Modulus of Elasticity and Mechanical Properties Assessment of Historical Masonry Elements After Elevated Temperature: Experimental Study and Numerical Analysis

Abstract

Historical masonry structures deteriorate over time, requiring restoration and strengthening. Hydraulic lime-based mortars (HLMs), due to their compatibility with historical materials, are commonly used for this purpose. This study examines the fire resistance of masonry walls constructed with HLMs. Masonry prisms with clay bricks were prepared using HLMs in accordance with material testing standards. Specimens were subjected to high temperatures ranging from 200 °C to 800 °C, followed by flexural–compression tests for mortar and compression tests for masonry prisms. A total of 20 masonry prism specimens, 15 brick specimens, and 15 mortar specimens were tested, including reference specimens at room temperature. Experimental results indicate that masonry prisms, clay bricks, and HLMs progressively lose their mechanical properties as temperature increases. The elastic modulus of masonry prisms was evaluated according to relevant standards, and Finite Element Analysis (FEA) was conducted to validate temperature-dependent material properties. The stress–strain response of M15 HLM masonry prisms was determined, addressing the absence of such data in EN 1996-1-2. Additionally, compression test results were compared with digital image correlation (DIC) analyses to enhance measurement accuracy. This study provides critical insights into the thermal performance of masonry walls with HLMs, contributing to the development of fire-resistant restoration materials.

1. Introduction

Historic masonry structures are exposed to a wide range of adverse conditions such as constant ravages of time, natural disasters, and human intervention. In addition to the impact of atmospheric conditions and aging, historic mortars are exposed to harsh conditions during natural disasters, such as earthquakes, landslides, air pollution, extreme temperatures, floods, and fires. Fire is a major threat to these buildings, which can destroy centuries of history and cause permanent damage. The performance of structural elements during and after a fire is critical in historical buildings. It is crucial to understand how construction materials behave during and after a fire and how this affects their physical and mechanical properties, because this has a significant impact on the performance of a building. In the context of the restoration of historical buildings, hydraulic lime-based mortars (HLMs) are materials commonly used for repair and reconstruction. This is due to the fact that the use of cement-based mortars is prohibited in such cases. Despite the significance of research pertaining to the fire resistance and physical and mechanical performance of hydraulic lime-based mortars (HLM) post-fire, the extant literature on this subject is limited. Studies in the literature have predominantly focused on cement-based mortar.

The fire resistance in masonry is influenced by the material composition, geometric design, and moisture content. Understanding these factors facilitates the optimization of masonry systems under extreme conditions. Elevated temperatures can alter mechanical properties, such as compressive strength, elasticity, and thermal conductivity, thereby impacting the load-bearing capacity and deformation resistance. Leal et al. explored the temperature gradients in concrete masonry blocks during fire exposure and highlighted the effect of moisture content on thermal behavior [1]. This study combines experimental and numerical analyses to examine the thermal properties and temperature distribution under fire conditions. Notable results include the minimal effect of block strength on temperature progression, quick temperature rises due to thin face shells, and a 60 min fire resistance for thermal insulation. In their analysis of fire resistance in hollow clay bricks, Nguyen and Meftah elucidated the influence of geometric and material properties on heat transfer [2]. Their paper presented the second part of a study on hollow clay brick masonry wall behavior during fire exposure, focusing on 3D finite element modeling and spalling assessment. This research examines the thermal and mechanical material characteristics, proposes spalling criteria, and conducts simulations for walls with and without a load-bearing capacity. Andreini and Sassu investigated the mechanical degradation under fire exposure and proposed simplified predictive models for masonry resilience [3]. Their study examined 200 cylindrical specimens of various materials from 20 °C to 700 °C to determine thermal strain and axial strength decay. This research proposes stress–strain curves and the secant modulus of elasticity to aid in the assessment of masonry panels under fire conditions. Prieler et al. employed a finite element method to analyze the heat transfer and deformation in masonry brick walls and an embedded fire safety steel door during fire exposure [4]. Their research demonstrated accurate temperature predictions and significant deformation influenced by door position and wall boundary conditions.

High temperatures have a profound and multifaceted impact on the mechanical properties of masonry, thereby influencing its strength, stiffness, and overall structural integrity. Exposure to elevated temperatures results in thermal strain, increased cracking, and material degradation, which can significantly alter the performance of the masonry systems. These effects vary depending on factors such as material composition, mortar modifications, and the type of bricks utilized, underscoring the need for tailored approaches to enhance the thermal resilience of masonry. Yang et al. investigated polymer-modified mortars and demonstrated that EVA polymer added to CSA cement enhances the thermal stability and mechanical properties of polymer-modified sulfoaluminate cement mortar (PMSCM) at elevated temperatures (20 °C to 600 °C) [5]. Despite the increased cracking, the PMSCM with EVA retained a higher strength, meeting the lightweight mortar standards. Bošnjak et al. assessed the residual behavior of masonry after heat exposure using clay and calcium silicate bricks [6]. Experimental and numerical analyses revealed that temperature moderately affected strength but significantly reduced stiffness, with numerical models effectively simulating post-heating performance. Similarly, Bamonte et al. examined cement–lime mortars, highlighting higher mass loss, lower thermal diffusivity, and greater ductility compared to concrete, aligning with the existing data on heat-induced mechanical decay [7]. Innovative mortar compositions were investigated by Singh et al. [8] and Darmayadi and Satyarno [9]. Singh identified geopolymer bricks with 30% red mud as optimal, whereas numerical modeling by Darmayadi and Satyarno accurately predicted the masonry wall performance under thermal effects [9]. Thamboo and Dhanasekar established correlations between prism and wallette compressive strengths and proposed a simplified deformation model [10].

