## 4 Test Setup, Instrumentation, and Crack Formation
### The Effects of Investigated Parameters on the Behavior of Anchors
#### 5.3.1 The Effect of Mortar Strength
Finally, (e) it is interesting to observe that if one assumes that the tensile strength of mortars (as is the case for concrete) is proportional to the square root of their compressive strength, the ratio \(\sqrt{3.2}/\sqrt{20.4}\) is equal to 0.40, i.e., rather close to the ratio obtained for the corresponding tension resistances (0.48 on average). This result is attributed to the fact that the tension resistance of anchors, in case of the governing pull-out failure, depends on the bond properties between the anchor and the masonry mortar. It is known that the tensile strength of the substrate (i.e., the mortar of the joint) is an estimator of the bond resistance. To evaluate the effect of the mortar strength in case a crack crosses the anchor installed in a mortar joint (L\({}_{\text{TT}}\) location), relevant load-displacement curves are presented in Figure 10. The comparison of Figures 9 and 10 shows the significant effect of the 0.50 mm crack on the tension resistance of the anchors. This effect (further discussed in Section 5.3.2) is governing and, thus, the influence of mortar strength is attenuated (the ratio of tension resistances for weak and strong mortar is equal to 0.75 on average). Furthermore, the crack through the mortar joint leads to a significantly larger scatter of the experimental results in terms of both tension resistance and respective displacement (Table 6). Figure 9: Load–displacement curves for anchors installed in an uncracked T-mortar joint (L\({}_{\text{ST}}\)) made with strong or weak mortar. The effect of mortar strength is also investigated for brick locations, in uncracked (L1) and cracked (L6) conditions (Figures 11 and 12). When anchors are installed in bricks within a wallette free of cracks, the mortar compressive strength affects the tension resistance, but this effect is less pronounced than for anchors installed in mortar joints. The curves in Figure 11 and the data in Table 6 show that for location L1 the reduction of the tension capacity is on average equal to 30% for weak mortar joints, as compared to an average reduction of almost 50% observed in mortar joint location L3T. The presence of a 0.50 mm wide crack at the vicinity of the anchor embedded in the middle of the brick (L6) leads to a substantial reduction of the tension resistance both for strong and weak mortar, with the tension resistance in case of weak mortar remaining by almost 30% lower than for strong mortar (Figure 12 and Table 6). The reduction coefficients calculated based on the test results are valid for the materials used within this experimental campaign and, hence, they cannot be of general applicability. Nonetheless, the test data prove the preponderance of brick against mortar locations for the installation of anchors, at least for the short embedment depth tested within this part of the campaign. Further caution is needed in the design of anchors in mortar locations, due  
Figure 11: Load–displacement curves for anchors installed in uncracked bricks with an adjacent uncracked joint (L1) made with strong or weak mortar. Figure 10: Load–displacement curves for anchors installed in a cracked (c\({}_{\mathrm{w}}\) = 0.50 mm) T-mortar joint (L4T) made with strong or weak mortar. to the inevitable scatter of the mortar properties. Even under laboratory conditions, with continuous control of mortar mix proportions, when curing the walls and the conventional mortar specimens, there was quite a scatter of the measured compressive values (Table 1). The effect of this variability on the compressive strength of mortar is illustrated in Figure 13. This figure presents sets of tension load vs. displacement curves for anchors differing only in the compressive strength of mortar (4.53 MPa vs. 2.89 MPa). Site conditions are not expected to be more favorable in this respect. Moreover, in the case of existing masonry structures, where the mortar strength is estimated based on a limited number of in situ and/or laboratory tests, a more pronounced variability of mortar properties is expected.