# Shear Tests of Deep Hollow Core Slabs Strengthened by Core-Filling
## 3 Test Results  
### Load-Displacement Behavior and Shear Strengths of Test Specimens  
Figure 5(a) compares the behavior of NR-0/4, NR-2/4, and NR-4/4 specimens to analyze the shear reinforcement effects according to the number of filled cores. Since two tests were performed at both ends for each specimen, the first and second tests were distinguished by adding '(1)' and '(2)' after the specimen name. Figure 5(b)-d shows crack patterns observed from each specimen, while the numbers shown in Figure 5(c),d indicate the order in which the cracks occurred with increasing loads. As shown in Figure 5(a), the initial stiffness of the NR-0/4, NR-2/4, and NR-4/4 specimens was almost similar with and without filled cores, and the NR-0/4 (1) and (2) specimens underwent brittle failure simultaneously when web-shear cracking occurred at shear forces of 351.0 and 331.0 kN, respectively (341.0 kN on average). In the case of the NR-2/4 (1) and (2) specimens with two hollow cores strengthened by normal strength concrete, the stiffness started to decrease when flexural cracks occurred prior to the shear cracking; and these specimens showed shear failure as web-shear cracks occurred at the shear forces of 472.0 and 451.2 kN, respectively (461.6 kN on average). The NR-4/4 (1) and (2) specimens in which all the hollow cores of the PHCS unit were strengthened by in-fill concrete showed a sharper decrease in stiffness, as compared to the NR-2/4 specimens, after the flexural cracks were observed, and they underwent web-shear failure at the shear forces of 426.9 and 422.3 kN, respectively (24.6 kN on average). Figure 5: Locations of strain gauges. Figure 6: _Cont._Table 4 compares the shear strengths of the specimens. The shear strengths of the NR-2/4 (1), (2), and NR-4/4 (1), (2) specimens increased by 32%-38% and 24%-25%, respectively, compared to the shear strength of the NR-0/4 specimen. However, in the case of the NR-4/4 specimen, with all four hollow cores strengthened, the shear reinforcement effect was smaller than that of the NR-2/4 specimen. This is because as shown in Figure 7, 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 failed to resist external shear forces. The crack in the interface between the PHCS unit and the filled core initially occurred near the top flange opened for core-filling, and spread towards the bottom of the section along the interface. Figure 7b shows that at the time of shear failure of the member, the PHCS unit, and filled core were completely separated. Despite this, considering the fact that the PHCS units are continuously placed in an actual construction site, and cast-in-place concrete or non-shrink mortar is poured between the members, the separation of the PHCS unit web from the filled core would be delayed. However, additional experimental research is needed to verify such estimation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen** & \(\mathbf{V_{f}}\)**(kN) * & \(\mathbf{V_{n}}\)**(kN)** & **Strength Ratio ** ** \\ \hline NR-0/4 (1) & - & 351.0 & 1.0 \\ NR-0/4 (2) & - & 331.0 & 1.0 \\ NR-2/4 (1) & 436.7 & 472.0 & 1.38 \\ NR-2/4 (2) & 412.8 & 451.2 & 1.32 \\ NR-4/4 (1) & 371.0 & 426.9 & 1.25 \\ NR-4/4 (2) & 387.4 & 422.3 & 1.24 \\ LR-2/4 (1) & 442.1 & 444.7 & 1.30 \\ LR-2/4 (2) & 412.8 & 428.7 & 1.26 \\ HR-2/4 (1) & 392.1 & 436.6 & 1.28 \\ HR-2/4 (2) & 392.1 & 446.1 & 1.31 \\ \hline \hline \end{tabular}  
* \(V_{f}\): Shear force at flexural cracking. ** The ratio between shear strength of the strengthened specimen and the average strength of both ends of the reference specimen NR-0/4 (341 kN).  
\end{table} 
Table 4: Cracking loads and web-shear strengths of test specimens. Figure 6: Effect of filled cores on shear strength of prestressed hollow core slab (PHCS). (**a**) Shear force-deflection responses. (**b**) Crack patterns of the NR-0/4 specimen. (**c**) Crack patterns of the NR-2/4 specimen. (**d**) Crack patterns of the NR-4/4 specimen.