# Bond Characteristics of a Deformed Steel Bar Embedded in Donut-Type Voided Slab
## 3 Experimental Results and Analysis  
### The Bond Stress-Slip Relationship of the Donut-Type Voided Slab  
Tables 4 and 5 and Figure 6, indicating the changes of the bond stress-slip relationship by inner cover thickness according to bond regions is depicted. The bond stress of the entire specimens increased by the initial chemical adhesive strength until slip occurred, and when slip occurred, the typical bond behavior in which the bond stress gradually increased due to interlocking between the rib of deformed steel bar and concrete was observed. However, the residual bond stress and failure behavior after the maximum bond stress varied according to the inner cover thickness and bond regions. As shown in Figure 6c, the GB-50-26 series specimen, which ensured sufficient inner cover thickness (2.5 \(d_{b}\)), showed similar maximum bond stress regardless of the location of bond regions. Moreover, after the maximum bond stress, the typical pull-out failure behavior in which the residual bond stress, which was maintained through the friction between the deformed steel bar and concrete, gradually decreased to 40-60% of the maximum bond stress that was observed. These results indicate that when the inner cover thickness of rebar was sufficient in the donut-type voided slab, the bond stress-slip relationship are same regardless of the bond regions. In other words, when the donut-type voided slab has sufficient inner cover thickness more than 2.5 \(d_{b}\), the bond characteristics of the rebar can be constant over the entire regions as in general RC slabs. \begin{table}
\begin{tabular}{c c c c c} \hline
**Specimen** & \(P_{u}\) [kN] & \(\tau_{max}\) [MPa] & \(s_{max}\) [mm] & **Failure Mode** \\ \hline GB-50-0-A & 28.74 & 14.41 & 1.29 & Pullout \\ \hline GB-50-10-A & 35.14 & 17.61 & 1.74 & Pullout \\ \hline GB-50-26-A & 30.41 & 15.24 & 0.78 & Pullout \\ \hline GB-50-0-B & 12.20 & 6.12 & 0.35 & Splitting \\ \hline GB-50-10-B & 21.84 & 10.95 & 0.85 & Pullout \\ \hline GB-50-26-B & 35.01 & 17.55 & 1.14 & Pullout \\ \hline GB-50-0-C & 14.37 & 7.20 & 0.38 & Splitting \\ \hline GB-50-10-C & 29.81 & 14.94 & 0.38 & Splitting \\ \hline GB-50-26-C & 33.00 & 16.54 & 0.93 & Pullout \\ \hline \end{tabular} Note: \(P_{u}\): the ultimate bond force, \(\tau_{max}\): bond stress, \(s_{max}\): maximum slip.  
\end{table} 
Table 4: Test results of the short specimen. Figure 5: LVDT locations. C-type, and B-type specimen when compared by the bond regions; and the maximum bond strength increased as the inner cover thickness increased. Figure 6a,b indicate the experimental results for the GB-50-10 and GB-50-0 series specimen whose clear inner cover thickness was 10 mm and 0 mm, respectively. The experimental results showed that the B-type specimens (GB-50-10-B and GB-50-0-B), which had a constant clear inner cover thickness at 10 mm and 0 mm, showed the lowest maximum bond strength of 10.95 MPa and 6.12 MPa, respectively. Regarding A-type and C-type specimens whose directions of thickness changes were symmetrical in the transition region in which the inner cover thickness changed, the maximum bond strength of the C-type specimen, which had the thin inner cover on the loaded end, is lower than that of the A-type specimen. The difference of A-type and C-type specimens was greater in the case of the GB-50-0 series specimen whose minimum inner cover thickness was lower. In the case of GB-50-10 series specimen whose clear inner cover thickness was 10 mm, the maximum bond strength of the C-type specimen, which had the thin inner cover on the loaded end, reduced by 15% compared to that of the A-type specimen as shown in Figure 6b. Meanwhile, in the case of GB-50-0 series specimen whose clear inner cover thickness was 0 mm, although the C-type specimen had same inner cover conditions of the concrete surrounding the rebar as those of the A-type specimen, the maximum bond strength of the former decreased by 50% according to the directions of inner-cover thickness change as shown in Figure 6a.