# State-of-the-Art Review on Determining Prestress Losses in Prestressed Concrete Girders
## 4 Concluding Remarks  
A state-of-the-art review of some important research works conducted worldwide on determining prestress losses in PC girders allowed to analyze various information and trace future developments. Some references affirm that a variation in prestress force does not significantly affect the vibration responses of PC girders. Accordingly, this makes uncertain the use of natural frequencies as appropriate parameters for prestress loss determinations. Vice versa, most of laboratory works show a slight increment in the eigenfrequencies under the increase in prestress force. This behavior is related to the concrete mechanics, and is a main consequence of the effect of crack and microcrack closure along PC girders. However, vibration-based identification methods require an accurate selection of the mode shape because different natural frequencies provide varying degrees of accuracy in prestress force evaluations. By considering these characteristics, the manuscript focused then on the static NDT method preliminary proposed by [PERSON] et al. [80] through laboratory simulations on an uncracked PC beam specimen. The procedure can accurately and instantaneously determine the effective prestress force using vertical deflection measurements, of 0.01 mm in accuracy, under actual ambient conditions [51; 81]. The precision of estimation improved when the PC beam was subjected to a high prestress force and, moreover, when midspan deflections were taken into account. Information regarding the flexural rigidity of a PC girder under investigation are also necessary. With regards, an average value of the chord elastic modulus, of 1 MPa in accuracy, must be estimated by compression tests on a set of concrete cores, drilled along its span, at the time of deflection measurements. Besides, the static NDT method does not require any direct measure of the tension force in the tendon and, mostly, in contrast to dynamic NDT ones, does not request selecting experimental data for use in the algorithms (Table 5). Figure 4: Prestress force determinations \(N_{a}\) based on (**a**) Equation (8a) (Test 1) and (**b**) Equation (8b) (Test 2). Symbols + refer to the comparison between determined \(N_{a}\) and measured values of prestress force \(N\) for all the 27 test combinations. The dashed lines with symbol \(\times\) represent the sensitivity analyses. \begin{table}
\begin{tabular}{c c} \hline \hline
**Advantages** & **Disadvantages** \\ \hline \hline \begin{tabular}{c} (1) Precise determinations by vertical deflection \\ measurements of 0.01 mm in accuracy \\ \end{tabular} & \begin{tabular}{c} (1) Determination of the concrete elastic modulus by \\ compression tests on a set of drilled cores \\ \end{tabular} \\ \hline \begin{tabular}{c} (2) No requirement of direct measure of the tension \\ force in the tendon \\ \end{tabular} & \begin{tabular}{c} (2) Vertical deflections, of 0.01 mm in accuracy, are not \\ always easy to measure in situ \\ \end{tabular} \\ \hline \
\begin{tabular}{c} (3) No requirement of any selecting experimental data \\ \end{tabular} &
\begin{tabular}{c} (4) Determinations take into account the combined \\ effects of tendon relaxation, concrete creep and \\ shrinkage, and parameters of the real environment \\ \end{tabular} & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Characteristics of the static NDT method preliminary proposed by [PERSON] et al. [80] for determining prestress losses in PC girders. In conclusion, to make the NDT method applicable in situ, further studies should focus on the measurement of vertical deflections induced by bending tests with vehicle loading along PC bridge girders [88; 89; 90], in which their constraint stiffness should be evaluated with unknown boundary conditions.