Their research revealed that moisture diminishes the compressive strength of fired-clay bricks and hydraulic lime mortars; however, confinement techniques can mitigate this effect. Similarly, Ramirez et al. developed a hygro-thermo-mechanical model to assess the effects of temperature and moisture on masonry walls [11]. The study found that cement mortar walls experienced higher stress levels than lime mortar walls, owing to their differing hygro-thermal properties, providing valuable insights into material selection and design for varying environmental conditions.

Recent advances in reinforcement methodologies significantly enhanced the resilience of masonry structures. These innovations have facilitated the development of materials and techniques for improving the structural integrity and durability of masonry under diverse loading and environmental conditions. Estevan et al. demonstrated the efficacy of textile-reinforced mortar (TRM) in enhancing shear strength, particularly for brick masonry panels [12]. Their study revealed that carbon fiber TRMs restored the wall capacity and improved ductility, whereas glass fiber TRMs exhibited increased susceptibility to heat exposure.

Cree and Pliya [13] investigated eggshell powder (ESP) as a sustainable alternative in cement mortar and found that specimens with up to 20 wt.% ESP, when tested at temperatures ranging from 20 to 450 °C, exhibited significant color changes and strength reductions, while still meeting ASTM’s 28-day compressive strength requirements. This study highlights the potential of ESP as a supplementary cementitious material (SCM) for improving the sustainability of cement-based mortars. Tian et al. emphasized the historical significance of sticky rice lime mortar in heritage structures and demonstrated its superior shear properties [14]. The study found that adding 2 wt% Na-SiO₂ and 0.5 wt% fiber to sticky rice lime mortar significantly improved its shear strength, plasticity, and ductility, rendering it more suitable for structures with significant shear deformation. Parent et al. investigated post-fire structural behaviors using advanced 3D modeling techniques for the analysis of the Notre-Dame de Paris Cathedral [15]. The study compared three modeling strategies—two discrete 3D approaches and one continuum 3D approach—highlighting their computational strengths and weaknesses and proposing future research on thermomechanical behavior models and hybrid calculation tools. Soleymani et al. examined the mechanical properties of clay brick masonry using traditional mortars, including gypsum and three types of lime-based mortars used in Iranian traditional and historical monuments [16]. The results indicated that the masonry with gypsum mortar exhibited the highest compressive strength, elastic modulus, and diagonal shear strength, highlighting its superior performance in structural applications. These studies contribute to the growing body of knowledge on sustainable materials and conservation techniques, and they provide insights into the benefits of integrating traditional and modern materials in masonry research.

Bowen Liu at al. compared the test result displacements (LVDT) with Digital Image Correlation (DIC) in the study of masonry arc bridges where the level of precision in detecting crack behavior is sufficient [17]. In an experimental study on RC beams, Cakir et al. highlighted the reliability of digital image correlation during the testing [18].

Although studies have been conducted on the fire resistance or post-fire mechanical properties of cement-based mortars or mortars reinforced with other materials as a result of the literature research, studies on HLMs and masonry prisms are quite scarce and insufficient. Therefore, in this study, the effects of different fire exposure levels on the mechanical strength of HLM masonry prisms were investigated. As part of the investigation, the HLM was formulated following material testing standards, and prism-shaped specimens were prepared using this mortar. Then, these specimens underwent exposure to the elevated temperatures ranging from 200 °C to 800 °C, followed by subsequent bending and compression tests. After the elastic modulus of the masonry prism under compression tests was reported, the stress–strain results were evaluated, and the maximum displacement values were compared with Digital Image Correlation results. Finite element transient thermal analysis was performed and compared with actual fire test results using the ISO 834 temperature–time curve [19]. It is believed that conducting research and experimental studies will fill this gap in the literature by presenting an original study.

