# Effect of Prestress Levels and Jacking Methods on Friction Losses in Curved Prestressed Tendons
## 3 Experimental Program
### Prestressing  
Figure 4 shows the systems used to prestress the steel tendons. In general, the load cells are installed between the hydraulic jack and anchorage to measure the prestress force at the jacking end. However, in this experiment, the prestress force was calculated from the strains, which were measured using strain gauges installed at the center of the tendon and at one end (jacking end). The reason is that it can be difficult to determine the prestress force accurately when using a load cell because of the unpredictable stresses in the cell itself.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Steel rebar**} \\ \hline
**Type** & **Diameter** & **Yield strength** & **Tensile strength** & **Modulus of elasticity** & **Elongation** \\  & **(mm)** & **(MPa)** & **(MPa)** & **(MPa)** & **(\%)** \\ \hline D-10 & 9.53 & 475 & 766 & \(2.01\times 10^{5}\) & 14.4 \\ \hline D-16 & 15.9 & 466 & 679 & \(2.11\times 10^{5}\) & 17.2 \\ \hline D-25 & 25.4 & 471 & 723 & \(2.06\times 10^{5}\) & 15.8 \\ \hline \multicolumn{4}{c}{**Sheath**} \\ \hline
**Diameter** & **Thread** & **Profile height** & **Profile width** & **Wall thickness** \\ \hline Inside: d\({}_{1}\) & Outside: d\({}_{2}\) & e & a & b & s \\ \hline (mm) & (mm) & (mm) & (mm) & (mm) & (mm) \\ \hline
65 & 72 & 20 & 3.5 & 12.5 & 0.3 \\ \hline
**Weight** & **Bending behavior** & **Relative volume of profile** & **Bending radius** & **Bending radius** \\ (**kg/m**) & **(N)** & **(cm\({}^{3}\)/cm\({}^{3}\))** & **(m)** & **(m)** \\ \hline
0.59 & 222 & 0.12 & 0.75 \\ \hline \hline \end{tabular}
\end{table}
Table 4: _Cont._  
Figure 4: _Cont._Only one among the four tendons of each girder was tensioned twice to measure the friction losses accurately, and the tendons were not fixed after the stretching was complete. There was a 1-h difference between the first and second measurements. In the case of jacking at both ends, the prestressing devices had to be moved and set for jacking at the other end after the jacking at one end was completed, and this task required about 1 h to complete. In the case of jacking at one end, the prestressing devices did not need to be moved and set, but jacking was performed with 1-h intervals to maintain the same experimental conditions as those for jacking at both ends.  
The KSCE design code [15] recommends that the smaller value between 0.8\(f_{pu}\) and 0.9\(f_{py}\) be chosen for the maximum jacking stress of the prestressing tendon, where \(f_{py}\) and \(f_{pu}\) denote the yield stress and ultimate stress of the prestressing tendon, respectively. There was a concern, however, that the tendon may be broken if the tension work was performed using one of these values, owing to various factors, such as fabrication error of the tendon or corrosion from exposure to the external environment. Therefore, to achieve stability in the jacking work, the maximum prestress level was determined based on the serviceability limit state in this experiment. The KSCE design code [15] recommends that the maximum jacking stress of the tendon at the serviceability limit state should not exceed 0.45\(f_{pu}\) (\(\approx\)0.65\(f_{py}\)). Prestress forces corresponding to 13%, 18%, 24%, 29%, 34%, 39%, and 45% of the ultimate tensile strength of the prestressing tendon were applied to each of the two girders. These prestress forces ranged from 303.9 to 1033.3 kN. Strain gauges were installed on the surface of the tendons, embedded at the midspan bottom of the girder. All of the strain data were recorded using the data-logger EDX-1500A (Kyowa, Chofu, Japan).