## 4 Test Results and Analysis
### Additional Confinement Effect of Stirrup Rebars (A12H-1 and -3, A12H-8 --11)  
#### 4.3.1 Fracture Mode  
The dimensions of the concrete blocks in A12H-1, -8, -9 and A12H-3, -10, -11 were 350 \(\times\) 350 mm and 800 \(\times\) 800 mm, respectively. A12H-1, -8, -9 and A12H-3, -10, -11 all had identical spirals with \(D\) = 295 mm and \(\varnothing\)16, but different stirrup rebars. A12H-9 and -11 had \(\varnothing\)10 stirrups, A12H-8 and -10 had \(\varnothing\)13 stirrups, and A12H-1 and -3 had \(\varnothing\)16 stirrups. The A12H-1, -8, -9 specimens had \(A/A_{g}\) = 1.82 and were fully confined by the spiral and stirrup, as shown in Table 5. That may be why they exhibited similar load-displacement curves, as shown in Figure 10. The crack patterns shown in Figures 6 and 11 might imply that their behavior was different. Obviously, more cracks and a greater cracked area were observed on the side surface of the concrete block in the specimen with the smallest stirrup rebar. This confirms that for the specimen fully confined with the spiral with \(A/A_{g}\) = 1.82, the load-displacement curve and ultimate behavior were rarely affected by the stirrup, but crack occurrence and pattern were highly related to the stirrup rebar reinforcement. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**No.** & **Spiral** & \(f_{lat,sp}\) (MPa) & \(A_{core}\) (mm\({}^{2}\)) & \(4.1\)/\(f_{lat,sp}A_{core}\) (kN) & **Difference** (kN, (\(\frac{1}{2}\))) & **Test Results** (kN, (\(\frac{1}{2}\))) & **Difference** (kN, (\(\frac{1}{2}\))) & **(\(\frac{1}{2}\))/\(\frac{1}{2}\)** \\ \hline A12H-5 & \(D295\), \(\varnothing\)10 & 4.40 & 36,666 & 662 & 1974 & 3947 & 184 & 14.4\% \\ A12H-7 & \(D745\), \(\varnothing\)10 & 1.74 & 368,890 & 2636 & 1947 & 5228 & 5228 & 793 & 9.3\% \\ A12H-3 & \(D295\), \(\varnothing\)16 & 12.25 & 36,666 & 1842 & & & & \\ A12H-6 & \(D745\), \(\varnothing\)16 & 4.85 & 368,890 & 7338 & 5496 & 6022 & 793 & 9.3\% \\ \hline \end{tabular}
\end{table} 
Table 4: Test strength and \(P_{s}\) of Equation (8) according to the spiral diameter. The A12H-3, -10, -11 specimens had \(A/A_{g}=9.52\) and were fully confined with the spiral and stirrup, as shown in Table 5. However, differences in the crack patterns were observed between specimens. As mentioned previously, a negligible amount of cracking was observed in A12H-3 with \(\varnothing\)16 stirrups, but a few horizontal cracks were observed in A12H-10 with \(\varnothing\)13. In A12H-11 with \(\varnothing\)10 stirrups, a few more cracks were observed in a wider area, as shown in Figure 10. This result implies that for the specimen with \(A/A_{g}=9.52\), crack occurrence and pattern are also highly related to the stirrup rebar reinforcement. In contrast to the A12H-1, -8, -9 specimens, the load-displacement curves of A12H-3, -10, -11 differed depending on the stirrup rebar size. Especially in A12H-11 with \(\varnothing\)10 stirrups, a sudden drop in load after the proportional limit was observed, as shown in Figure 11a. However, all curves showed load fluctuations at around 4500 kN, which were also seen in other specimens with \(A/A_{g}=9.52\). This confirms that all specimens with \(A/A_{g}=9.52\) except for A12H-7, which is reinforced with a negligibly small amount of lateral rebar, first experienced fracture of the anchorage bearing plate, and their later behavior was determined by the lateral confinement. The difference between A12H-7 and -11 lies in the spiral diameter and spiral rebar size. The local confinement due to the spiral in A12H-11 was sufficient to exceed the maximum effective lateral confinement, but the load suddenly dropped as the bearing plate fractured and did not recover. This means that the lateral confinement of locally damaged concrete is not sufficient to carry all load acting on the bearing plate.