Introduction

The pursuit of cost minimization in structural masonry construction, coupled with inadequate quality control in components and construction procedures, and the absence of specific design codes, can lead to a range of pathologies and accidents in this construction system (Oliveira et al., 2018). Masonry structures primarily experience compressive stresses, making the compressive behavior of masonry crucial for design and safety assessment purposes (Mohamad; Lourenço; Roman, 2007; Barbosa; Lourenço; Hanai, 2010). One of the challenges in structural masonry design codes is how to evaluate the strength and strain capacity of hollow masonry elements for structural purpose. Mainly due to their difficulties to manufacturer, transport, costs, requiring considerable capacity of reaction frames for applying and measuring the compression capacity of masonry specimens in prototype dimensions (Milani et al., 2021). The compressive strength of masonry not only has considerable effects on structural safety but can also significantly impact construction costs (Zhou; Wang; Zhu, 2016). It is influenced by various factors, including the strength of the mortar and units, the height-to-thickness ratio of the units, the orientation of the units concerning the direction of applied load, and the thickness of mortar joints (Fortes et al., 2017; Ravula; Subramaniam, 2017; Garzón-Roca; Marco; Adam, 2013).

Masonry prisms have been widely used in research and quality control of masonry structures due to their simplified models, ease of construction and testing, considering operational and economic aspects, which can represent the interaction between different masonry components (Nalon et al., 2021; Abasi et al., 2020). The mechanical performance of prisms under compression is directly affected by failure modes associated with the mechanical properties of their constituent materials. Factors such as the relative strength between their components, preparation/test conditions, arrangement of the mortar joint, curing procedures, loading rate, geometric properties, also influence their performance (Parsekian et al., 2012; Gumaste et al., 2007; Sumanthi; Mohan, 2014). Numerous studies have been conducted on the behavior of masonry prisms under axial compression (Ewing; Kowalsky, 2004; Kaushik; Rai; Jain, 2007a; 2007b; Caldeira et al., 2020; Padalu; Singh, 2021; Baghi et al., 2018), however, few studies consider the mortar coatings on prisms.

Milani et al. (2021), for example, studied the failure mode of masonry specimens, which was found to be highly similar between full and small scales. This suggests that, for blocks, prisms, and wallettes, these components could be utilized to represent the strength and strain behavior of masonry under compression. Oliveira et al. (2018) discusses masonry buildings constructed in Pernambuco and their relationship to several failures that have occurred, some of which included the death of inhabitants. Its content analyses in depth, experimentally, the behaviour of single bricks, prisms and wallets that allow the identification and quantification of the influence of several mortar coating layers on the load capacity of elements tested. Azevedo et al. (2019) describes an experimental study carried out prisms of two and three ceramic blocks, with and without cement mortar coating and some samples reinforced with mesh, subjected to axial compression in order to enhance the capacity of masonry. The experimental results indicate an increase both in the compressive load capacity of the coated prisms and in those that use coatings based on reinforced mortar.

Despite their extensive historical use, mortar coatings, widely used in masonry walls, particularly in Brazil, for both internal coatings and facades (Botas; Veiga; Velosa, 2017), still exhibit several pathological manifestations, including cracking and detachment due to a lack of adhesion between the constituent layers of a building’s coating system, contributing to a decrease in the useful life of structures (Nogueira; Pinto; Gomes, 2018). It is crucial for the mortar used in wall rendering to possess adequate strength to ensure durability and impermeability while contributing to stability (Álvarez-Pérez et al., 2020). The conditions of mortar preparation and application to the substrate significantly impact the hardening process and characteristics relevant to in-service performance (Starinieri; Hughes; Wilk, 2015; Válek; Skruzná, 2019). Additionally, the interaction between applied mortar and the substrate, through the adhesion mechanism, also plays a role in mortar performance (Starinieri; Hughes; Wilk, 2015; Torres; Veiga; Freitas, 2018).

Adhesion is a very complex property that is dependent on several factors, namely:

1. the characteristics and properties of the porous material in contact with the mortar;

2. the characteristics of mortars and their constituent materials;

3. the mortar’s application technique;

4. the climatic conditions at the time of application and throughout the life period of the cladding or rendering; and

5. the time span after mortar application (Carasek, 1996).

Several factors influencing mortar behavior have been studied by different researchers, including:

1. substrate characteristics and substrate preparation (Starinieri; Hughes; Wilk, 2015; Stolz et al., 2016);

2. mortar curing environmental conditions (Tongyuan et al., 2018);

3. number and thickness of mortar layers (Starinieri; Hughes; Wilk, 2015; Zanelatto et al., 2013; Silveira et al., 2021); and

4. mortar application techniques (Válek; Skruzná, 2019; Zanelatto et al., 2013).