2. Materials and Methods

In this study, the fire resistance of HLMs, clay bricks, and masonry prisms was investigated. In this study, mortar and masonry prisms were prepared according to material testing standards, and prism samples were created using this mortar [20,21,22,23]. These samples were then subjected to bending and compression tests at temperatures ranging from 200 °C to 800 °C after exposure to high temperatures. Fifty samples were included in the high-temperature tests, with three experimental samples, one temperature-monitored sample, and three reference samples for each temperature. During the elevated temperature tests (ETTs), 20 specimens were prepared for the masonry prism, 15 specimens for the brick, and 15 specimens for the masonry mortar, including three test specimens, one temperature monitoring specimen for each temperature of the masonry prism, and three reference specimens. The reference samples were tested at room temperature, whereas the other samples were held at room temperature after completing the elevated-temperature test and were then tested under the same conditions as the reference samples. No tests were conducted on the temperature-monitoring samples; they were only used to monitor the internal temperatures of the samples using the embedded thermocouples. In this study, all steps were performed according to the flowchart (Figure 1).

In summary, in the test program, the following steps were applied to specimens:

Step 1: Specimen preparation for mortar prism and masonry prism.

Step 2: Specimen conditioning for 28 days after specimen preparation.

Step 3: Elevated temperature testing in the fire resistance furnace of all specimens at elevated temperatures from 200 °C to 800 °C.

Step 4: Flexure and compression testing for mortar ambient and after elevated temperature. Compression testing for masonry prism ambient and after elevated temperature. Digital image correlation was performed during masonry prism testing in order to compare the actual displacement values.

Step 5: Evaluation of test results.

2.1. Hydraulic Lime-Based Mortar, Brick, and Masonry Prisms

Mortar serves as an adhesive substance that binds stones or bricks, and it plays a crucial role in masonry construction. It fills the spaces between the building blocks (mortar joints), thereby enhancing structural integrity. Additionally, mortar provides resistance to air and water infiltration, which bolsters the durability of masonry structures. The key properties of mortars include their workability, cohesiveness, resistance to external forces, and ease of spreading. The strength and durability of masonry structures are significantly influenced by the physical properties of the mortar. The bond strength and binding characteristics of mortar play a crucial role in determining the overall performance of a building. A well-formulated mortar with a strong bond strength enhances the structural integrity of the masonry, ensuring stability and resilience against various external forces and environmental factors. Consequently, understanding and optimizing these mortar properties are essential for ensuring the longevity and effectiveness of masonry structures. The required volume of mortar varies based on the type of masonry, and it typically ranges from 0 to 20 percent of the total volume. Despite its relatively small volume, mortar plays a crucial role in masonry structures [24]. Initially, the mortar was in a plastic state after preparation but quickly began to stiffen. Mortar is typically produced by blending sand, lime, cement, and water. Previously, traditional mortars were composed of mud, clay, and lime. Lime mortars, among the earliest variants, trace back to Babylonian origins, although Egyptians had advanced mortar technology that incorporated lime and gypsum to construct pyramids. Historical masonry structures commonly used HLM. However, Romans revolutionized mortar production by introducing cement, which subsequently became the predominant material for mortar applications [25].

HLMs are most commonly used to repair and restore historical masonry structures, and there are special mortars prepared for historical structures without cement. They provide breathability and high water vapor permeability. They are compatible with historical structures and allow for the production of mortars with different properties. Natural hydraulic lime (NHL) acquires its characteristics by calcining argillaceous or siliceous limestones, followed by pulverization via slaking with or without grinding. HLMs solidify via a dual process involving hydration, which forms a calcium–silicate–hydrate structure, and carbonation, which occurs when calcium hydroxide reacts with carbon dioxide in the air. Commercial binders are categorized based on their hydraulicity levels, which correlate with the strength development. Natural hydraulic limes are categorized based on their compressive strengths. For example, the compressive strengths of NHL5, NHL10, and NHL15 mortars were 5, 10, and 15 MPa, respectively.

In this study, a Teknorep 520 EX mortar (TEKNO Construction Chemical, İstanbul, Turkey), specially developed for historical buildings in accordance with the EN 998-2:2011 “Specification for mortar for masonry—Part 2: Masonry mortar” standard, was used for utilizing the M15 class natural hydraulic lime [20]. The technical data for Teknorep 520 EX are presented in

Table 1. Technical data of Teknorep 520 EX.

General Information
Material Structure	Special blend with natural hydraulic lime base and adjusted gradients
Cement Content (%)	0%
Appearance	Off-white and white coffee
Sheff Life	12 months in dry place in unopened packaging.
Package	20 kg kraft bag
Application Information
Implementation Process	Min. 30 min at 20 °C
Application Ground Temperature	(+5 °C)–(+35 °C)
Grain Size	0–8 mm
Application Thickness	Each storey 1–5 cm
Performance Information
Flexural Strength	4.0 N/mm2
Compressive Strength (EN 1015-11) [21]	15–20 N/mm2 (M15 Class)
Elastic Modulus	7000 N/mm2
Water Vapor Permeability (EN 1745) [26]	μ < 35
Capillary Water Absorption (EN 1015-18)	0.2 kg m−2 h−0.5
Bond Strength	>0.15 N/mm2
Reaction to Fire	A1

