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Review

Review of Recent Progress on the Effects of High Temperatures on the Mechanical Behavior of Masonry Prisms

by
Gustavo Henrique Nalon
1,*,
José Carlos Lopes Ribeiro
1,
Leonardo Gonçalves Pedroti
1,
Roberto Marcio da Silva
2,
Eduardo Nery Duarte de Araújo
3,
Rodrigo Felipe Santos
1 and
Gustavo Emilio Soares de Lima
1
1
Department of Civil Engineering, Universidade Federal de Viçosa, Viçosa 36570-900, Brazil
2
Department of Structural Engineering, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazil
3
Department of Physics, Universidade Federal de Viçosa, Viçosa 36570-900, Brazil
*
Author to whom correspondence should be addressed.
Infrastructures 2023, 8(7), 112; https://doi.org/10.3390/infrastructures8070112
Submission received: 9 May 2023 / Revised: 30 June 2023 / Accepted: 13 July 2023 / Published: 14 July 2023
(This article belongs to the Topic Resilient Civil Infrastructure)
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Abstract

:
The structural performance of civil engineering infrastructures exposed to elevated temperatures has been investigated in many recent works. Some of these studies evaluated the residual mechanical behavior of masonry prisms subjected to high temperatures, as these specimens are simplified models (2–5 units in height) that can be easily produced and tested, in terms of operational and economic factors. However, there is no previous literature review on the mechanical properties of fire-damaged masonry prisms. Therefore, this paper presents an investigation of the current state-of-the-art on this topic. It provides a careful review of recent knowledge on the failure mechanisms, residual compressive strength, modulus of elasticity, and stress–strain behavior of masonry prisms made with different types of units, mortars, and/or grout after exposure to different types of thermal treatments. Based on the revised information, future research directions on the scientific field of masonry infrastructures are reported.

1. Introduction

The high complexity of the structural behavior of masonry elements is significantly increased when they are exposed to high temperatures. The combination between the mechanical behavior of fire-damaged units and mortars governs the overall performance of masonry elements under fire conditions [1,2]. The importance of studying the effects of fire on structural elements is justified by the fact that high temperatures change the physical, chemical, and mechanical properties of the materials, leading to possible strength/stiffness losses and building collapse risks [3,4,5].
Since the masonry elements are mainly subjected to compressive loads, the determination of the compressive behavior of fire-damaged masonry provides a valuable dataset for the development of strategies for the preservation of human lives and the structural diagnosis of masonry infrastructures. Based on the residual strength and stiffness of fire-damaged structures, the engineers can check their loadbearing capacity and decide whether they can be repaired for reuse or not [6,7,8].
Some studies on the effects of high temperatures on masonry structures have been developed over the last decades and reviewed by some authors [9,10,11]. Other works have focused on investigations of masonry components (e.g., units, mortar, and grout) exposed to elevated temperatures [12,13,14,15,16,17,18]. Despite this, scarce information is still available that provides quantitative data regarding the effects of fire on the structural performance of masonry elements.
In fact, the post-fire behavior of masonry elements has been rarely investigated due to the peculiarities of masonry as a composite material, the high variability of geometry and type of masonry components, and the high complexity of testing programs [1,9]. Fire tests of walls and small walls are expensive and time consuming, require complex equipment, and must be developed by qualified personnel with appropriate training. The lack of standard test methods and reliable equipment has led to a large scatter of thermal and mechanical results of masonry components [19]. In order to solve some of these issues, fire tests of masonry prisms have been carried out due to the operational and economic benefits provided by these simple types of specimens.
The mechanical behavior of masonry prisms can simulate many important mechanisms that characterize the complex structural behavior of full-scale masonry elements. They are simplified models (2–5 units in height) that can be easily produced and tested [20,21,22]. Although many review papers have already revised the behavior of masonry walls under fire conditions [9,10,15,17,19,23], there is no previous review focused on reviewing the residual compressive behavior of masonry prisms after exposure to elevated temperatures. In order to bridge this knowledge gap, the present work revised the current state-of-the-art on this topic, summarizing the results of residual compressive strength, static elastic modulus, stress–strain curves, and failure modes of masonry prisms made with different types of units, mortars, and/or grout after exposure to different types of thermal treatments.
The present paper provides important contributions for the areas of the design, diagnosis, and maintenance of masonry infrastructures subjected to fire events, such as: the (i) evaluation and comparison of the post-fire performance of masonry prisms produced with different types of masonry components (e.g., clay bricks, solid calcium silicate bricks, concrete blocks, cement mortar, lime mortar, pre-blended mortar, cement-lime mortar, conventional grout, etc.); (ii) recommendations for improvement in masonry design codes, based on the effects of the mutual dependency of different variables that affect the post-fire behavior of masonry prisms (e.g., maximum exposure temperature, type of unit or mortar, specimens rendering type, cooling method, etc.); (iii) a compilation of new test methods and masonry components investigated in recent research dealing with masonry prisms exposed to elevated temperatures; (iv) recommendations for the practical application of fire tests on masonry prisms as cost-effective alternatives for estimating the residual mechanical performance of fire-damaged masonry; and (v) recommendations for future research in this scientific field.
This article was structured in four sections, as indicated in Figure 1. Section 1 presents the introduction of the paper, which covers the research context and the original contributions of this work. Section 2 presents the literature review about the effects of high temperatures on the failure mode, strength, and stiffness of masonry prisms. Section 3 summarizes the conclusions obtained from the content revised in this paper. Finally, Section 4 shows the limitations and recommendations for future works.

