This paper proposes a novel method for in-service evaluation of prestress forces of 7–wire prestressing strand which is frequently employed for prestressed concrete (PSC) bridge girders. The proposed method incorporates the fiber Bragg grating (FBG) sensor system. The straight king wire of the 7-wire prestressing strand is replaced with an instrumented steel tube in which the FBG sensor is embedded. Since the strain of the strand can easily be measured using the in-strand FBG sensor, it is possible to monitor the prestress force of the strand during the in-service state as well as during the jacking process. To demonstrate the effectiveness of the proposed strand with the FBG sensor system, the proposed method is applied to 1 m, 7 m, 64 m and 43 m long strand prototypes to evaluate the prestress forces, and they are respectively tested under different conditions. The experimental results show that the proposed in-strand FBG sensor system is viable for monitoring the prestress forces of the prestressing strand.
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