. The mean density of the hardened mortar was approximately 1850 kg/m³. A 190 mm × 90 mm × 50 mm clay brick with voids was used, and the net dry density of the clay brick was 2000 kg/m³. Masonry prisms were prepared with these bricks, and mortar with a 10 mm joint thickness (filled voids) and three bricks was used. (Figure 2)

2.2. Preparation of Specimens

To prepare the specimens, Teknorep520 EX was mixed at a 20/3,1–3,3 water content ratio in the laboratory. Subsequently, the mixed mortar was poured into molds measuring 40 mm × 40 mm × 160 mm according to the specifications outlined in EN 1015-11 [21]. These specimens were then enclosed in polyethylene bags to maintain a relative humidity of 95% ± 5%, following the guidelines provided in EN 1015-11 [21] (Figure 2). After a period of five days, the specimens were extracted from the molds and kept in polyethylene bags for an additional two days for a 7-day duration. Subsequently after this, the specimens were transferred to laboratory conditions of 65% ± 5% relative humidity and 20 °C for further testing. Masonry prisms were prepared with a 10 mm mortar joint with 3 bricks, as shown below detailed in Figure 2 and Figure 3, and they were tested under compression as per ASTM 1314 [22].

3. Elevated Temperature Tests (ETTs)

Experimental studies were conducted at the Construction Materials, Fire, and Acoustic Laboratory of the Turkish Standards Institution (TSE) in Istanbul, Turkey. The specimens were subjected to a designated constant temperature (200 °C, 400 °C, 600 °C, and 800 °C) using the ISO 834-1 temperature–time curve [19]. A specialized furnace powered by natural gas burners was used to achieve these temperatures. To adhere to the EN 1363-1 [27] standard and prevent direct flame contact, the specimens were positioned 750 mm away from the furnace burner. Two-plate K-type thermocouples were deployed throughout the tests to regulate the furnace temperature and were positioned 100 mm from the specimens from the four sides, following the guidelines outlined in EN 1363-1 [27]. In addition, a K-type Inconel thermocouple was used to gauge the temperature of the specimens accurately (Figure 3 and Figure 4). This controlled specimen was used solely to monitor the temperature levels.

This study conducted six ETTs spanning temperatures ranging from 200 °C to 800 °C in increments of 200 °C. Four specimens were used for each test, with one designated for measuring the mortar temperature and the remaining three for assessing the mechanical properties. Temperature control was performed for the prism only; thus, the remaining mortar and brick specimens were assumed to reach the proposed temperature. Upon reaching the target temperature for each specimen, the furnace cover was removed, both the furnace and specimen were allowed to cool to approximately 100 °C, and the cooling phases were meticulously recorded (Figure 5).

4. Mechanical Tests After ETTs

4.1. Mortar

The determination of the compressive strength of the brick units is described in EN 1015-11 [21]. To determine the compressive strength, three specimens were used at each ambient and elevated temperature, and the average values of their compressive and flexural strengths were calculated. A Zwick brand hydraulic testing machine with a capacity of 100 kN was used to determine the compressive strength of the mortar units with dimensions of 40 × 40 × 160 mm according to the specifications outlined in EN 1015-11 [21]. After the completion of the flexural strength of the three specimens, the compressive strength was measured in the cracked six specimens, as described in EN 1015-11 [21], and the average value of the results was calculated (Figure 6).

4.2. Brick

The determination of the compressive strength of the brick units is described in EN 772-1 [23]. To determine the compressive strength, three specimens were used at each ambient and elevated temperature, and the average compressive strength was calculated. A Besmak brand hydraulic testing machine (manufactured by Besmak, Ankara, Turkey) with 1000 kN capacity was used to determine the compressive strength of the brick units with dimensions of 190 mm × 90 mm × 50 mm (Figure 7).

4.3. Masonry Prism

The compressive strength of the masonry units was determined according to ASTM C1314 [22]. To determine the compressive strength, three specimens were used at each ambient and elevated temperature, and the average compressive strength was calculated. A Besmak brand hydraulic testing machine with 1000 kN capacity was used to determine the compressive strength of the masonry units, similar to a clay brick masonry prism with dimensions of 190 mm × 90 mm × 170 mm (Figure 8). A two-point displacement measurement was recorded, as illustrated in Figure 8, and the average value of these measurements was considered to determine the elastic modulus and strain value of the masonry prism.

