## 4 Overview of Experimental Research in the Literature
### Effect of Strength of Components and Loading Orientation
[80] carried out a test on 23 URM wallettes--590 mm \(\times\) 990 mm \(\times\) 190 mm their nominal sizes--using different block-mortar strength combinations, i.e., HCB (\(f_{\text{b}}\) = 14.08 MPa, 25.73 MPa, and 31.18 MPa) and mortars (\(f_{\text{m}}\) = 6.31 MPa and 15.55 MPa). An increment of about 80% in the compressive strength of the block was found to have caused a corresponding increase in the compressive strength of the wall panels by approximately 60%. On the other hand, an increment in mortar strength by about 150% was found to have caused an increase in the masonry compressive strength by only approximately 2%, indicating--and in accordance with prior studies [85; 86]--that the strength of mortar has minimal influence on the strength of URM wall. The negligible effect of mortar strength on the compressive strength of unreinforced masonry was also observed in the study by [PERSON] and [PERSON] [74]. The authors observed that an increase in mortar strength by 100.4% only amounted to a 19% increase in the compressive strength of the wall. In contrast, an increase in the unit strength by 94.9% increased the strength of the masonry by about 155%. However, the influence of mortar joint thickness on full-scale URM walls constructed with different combinations of unit strength and mortar strength is unavailable in the experimental literature, thus drawing up possible lines for future research. In recent years, the need to construct taller masonry structures has driven the construction industry to develop a robust capability for producing high-quality units with varying degrees of strength and geometric characteristics [14; 87; 88]. Consequently, there has been a growing importance placed on advancing the understanding of URM walls constructed using high-strength blocks [87]. However, it is worth highlighting that existing masonry codes do not include design procedures that account for the use of high-strength units. In this regard, [PERSON] et al. [14] conducted an articulated experimental campaign to study the performance of high-strength URM walls subjected to axial compressive loading. Grounded wall panels (2.2 m high, 1.2 m wide) and ungrouted wall panels (2.2 m high, 0.8 m wide) constructed with and without a mid-height bond beam were included in the set of experiments. Three different strengths of hollow concrete block and mortar were used: \(f_{\text{b}}=18\) MPa, 27.3 MPa, 34.5 MPa and \(f_{\text{m}}=12\) MPa, 18 MPa, 24 MPa. Observations from the experiments indicated an increment in compressive strength of grouted wall panels by about 50% in all the grouted wall panels in comparison to the un-grouted wall panels. Test observations also indicated that the use of a bond beam at the mid-height of the wall prevented a decrease in the compressive strength of the wall panels. However, the use of such special schemes altered the failure pattern of tested walls. In addition, results from the set of experiments also differ from code recommendations specified in [89], and this may be due to the geometry of blocks used in the study. A number of studies have also indicated that the compressive performance of URM walls may be influenced by the loading orientation. Specifically, loading perpendicular to the bed joints is known as vertical compression and loading parallel to the bed joints is referred to as horizontal compression (e.g., see Figure 6). For example, recent experimental comparisons conducted by [PERSON] et al. [69] revealed a higher compressive strength of masonry in the case of O1B1M1 and O1B2M2 wall specimens loaded in the vertical direction compared to P1B1M1 and P1B2M2 wall specimens loaded in the horizontal direction. Similarly, the experiments carried out by [PERSON] and [PERSON] [74] indicated that the compressive strength of hollow brick masonry is significantly affected by the strength of both the units and mortar.