Although significant progress has been made in the study of the behavior of masonry prisms subjected to compression in recent years, there is a fundamental need for further research to enhance the understanding of the behavior of rendering under this condition. Therefore, the objective of the study presented here is to evaluate the impact of various properties of coating mortar on the compressive behavior of masonry prisms constructed with two structural ceramic blocks, as commonly utilized in rationalized masonry.

Materials and methods

Masonry prisms, constructed with two ceramic blocks and stabilized laying mortar, were coated with three mortar types (mixed, industrialized, and stabilized). These coatings were selected based on their suitability for rationalized masonry in the Southern region of Brazil. The mortars were characterized before and after application to the prisms, which were then subjected to a compression strength test.

Blocks

The ceramic blocks utilized in this study were of the EST60 structural type, featuring vertical perforations and nominal dimensions of (140 x 190 x 290) mm (length x thickness x height) (Figure 1a). They were stored in a laboratory environment, maintaining covered, dry, and well-ventilated conditions until use (Figure 1b). The blocks were subjected to geometric (effective dimensions, wall thickness), physical (water absorption), and mechanical characterizations, including characteristic compressive strength (f_bk) and average compressive strength (f_bm), as specified by NBR 15270-2 (ABNT, 2023).

Mortars

The mortars utilized in this study included laying mortar, plastering mortar, and three types of coating mortars (mixed, industrialized and stabilized).

Laying mortar

The laying mortar, a stabilized batch type, underwent a curing period of thirty-six hours and was sourced from a reputable company. Transported from the batching plant to the laboratory’s outdoor area using a concrete mixer truck, it was discharged into metal boxes with a 0.5 m³ capacity, ensuring adequate protection against external elements.

Plastering mortar

The adhesive industrialized mortar used for the rolled rendering was composed of cement, polymers, mineral aggregates, and special additives, according to the manufacturer. The recommended water-to-dry material ratio was 4.5 liters of water per 20 kg bag. It exhibited a coverage of ± 2.80 kg/m², fresh density ranging from 1.6 to 2.0 g/cm³, and laboratory tensile bond strength ≥ 0.4 MPa at 3 days and ≥ 0.6 MPa at 28 days.

Coating mortars

Three types of mortars were utilized for the coating of the prisms:

1. mixed mortar (MM);

2. industrialized mortar (IM); and

3. stabilized mortar (SM).

The mixed mortar had a mix ratio of 1:1:6, by volume, and was commonly used as a coating material in civil construction. It was produced with Portland cement CP II F-32, with a unit weight of 1053.20 kg/m³ (ABNT, 2021a) and density of 3.09 g/cm³ (ABNT, 2017); hydrated lime CH III, with a unit weight of 593.51 kg/m³ (ABNT, 2021a) and density of 2.66 g/cm³ (ABNT, 2017); and natural sand of quartzous origin sourced from the Paraná River, Brazil. The granulometric composition and physical characteristics of the sand are presented in Table 1, and the CP II-F-32 specifications, as provided by the manufacturer, are outlined in Table 2.

The industrialized mortar (IM) used for coating the prisms was a pre-mixed type, composed of Portland cement, silica sand, and calcium carbonate sand in concentration ranges of 5-25%, 10-35%, and 45-74%, respectively, according to the manufacturer. The batched mortar used for coating the prisms was a stabilized type with a curing period of thirty-six hours, supplied by an established company, and the composition details were not provided by the manufacturer.

Production and characterization of mortars

Mixed and industrialized mortars were prepared using a planetary mortar mixer according to NBR 16541 (ABNT, 2016a). The batched mortar, supplied ready for use, required no additional mixing. Fresh-state characterization included consistency index measurements (ABNT, 2016b) (Figure 2a), bulk density, and air content (ABNT, 2005a). Mortar rheological behavior was assessed via the squeeze-flow method at 1 mm/s (ABNT, 2010) (Figure 2b). Tests were replicated on both the ceramic block and the block with plastering (Figure 2c and 2d), with the lower plate of the testing equipment replaced. Test specimens were molded for evaluating hardened-state properties after 28 days, including water absorption, void index, dry bulk density (ABNT, 2005b), flexural strength (Figure 2e), compressive strength (ABNT, 2005c) (Figure 2f), dynamic modulus of elasticity (ABNT, 2005c) (Figure 2g), and dimensional variation (ABNT, 2005d) (Figure 2h).

