# Shear Tests of Deep Hollow Core Slabs Strengthened by Core-Filling
## 5 Conclusions  
In this study, experimental research was performed to investigate the shear resistance mechanism of the PHCS strengthened by the core-filling method. The number of filled cores and the shear reinforcement ratio were set as test variables, and the behavior of the specimens, as well as the patterns and angles of cracks that occurred in the PHCS units and filled cores, were analyzed in detail. In addition, this study examined whether the shear strength equations specified in the ACI 318-14, CSA-A23.3-14, and BS EN1168 codes provide reasonable evaluations of the shear strengths of the PHCS with filled cores, and proposed a modified equation that is capable of properly reflecting the shear strengthening effects of the core-filling method. Based on the findings of this study, the following conclusions can be drawn:  
1. The NR-2/4 specimen which has two hollow cores filled with normal strength concrete, showed a 32%-38% increase in shear strength compared to the NR-0/4 specimen, while the shear strength of the NR-4/4 specimen with four hollow cores strengthened increased by only 24%-25%, compared to that of the NR-0/4 specimen. This is because the web of the PHCS unit at the edge of the NR-4/4 specimen was separated from the filled core before reaching the ultimate load, and thus the edge web failed to exert effective shear resistance. 2. The shear crack angle of the PHCS unit ranged from 30\({}^{\circ}\) to 34\({}^{\circ}\), whereas a very steep crack angle ranging from 53\({}^{\circ}\) to 76\({}^{\circ}\) occurred in the filled core. In addition, cracks were observed along the interface between the PHCS unit and the filled cores, resulting in the slip between the PHCS unit and the filled cores, which in turn led to a failure of full composite behavior. As a result, the filled core made a partial contribution to the shear resistance of the member, but did not exert its shear capacity due to the slip between the PHCS unit and the filled cores. 3. The shear strengths of the LR-2/4 and HR-2/4 specimens strengthened with helix reinforcing bars were almost similar to the shear strength of the NR-2/4 specimen without shear reinforcement. In other words, the shear reinforcement did not exert a proper shear resistance, which is because the shear crack did not cross the helix reinforcing bars. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & **Test Results** & **Proposed Equation (2)** \\ \cline{2-4}  & **kN** & **kN** & **Ratio \({}^{*}\)** \\ \hline NR-0/4 (1) & 351.0 & 331.4 & 1.06 \\ NR-0/4 (2) & 331.0 & 331.4 & 1.00 \\ NR-2/4 (1) & 472.0 & 458.3 & 1.03 \\ NR-2/4 (2) & 451.2 & 458.3 & 0.98 \\ NR-4/4 (1) & 426.9 & 452.7 & 0.94 \\ NR-4/4 (2) & 422.3 & 452.7 & 0.93 \\ LR-2/4 (1) & 444.7 & 458.3 & 0.97 \\ LR-2/4 (2) & 428.7 & 458.3 & 0.94 \\ HR-2/4 (1) & 436.6 & 458.3 & 0.95 \\ HR-2/4 (2) & 446.1 & 458.3 & 0.97 \\ Average & - & 0.98 \\ STD & - & 0.04 \\ COV & - & 0.04 \\ \hline \hline \end{tabular} \({}^{*}V_{n,test}/V_{n,test}\).  
\end{table} 
Table 6: Verification results of proposed equation. 4. The ACI 318-14 code provided conservative evaluations of the shear strengths of the specimens, while the CSA-A23.3 code tended to estimate the shear strengths of the NR-4/4, LR-2/4, and HR-2/4 specimens to be on the unsafe side. In particular, the BS EN 1168 code provided shear strength estimation results on the unsafe side for all of the specimens. 5. The separation of the web of the PHCS units and the filled cores at the edge of the section should be considered when applying the code equations, in order to avoid unsafe shear design of the PHCSs reinforced by the core-filling method. 6. To understand the shear transfer mechanism of the PHCS units and core-filling concrete more clearly, further research is still needed: (1) Push-off tests of PHCS with filled cores to obtain the bond properties between PHCS and core-filling concrete; (2) Finite element analysis considering the bond-slip relationship; (3) Shear tests of PHCS with core-filling and topping concrete.