# 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
#### 2.3.2 Stress and Strain of Rebar  
The strain of the rebar can be obtained by the stain gauge 1 and 4. The force \(P\) applied to the rebar can be measured by the 600 kN capacity electro-hydraulic servo universal testing machine. Thus, the stress of the rebar can be calculated as:  
\[\sigma=\frac{4P}{\pi d^{2}} \tag{1}\]  
The rebar stress-strain curves for fully compact cases are shown in Figure 10. It can be seen that the stress-strain curves of the rebar present a linear growth trend. The curves appear jagged at both the initial stage and the ultimate tensile capacity stage. This phenomenon is caused by the fixture-reinforcement slip at the initial loading stage, the breakage of rebar-grout teeth at the final stage. The curves suddenly decline after reaching the peaks. However, the decline stage is not detected due to the limitation of test equipment or instruments. In accordance with mechanical properties of rebar (as shown in Table 4), the stresses of rebars are beyond the tensile yield stresses and amount to the tensile ultimate strengths for all four cases. This indicates that the rebars have yielded, and finally, the rebars rupture when the loads amount to the tensile ultimate load-bearing capacity.  
Figure 11 shows D18 rebar stress-strain curves with different compactness. It was found that rebars of GSS connectors (D18-70%, D18-50%) have lower tensile stresses and tensile strains than those of other cases. More specifically, the maximal tensile stresses and tensile strains are all less than the yield strength and yield strain of rebar (shown in Table 4, namely 375 MPa and 2.206 \(\times\) 10\({}^{-3}\), respectively, for GSS connects with grout compactness of 70% and 50%. The reason for this is that the connectors with lower grout compactness experience the bond-slip failure modes of rebar-grout, while other cases experience the rebar rupture failure modes (shown in Table 4). Since the bond strength of rebar-grout is less than the tensile strength of rebar, the rebars are being slowly pulled out from the GSS connectors and the connectors fail, while the stresses and strains of rebars have not fully developed and changed. Consequently, the tensile stress and strain of spliced rebars for GSS connectors with lower compactness are much lower than those of connectors with higher compactness.  
The bond-slip failure mode includes the grout-pipe bond slip failure and the bar-grout bond slip failure. When the stress of the rebar is small than the strength of the bar-grout bond slip, the failure mode can be considered as the grout-pipe bond slip failure. Otherwise, it is the bar-grout bond slip failure. To clarify the failure mode of slip without breaking the sleeve, the bond strength was calculated and compared with the stress of rebar.  
Figure 10: Rebar stress-strain curves for fully compact connectors.  
The theoretical strength of rebar-grout bond slip can be calculated as:  
\[\tau_{\text{bg}}=\frac{P_{\text{u, avg}}}{\pi dCL} \tag{2}\]  
where \(P_{\text{u, avg}}\) is the tensile load-bearing capacity, \(d\) is the nominal diameter of rebar, \(L\) is the embedded length of the rebar, and \(C\) is the grout compactness.  
The theoretical strength of the bar-grout bond slip was calculated and obtained in Table 5.  
To verify the bond-slip failure mode of GSS connectors (D18-70%, D18-50%), the relevant maximum reinforcement strains were compared with the strength of the bar-grout bond slip. The results showed that the stress of the two rebars were 149.2 MPa and 52.3 MPa, respectively, and were larger than the strength of the bar-grout bond slip. This indicates that the failure mode of GSS connectors (D18-70%, D18-50%) were the bar-grout bond slip.