## 6 Recommendations for Future Research  
Existing experimental works focused on various influencing parameters such as unit strength, mortar strength, loading conditions, slenderness ratio, openings, etc. Observations from the experimental literature indicate that while some aspects have been given substantial consideration, some others require further detailed investigation. In most of the tests with hinged support both at the top and bottom of the wall panels, a reduction in the slenderness ratio can cause a corresponding increase in the compressive strength of the walls. However, this may not be true when the lateral and rotational restraint conditions are considered, but the sample size of experiments is too small to make a proper comparison. More experimental tests utilizing different boundary conditions, tested under axial compression--and in particular for full-scale concrete block masonry walls--are highly recommended. In addition, most of the experimental campaigns have only been dedicated to investigating the different parameters which influence the compressive performance of URM walls built using normal strength units. Relevant future research in this area should focus more on full-scale wall specimens constructed with high-strength units to obtain representative strength values, considering different combinations of unit strength, mortar strength, joint thickness, and boundary conditions, among others. Several inconsistencies have also been observed in the stress-strain state of URM walls under axial compression. In particular, future research should focus on developing appropriate stress-strain curve models, as well as strength/stiffness prediction, taking into account the effects of the mismatch of elastic properties of masonry components. Thus, future research should prioritize enhancing the predictive accuracy of URM walls built with a combination of high-strength units and high-strength mortar. Observations from the literature have shown that the load capacity of URM wall structures is also significantly affected by the size of openings. The load capacity of the walls also depends on the position of the opening. However, very little experimental research has focused on URM walls with openings, tested squarely under axial compressive loading. In particular, systematic experimental studies on crucial geometric variability of the opening, such as the shape, size, and positions, are limited in quantity in the experimental database. Thus, reliable conclusions cannot be drawn on the effect of the different opening parameters on the axial compressive behaviour of URM wall structures due to the limited number of experimental studies. In general, it appears that all the aspects previously stated require further investigation to adequately understand the behaviour and performance of URM wall panels under compressive loading. However, two major aspects should be given priority: the strength of URM walls and the slenderness ratio. After a detailed knowledge of the influence of these two aspects, determining the effect of the other aspects becomes much easier. Hence, more experimental studies considering wall specimens--possibly in full scale--should be increased for URM walls, considering different strengths of units and mortar and featuring an adequate number of test specimens. Also, in-depth experimental research on the durability of URM walls is highly recommended since existing literature efforts have majorly fixated on their compressive behaviour. Conceptualization, A.A.A.B., B.H.A.B. and U.J.U. ; methodology, A.A.A.B., L.Y., B.S.A.D. and U.J.U. ; formal analysis, A.A.A.B., B.H.A.B., L.Y., B.S.A.D. and U.J.U. ; investigation, A.A.A.B. ; resources, A.A.A.B., A.A.J., L.Y. and M.R. ; writing--original draft preparation, A.A.A.B. ; writing--review and editing, B.H.A.B., U.J.U., A.A.J., L.Y., I.A.A.B., M.R. and B.S.A.D. ; supervision, B.H.A.B. and I.A.A.B. ; funding acquisition, B.H.A.B. All authors have read and agreed to the published version of the manuscript.