A masonry wall is the structural assemblage of masonry elements, i.e., bricks or blocks with mortar as the binding material. Masonry walls and the building as a whole are designed to be durable, stable, and resilient under sustained loading. In masonry construction, the imposed loads are supported and transmitted to the foundations by the bricks or blocks joined together by mortar and, in some cases, are filled with grouts and reinforcements. One significant benefit of masonry building—aside from the ease of construction with readily available basic materials—is the durability of masonry materials such as the unit, which, with proper selection, could be expected to last for several decades, if not centuries, with very little maintenance [1]. Despite the ease with which masonry buildings are constructed, the analysis of its structural behaviour still remains a difficult challenge.

Unreinforced Masonry (URM) walls are largely non-homogeneous, non-elastic, and anisotropic elements made up of two materials with very different characteristics—one element of either mortar or unit considerably stiffer than the other—and a relatively weak bond between them ^[2][3][4][2,3,4]. Thus, masonry walls are relatively weak in tension. As a result, the walls are usually designed and expected to resist only compression loads ^[5][6][7][8][5,6,7,8]. Over the last several decades, research studies have been dedicated to investigating the structural behaviour of masonry walls and improving design standards ^[9][10][11][9,10,11]. In particular, significant attention has been focused on masonry components and the interaction between them. The walls of a structural masonry building serve as the primary resisting component against dynamic or static loads exerted on the structure, and as such, its compressive strength is of utmost importance when designing masonry walls in different loading conditions. In addition, the compression behaviour of masonry walls is a key factor in the design of masonry structures for other actions, such as in-plane shear and out-of-plane flexural behaviour [12]. However, the analysis of masonry walls under axial compressive load has been largely overlooked in the past.

In recent years, several research studies have been dedicated to investigating the axial compressive behaviour of URM walls ^{[13][14][15][16][17][18][19][20][21][22][23]}[13,14,15,16,17,18,19,20,21,22,23]. In general, the mechanical properties of each masonry element and bonding properties between mortar and units are of major significance in the compressive behaviour of URM walls as a composite material. As a result, the behaviour of axially loaded URM walls becomes very complicated because it is dependent on several factors relating to the slenderness ratio and eccentric loading conditions of the walls. Moreover, multiple provisions in the masonry design standards are stipulated in predicting the strength of URM walls under axial compressive loading ^[24][25][26][24,25,26]. Nonetheless, there are discrepancies observed in existing design standards to predict the compressive strength of URM wall structures, particularly in the guidelines in choosing and testing of wall configurations.

2. Masonry Units in Construction

Masonry wall units such as blocks or bricks are primarily made of concrete, calcium silicate, and clay. Blocks and bricks come in a variety of forms, shapes and sizes, including solid, hollow, and interlocking (e.g., see a selection in Figure 13). All these units have roughly comparable functionalities, but their properties vary markedly and are dependent on their constituent raw materials employed and their mode of production. Besides solid block and brick units, hollow concrete blocks (HCB) are widely attractive nowadays in low- and medium-rise constructions due to their advantages like light weight, ease of construction, and higher load-bearing capacity, as well as their ability to facilitate conduits for concealing electrical units, sewer, and water pipes ^[9][27][28][9,41,42]. Under similar conditions, the quantity of masonry mortar used to lay HCB can be lowered by more than 50% when compared to solid concrete blocks, thereby significantly lowering construction costs. With regard to masonry unit characteristics, its compressive strength is the most important. In addition to being closely associated with the strength of the wall, the properties of the unit provide an overall indicator of the masonry characteristics ^[29][43]. It is evaluated using standardized tests, and the results rely largely on the conditions and recommendations specified in the relevant standards in use. It can be said that the axial compression behaviour of masonry blockwork differs considerably from brick masonry since not only does the mortar strength influence the axial compression response but also the type and shape of block unit and strength of grout can inadvertently influence the overall response of the masonry wall ^[6][28][30][6,36,42]. Clay bricks can be made with compressive strength as high as 100 N/mm², but modest strength, i.e., 20–40 N/mm², is usually adequate for low-rise residential construction and the cladding of walls in medium-rise constructions. Concrete block units have lesser normalized strength, which ranges between 2.8 and 35 N/mm², but the effect mentioned previously must be considered if comparisons are to be made. In addition, the strength of brickwork will be lower than that of the corresponding blockwork if made from clay units with a similar strength value ^[31][44]. Although mortar represents only about 7% of total masonry volume, it has a far greater impact on performance than this fraction suggests. Conventional mortar mixtures are composed of cement, sand, plasticizer, or lime, and the mortar mix is graded based on its compressive strength ^[32][45]. The stiffer the mixture, the less it can facilitate movement, and thus, it is not recommended to employ a stronger mixture than required to satisfy performance requirements ^[31][44]. Various types of concrete blocks are also available, such as aerated autoclaved concrete (AAC) ^[33][34][46,47], whose material density ranges from 450 to 850 kg/m³ and enables the sizeable solid unit to be handled without mechanical support. The use of core-aligned blocks with special geometries, such as the Double H block ^[35][48], interlocking bricks ^{[36][37][38][39][40]}[49,50,51,52,53], cellular lightweight concrete (CLC) ^[41][42][43][54,55,56], and three-cell concrete block ^[44][57], have also garnered a lot of attention in recent years. The development of these newer types of blocks necessitates corresponding research into the adequacy and durability of such blocks for continued usage in masonry construction, and in particular to walls subjected to compressive loads.