2. Effects of High Temperatures on Masonry Prisms

This section presents a literature review about the effects of high temperatures on the compressive behavior of masonry prisms. In order to ensure the reliability and accuracy of the content of this review article, only research papers, dissertations, and theses with high standards of critical appraisal were revised. For this purpose, research was carried out using reliable search engines (Scopus, Google Scholar, and Periódicos CAPES). Various combinations of keywords were investigated in each database. Keywords related to the structural masonry field (e.g., masonry, masonry prisms, masonry wallettes, and masonry walls) were combined with keywords often used in studies dealing with structural fire performance (e.g., fire, high temperatures, and elevated temperatures). After a careful reading, the relevant studies (experimental works of fire-damaged masonry prisms) were selected, important information was extracted for review and Table 1 was constructed to summarize the main findings of the present review.
However, it is important to highlight that the previous research on the compressive behavior of masonry prisms exposed to elevated temperatures is still limited. Great efforts were made to select relevant references for the present review, avoiding a lack of transparency and scientific evidence in the revised studies. Another limitation of the review process was the lack of presentation of some of the methodological details in some papers, such as standard deviation (SD) values, heating rate used in fire tests, and prism capping procedures. Consequently, it was not possible to present some of these results in Table 1. The authors of some papers did not provide the ratio between net and gross area of the blocks. Then, this ratio was estimated as 0.50 [24] when the net area strength of hollow masonry blocks and prisms was calculated. Another limitation that must be highlighted is the use of different types of specimens for the determination of mortar compressive strength. When cylindrical specimens were used for mortar characterization, a cube/cylinder strength ratio equal to 1.25 [25] was applied as a correction factor.
Different characteristics (e.g., material type, length, width, height, strength, etc.) of masonry units, mortar, grout, and prisms used in previous works dealing with this topic [1,26,27,28,29,30,31,32,33,34] are displayed in Table 1. The residual compressive behavior of prisms exposed to high temperatures was mainly investigated in these experimental studies based on their residual stress–strain relationships, failure mechanisms, residual compressive strength (fc), and residual static elastic modulus (Es).
Table
Table 1. Previous investigations of residual compressive behavior of fire-damaged masonry prisms.
Table 1. Previous investigations of residual compressive behavior of fire-damaged masonry prisms.
AuthorsUnits (a)MortarGroutPrisms
TypeLength ×
Width
× Height (mm)		fb,net (b, c) (MPa)Materialfm,cub (c, d) (MPa) Materialfg,cyl (c, e) (MPa)RenderingJoint Thickness (mm)Heating Rate (°C/min)Time at Maximum Temperature (h)Maximum Exposure Temperature (°C)Cooling MethodInvestigated Mechanical Properties Capping of Prisms after Fire and before Mechanical Tests
Gao et al. [35]Solid clay brick240 × 115 × 53 16.32
(0.47)		Cement mortar29.38
(0.73) 		--Various reinforced layer types (f) (15 mm)10162200; 400; 600Natural coolingfc, load-displacement curvesQuick-hardening cement layer (3 mm)
Kiran et al. [34]Solid clay brick255 × 100 × 1007.85
(SD was not presented)		Cement mortar13.65
(SD was not presented)		--Manufactured sand, vermiculite or perlite mortar (20 mm)10Standard ISO 834 curve [36]
(30; 60; 90; 120 min)		Natural cooling (outside the oven)fc, Es, stress
–strain curves		Gypsum plaster (10 mm) and plywood sheets (20 mm)
Leal et al. [33]Hollow concrete blocks390 × 140 × 1907.7
(SD was not presented)		Cement–lime mortar6.13
(SD was not presented)		---10Standard ISO 834 curve (1999)
(70 min)		Natural cooling (inside the oven)fc (tests at 7 days after fire) Mineral fiber capping board (20 mm)
