# Verification of Actual Prestressing in Existing Pre-Tensioned Members
## 3 Experimental Program
### Saw-Cut Method  
Overall, three saw-cuts with an identical axial distance of 120 mm were applied on the upper edge in the middle area of the prestressed concrete sleepers. The sawing was performed in three increments, whereas the depth of the saw-cuts was 10, 20, and 30 mm. The measurement is shown in Figure 7. Maximal depth was chosen with respect to specimens' reinforcement. We intended to avoid cutting the sleepers' prestressing wires, which could affect the integrity of specimens. After each step, we waited for the stabilization of the recorded stress (strain) reading. In the experimental measurements, linear foil strain gauges HBM LY41-50/120 made of ferritic steel (temperature matching code \"1\": \(10.8\times 10^{-6}/\mathrm{K}\)) with a measuring grid length of 50.0 mm and a total length of 63.6 mm were used to record the strain. The linear foil strain gauges were installed in the middle of the axial distance between the saw-cuts. Transversely, they were applied to the centre of the upper edge of the sleepers. The scheme of saw-cuts and position of strain gauges can be seen in Figure 8.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & \(\mathbf{f_{p0.1}}\) **(MPa)** & \(\mathbf{f_{p}}\) **(MPa)** \\ \hline W1 & 1563.56 & 1742.15 \\ W2 & 1531.13 & 1729.94 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Materials properties of prestressing steel.  
Figure 5: Sampling procedure of steel. Exposed prestressing wires (a); samples (b).  
Figure 6: Stress–strain curve of tested samples of prestressing wires.  
Quality of the specimens' surface and grain size are important factors for stress (or strain) measurements. Thus, a foil strain gauge should be selected considering these parameters. It is necessary to identify the aggregate grain size of the examined prestressed concrete structure and then prepare a smooth surface before installing a strain gauge. These difficulties result from the inhomogeneity of the concrete. Generally, in the case of precast concrete members, such as prestressed concrete sleepers, the better quality of used concrete can be expected. Nevertheless, some aged monolithic structures can be, in terms of concrete quality, very problematic. For the strain measurements, as can be seen in Figure 9, the measuring grid length of the strain gauge should be at least five times as long as the largest aggregate grain size. In the case of the precast concrete sleepers in question, the 50 mm long measuring grid was sufficient.  
Analysed specimens were supported by two lines at a distance of 100 mm from the ends, see Figure 10. Supports consisted of steel rollers, thus the static scheme of the simply supported beam with the effective length of 2400 mm was created.  
Recorded stress readings from testing of all seven specimens are presented in Figure 11. Evaluation of obtained results was based on the real value of the concrete's modulus of elasticity determined via removed cylindrical samples in the experimental program.  
Figure 8: Position of saw-cuts (SC) and strain gauges (SG).  
Figure 7: Application of the saw-cut method. Sawing (**a**); final saw-cuts (**b**).  
Figure 9: Length of measuring grid of strain gauges considering aggregate grain size.  
Figure 11: Cont.  
Figure 10: Supported prestressed concrete sleeper during the experiment (**a**) view, (**b**) detail.