5. Finite Element Modeling for Thermal Properties Validation

The ambient temperature thermal values can be determined from tabulated data, measurements, calculations, or a combination of these values as per EN 1745 [26]. For the masonry mortar, thermal conductivity (1850 kg/m³ density) can be taken as 0.89 W/(m·K) as per EN 1745 Table A.12 (defined in the standard)—Mortar (masonry mortar and rendering mortar), and for the clay brick, thermal conductivity can be taken as 1.0 W/(m·K) for density 2000 kg/m³ as per Table 3 (defined in the standard) EN ISO 10456 [28] Building materials and products—Hygrothermal properties—Tabulated design values and procedures for determining declared and design thermal values (referenced in EN 1745) [26]. In this study, voids of masonry prism filled with lime-based mortar, for the combination of the overall thermal conductivity of masonry prism, can be calculated using weight ratio voids of masonry elements if voids are filled with mortar, as follows, in EN 1745 [26]:

Calculate the λ_design,mas–values using the following formula:

λ_design,mas = a_mor λ_design,mor + a_unit λ_design,unit

where

• a_mor is the percentage area of mortar joint, in %;
• a_unit is the percentage area of units, in %;
• λ_design,mor is the design equivalent thermal conductivity of the mortar joint (W/(m·K));
• λ_design,unit is the designed thermal conductivity of the units in W/(m·K).

The area of the masonry unit was approximately 30% of the total area. Therefore, the thermal conductivity of the masonry prism can be calculated as follows:

λ_design,mas = 0.89 × 0.30 + 1.0 × 0.70 = 0.967 W/(m·K)

From the national regulation point of view, in Turkiye, TS 825 Thermal Insulation Requirements for buildings [29] are used, and the thermal conductivity can be taken as 0.96 W/(m·K) for a density of 2000 kg/m³. Therefore, the thermal conductivity of the National Annex, TS 825 0.96 W/(m·K), is compatible with 0.967 W/(m·K), which is the calculated design value as per EN 1745 [26] and EN ISO 10456 [28]. Thus, in this study, the thermal conductivity of the masonry prism was taken as 0.97 W/(m·K) for ambient temperature.

The specific heat values of brick and mortar can be taken as 1000 J/(kg⋅K), as per EN 1745 and EN ISO 10456 [28]. For the overall density of the masonry prism, 1900 kg/m³ was used for thermal analysis.

The same masonry prism was used for thermal analysis validation, as detailed above. Two thermocouples were fixed to the middle part of the mortar and brick, as described in Section 5.3. The prism was insulated from the top and bottom parts in order to observe the actual thermal behavior. The masonry prism was exposed to the ISO 834 temperature–time curve [19] for one hour, and the temperature values were recorded for comparison with the FEA transient thermal analysis. The calculated values of the temperature-dependent material properties of clay units are given in EN 1996-1-2 [30] over the density range of 900 kg/m³–1200 kg/m³. These coefficients were used for the masonry prism transient finite element analysis as the input value of the masonry prism.

The specific heat, density, and thermal conductivity of the clay units described in EN 1996-1-2 [30] are shown (Figure 9).

5.1. Elevated Temperature Material Properties

The development of the thermal FE modeling of masonry prisms depends on the temperature-dependent thermal properties of the material by means of conduction input parameters such as thermal conductivity, specific heat, and density at elevated temperatures. In addition, thermal boundary conditions are important for precisely determining the thermal behavior of masonry walls. The ISO 834 time–temperature curve was applied to the masonry prism from four sides. Radiation and convection boundary conditions were applied on the fire side. A convection coefficient of 25 W/(m²·°C) was applied to the fire side and a 0.7 radiation emissivity was applied from the fire side. No boundary condition was applied for the top and bottom parts of the masonry prism, and the FEA analysis was assumed to be an adiabatic boundary condition to simulate the test.

The thermal properties at elevated temperatures, such as the thermal conductivity, specific heat, and pumice density, are described in EN 1996-1-2 [30] as follows (Figure 10, Figure 11 and Figure 12).

5.2. Transient Thermal Finite Element Analysis (FEA) with ISO 834 Temperature–Time Curve

The aim of the transient thermal analysis is to calculate the thermal gradient in elevated temperature and to validate with actual test results; thus, once the thermal gradient is calculated precisely, and if the compassion satisfies with actual test values, then these input values can be used for the future calculations without testing. Total calculation times for validation for 3600 s (60 min) were used with 30 s time interval. A 20 mm mesh size was applied to the elements.

The ISO 834 temperature–time curve was assigned as the boundary condition for masonry from the four sides. A 30 s time interval was input for the transient thermal analysis by following the ISO 834 temperature–time curve [19]. In addition, convection and radiation boundary conditions were applied to the fire side to illustrate the actual conditions, and modeling was performed using ANSYS Workbench 19.2 with all temperature-dependent material properties and boundary conditions. The boundary condition for convection was set at 25 W/(m²·°C) on the exposed side. The boundary conditions for the radiation were assigned 0.7. A temperature probe was assigned to assess the maximum temperature after the FEA transient heat transfer analysis was performed in ANSYS, as illustrated in Figure 13.

5.3. Fire Testing for Masonry Prism with ISO 834 Temperature–Time Curve

To validate the FEA results, a masonry prism was exposed to an ISO 834 [19] fire for 60 min from four sides in the furnace described in EN 1363-1 [27]. The top and bottom parts of the masonry prism were insulated with ceramic wool to meet the FEA results, which were assumed to be adiabatic in the transient thermal analysis. Two thermocouples (TC1 and TC2) were fixed inside the masonry prism, as illustrated in Figure 14 and Figure 15.