Bošnjak et al. [1,30]Solid clay brick240 × 115 × 7132.6
(SD was not presented)		Cement mortar23.4–25.9
(SD was not presented)		---1022100–1100Natural cooling (inside the oven)fc, Es, stress–strain curvesWithout capping (bricks’ surface polished before heating)
Solid calcium silicate brick28.3
(SD was not presented)		25.9–27.5
(SD was not presented)
Neto [26]Hollow clay blocks290 × 140 × 19024.3 (3.5); 27.9 (5.5)Cement–lime mortar4.38 (0.9)---10Standard ISO 834 curve [36]
(120 min)		Fan coolingfc, Es, stress–strain curvesMineral fiber capping board
Bitten-court and Antunes [31]Hollow clay blocks290 × 140 × 19014.9 (3.6)Not reported11.4 (0.5)Conventional grout67.7 (23.2)--30.5300; 600; 900Natural cooling (inside the oven)fcNot reported
Dupim [27]Hollow concrete blocks290 × 140 × 19042.2 (2.4); 14.3 (1.2)Cement–lime mortar11.5 (1.0)--Gypsum plaster layer (5 mm)10Standard ISO 834 curve [36]
(70; 120 min)		Fan coolingfcMineral fiber capping board
290 × 190 × 19038.1 (2.0); 11.6 (0.8)
Zhang et al. [32]Perforated clay brick240 × 115 × 9012.9 (0.3)Cement–lime mortar2.5 (0.2);
5.6 (0.3);
10.0 (0.1)		---15Standard ISO 834 curve [36]
(90; 150 min)		Air cooling (opened oven door if <400 °C); water coolingfc, stress–strain curveNot reported
Rigão [28]Hollow clay blocks290 × 140 × 19011.5 (1.1)Pre-blended mortar4
(SD was not presented)		-----0.5400; 900Natural cooling (inside the oven)fcNot reported
Russo et al. [29]Solid clay brickAround 60 × 60 × 6011.4–39.2
(SD was not presented)		Lime mortar3.0–31.5 (SD was not presented) (g)----Real fire event in a historic brick masonry structure in Venice, 2003fc, Es, stress–strain curvesNot reported
(a) Masonry unit: preformed component used in masonry construction [37]. (b) fb,net is the average value of the net area compressive strength of the units. (c) Standard deviation (SD) values (in MPa) are provided in parentheses. (d) fm,cub is the average value of the compressive strength of the mortar obtained from equivalent cubic specimens. (e) fg,cyl is the average value of the compressive strength of grout obtained from cylindrical specimens. (f) Reinforced layer types: PVA (polyvinyl alcohol) fibers, mixed PVA/basalt fibers reinforced layer, mixed PVA/steel fibers reinforced layer, and ordinary plaster mortar. (g) Predictions of mortar compressive strength based on non-destructive tests with energy dissipation penetrometer.
Different methodologies were used to test the masonry prisms at high temperatures. In some studies [26,27,32,33,34], they were exposed to a standard temperature increase defined by the ISO 834-1:1999 fire curve [36], which is represented in Figure 2a and Equation (1). In this formulation, θg is the gas temperature in the fire compartment, θ0 is the initial room temperature, and t is the time of the heating process. In other works [1,28,30,31], the masonry prisms were subjected to a heating ramp (constant heating rate) up to a maximum temperature, as indicated in Figure 2b. After reaching the maximum test temperature, this was maintained for a period of 0.5–2 h (Table 1). Table 1 shows that different maximum temperatures levels have been used to cause severe damage conditions to masonry prisms: 900 °C [28,31], 1100 °C [1,30], or application of the ISO 834-1:1999 fire curve [36] up to ∼970 °C [33], ∼1050 °C [26,27,34], or ∼1080 °C [32].
θg = θ0 + 345 log (8t + 1)
In addition, Table 1 shows that different types of cooling methods have been applied to masonry prisms, such as natural cooling inside the furnace, natural cooling outside the furnace, fan cooling, and water cooling. The effects of these different methodologies on the residual mechanical behavior of the specimens are discussed in the following sections.
In most of the cases, the fire tests of masonry prisms were carried out in computer-controlled electrical furnaces, as indicated in the experimental setup of Gao et al. [35] of Figure 3a. In modern experimental programs, masonry prisms were also subjected to heating/cooling procedures in gas furnaces with high heating capacity. For example, Figure 3b shows the gas furnace used by Leal et al. [33] to heat masonry prisms and small walls.

