Abstract
In post-tensioned structures, it is of the utmost importance that the correct force is transferred to the strand during stressing. The industry standard in South Africa for checking this force is to compare the actual extension measured on-site with the theoretical calculation of the extension at the design stage. This comparison must fall within a specified tolerance. This can be problematic as the elongation is heavily dependent on the amount of friction within the post-tensioning system and the calculation of the friction is dependent on a wide range of variables. As a result, the actual extension quite often falls outside this tolerance, leaving the engineer unsure whether the strand has been under or over tensioned. Verifying the force utilizing Fibre Bragg Grating (FBG) sensors could eliminate this problem.
An exploratory investigation was conducted to determine the viability of attaching these FBG sensors to the bare strand with different adhesives. Several tensile tests were performed. This was done to compare the data obtained from the tensile test with the FBG sensors. These tests determined that the best attachment method was attaching the sensor to the surface of one of the seven wires of the strand with X60 methacrylate. This technique was able to get within 86% of the actual strain measured with a tensile machine. Using the results from the tensile tests, full-scale tests on four concrete beams were done. Although there were difficulties in protecting the FBG sensors during the full-scale tests, the successful beams demonstrated that the data obtained by the FBG was within 4% of the actual extension, which is within the specified tolerance. In contrast, comparing the actual and theoretical extension, the results are unfavourable as they exceeded the specification of 6%. Although the FBG needs better protection techniques due to its fragility, the results indicated that monitoring the short- and long-term losses with FBG sensors could be possible.