## 3 Test Results and Discussions  
### Crack Propagation and Failure Mode  
Figure 6 shows the crack propagation behavior of the specimens at different load levels. The crack propagations generated in the negative direction are plotted using dotted lines. The first flexural cracks occurred at the first cycle of \(d_{r}\) = 0.375% within the sagging and hogging moment zones of the beam in each loading direction, irrespective of the governing failure mode of the beam. With the increase in the applied load, different crack propagation in the beam was observed in both the F-series and S-series specimens. Except for the F-N-G specimen, the failure mode for the rest of the F-series beams was dominated by the flexure with the crushing of concrete in compression at the interface between the beam and column, and no shear cracks or diagonal tensile cracks were observed in any of the beam elements. Table 3 also summarizes the flexural strength and ductility measured in the beam of the F-series specimens. The yielding displacement (\(\Delta_{y}\)) in the beams with GFR bars was determined from the intersection of the secant line at 75% of the peak load and the horizontal line at the peak load [13]. For the H-N-C specimen, the flexural crack zone in the beam extended up to 0.41\(L\) from the maximum moment zone until the failure of the beam. At the first cycle of \(d_{r}\) = 0.75%, several diagonal cracks occurred in the column; however, no concrete crush in the column was developed until the beam failure. The spalling of the concrete cover in compression started with severe flexural cracks at \(d_{r}\) = 3.3%, which corresponded approximately to the peak load, beyond which the applied loads gradually decreased with the increasing displacement history. The loss of the concrete cover was expanded and longitudinal steel bars were then buckled with the opening of a 90-deg hook of the shear reinforcement at \(d_{r}\) = 5%, as shown in Figure 7a. The crack pattern in the beam of the specimen F-N-G was similar to that of the companion specimen F-N-C, but the splitting crack that was observed along the GFR bars and not any diagonal cracks in the column were an exception. At \(d_{r}\) = 3.4%, the pull-out failure of GFR bars was observed with the spalling of concrete cover at the interface between the beam and column, as shown in Figure 7b. Because of the premature failure of the beam by the pull-out of GFR bars, no diagonal cracks occurred in the column. The specimen F-H-C exhibited more severe crack propagation and concrete crushing areas when compared with those observed in the specimen F-N-C. This implies that the cover spalling in compression occurs more rapidly in concrete with higher strengths. The buckling of the longitudinal steel bars in this specimen was observed at \(d_{r}\) = 4%, but no splitting cracks along the longitudinal reinforcement were observed until the end of the tests. The specimen F-H-G exhibited a similar trend in relation to the crack propagation observed in the companion specimen F-H-C. However, the post-peak behavior of the specimen F-H-G was governed by the tensile rupture of GFR bars at the column interface, as shown in Figure 7b. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Specim-en**} & \multicolumn{3}{c}{**At Initial**} & \multicolumn{3}{c}{**At Yielding of**} & \multirow{3}{*}{**At Ultimate**} & \multirow{3}{*}{**Displacement**} & \multirow{3}{*}{**} & \multirow{3}{*}{**} \\  & \multicolumn{2}{c}{**Flexural**} & & & & & & & & & & \\  & \multicolumn{2}{c}{**Cracking**} & & & & & & & & & & \\ \cline{2-10}  & \(P_{\sigma}^{+}\) & \(P_{\sigma}^{-}\) & \(P_{\sigma}^{+}\) & \(P_{\mu}^{-}\) & \(P_{\mu}^{+}\) & \(P_{\mu}^{-}\) & \(\Delta_{y}\) & \(\Delta_{n}\) & \\  & **(kN)** & **(kN)** & **(kN)** & **(kN)** & **(kN)** & **(kN)** & **(kN)** & **(mm)** & **(mm)** & & \\ \hline F–N–C & 29.5 & 36.2 &Figure 6: Crack propagation and failure mode of specimens. (Numbers shown in figures indicate the applied load (in kN) at which crack occurred).