FBG sensors offer many advantages, such as a lack of sensitivity to electromagnetic waves, small size, high durability, and high sensitivity. However, their maximum strain measurement range is lower than the yield strain range (about 1.0%) of steel strands when embedded in steel strands. This study proposes a new FBG sensing technique in which an FBG sensor is recoated with polyimide and protected by a polyimide tube in an effort to enhance the maximum strain measurement range of FBG sensors embedded in strands. The validation test results showed that the proposed FBG sensing technique has a maximum strain measurement range of 1.73% on average, which is 1.73 times higher than the yield strain of the strands. It was confirmed that recoating the FBG sensor with polyimide and protecting the FBG sensor using a polyimide tube could effectively enhance the maximum strain measurement range of FBG sensors embedded in strands.

This paper presents the results of the performance test and long-term monitoring of the prestressing force inside concrete performed on a pretensioned Ultra-High Performance Concrete (UHPC) deck. The force is measured by applying a 7-wire strand embedded with an FBG (Fiber Bragg Grating) sensor. The performance test was conducted on a 3.7 m × 1.8 m pretensioned deck specimen through wheel loading tests to verify the applicability of the measurement method. In addition, a 12.3 m long and 4.8 m wide bridge with a pretensioned UHPC deck was erected and long-term monitoring was conducted over three years to verify the applicability of the method to real bridges. The effectiveness of the measurement method of the prestressing force inside concrete is verified, and the long-term monitoring data are used to investigate various temperature compensation methods. The results show that the proposed method enables effective measurement of small changes in the prestressing force inside the concrete. These changes are caused by the external forces acting on the bridge in service and provide sufficient durability for long-term sensing. The analysis of the prestressing force obtained through long-term monitoring reveals the necessity of conducting temperature compensation for the consistency of the data acquired using the FBG sensor. Moreover, the selection of the thermal expansion coefficient appears also to be of critical importance for temperature compensation.

In this paper, an experimental program has been conducted to investigate transfer length in high strength concrete members pretensioned through a seven-wire strand with FBG sensors. To measure transfer length, five members were fabricated, which had a length of 3 m and a cross-section of 150×150mm. It was measured that the concrete compressive strength was 58MPa at pretensioning. Test results indicated that more precise and reliable measurement on the transfer length was attained with FBG sensors than conventional gauges attached on concrete surface. Through comparing the measured transfer length and predictions, applicability of several transfer length models in literature was investigated. This paper can be useful for relevant research field such as investigation on the bond mechanism of a seven-wire strand in concrete members.

This paper presents results of one-year monitoring on prestressing force of a 7-wire steel post-tensioning strand which is installed in a UHPC(ultra high performance concrete) bridge with 11.0 m long, 5.0 m wide, and 0.6 m high by using a FBG-encapsulated 7-wire steel strand. The initial prestressing forces and the prestress changes during a vehicle load test were measured using the FBG-encapsulated strand. The results show that the FBG-encapsulated 7-wire strand is very effective for monitoring the prestress forces even the change in the tension force is very small. Additionally, it was indicated that selection of the thermal expansion coefficient which is used for the temperature correction shall be carefully carried out.