## 1 Introduction  
Loss of prestress force jeopardizes the structural integrity, and in extreme cases could lead to catastrophic collapse of prefabricated segmented structures. As prefabrication has been becoming more and more popular in construction, effective monitoring of the prestress force is one of the primary concerns for management of prefabricated structures. Many studies have been conducted to investigate the effect of prestress force on the vibration characteristics of prestressed concrete beams for developing methods to monitor the prestress force through vibration tests. The researchers [1; 2; 3] reported that an increase in the prestress force could lead to a decrease in the natural vibration frequencies of prestressed beams. However, different, or even opposite observations and conclusions have also been reported, with respect to the effect of prestress force on the natural vibration frequency of the prestressed concrete beams. The studies [4; 5; 6; 7] concluded that the prestress force has a negligible effect on the vibration frequencies of prestressed beams, while [8; 9; 10; 11] have demonstrated that prestress force and natural vibration frequency are positively correlated. Besides the mixed observations of the relationship between prestress force and vibration frequency, the vibration frequency of a structure is not always sensitive to the change in prestress force [12]. The effectiveness of using the natural vibration frequency as a feature to evaluate prestress force is also affected by other factors, e.g., measurement noise [13; 14], temperature, and humidity [14; 15], which may cause even more substantial variations in vibration frequencies of a structure than the change in prestress force. Therefore, the natural vibration frequencies of prefabricated structures, although they are relatively easily obtained through vibration tests, are not a reliable indicator for identification of the changes of the prestress force. The neutral axis is a universal parameter of beam-like structures that is related to the centroid of stiffness in a cross-section. [PERSON] et al. [14] reported that temperature changes in a concrete beam do not have a measurable influence on the neutral axis location, which is directly related with bending stiffness of the structural cross-section. The neutral axis location is only dependent on the material and geometric properties of the beam cross-section [16]. It remains stable under changing temperature, and it is insensitive to environmental noise, therefore making it a promising feature for structural condition identification. This characteristic of the neutral axis implies that it has the potential to be used for damage detection [17; 18; 19; 20], and condition assessment of the structures due to typical damage such as cracking, delamination, and loss of materials, which affects the cross-section, and may lead to a change in the location of the centroid [17]. The above studies focused on using the neutral axis location to detect whether damage has occurred. However, the possibility and feasibility of using the neutral axis location to quantify the prestress force, to the best of our knowledge, is yet to be reported. Since the neutral axis location depends on the force equilibrium over the cross-section, identification of the neutral axis location would lead to the determination of prestress force. [PERSON] and [PERSON] [21] reviewed the currently available prestress force monitoring methods for prestressed concrete structures, and found that most studies focused on monitoring the occurrence of prestress force loss rather than the quantification of the prestress force level. In recent years, precast prestressed segmental beams have been widely used in bridge constructions around the world, owing to the merits of better quality control and efficient construction on site.