Ceramic block prisms

Prisms were constructed by placing two hollow ceramic blocks in a plumb orientation, with a height/thickness ratio of 2.8 (ASTM, 2022). Full mortar bedding was utilized, maintaining a (10 ± 3) mm joint thickness (ABNT, 2020) (Figure 3a). A 3 mm layer of plastering mortar was applied using a high-texture roller. The coating mortar type varied among mixed, industrialized and stabilized mortars, as detailed in Table 3, and was applied with an approximate thickness of 20 mm, ensuring uniformity with templates (Figure 3b). Prisms were molded by a qualified professional to ensure execution standardization, and the curing process occurred in a laboratory environment for 28 days (Figure 3d). The prisms underwent the compression strength test, with a compression load applied at a rate of (0.05 ± 0.01) MPa/s, on the EMIC universal testing machine, which had a load capacity of up to 300 kN (ABNT, 2023) (Figure 3f). The coating mortars applied to the prisms were evaluated for their tensile bond strength (ABNT, 2021c) (Figures 3c and 3e).

Results and discussions

Table 4 presents the geometric, physical and mechanical properties of the ceramic blocks. Table 5 presents the properties of coating mortars in the fresh state. The curves obtained through the squeeze-flow test are shown in Figure 4.

Statistical analysis was conducted (ANOVA) to verify the differences between all parameters evaluated in experiments. The results indicate that the mortars significantly differ from each other concerning fresh properties such flow, bulk density and air content (the p-values are shown in the table 5). The analysis of the curves reveals a similar behavior among the mixed, industrialized, and stabilized mortars when tested on three distinct substrates. The industrialized and stabilized mortars demonstrated increased displacement on all three substrates compared to the mixed mortar. In terms of the mixture composition, this can be attributed to the incorporation of chemical additives in the industrialized and stabilized mortars, enhancing material fluidity. The introduction of these additives likely influenced air incorporation during mixing, resulting in a higher air content in the industrialized and stabilized mortars, consequently leading to lower mass density compared to the mixed mortar. The industrialized mortar showed a 400% increase in the air content compared to the mixed mortar. Commercial mortars displayed diverse compositions, contributing to substantial variability in phase distribution in the fresh state (Grandes et al., 2021). The consistency indices of the industrialized and stabilized mortars increased by 3.6% and 6.3%, respectively, compared to the mixed mortar. In terms of substrate characteristics, notably, there is a greater spreading capacity of the mortars on the metal base compared to the ceramic block, followed by the plastered substrate on the ceramic block, which exhibited more restricted spreading. This phenomenon can be partially attributed to the irregular surface of the plaster hindering the mortar’s flow, while the non-absorbent and smooth metal surface facilitates such behavior. Additionally, potential water percolation through the porous structures of the ceramic block and plaster may have contributed to a reduction in the total water content in the sample. Substrate water absorption enhances resistance to mortar deformation, consequently reducing spreading capacity during the test. Similar results were observed in previous studies involving mortars and cement pastes (Costa et al., 2020; Barbosa; Lourenço; Hanai, 2010). The physical and mechanical properties of coating mortars in the hardened state are presented in Figure 5 and Table 6, respectively. The shrinkage curves of the mortars are illustrated in Figure 6.

The results indicate that the mortars significantly differ from each other concerning physical properties such as water absorption (ρ = 2.62196E-06), void index (ρ = 9.26177E-08), and dry bulk density (ρ = 1.879E-11). Significant differences were also found among the mortars for the mechanical properties, as indicated by the p-values in Table 6. The results indicate that the industrialized mortar exhibited the highest values for compressive strength, flexural tensile strength, tensile bond strength, and dynamic modulus of elasticity, followed by the stabilized and mixed mortars. Although the calculation of the average potential tensile bond strength excluded values deviating by 30% from the mean, following NBR 15258 (ABNT, 2021c). The coefficient of variation in the tensile pull off results was approximately 13-16%, which is typical for such experiments and were findings from other authors (Carasek et al., 2014; Santos et al., 2020). The highest values of tensile bond strength were observed in industrialized and stabilized mortars. More water facilitated the flow to the substrate, which increased the wetting capacity and consequently the contact area at the interface. Mortars with faster water loss, either to the substrate or the environment, accelerate the approach of mortar particles, reducing the fluid amount to mitigate friction forces between them. Consequently, this limitation in mortar mobility may increase the occurrence of defects at the interface with the substrate, diminishing tensile bond strength (Bernardo et al., 2020). On the other hand, increases in compressive strength do not proportionally impact tensile bond strength. While the increases in the compressive strength of IM and SM were in the order of 120% and 56%, compared to MM, in terms of bond strength, the gains were 45% and 25%, respectively. Other authors did not find a correlation between bond strength and the corresponding average compressive strength of the mortars (Costa et al., 2020). Additionally, shrinkage was more pronounced in the mixed mortar, especially at early ages, compared to the other mortars, which exhibited more similar behaviors. Table 7 presents the average compressive strength results and analysis of the prisms, including the enhancements in compressive strength observed after the application of the coating mortars.

