# Experimental Study on Shear Strengthening of RC Beams with an FRP Grid-PCM Reinforcement Layer
## 3 Experimental Study on Reinforcement Amount
### Results and Discussion  
#### 3.2.1 Failure Modes  
The summary of the experimental results and the crack patterns of beam specimens regarding the ultimate load are presented in Table 8 and Figure 9, respectively. Similar to the analysis of Series A (Figure 6), the PCM material was stripped off from strengthened beam specimens as completely as possible after loading tests were finished. All beams failed in a typical shear-compression failure, which is concrete crushing after the main shear cracks developed completely, as shown in Figure 9. During the whole loading process, although fine flexural cracks were observed on the lateral surface at a load level of approximately 240 kN, the critical shear crack along the diagonal direction connecting the support and loading point appeared at various loads with the changes in the reinforcement amount of the CFRP grid. In addition, as load increased, the length and width of the flexural and shear cracks developed gradually for all beam specimens, but an increase in the number of cracks with smaller width were observed on the outside surface when RC beams were strengthened by PCM as a thin covering. As predicted, the local concrete crushing occurred on the upper edge of the RC beam.  
Figure 9: Crack patterns: (**a**) RHWC4; (**b**) RHWC6; (**C**) RHWC8; (**d**) RHWC4; (**e**) RHWC64. The pictures on the right correspond to the marked areas (blue dashed rectangle) on the specimens, showing the grid.  
As exhibited in Table 7, the load value at which the first flexural cracking happens (\(P_{\text{fc}}\)) stays similar for all specimens, as expected. For the specimen RHWP reinforced by PCM only, the ultimate load value (\(P_{\text{u}}\)) shows little increase in comparison with specimen N0\({}^{\prime}\), but the value at which the shear cracks appear (\(P_{\text{sc}}\)) is higher due to the existence of the PCM layer. The enlargement of the cross-sectional areas of CFRP grid (i.e., specimens RHWC4 and RHWC6) causes the magnitudes of both \(P_{\text{sc}}\) and \(P_{\text{u}}\) to increase gradually with the cross-sectional areas of CFRP. The same strengthening effect due to the reduction in grid spacing is observed comparing specimens RHWC4 and RHWC4\({}^{\prime}\) (with one-third the transverse grid spacing). The specimen RHWC8 shows, on the one side, an increase of the \(P_{\text{sc}}\) value in respect to specimen RHWC6, but almost the same ultimate load level \(P_{\text{u}}\), caused by the debonding phenomenon, and despite the largest cross-sectional area involved. Specim RHWC64 is similar to specimen RHWC8 in terms of CFRP cross-sectional area involved. However, the debonding failure occurs prior to the upper concrete crushing, leading to an ultimate load value lower than any other CFRP grid-PCM layer strengthened specimen. The delamination failure type and the rupture of the CFRP grid has not been observed in Series B, except for specimen RHWC4 (see Figure 9a), which shows the delamination failure of the partial CFRP grids (red dashed rectangular pointed by an arrow) that were near the critical shear crack.