2.1. Failure Mechanisms of Prisms Exposed to High Temperatures

Bošnjak et al. [1,30] reported an experimental evaluation of ungrouted prisms fabricated with solid clay and calcium silicate units, after exposure to temperatures up to 1100 °C and slow cooling. The failure mode of fire-damaged prisms made with clay bricks (Figure 4a) and calcium silicate bricks (Figure 4f,g) was governed by the mortar joint. It happened because the mortar was the least stiff and weakest component of the prisms, regardless of the type of unit (Table 1). At first, the joint was compressed by the units, leading to increased deformation in the mortar joint. Consequently, high transversal tensile stresses happened in both types of bricks, so that they failed after the propagation of vertical cracks.
The exposure to high temperatures increased the difference in the stiffness of bricks and mortar, especially for clay brick prisms. This increase may be attributed to the significant thermal degradation of the mortar joint (physical and chemical changes happened in the hydrated cement paste, aggregates, and interfacial transition zone) and the minor damage in the clay and calcium silicate units after exposure to high temperatures. After exposure to 500 °C, the crushing of mortar joints was visible in the outer portions of the prisms. The significant degradation of the mortar joint caused significant losses of compressive strength and modulus of elasticity with increasing temperature, which are discussed in the next section.
Masonry prisms produced with fire-damaged clay bricks were tested by Russo et al. [29]. The authors constructed these prisms with fire-damaged units collected from a historic brick masonry structure in Venice after a fire event. The typical collapse of the prisms is reproduced in Figure 4b. The localized spalling of mortar joints and vertical cracks in the clay bricks can be observed in this image, as also reported by Bošnjak et al. [1,30].
Kiran et al. [34] investigated the post-fire behavior of clay brick prisms. After exposure to the ISO 834-1:1999 standard fire curve [36] for different periods (30, 60, 90, or 120 min), the prisms’ plastering portion was chipped-off in order to observe existing crack patterns. In specimens subjected to 30 or 60 min of exposure (Figure 4c), cracking was only visible at the point of the ultimate compressive load. In prisms subjected to 90 min of exposure (Figure 4d), most of the cracks developed at the brick/mortar interface, at 70–85% of the prisms’ compressive strength. In these prisms, the final failure was observed at the mortar joints. After a 120 min exposure (Figure 4e), vertical cracks close to the vertical joints were visible at the loading stage, due to the de-bonding at the weak brick-mortar interface.
The rupture modes of ungrouted prisms made of hollow clay blocks were investigated by Rigão [28]. Prisms that were not exposed to high temperatures and prisms exposed to 400 °C failed by transverse block splitting. In contrast, mortar crushing was observed in prisms exposed to 900 °C. These results evidence that the significant degradation of the joints at higher temperatures was able to change the masonry rupture mechanisms. Therefore, the failure mode of clay masonry prisms was remarkably affected by the thermal treatments.
Failure mode modifications due to increased exposure temperatures were also verified by Zhang et al. [32] in clay brick prisms exposed to different times of exposure of the ISO 834-1:1999 standard fire curve [36]. According to the authors, the first crack appeared at 65–75%, 30–40%, and 15–20% of the failure load in specimens subjected to 0, 90, and 150 min of standard fire exposure, respectively. These authors also reported the first experimental investigation of the effects of different cooling methods (air cooling vs. water cooling) on the failure mode of masonry prisms. As previously observed in plain concrete specimens subjected to fast cooling procedures, the thermal shock associated with the application of the water-cooling method generated tensile stresses in the bricks, resulting in severe cracks and permanent deformation in the center of the specimens.
The application of a reinforced layer to the surface of masonry prisms also changed their failure mechanisms. According to the recent work developed by Gao et al. [35], the different types of reinforced layers (PVA fibers reinforced layer, mixed PVA/basalt fibers reinforced layer, mixed PVA/steel fibers reinforced layer, and ordinary plaster mortar) delayed the failure of the internal clay–brick structure. Prisms produced with the PVA/steel fibers reinforced layer presented a lower amount of cracks after exposure to 200 °C. After 400 °C and 600 °C, the brittleness and the damage of the different types of specimens increased significantly. The use of steel fibers improved the bonding performance between the reinforced layer and the masonry prism.
Leal et al. [33] recently compared the compressive behavior of masonry blocks, prisms, and small walls after exposure to high temperatures. These different types of specimens were subjected to the ISO 834-1:1999 [36] standard fire for 70 min. Different from the minor thermal damage observed in the clay and calcium silicate units previously mentioned in this section, Leal et al. [33] observed that fire-damage concrete blocks showed superficial cracks (Figure 4h) and noticeable degradation, so that some units could be easily broken when handled after fire exposure. A similar cracking pattern was observed in masonry prisms and small walls, which was mainly characterized by the formation of vertical cracks at the unit’s webs (Figure 4i). Localized mortar crushing and spalling of block portions were observed by Dupim [27] in masonry prisms produced with hollow concrete blocks and subjected to the ISO 834-1:1999 standard fire curve [36] for 70 min.