5.4. Comparison with Test and FEA Transient Thermal Analysis Results

At the 30 min fire exposure, as illustrated in Figure 16, temperatures were a bit different from each other; however, for the masonry prism described above elevated temperature, compression results were close to each other at 200 and 400 °C, and it can be assumed that FEA can be used for thermal gradient.

At the end of the 60 min fire exposure, temperature results were 679.1 °C for TC1, 649 °C for TC2, and 661.4 °C for FEA results (Figure 16). It can be concluded that the test results for TC1 668.45 °C, TC2 679.1 °C, and 649 °C (FEA results) for 60 min fire exposure are close to each other by means of engineering calculation point of view. According to the thermal coefficients, as per EN 1996-1-2 [30], they can be used for thermal gradient calculations, and the FEA results are compatible with the test results. As a result, even if thermal coefficients are given for different masonry densities, such as 900–1200 kg/m³ as per EN 1996-1-2 [30], these coefficients are compatible with and are used as input data for the masonry prism for a density of 1900 kg/m³.

6. Results and Discussion

6.1. Mortar

Significant trends over different temperature ranges were observed in the compressive strength analysis. Initially, up to 200 °C, the compressive strength decreased to 80% and remained almost constant up to 400 °C. However, a significant decrease was observed at 600 °C, where the strength decreased to 30% compared to ambient temperature conditions. This trend continued as the temperature increased, with compression strength reaching a mere 30% of the ambient level at 600 °C. These results suggest a critical temperature threshold, above which the structural integrity of the material is significantly compromised (

Table 2. Compression strength test results for M15 lime-based mortar.

Test Sample No	Ambient N/mm2	200 °C N/mm2	400 °C N/mm2	600 °C N/mm2	800 °C N/mm2
No:1	15.73	10.45	12.03	5.64	0.00
No:2	15.31	14.16	11.78	5.76	0.00
No:3	14.35	11.82	12.44	4.47	0.00
No:4	15.19	11.17	11.96	4.60	0.00
No:5	15.23	14.76	12.89	4.96	0.00
No:6	15.50	11.91	13.00	5.11	0.00
Average	15.22	12.38	12.35	5.09	0.00

and Figure 17). At 800 °C, the three specimens were totally collapsed and reported zero accordingly.

Table 3. Flexural strength test results for M15 lime-based mortar.

Test Sample No	Ambient N/mm2	200 °C N/mm2	400 °C N/mm2	600 °C N/mm2	800 °C N/mm2
No:1	4.73	2.40	1.05	1.42	0.00
No:2	4.84	2.70	0.97	0.88	0.00
No:3	4.31	2.19	1.06	1.09	0.00
Average	4.63	2.43	1.03	1.13	0.00

and Figure 18 show the results of the flexural tests performed on HLM specimens subjected to different temperatures. As can be seen from the data, there is a clear trend in the flexural strength of the mortar with increasing temperature. At lower temperatures (20 °C), the control samples exhibited the highest flexural strength, which gradually decreased with the increasing temperature. This decrease became more pronounced at higher temperatures, with a significant decrease observed at 400 °C and above; at 800 °C, the sample completely collapsed after elevated temperature. The average flexural strength of all specimens followed a similar pattern, indicating that the mortar responded uniformly to thermal loading. These results suggest that HLMs experience a reduction in flexural strength when subjected to thermal stress, which has implications for their suitability for applications at elevated temperatures (Table 3 and Figure 18). In a similar way, Pachta et al. (2018) [31] found that the flexural strength of the lime-based mortars gradually decreased up to 800 °C, and at 1000 °C, it was minimized. Pachta and Stefanidou (2021) [32] found that the flexural strengths of the lime- and cement-based mortars declined around 85–90% at 1000 °C.

6.2. Brick

Similarly, trends over different temperature ranges were observed in the compressive strength analysis of brick units. Initially, up to 200 °C, the compressive strength decreased 20% and remained almost constant up to 600 °C. However, after 600 °C, brick units started to crack compared to ambient temperature conditions; thus, the results of the two specimens at 600 °C were disregarded. These results suggest a critical temperature threshold, above which the structural integrity of the material is significantly compromised (

Table 4. Compression strength test results for 190 × 90 × 50 clay brick.

Test Sample No	Ambient N/mm2	200 °C N/mm2	400 °C N/mm2	600 °C N/mm2	800 °C N/mm2
No:1	45.39	36.88	32.73	32.90	0.00
No:2	42.66	35.19	32.80	-	0.00
No:3	42.97	36.94	32.64	-	0.00
Average	43.68	36.34	32.76	32.90	0.00

and Figure 19). At 800 °C, all three specimens were totally cracked.