The analysis of variance (ANOVA) conducted with a confidence level of 95% showed statistically significant differences in the compressive strength among the prisms produced with different types of mortar coatings. However, it is important to note that the coefficients of variation observed, ranging from 13.01% to 32.54%, indicate that the observed values should be examined with caution. Notably, the mortar with the highest mechanical strengths (IM) exhibited superior performance for the prism, followed by the stabilized mortar (SM), corroborating with Milani et al. (2021). A comparison of the average compressive strength values for prisms coated with mixed, industrialized, and stabilized mortar with the reference prism (uncoated) reveals increases of approximately 19%, 33%, and 31%, respectively. Similar behaviors have been reported in previous studies (Oliveira et al., 2018). The coating of mixed mortar provided the smallest increase in strength for the prisms compared to the other coatings. The individual increases of IM and SM, in relation to MM, were 46% and 25% for tensile bond strength (bs); 166% and 107% for flexural strength (σm); 120% and 56% for compressive strength (fm); and 171% and 42% for dynamic modulus of elasticity (Em), respectively. This resulted in increments of 12% and 10% in prism strength, showing that the mortars with better mechanical performance contributed more significantly to this; however, the increases did not occur proportionally, as shown in Figure 7.

Response surfaces and contour graphs illustrating the relationships between compressive strength and modulus of elasticity (Figure 8) and flexural strength and tensile bond strength of coating mortars (Figure 9) on the compressive strength of prisms.

It is possible to observe a trend of increasing compressive strength in the prisms as the compressive and flexural strength of the mortar rises. However, regarding the modulus of elasticity, there is noticeable variability where an increase in this parameter does not always reflect an increase in prism strength. This may be related to the fact that less deformable mortars may lack the capacity to absorb deformations, leading to their rupture. Figure 8a highlights a better performance of the prism when mortars have higher values of flexural and tensile bond strength; however, it is possible to observe that there are no direct relationships between these properties. In Brazil, a large portion of the buildings is constructed using clay block structural masonry. Advancements in research are necessary to enhance mechanical properties and devise new procedures for evaluating stress and strain in multi-story structures at a smaller scale. Figure 10 illustrates the failure modes observed in the ceramic block prisms, characterized by brittle failures with an immediate loss of the system’s strength capacity shortly after reaching the maximum load.

The relationships between the laying mortar strength and the ceramic block strength were within the recommended parameters for use (Parsekian et al., 2012), therefore, the failure mechanism of the uncoated prism occurred due to lateral tension of the block (Figure 9a). This type of failure, considered most desirable, is characterized by the appearance of a vertical crack in the block, preceded by signs of joint mortar failure through its cracking (Cheema; Klinger, 1986). In this condition, the mortar joint has a greater tendency to expand laterally in relation to the blocks since they have higher rigidity. The mortar is laterally confined at the block/mortar interface by the blocks; therefore, shear stresses at the block/mortar interface result in an internal stress state consisting of triaxial compression in the mortar and bilateral tension and axial compression in the blocks. This stress state initiates vertical cracks in the blocks, leading to the failure of the walls (Fonseca et al., 2019). In the coated prisms, the cracking process initiated in the blocks, and subsequently, the two layers of mortar coating primarily assumed the load. The mixed coating mortar showed signs of deformation under lower compression loads, compared to the others (Figure 9b). This may also be related to the fact that this mortar exhibited the lowest individual values of mechanical strength. The failure mode of industrialized and stabilized mortars occurred through the detachment of the coating layers from the block (Figures 9c and 9d). It should be noted, however, that the mortar coating layer prevented the complete visualization of the block’s behavior. After the rupture and complete detachment of the coating layers from the block, cracks in the blocks preceding the failure became visible (Figure 9e). Moreover, various factors can influence the failure process, including the quality of the workforce used in the construction of specimens, the thickness, and uniformity of the laying mortar joints, among other factors (Oliveira et al., 2018).

Conclusions

This study aimed to investigate the impact of various coating mortars on ceramic block prisms to anticipate the performance of rationalized masonry walls. The obtained results lead to the following conclusions: the greater spreading capacity on the substrate did not directly result in higher bond strength after the application of the coating on the prism. Prisms produced with coating mortars with the highest mechanical strengths showed superior performance; however, the increments do not occur proportionally. The increased adhesion of the coating to the substrate does not guarantee a delay in the rupture of the prisms, as mechanical properties can also influence the behavior of the assembly. Semi-prepared or factory-produced mortars exhibited favorable behavior when applied as coatings, indicating that technological control and the use of additives lead to performance improvement. The limited technical information on the influence of this layer on the compression load capacity of masonry elements emphasizes the importance of the obtained results. Furthermore, these findings have potential applications in assessing the structural safety of existing buildings and facilitating safety analyses for construction structures.

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