Long-Term Characteristics of Prestressing Force in Post-Tensioned Structures Measured Using Smart Strands  
[PERSON]  
1 Department of Infrastructure Safety Research, Korea Institute of Civil Engineering and Building Technology, 283, Goyang-daero, Ilsangeo-gu, Goyang-si, Gyeonggi-do 10223, Korea; [EMAIL_ADDRESS] (S.-H.K. ); [EMAIL_ADDRESS] (S.Y.P)  
[PERSON]  
1 Department of Infrastructure Safety Research, Korea Institute of Civil Engineering and Building Technology, 283, Goyang-daero, Ilsangeo-gu, Goyang-si, Gyeonggi-do 10223, Korea; [EMAIL_ADDRESS] (S.-H.K. ); [EMAIL_ADDRESS] (S.Y.P)  
[PERSON]  
2  
Footnote 1: email: {sang-hyun  
distribution. Although many attempts have been made to validate the theoretical distribution of the PF by using conventional sensing technologies, such as electrical resistance strain gauges (ERSGs), their outcomes were not very successful due to several drawbacks of the ERSGs that led to unreliable data. For example, ERSGs attached to a surface of a strand and lead wires for data acquisition are susceptible to damage during the insertion of strands into a duct, tensioning, and grouting in post-tensioned members [2; 3], and at transfer and during the casting of concrete in pre-tensioned members [4; 5]. In particular, ERSGs exhibited poor performance for long-term measurement due to inherent low durability when compared to fiber optic sensors [6]. The lifetime of ERSGs is typically much shorter than that of infrastructures. Therefore, alternative experimental methods, instead of measuring the strains of a strand using ERSGs, have also been adopted to indirectly predict the long-term prestress losses, although the accuracy is decreased. [PERSON] et al. [7] employed the crack opening method to estimate the losses in full-scale PSC beams removed from an actual bridge. [PERSON] et al. [8] used vibrating wire gauges to measure the prestress loss in full-scale girder specimens. However, their calculated loss may have been less accurate because the strand strains were derived from the concrete strains obtained from the vibrating wire gauges embedded in concrete at mid-span. The study only provided limited information on the prestress loss at the mid-span. [PERSON] and [PERSON] [9] monitored long-term prestress loss in a pedestrian bridge using fiber Bragg gratings (FBGs) embedded in a concrete section. As in the case of [PERSON] et al. [8], the strand strains were indirectly derived from these concrete strains. [PERSON] and [PERSON] [10] compared several predictive equations and field data in terms of the long-term prestress losses in some PSC box girder bridges. However, the field data were not modified by temperature correction, even though the data fluctuated temporally. Furthermore, although the detailed methodology to obtain the field data was not stated, the data showed an unstable trend. [PERSON] and [PERSON] [11] also compared the theoretical prestress losses obtained by several predictive equations with the measured losses. However, the comparison was made only at the anchorage of unbonded tendons because of the absence of any data measured inside the tendons using any sensors. In summary, prestress losses were estimated indirectly or measured only at a specific point on a tendon in most of the previous studies due to the limitations of conventional sensing technologies. The complete measurement of the PF distribution along a tendon has very rarely been reported for full-scale specimens or actual infrastructures. In order to overcome the abovementioned drawbacks and limitations of existing sensing technologies in the estimation of the PF distribution, Smart Strands with embedded fiber optic sensors have been developed recently [2; 3]. The Smart Strands were applied to a 20 m-long full-scale specimen and a 60 m-long PSC girder bridge to investigate the long-term characteristics of the PF distribution affected by long-term prestress losses.