# State-of-the-Art Review on Determining Prestress Losses in Prestressed Concrete Girders
## 2 Works Conducted by Researchers Worldwide
### Testing Methods
This is not possible when the tendon is too close to the concrete surface because it could be damaged by the deep hole-cut. Notably, further studies in a controlled environment with monitoring of actual prestress forces were suggested in order to validate its applicability for PC girders with a parabolic tendon [55]. **Method 5** represents a series of NDT techniques which require local vibration measurements along the PC girder. With regards, [PERSON] et al. [66], [PERSON] et al. [67], and [PERSON] et al. [68] performed numerical simulations using the dynamics of PC girders to moving vehicular loads. In their methods, prestress force identifications were obtained using vibration measures if the prestress force in the tendon is assumed as equivalent to an external compressive force applied to the beam ends [69; 70; 71; 72]. Consequently, the natural frequency of the PC girder tends to increase with a decrease in prestress force according to the \"_compression-softening_\" theory. Notably, whether prestressed beams are subjected to the \"_compression-softening_\" effect was extensively discussed as, e.g., in the literature review by [PERSON] et al. [49; 83]. Most of the dynamic methods, cited therein, are based on the identified modal characteristics of a PC girder, id est, natural frequencies and/or mode shapes. The modal characteristics depend on the PC girder's stiffness and, accordingly, are affected by the prestress force. Particularly, such vibration-based identifications require an accurate selection of the mode shape to be utilized in the procedures. In fact, selecting the optimal frequency a priori is challenging, and different frequencies yield varying degrees of accuracy in prestress force determinations. **3. Static NDT Methods**  
Amongst vibration-based techniques, static identifications using vertical deflections have proved to be reliable methods for axial force identification in beam elements. Indeed, measured vertical deflections take accurately into account the changes of structural geometry of the members due to axial force variations on the equilibrium conditions [40; 41; 71; 74; 77; 84] (Figure 1a,b). Experimental simulations were as well conducted on members belonging to space frames and trusses [78; 79; 85]. Likewise, [PERSON] et al. [80] verified the feasibility of estimating prestress force in a PC girder using vertical deflections measured by three-point bending tests. It is also worth noting that this approach only adopts static parameters, thus, in contrast to vibration-based techniques, does not require selecting experimental data for use in the algorithms. \begin{table}
\begin{tabular}{c c c c} \hline \hline (3) & [54; 57] & Relating the tension in the tendon & \\ Semidestructive & [54; 57] & to a vertical deflection recorded & It causes partial damages \\  & & when weights are suspended from & \\  & & it on an exposed length & \\ \hline
**Method** & **Reference** & **Testing on a PC Girder** & **Result and Finding** \\ \hline (4) & [55; 58] & Determining the side pressure to & The embedded tendon, \\ Nondestructive & [55; 58] & close the induced crack in a small & if is too close to the \\  & & cylinderical hole drilled adjacent to & concrete surface, \\  & & the tendon in the bottom flange & could be damaged by \\ \hline (5) & [59; 60; 61; 62; 63; 64; 65; 66; 67; 68] & Vibration test to determine natural & It requires an accurate \\ Nondestructive & [59; 60; 61; 62; 63; 64; 65; 66; 67; 68] & frequencies and/or & selection of the \\  & & dynamic responses & mode shape \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._