## 3 Test Results
### Test for Bond Strength  
The maximum loads from the bond strength tests are summarized in Table 6. The damage state of the specimens after failure is shown in Figure 10. As shown in Figure 10a, the bare prism failed at the bed joints between the masonry units through which the test load was transmitted. Figure 10b shows that the prism strengthened on one side failed at both the bed joints and the overlay-to-masonry interface. Meanwhile, Figure 10c shows that the prism strengthened on both sides failed in the masonry unit at the side with a relatively low strength material, as the load transmitted by the overlay increased and the hole in the inside brick was partially filled during bricklaying. The load transfer and stress at the overlay-to-masonry interfaces are illustrated in Figure 11. The overlay-to-masonry interfaces can be classified into side and middle interfaces. The test load was transmitted from the middle interface to the two side interfaces through the overlay. The area of the interface considered in the calculation of the bond strength was doubled for strengthening on both sides, as shown in Figure 11b. To calculate the bond strength at the overlay-to-masonry interface, the load transmitted through the bed joints directly was subtracted from the total load resisted by the strengthened specimen as follows:  
\[P_{\tau}=P-P_{j}, \tag{1}\]  
where \(P\), \(P_{\tau}\), and \(P_{j}\) denote the total load (kN), load shared by the overlay-to-masonry interface (kN), and load shared by the bed joint (kN), respectively. The bond strength at the overlay-to-masonry interface was calculated considering the number of interfaces, as follows:  
\[\tau_{aver,S}=\frac{P-P_{j}}{2A_{o}}=\frac{P_{\tau}}{2A_{o}} \tag{2}\]  
\[\tau_{aver,B}=\frac{P-P_{j}}{4A_{o}}=\frac{P_{\tau}}{4A_{o}}, \tag{3}\]  
where \(\tau_{aver,S}\) and \(\tau_{aver,B}\) denote the bond strength (MPa) at the overlay-to-masonry interface for strengthening on a single side and on both sides, respectively. \(A_{o}\) denotes the interface area (mm\({}^{2}\)) between the adjacent masonry units.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Type of** \\ **Bricks** \\ \end{tabular} } & \multirow{2}{*}{**Specimen**} & \multicolumn{2}{c}{**Maximum Load (kN)**} \\ \cline{3-5}  & & **B-P** & **B-SF-30S** & **B-SF-30B** \\  & & **(Plain)** & **(One-Side Overlay)** & **(Two-Side Overlay)** \\ \hline \multirow{4}{*}{Clay brick} & 1 & 15.3 & 33.5 & 55 \\  & 2 & 17.3 & 27.3 & 52.8 \\ \cline{1-1}  & 3 & 20.1 & 38.6 & 59.7 \\ \cline{1-1}  & Average & 17.6 & 33.1 & 55.8 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Bond strength test results for masonry prisms.  
Figure 10: Damage state after termination of bond strength tests: (**a**) B-P; (**b**) SF-30S; (**c**) SF-30B.  
The bond strengths of the overlay-to-masonry interface calculated using Equations (1)-(3) are summarized in Table 7. The bond strengths for strengthening on one side and both sides were 0.716 and 0.884 MPa, respectively. The lower bond strength by strengthening on one side is attributable to the normal stress exerted on the interface due to the eccentricity between the interface and the line of action for loading. In addition, bond strength tests were conducted for concrete masonry strengthened with the same ASFRM overlay in another study [22], the results of which are presented in Table 7 and used to predict the diagonal tension strength in the following section.