# Verification of Actual Prestressing in Existing Pre-Tensioned Members
## 6 Conclusions  
This paper focused on the comparison of the standard value of prestress losses with the experimentally determined actual state of prestressing performed on precast pre-tensioned sleepers. Based on the experimental and numerical analyses, the following conclusions can be summarized:  
An essential fact resulting from our investigation is that the presented methods can be performed without the application of an external load. Therefore, the application of these techniques on existing prestressed concrete bridges (or structures in general) can be performed quickly and easily and does not require expensive tools. In the case of saw-cuts with a depth of 20 mm, obtained stress relief curves show some discrepancies from the linear association between stress change and the depth of saw-cuts. These differences could be attributed to unsatisfactory stabilization of strain reading and should be studied in the future. Therefore, it is suitable to apply saw-cuts gradually to check the expected linearity of the stress relief curve. On the other hand, 10- and 30-mm deep saw-cuts showed good conformity with the numerical analysis. Residual prestressing force determined using the saw-cut method after one year in almost all tested prestressed concrete sleepers varied from 80 to 90% (in some cases even lower). Consequently, according to numerical and experimental analysis, the value of prestress losses was mainly from 10 to 20%. These values are related to the standard maximal value of prestressing force (P\({}_{\text{max}}\)). As a result, obtained prestress losses show good agreement with the Eurocodes (14.8%). In practical use, it would be necessary to use a conservative value to determine the load-carrying capacity of the prestressed concrete structure. Obtained results suggest that a relatively small intervention into the concrete structure could lead to notable local stress relief. In the case of 30-mm deep saw-cuts in the axial distance of 120 mm, it was possible to evoke approximately 70% stress change. Such intervention was negligible in comparison to the cross-section of the prestressed concrete structure. Hence, the integrity of the structure was preserved, and simultaneously, important data about residual prestressing force could be recorded. In practice, the underestimation of the value of prestressing losses affected, for example by corrosion, can lead to failure of the prestressed concrete structure. An important factor for the successful indirect determination of residual prestressing force using the saw-cut method is knowledge of real material properties. The modulus of elasticity plays an especially key role in the presented method. Unreliable values of material properties can lead to dangerous discrepancies, which would ensue in an \begin{table}
\begin{tabular}{c c c} \hline \hline
**Point** & \(\boldsymbol{\sigma_{\text{p}}}\) **(MPa)** & **Prestress Losses (\%)** \\ \hline I & 1011.64 & 26.4 \(\pm\) 5.7\% \\ II & 969.28 & 29.6 \(\pm\) 5.7\% \\ III & 893.37 & 35.9 \(\pm\) 6.4\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results from the MBN measurement (Specimen 4; Wire 1). incorrect evaluation of load-carrying capacity and remaining service life of the investigated prestressed concrete structure. In our opinion, the main outcome of our study is that the measurements confirmed the assumption of linear association between the depth of saw-cuts and stress relief. This fact could help engineers in cases when it is not possible to cause 100% stress change in the monitored area. The axial distance between saw-cuts of 120 mm seems to be sufficient, as the installed strain gauges should not be damaged during sawing. The application of the magnetic Barkhausen noise technique confirmed the results obtained from saw-cut testing. However, values of prestress losses determined using the MBN method seemed to be more conservative. The interval of prestress losses after the evaluation is wide.