## 2 Smart Strand with Fiber Optic Sensor  
The dimensions of the Smart Strand are almost identical to those of a regular seven-wire strand [12] widely used in PSC structures, as shown in Figure 1. The steel core wire of the regular strand is replaced with carbon fiber reinforced polymer (CFRP) and a fiber optic sensor with several embedded FBGs, while the CFRP core wire is manufactured. The Smart Strand can also play the role of a structural component as a strand because the mechanical properties of the Smart Strand are similar to those of a regular strand in service. The Smart Strand has the following advantages in the measurement of PF when compared to a conventional technique where ERSGs are attached to helical wires of a strand: highly accurate and stable measurement, durability due to the protection provided by embedment, and the measurement of the actual axial strain of a strand. The reliability of the data obtained by Smart Strands was validated by the comparison with those of ERSGs in laboratory tests and a full-scale specimen [2]. If the scope is extended to relatively advanced techniques to measure the PF, other than ERSGs and those introduced in Section 1, the following attempts have been made: vibration and modal analysis [13], ultrasonic waves [14; 15], acoustic emission and stress waves [16; 17], elasto-magnetic (EM) sensors [18; 19], vibrating strings [20], and FBGs encapsulated inside a strand or attached to the surface of a strand [21; 22; 23; 24; 25; 26; 27]. However, many of these methods suffer similar drawbacks to those mentioned earlier in Section 1: they were mainly applied only to laboratory test specimens and were not sufficiently validated through the field measurement of actual full-scale structures. Furthermore, they can only provide approximate and limited data of PF at a specific location. It is noted that a similar type of strand with the FBG-embedded steel core wire was developed in other studies [21; 22] and efforts were made to apply this strand to actual structures. Although [PERSON] et al. [27] criticized a type of Smart Strand [2; 3; 21; 22] by mentioning the difficulty in positioning the FBGs at predetermined points and in connecting the fiber optic sensor to an optical cable for data logging, these aspects did not matter, at least in the Smart Strand developed in this study [2; 3], according to the authors' experience. That is, the position of the FBGs on fiber optic sensors can be easily traced by a proper marking technique when the CFRP core wire is manufactured through a pultrusion process and helical wires are assembled around the core wire. Besides, the connection of the fiber optic sensor to the optical cable does not involve much difficulty if the core wire is extruded in advance, as shown in Figure 1.  
The wavelength of a reflected light wave measured at each FBG can be converted to the strain at the FBG by Equation (1) when the effect of temperature can be ignored, such as in short-term measurements. However, the temperature should be compensated, as shown in Equation (2), to separate the temperature effect from the total strain. The resulting actual mechanical strain is obtained from long-term measurements over a wide range of ambient temperatures [28; 29; 30]. \[\varepsilon=\frac{1}{1-p_{e}}\frac{\Delta\lambda}{\lambda_{B}}, \tag{1}\]  
\[\varepsilon=\frac{1}{1-p_{e}}\bigg{[}\frac{\Delta\lambda}{\lambda_{B}}-( \alpha+\xi)\Delta T\bigg{]}, \tag{2}\]  
where \(\varepsilon\): strain, \(P_{e}\): photo-elastic coefficient, \(\Delta\lambda=\lambda-\lambda_{B}\): wavelength shift, \(\lambda\): measured wavelength, \(\lambda_{B}\): base wavelength at the start of measurement, \(\alpha\): thermal expansion coefficient, \(\xi\): thermo-optic coefficient, \(\Delta T=T-T_{B}\): temperature change, \(T\): measured temperature, and \(T_{B}\): base temperature at the start of measurement.