# Effects of Grout Compactness on the Tensile Behavior of Ground Splice Sleeve Connectors
## 2 Tensile Tests on GSS Connectors with Different Compactness
### Results and Analysis
#### 2.3.3 Surface Stress and Strain of Sleeve  
(1) Longitudinal stress and strain  
The longitudinal strain of the rebar can be obtained by the stain gauge 3. The two rebars at the upper and lower ends of the sleeve transmit force through the sleeve. Thus, the force applied to the sleeve is equal to the force \(P\) applied to the rebar. Hence, the stress of the rebar can be calculated as:  
\[\sigma=\frac{4P}{\pi(D-d_{si})^{2}} \tag{3}\]  
The longitudinal stress-strain curves for all four cases with full compactness are shown in Figure 12. It can be seen that the longitudinal stress-strain curves show a linear growth trend. As the tensile capacity of the sleeve is approached, the curves do not present plateau like those for the rebars. The maximal values of the longitudinal tensile stresses are less  
Figure 11: Rebar stress-strain curves for D18 connectors with different compactness.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Types** & \(P_{\text{u,avg}}\)**/**kN** & \(L\)**/mm** & \(\tau_{\text{bg}}\)**/MPa** & **Failure Mode** \\ \hline D18-70\% & 102.2 & 348 & 29.7 & Bond-slip \\ D18-50\% & 79.33 & 348 & 32.3 & Bond-slip \\ \hline \end{tabular}
\end{table}
Table 5: Theoretical strength of the rebar-grout bond slip.  
than 300 MPa for all four kinds of sleeves. The rated tensile strength of the sleeves is 550 MPa according to the manufacturer, which means the stress of the sleeve is still within its tensile strength. As can be seen, there is still a great surplus of longitudinal tensile stress. However, the decline stage is not recorded due to the limitation of test equipment and instruments. After a comparison is made with the longitudinal stress of the sleeve (Figure 12) and the stress of rebars (as seen in Figure 10 and Table 4), it was found that the maximal stress cross-sections are not within the length of the sleeves but within the length of outer rebars for all four kinds of sleeves. The reason for this is that the sleeve, grout, and spliced rebars together bear the tensile load at the GSS section, while the outer rebars uniquely bear the tensile load. Therefore, the failure modes are rebar rupture for all four kinds of sleeves.  
The stress-strain curves of sleeves for D18 connectors with different compactness are shown in Figure 13. The observed phenomenon and laws are similar to those of rebar for the connectors with different compactness (as shown in Figure 11). The sleeves of GSS connectors have much lower tensile stresses and tensile strains. The maximal tensile stresses and tensile strains are less than 100 MPa for sleeves with lower compactness (D18-70%, D18-50%), which means the sleeves are still in the elastic stage. The curves drop abruptly in their rise due to either the snap of the rebar or the bond-slip of the rebar-grout. It is noteworthy that the longitudinal stress-strain curves of D18-90% sleeves in Figure 13 show abnormality. One possible reason for this is that a closure of cracks in the grouting material has occurred at loading and it has a greater effect than the newly developed cracks in the grouting material.  
Figure 14: Load-hoop strain curves of sleeve for fully compact connectors.  
Figure 13: Longitudinal stress-strain curves of sleeve for D18 connectors with different compactness.