3. Overview of Experimental Research in the Literature

As previously stated, various construction and testing parameters can influence the compressive performance of URM walls. The primary factors are the strength of masonry units (brick/block) and mortar strength, as well as the slenderness ratio and eccentric load conditions of the walls [17]. Apart from these parameters, the presence of openings is another significant parameter which could affect the performance of the masonry structure and should be properly taken into account. All the above-mentioned parameters can be varied during the construction and testing of masonry walls, leading to a more accurate assessment of the design of load-carrying URM elements. When evaluating the structural response of URM walls, both shear capacity and bending moment should be considered by taking into account its interaction with compression forces due to lateral load and gravity ^[45][46][47][58,59,60]. In the context of masonry structures in general, substantial eccentricity loadings could be precipitated either by in-plane or out-of-plane lateral forces or a combination of both ^[48][61]. The latter are typically induced by thrusting components (e.g., vaults and arches), wind and soil pressures and seismic actions transmitted from the masonry floor. In contrast, the former can be seismic-induced shear force ensuing from box-type global seismicity of the building structures. As observed from the literature (e.g., see ref. ^[49][62]), axial loading differs from eccentrically loaded walls since the former is a statically determinate problem in which the strength of a given wall panel may be presumed to be maximum compression forces divided by the total cross section (Figure 24). On the contrary, eccentrically loaded wall panels are statically indeterminate problems in which the bending moment could be defined by integrating the normal stress and the corresponding axial strain over the given gross sectional area ^[50][51][52][63,64,65]. As a general recommendation in the design of URM structures, the capacity reduction factor (Φ)—expressed as a ratio of prism strength to the strength of the wall—can be introduced to accommodate and account for different characteristics and to obtain the permissible compressive strength of the masonry ^[24][53][54][24,40,66]. In evaluating the axial compressive performance of URM walls, experimental tests carried out on brick and concrete block masonry wall specimens available in the literature have been analyzed (e.g., see a selection in Table 1). However, it is important to highlight that previous research on the compressive behaviour of full-scale masonry walls is still limited. To augment the understanding, tests conducted on wallettes and partially grouted wall panels subjected to axial compressive loading have been used since the actual behaviour of these types of wall panels is identical to full-scale URM wall panels [12]. In addition, URM wall panels subjected to shear–compression loading have also been examined. However, the results from such experiments reported in the literature were not included in the database since these types of tests are closely associated with earthquake loadings. Based on a series of assumptions, scholars have introduced a wide range of predictive models to estimate the compressive strength of masonry. These models vary from simple linear ones with a single variable to complex nonlinear ones that consider the influence of multiple variables. Linear models were proposed in ^[55][56][67,68], while nonlinear expressions were introduced via standards such as Eurocode 6 [24]. These expressions established correlations between the compressive strength of masonry and both the compressive strength of the unit and the compressive strength of mortar. In many cases, the expressions proposed did not consider any distinctions based on the type of masonry. However, beyond the compressive strength of masonry constituents, additional factors were considered by researchers like Khan et al. ^[57][69]. The authors introduced a mathematical model that takes into account the slenderness ratio (height-to-thickness ratio, h/t) and the width-to-thickness ratio (l/t). A summary of the expressions proposed in the literature for calculating the compressive strength of masonry wallettes is provided in Table 2. The tabfolles belowowing sections present a review of the literature on the compressive performance of URM walls, summarizing the work and salient observations made by earlier researchers in different experimental campaigns. Code provisions have also been compared with results from experimental studies, and disparities are highlighted to identify and outline potential directions for future studies.