2.2. Strength and Stiffness of Prisms Exposed to High Temperatures

The experimental results of the compression tests carried out by Bošnjak et al. [1,30] indicated that the compressive strength of masonry prisms made with solid clay or calcium silicate units remained at least the same as the original compressive strength (before heating) up to temperature levels of 700 °C, which shows the excellent fire resistance of masonry produced with clay and calcium silicate bricks. For temperature levels higher than 700 °C, clay brick prisms presented strength reductions up to 22%, while strength reductions higher than 55% were observed in calcium silicate prisms. In contrast, stiffness reductions higher than 50% were observed in both types of prisms when the maximum exposure temperature was higher than 600 °C. The residual stiffness of the prisms reduced progressively with the increase in the maximum temperature, mainly for temperatures higher than 300 °C, due to the strong thermal degradation of the cement mortar joint of the prisms.
Kiran et al. [34] evaluated the residual mechanical performance of clay brick prisms plastered with the different lightweight mortars indicated in Table 1 (manufactured sand mortar, vermiculite mortar, or perlite mortar), after exposure to 30, 60, 90, or 120 min of the ISO 834-1:1999 standard fire curve [36]. The reference prisms (without plaster) could not resist exposure temperatures for periods ≥60 min. In contrast, interesting residual values of strength and stiffness were observed in specimens plastered with lightweight mortars. Prisms plastered with perlite mortar had better fire resistance, since they did not present strength reductions when subjected to any heating duration. After 120 min of fire exposure, prisms with vermiculite plaster and perlite plaster presented values of axial compressive strength 31.6% and 86.1% higher than that of prisms with sand plaster, respectively. After the same thermal treatment, prisms with perlite, vermiculite, and sand mortar presented elastic modulus losses of 15%, 25%, and 60%, respectively. These results indicated that lightweight minerals such as vermiculite and perlite behaved like sacrificial materials that can provide adequate thermal protection for clay brick masonry.
A trend of compressive strength decreasing with temperature increases was also observed by Russo et al. [29] in masonry prisms produced with fire-damaged clay bricks resulting from a real fire event in Venice. The joints of these prisms were made with mortar with mechanical properties close to those obtained from in situ non-destructive tests with an energy dissipation penetrometer. The compressive strength of these prisms was about 39% lower than that of reference prisms produced with units and mortar that were not exposed to high temperatures. In contrast, stress–strain curves revealed that reference prisms presented greater deformability than the fire-damaged prisms, which is not in agreement with the results of Bošnjak et al. [1,30]. Based on micro-destructive tests with flat jacks carried out in the field, the authors observed that the production of mortar in a laboratory to simulate fire-damage conditions can be the cause of those unexpected stiffness results.
The effects of different cooling methods on the residual compressive strength of clay brick prisms exposed to 90 min of the ISO 834-1:1999 standard fire curve [36] were reported by Zhang et al. [32]. Water-cooled masonry prisms presented a compressive strength loss 10% greater than that of the air-cooled prisms. This result was attributed to the sudden drop in temperature associated with the water-cooling process, which resulted in cracks in units and mortar joints, reducing the load-bearing capacity of the specimens.
