## 3 Results and Discussion
### Fiber Matrix Interaction  
During fiber pull-out the crack propagates along the embedded fiber with frictional slip in the debonded regions, as described in detail in Reference [72]. The first part of the force-displacement curve is characterized by linear elastic deformation, followed by the first crack during interfacial debonding that is usually revealed by a kink in the graph, especially when high-modulus fibers and low modulus polymer matrices are combined. As already discussed in previous works [58], typically, therefore, no \"kink\" does occur if \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**Young’s**} & \multicolumn{2}{c}{**Tensile**} & \multirow{2}{*}{**Strain at Break [\%]**} \\  & **Modulus [MPa]** & & \\ \hline unsized & 76.14 \(\pm\) 2.5 & 1018.48 \(\pm\) 379.23 & 1.51 \(\pm\) 0.58 \\ PTMO & 76.16 \(\pm\) 1.00 & 1534.47 \(\pm\) 433.88 & 2.35 \(\pm\) 0.71 \\ SB-MMT & 76.34 \(\pm\) 0.85 & 1520.79 \(\pm\) 392.86 & 2.29 \(\pm\) 0.62 \\ CSB & 77.06 \(\pm\) 0.89 & 1578.88 \(\pm\) 371.82 & 2.35 \(\pm\) 0.58 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical properties of the IPF AR-glass fibers, unsized, sized with PTMO, and coated with either SB-MMT or CSB. Figure 9: Microscope images of the glass SB fiber (**a**) and SB-MMT fiber (**b**) after immersion in cement solution overnight. AR-glass fiber is pulled out of a cementitious matrix (Figure 10). After debonding, the applied force increases, as results of both interfacial adhesion and friction, until it reaches a peak value (F\({}_{max}\)). At some point, the interfacial adhesion and friction are no longer in equilibrium, which leads to uncontrolled debonding until the frictional bond is reached, indicated by a characteristic force drop, which is particularly pronounced for high chemical fiber-matrix bonding. From this point to full fiber pull-out, the measured force value is determined by fiber-matrix friction that is here observed as the most dominant form of fiber-matrix interaction in the measured pull-out curves for all treated fibers. For this reason, the area under the graph that corresponds to the total pull-out work W\({}_{total}\) was considered to characterize the efficiency of the different surface treatments. Additionally, the interfacial shear strength (IFSS), defined by  
\[IFFS=\frac{F_{max}}{\pi\cdot d_{f}\cdot l_{e}}, \tag{5}\]  
where \(d_{f}\) is the fiber diameter, and \(l_{e}\) the embedded fiber length, was determined to evaluate the fiber-matrix interaction; see Table 4.  
For all the fibers, the maximum force was reached at low displacement levels due to the brittle nature of the fiber-matrix combination. The silane-treated fibers (PTMO) (Figure 10a) display the lowest W\({}_{total}\) but the highest F\({}_{max}\) value and IFSS. This means that the PTMO fibers have a stronger interfacial adhesion than the coated fibers. This can be attributed to the ability of the silane groups to build up chemical bonds to the OH-groups provided not only by the fiber surface, but also by the components of the cementitious matrix. By applying some coating on the fibers surface, the IFSS decreases, but, at the same time, a significant increase in the pull-out work can be observed for all the coated fibers. This can be attributed to the enhancement of the interfacial friction, which is particularly evident observing the shape of the second part of the graph. Here, the fiber is completely debonded and the measured force is the result of only frictional interaction between the fiber and the matrix. Comparing the pull-out graphs in Figure 10, it can be observed that, on average, after debonding, the coated fibers display higher pull-out load than PTMO fibers. During the pull-out, the coating is partially removed from the coating surface, and it accumulates at the fiber-matrix interface.