6.3. Masonry Prism

Similarly, trends over different temperature ranges were observed in the compressive strength analysis of the masonry prisms. Initially, up to 200 °C, the compressive strength decreased to 70% and remained almost constant up to 400 °C. However, after 600 °C, brick units started to crack compared to ambient temperature conditions, and at 800 °C, the three specimens cracked, and the results were disregarded (

Table 5. Compression strength test results for the masonry prism.

Test Sample No	Ambient N/mm2	200 °C N/mm2	400 °C N/mm2	600 °C N/mm2	800 °C N/mm2
No:1	18.30	13.75	11.05	8.17	-
No:2	18.67	12.82	11.60	8.62	-
No:3	20.34	11.82	8.04	7.68	-
Average	19.10	12.80	11.32	8.16	-

and Figure 20). The third compression result at 400 °C was disregarded due to a deviation of the test results and was not taken into account in the average compression strength. The first compression result at 400 °C was recorded, but the displacement measurement interrupted; therefore, the calculation of average compression strength was taken into account, but that for strain values was disregarded.

After applying the height-to-thickness correction factor as per ASTM C1314 [22], the compressive strength of the masonry prism with a correction factor of 0.97 was reduced and calculated, as shown in Figure 20.

If the compression strength of the masonry elements is known individually for brick and mortar, then the formula given below should apply for the characteristic compressive strength of the masonry prism as per EN 1996-1-1 [33], as follows:

fk_calculated = K fb^0.7 fm^0.3

The mean compressive strength of the brick is 43.7 Mpa for ambient temperature,

The mean compressive strength of the lime-based mortar is 15.2 Mpa. For ambient temperature, the characteristics of the masonry prism as per EN 1996-1-1 (EC6) [33] were applied to the formula above with the general purpose mortar coefficient (K = 0.55), and a comparison table for the ambient temperature and remaining elevated temperatures with the same coefficient is given in

Table 6. Compression strength comparison as per EC6 for the masonry prism.

Temperature	K	fb N/mm2	fm N/mm2	fkcalculated N/mm2	fktest N/mm2
Ambient	0.55	43.7	15.22	17.51	18.50
200 °C	0.55	36.3	12.38	14.47	12.40
400 °C	0.55	32.8	12.35	13.45	11.00
600 °C	0.55	32.0	5.09	10.34	7.90
800 °C	0.55	-	-	-	-

.

The calculated compressive strength per EC6 [33] was less than the test strength; hence, it can be concluded that EC6 [34] can be assumed to be conservative. However, at elevated temperatures, the calculated compressive strength was higher than that of the test results. It can be concluded that EC6 calculated the compressive strength estimates, as per brick-and-mortar strength, individually higher than the tested ones; therefore, the proposed formula should not be applied for elevated temperatures even if they are close to each other.

The modulus of elasticity should be determined by testing. In terms of the lack of test results, the short-term modulus of elasticity can be determined by the multiplication factor KE = 1000 with compression strength, unless the national annex gives a different value as per EN 1991-1-1 [33]. In addition, if the test results are available, then one-third of the ultimate compression test results can be considered with the displacement values. According to the Turkey Building Earthquake Regulation, 2018, as National Annex, the multiplication factor KE can be assumed to be 750 in the absence of test results. However, in most cases, these values deviate. In this study, one-third of the ultimate compressive strength values versus the strain values are listed in Table 8. The modulus elasticity can be approximately calculated for this study as multiplied by KE = 170 with the compression test results for the M15 lime-based mortar. The modulus of elasticity test results for ambient and elevated temperatures are listed in

Table 7. Elastic modulus at elevated temperatures.

Test Sample No	Ambient N/mm2	200 °C N/mm2	400 °C N/mm2	600 °C N/mm2	800 °C N/mm2
No:1	3068	2441	-	538	83
No:2	2725	2096	911	388	167
No:3	2440	1405	-	263	0
Average	3047	1980	911	337	125

. Accordingly, the stress–strain relations after the calculation can be found in

Table 8. Stress and strain relationship.

Temperature	Ambient Temp. Mean	200 °C	400 °C	600 °C	800 °C
Mean Stress (Mpa)	18.5	12.4	11.0	7.9	-
Mean Elastic Strain (mm/mm) (EC6)	0.0021	0.0022	0.0065	0.0083	-
Mean Ultimate Strain (mm/mm)	0.0061	0.0065	0.0143	0.0239	-

and Figure 21.

When test data are used in the calculation of the modulus of elasticity, strain values are calculated by displacement values at one-third of the ultimate compressive strength as per EN 1991-1-1 [33]. The average of measurement values is taken into account, calculated, and tabulated for each measurement in Table 8 and Figure 21.

6.4. Digital Image Correlation (DIC) Comparison with Test Results

The purpose of digital image correlation (DIC) is to provide a non-contact measurement technique that enables the evaluation of the deformation and movement of objects. In certain instances, traditional measurement methodologies can be laborious and subject to interruption. In such scenarios, the implementation of Digital Image Correlation (DIC) can be a viable solution. This technique involves the straightforward integration of a digital camera with the test specimen, facilitating the acquisition of data. Subsequent analysis of the captured images is then conducted through the utilization of specialized software, enabling the extraction of quantitative insights.