The residual mechanical properties of ungrouted prisms produced with hollow clay blocks were studied by Rigão [28]. The residual strengths of clay block prisms exposed to 400 °C and 900 °C were 73.0% and 48.7% of their original compressive strength (before the heating treatments), respectively. The different strength losses observed by the authors are attributed to the changes in the rupture mode from tensile block splitting to mortar crushing, as discussed in the previous section.
Neto [26] also evaluated the residual mechanical properties of ungrouted prisms constructed with hollow clay blocks. However, they were subjected to the ISO 834-1:1999 standard fire curve [36] for 120 min. After being exposed to temperatures higher than 900 °C, the prisms exhibited reductions in compressive strength and elastic modulus up to 93.5% and 98.5%, respectively. Comparisons between the results of Neto [26], Rigão [28], and Bošnjak et al. [1,30] suggest that prisms made with hollow clay blocks seem to be more affected by the fire exposure than prisms constructed with solid clay bricks.
The residual compressive strength of hollow clay blocks filled with conventional grout was recently investigated by Bittencourt and Antunes [31]. Considering a 5% significance level, no statistical difference was observed between the original strength of the grouted prisms and their residual strength after exposure to 300 °C, 600 °C, and 900 °C. Although deleterious effects on the grout/unit adherence and decreases in grout strength were observed after exposure to temperatures of 300 °C, the high thermal resistance of hollow clay units provided excellent residual mechanical properties for masonry prisms.
All types of reinforced layers (PVA fibers reinforced layer, mixed PVA/basalt fibers reinforced layer, mixed PVA/steel fibers reinforced layer, and ordinary plaster mortar) investigated by Gao et al. [35] also increased the residual compressive strength of clay brick prisms exposed to temperatures of 200 °C, 400 °C, or 600 °C. The best residual strength improvements were provided by the PVA/steel fibers reinforced layer. In addition, the load-displacement curves obtained from the compression tests of these specimens revealed that the specimens with fiber-reinforced layers presented the highest initial residual stiffness after exposure to 200 °C or 400 °C. On the other hand, prisms covered with ordinary plaster mortar exhibited the largest initial stiffness after exposure to temperatures of 600 °C.
Dupim [27] carried out an experimental program that investigated the residual compressive strength of prisms made with hollow concrete blocks. After exposure to the ISO 834-1:1999 standard fire curve [36] for 70 min, their compressive strength was only 14% of their original compressive strength, since both concrete and mortar joints presented significant degradation due to the exposure to elevated temperatures. Other prisms were covered with a 5 mm plaster layer and then exposed to the ISO 834-1:1999 standard fire curve [36] for 120 min. They presented residual compressive strength values ranging between 6% and 9% of their initial compressive strength. Comparisons between the results of Dupim [27] and Neto [26] indicated that prisms made with hollow concrete blocks were more affected by the fire exposure than prisms constructed with hollow clay blocks.
Values of compressive strength of masonry prisms and small walls of hollow concrete blocks, after exposure for 70 min to the ISO 834-1:1999 curve [36], were recently compared by Leal et al. [33]. The fire-damaged prisms and small walls showed a compressive strength of approximately 14% of their initial strength. According to the authors, these results suggest that using simpler specimens (such as stack-bond prisms) can provide satisfactory results in the evaluation of the residual masonry compressive strength, considering slenderness effects separately. Since prisms are specimens that are easier to construct and test, the results of Leal et al. [33] indicate that prisms are promising candidates for future experimental analyses of masonry exposed to elevated temperatures.

