## 4 Mechanical Properties
### Ductility and Residual Deformation
#### 4.3.2 Residual Deformation  
The residual deformation of a specimen refers to the unrecoverable deformation of the specimen after the tension, that is, the deformation could not return to the initial state after unloading to zero in the plastic stage. In the test, the values of the residual deformation \(u_{0}\) after uniaxial tension and those of the residual deformation \(u_{20}\) after repeated tension and compression loading of each specimen are shown in Table 4. According to the specifications of grade-I connections, \(u_{0}\) should be no more than 0.1 mm and \(u_{20}\) should be no more than 0.3 mm. The relationship between the average residual deformation of the specimen and the lapping length is shown in Figure 17.  
Figure 17: Relationship between residual deformation and lapping length under uniaxial tension and high-stress repeated tension and compression loading. Under the anti-deflection measures, the residual deformation values \(u_{0}\) and \(u_{20}\) of the specimens decreased with the increase in the lapping length. In the case of uniaxial tension, \(u_{0}\) did not meet the requirements of the specification when the lapping length was \(6d\)[30], whereas \(u_{0}\) met the requirements of the specification when the lapping length was no less than \(8d\)[30]. In the case of repeated tension and compression, \(u_{20}\) was much larger than the requirement of the specification when the lapping length was \(6d-8d\)[30], whereas it was very close to the requirement of the specification when the lapping length was no less than \(10d\)[30]. For specimens with anti-deflection measures, \(u_{0}\) consisted of the residual deformation of the steel bar with the lapping length of \(4d\) and the micro-slip between the steel bar and the grouping material. The residual deformation of the steel bar is the unrecoverable deformation of the steel bar after tension. Micro-slip between the steel bar and the grouting material refers to chemical cementation strength losses and the friction and mechanical bite force between the steel bar and grouting materials. It increased gradually for the specimens after tensile loading, so a relatively small slip between the steel bar and grouting material was produced. Micro-slip occurred at the loading end in the initial stage of loading, and it also occurred at the free end of the steel bar with the increase in the load [32]. However, when the lapping length was long, there was no micro-slip at the free end, so the residual deformation decreased with the increase in the lapping length. The \(u_{0}\) value of the specimen without anti-deflection measures consisted of residual deformation of the steel bar, micro-slip between the steel bar and grouting material [32] and bending deformation of the steel bar. As shown in Figure 17, for specimens with anti-deflection measures and the same lapping length, the residual deformation of the specimen under a high-stress repeated load  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen** & \(A_{\text{sgt}}\) & \(u_{0}\)/\(u_{20}\) **(mm)** & \begin{tabular}{c} **Whether the Residual Deformation** \\ **Met the Requirements of [30]** \\ \end{tabular}  & \(R_{d}\)=\(\delta_{d}\)/\(\delta_{g}\) \\ \hline L-1-1-N & — & 0.22 & N\({}^{c}\) & — \\ L-1-2-N & — & 0.33 & N & — \\ L-1-3-N & — & 0.52 & N & — \\ L-1-4-Y & — & 0.  
was larger than that under uniaxial tension, which was caused by the further development of the cracks and an increase in the micro-slip between the steel bar and grouting material during repeated tension and compression loading. The residual deformation of the specimens without anti-deflection measures was larger than that of the specimens with anti-deflection measures in the case of uniaxial tension.