Digital Image Correlation (DIC) is an innovative non-contact measurement technique used to evaluate the deflection and strain of a test specimen. According to Cakir et al. [18] and Bowen Liu et al. [17], the fundamental principle of DIC is predicated on the comparison of a series of digital images of a sample captured in the sequence before, during, and after the application of the load. Hossain et al. [35] examined DIC and MATLAB Version R2024b image tools that meet each other up to large relative displacements of 100 mm in large in-plane displacements. Belliazzi et al. [36] defined advantages in the study that captured the mechanical shear sliding behavior of strengthened masonry. Wilson et al. [37] investigated the time-dependent behavior of clay brick and lime mortar that strain time history curves acquired by using commercial 2D DIC techniques. Bello et al. and Kumar et al. investigated strain for masonry brick with DIC [38,39]

In this study, the cameras were mounted on a tripod and placed in front of the prism specimen during the compression test. To ensure the accuracy of the measurements, it was necessary to utilize the reference points on the surfaces to compensate for any errors that may have occurred. In this study, 30 mm reference points were applied to the surface of the specimen to precisely evaluate and compare the ultimate displacement of the specimen during the compression test. To ensure the accuracy of the measurements, it was necessary to utilize the reference points on the surfaces to compensate for any errors that may have occurred (Figure 22). After the completion of DIC, the ultimate displacement was compared with the average displacement test results at the point of maximum force.

DIC was performed using Zeiss Inspect 2025 Correlate Trial version 2D, Oberkochen, Germany [40]. The results are illustrated in Figure 23, Figure 24, Figure 25, Figure 26, Figure 27, Figure 28, Figure 29, Figure 30, Figure 31 and Figure 32.

The ultimate displacement values in mm were compared with the two average test displacement results, as described in

Table 9. Comparison of the ultimate displacement between digital image correlation and test results.

Specimen	Digital Image Correlation (mm)	Test Results (mm)	Error (%)
Ambient Temperature 1	1.451	1.475	7.7
Ambient Temperature 2	1.771	1.932	8.3
Ambient Temperature 3	2.254	2.442	1.6
200 °C-1	1.649	1.728	4.5
200 °C-2	2.061	2.127	3.1
200 °C-3	2.179	2.317	6.0
400 °C-2	2.916	2.825	3.2
600 °C-1	4.693	4.700	0.2
600 °C-2	5.933	5.842	1.6
600 °C-3	6.080	6.353	4.3

. The results were close to each other and can be assumed to be satisfactory with the test results.

7. Conclusions

• The flexural strength lime-based mortar reduced to approximately 50% of the ambient value at 200 °C. Subsequently, from 200 °C onwards, there was a gradual decrease of approximately 30% until reaching 600 °C, resulting in zero as a final flexural strength due to total cracking at the 800 °C level. The compressive strength at 200 °C decreased to 80% of the compressive strength observed at ambient temperature. By the time it reached 600 °C, the compression strength had diminished to 5.09 N/mm², representing 30% of the strength recorded at ambient temperature. However, above 600 °C, the specimens completely collapsed. Despite the decline in strength, the compressive strength at 600 °C (5.09 MPa) still retained considerable strength. Therefore, it can be inferred that this aligns with class M5, according to TBDY 2018 [34], EN 1015-11 [21], and EC6 [30].
• Similarly, trends over different temperature ranges were observed in the compressive strength analysis of brick units. Initially, up to 200 °C, the compressive strength decreased 20% and remained almost constant up to 600 °C. However, after 600 °C, brick units started to crack compared with ambient temperature conditions.
• The multiplication factor of the modulus of elasticity in EC6 (E = 1000 fk) [33] and TBDY 2018 (E = 750 fk) [34] is not realistic in the absence of test results, and the proposed multiplication factor for this study is approximately 170 for masonry walls under compression.
• Masonry prism stress–strain relationships can be used for calculations of masonry walls with lime-based M15 mortar, which are lacking in EN 1996-1-2 [30].
• DIC measurements and readings from testing displacement measurements were compatible with each other under the compression of the masonry prism. The accuracy level of the validation between the DIC and test results was satisfactory and can be used in the compression test displacement measurements.
• Transient thermal coefficient temperature-dependent material properties as per EC6 [33] and EN 1996-1-2 [30] can also be used for a masonry wall density of 1900 kg/m³, which is given for only a density of 900–1200 kg/m³ masonry walls that can be used for the transient thermal analysis of bricks with M15 lime-based mortar. In future studies, temperature-dependent material properties can be used as input data for loading condition numerical analysis and can be validated by actual testing in masonry structures. Validated transient thermal coefficients can be used for thermo-mechanical analysis (coupled analysis) for structural analysis during fire as a future study direction.