3. Conclusions

This study reviews the effects of high temperature exposure on the compressive behavior of masonry prisms, which is an important topic in the research field of resilient civil infrastructures. The following conclusions were obtained from the content revised in this study:
  • Exposure to high temperatures can cause remarkable changes in the failure mode of prisms produced with clay or calcium silicate units, due to increases in the difference between the residual stiffness of the units and the mortar. This increase has been attributed to the significant thermal degradation of the joint compared to the units, so that a premature crushing of the fire-damaged mortar joint was usually observed.
  • The effects of fire on the failure mode of concrete block prisms were less remarkable compared to those verified in prisms produced with clay or calcium silicate units. In concrete block prisms, units and mortar joints were all severely affected by the elevated temperatures, leading to slight changes in the difference between their residual stiffness. Then, tensile block splitting, spalling of block portions, and localized mortar crushing were reported in prisms of concrete units.
  • Comparisons between the residual strength and stiffness of masonry prisms demonstrated the excellent fire resistance of prisms produced with clay and calcium silicate units, compared to prisms constructed with concrete units. The mechanical performance after exposure to high temperatures was improved when concrete grout was poured into the hollow block prisms. However, the mechanical properties of ungrouted hollow clay block prisms seemed to be more affected by fire exposure than prisms constructed with solid units.
  • Experimental tests of masonry prisms have been successfully used to evaluate the effects of different factors on the behavior of masonry exposed to high temperatures. The following factors have been investigated in previous studies: maximum expo-sure temperature of the prisms (using constant heating rate or standard fire curve), type of unit or mortar (solid clay bricks, solid calcium silicate bricks, hollow clay blocks, hollow concrete blocks, cement mortar, lime mortar, and cement–lime mortar), specimens’ rendering type (PVA fibers reinforced layer, mixed PVA/basalt fibers reinforced layer, mixed PVA/steel fibers reinforced layer and ordinary plaster mortar, manufactured sand mortar, vermiculite mortar, or perlite mortar), and cooling method (air cooling and water cooling).
  • The good agreement between the compressive strength results of masonry prisms and small walls (before and after exposure to fire) suggested that prisms can be considered simple types of specimens that may provide interesting insights into the residual strength of fire-damaged masonry.

4. Limitations and Future Directions

Based on the information revised in this study, some limitations and open questions could be identified. Many issues associated with the compressive behavior of masonry prisms exposed to high temperature need future studies.
  • The effects of numerous variables on the post-fire mechanical properties of masonry prisms have not yet been investigated in the previous literature. Thus, it is pertinent to evaluate the influence of the following factors on the structural behavior of prisms subjected to high temperatures: time and rate of heating, type of aggregates used in concrete units and mortar joint, geometry of hollow blocks, different slow and fast cooling procedures, moisture content of the prism, relative strength of units and mor-tar, bedding approach, and bonding arrangement.
  • Previous studies have only explored the mechanical performance of fire-damaged masonry prisms made with conventional types of units and mortars. Therefore, future studies are recommended to investigate the effects of high temperatures on mechanical properties of prisms produced with high-strength blocks, units and mortars incorporating wastes and recycled admixtures, nanomodified units and mortars, polymer adhesive mortars, fiber-modified units and mortars, and other unconventional masonry components.
  • The current knowledge regarding the mechanical behavior of grouted masonry prisms exposed to high temperatures is very limited. Thus, relevant future studies should focus on the compressive behavior of prisms produced with different grouting procedures, prisms made of hollow blocks filled with grout of different mechanical properties, and grout containing different types and dosages of shrinkage-compensating admixtures.
  • Future experimental works should be developed in order to increase the available dataset on the residual mechanical strength and elastic modulus of masonry prisms, which could be used to develop more representative stress–strain curve models and empirical formulations for the prediction of prism strength and stiffness.

Author Contributions

Conceptualization, G.H.N. and J.C.L.R.; methodology, G.H.N., J.C.L.R., R.F.S. and G.E.S.d.L.; validation, L.G.P., R.M.d.S. and E.N.D.d.A.; formal analysis, L.G.P., R.M.d.S. and E.N.D.d.A.; investigation, G.H.N., R.F.S. and G.E.S.d.L.; resources, G.H.N., J.C.L.R. and L.G.P.; data curation, G.H.N.; writing—original draft preparation, G.H.N.; writing—review and editing, J.C.L.R., L.G.P., R.M.d.S., E.N.D.d.A., R.F.S. and G.E.S.d.L.; visualization, L.G.P., R.M.d.S. and E.N.D.d.A.; supervision, J.C.L.R., L.G.P., R.M.d.S. and E.N.D.d.A.; project administration, J.C.L.R.; funding acquisition, J.C.L.R. and L.G.P. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001.

