this novel technique for future SHM (structural health monitoring) installations in civil engineering. The readout of the FBG sensors which are all arranged in an optical sensor network is done by a tunable laser approach. Preparation of the set-up has been accomplished recently, and data are now being collected. The observation of the prestress loss process is planned to run over several months, and results will be presented at the conference.