Data Availability Statement

The data presented in this study are available in Table 1 of the paper.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

Esresidual static elastic modulus of prisms
fb,netnet area compressive strength of units
fcresidual compressive strength of prisms
fg,cylcompressive strength of grout
fm,cubcompressive strength of mortar
PVApolyvinyl alcohol
SDstandard deviation
ttime of the heating process
Tmaxexposure temperature level
qggas temperature in the fire compartment
q0the initial room temperature

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Figure 1. Structure diagram of the present literature review paper (fb,net is the net area compressive strength of the units, fm,cub is the compressive strength of the mortar, and fg,cyl is the compressive strength of the grout).
Figure 1. Structure diagram of the present literature review paper (fb,net is the net area compressive strength of the units, fm,cub is the compressive strength of the mortar, and fg,cyl is the compressive strength of the grout).
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Figure 2. Temperature versus time curves used in different methods for tests of masonry prisms. Source: (a,b) adapted from Gao et al. [35] Copyright 2023, with permission from Springer.
Figure 2. Temperature versus time curves used in different methods for tests of masonry prisms. Source: (a,b) adapted from Gao et al. [35] Copyright 2023, with permission from Springer.
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Figure 3. Equipment used in tests of prisms exposed to high temperatures. Source: (a) adapted from Kiran et al. [34], Copyright 2022, with permission from Wiley Company; (b) adapted from Leal et al. [33,38] as permitted under the Creative Commons Attribution License Agreement.
Figure 3. Equipment used in tests of prisms exposed to high temperatures. Source: (a) adapted from Kiran et al. [34], Copyright 2022, with permission from Wiley Company; (b) adapted from Leal et al. [33,38] as permitted under the Creative Commons Attribution License Agreement.
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Figure 4. Failure mechanisms of masonry prisms subjected to different exposure temperature levels (Tmax). Source: (a,f,g) adapted from Bošnjak et al. [1], Copyright 2020, with permission from Elsevier; (b) adapted from Russo et al. [29] with permission from the International Masonry Society; (c–e) adapted from Kiran et al. [34], Copyright 2022, with permission from Wiley Company; (h,i) adapted from Leal et al. [33], as permitted under the Creative Commons Attribution License Agreement.
Figure 4. Failure mechanisms of masonry prisms subjected to different exposure temperature levels (Tmax). Source: (a,f,g) adapted from Bošnjak et al. [1], Copyright 2020, with permission from Elsevier; (b) adapted from Russo et al. [29] with permission from the International Masonry Society; (c–e) adapted from Kiran et al. [34], Copyright 2022, with permission from Wiley Company; (h,i) adapted from Leal et al. [33], as permitted under the Creative Commons Attribution License Agreement.
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MDPI and ACS Style

Nalon, G.H.; Ribeiro, J.C.L.; Pedroti, L.G.; Silva, R.M.d.; Araújo, E.N.D.d.; Santos, R.F.; Lima, G.E.S.d. Review of Recent Progress on the Effects of High Temperatures on the Mechanical Behavior of Masonry Prisms. Infrastructures 2023, 8, 112. https://doi.org/10.3390/infrastructures8070112

AMA Style

Nalon GH, Ribeiro JCL, Pedroti LG, Silva RMd, Araújo ENDd, Santos RF, Lima GESd. Review of Recent Progress on the Effects of High Temperatures on the Mechanical Behavior of Masonry Prisms. Infrastructures. 2023; 8(7):112. https://doi.org/10.3390/infrastructures8070112

Chicago/Turabian Style

Nalon, Gustavo Henrique, José Carlos Lopes Ribeiro, Leonardo Gonçalves Pedroti, Roberto Marcio da Silva, Eduardo Nery Duarte de Araújo, Rodrigo Felipe Santos, and Gustavo Emilio Soares de Lima. 2023. "Review of Recent Progress on the Effects of High Temperatures on the Mechanical Behavior of Masonry Prisms" Infrastructures 8, no. 7: 112. https://doi.org/10.3390/infrastructures8070112

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