Quasi-Static Testing of Unreinforced Masonry Walls Using Different Styles of Basalt Fiber Mortar Surface Reinforcements

Abstract

To investigate the reinforcement effects of different reinforcement methods including basalt fibers on unreinforced masonry walls (UMWs), this study examined three reinforcement methods: ordinary mortar, basalt fiber mortar, and basalt fiber mesh mortar. Three masonry wall specimens were designed: ordinary mortar surface-strengthened masonry wall (O-MW), basalt fiber mortar surface-strengthened masonry wall (BF-MW), and basalt fiber mesh mortar surface-strengthened masonry wall (BFM-MW). Quasi-static tests were conducted to analyze the failure phenomena, hysteresis curves, backbone curves, energy dissipation capacity, and stiffness degradation. The results show that, compared to O-MW, BF-MW exhibited a 10.3%, 1.5%, and 28.1% increase in cracking load, peak load, and energy dissipation capacity, respectively. Meanwhile, BFM-MW showed more pronounced improvements, with cracking load and peak load increasing by 41.6% and 3.9%, respectively, and initial stiffness rising by 32.8%. However, this method shifted the failure mode of masonry walls from flexural failure to shear failure. Both basalt fiber mortar reinforcement methods outperformed ordinary mortar, each demonstrating distinct characteristics that can be selected based on practical application requirements.

1. Introduction

Masonry is one of the oldest building techniques in the world. Due to its convenient materials and simple construction process, masonry structures are widely found across the world. Brick masonry structures play a significant role in China’s urban and rural construction and remain the primary structural system for residential buildings in certain regions [1]. However, many masonry buildings, due to factors like long construction periods, non-compliant seismic designs, poor construction quality, and the brittleness of masonry materials [2], have been severely affected during seismic events, even leading to complete structural collapses. Following the Wenchuan earthquake, Professor Li Bixiong from Sichuan University [3,4] conducted multiple field investigations in the affected areas, revealing that self-built unreinforced masonry structures exhibited the most severe damage among residential buildings. Notably, in earthquakes from various countries—including the Lushan Earthquake in China [5,6], the Kahramanmaraş Earthquake [7,8], the Ayvacik-Canakkale Earthquake [9], and the Elazig-Sivrice Earthquake [8,9,10] in Turkey, the Darfield Earthquake in New Zealand [11], the Durres Earthquake [12] in Albania, the 2010–2011 Canterbury Earthquakes [13], the 2016 Central Italy Earthquake [14] in Italy, the Kashmir Earthquake [15] in Pakistan, and the Kaki Iran Earthquake [13]—extensive damage to masonry structures was observed, with varying degrees of severity. This recurring pattern highlights the widespread vulnerability of unreinforced masonry under seismic loads. In recent decades, the demand for strengthening masonry structures to prevent potential damage and casualties has rapidly increased.

Fibrous materials generally possess high strength, toughness, and durability, which can significantly enhance the crack resistance, load-bearing capacity, and seismic performance of masonry walls. Currently, the main methods for strengthening masonry walls with fibrous materials include fiber-reinforced mortar, fiber mesh reinforcement, and the application of fiber-reinforced polymer (FRP) composites [16]. The FRP strengthening method typically employs epoxy resin as the bonding agent; however, its high cost, poor compatibility with masonry substrates, and thermal sensitivity significantly limit its applicability in low-temperature and high-humidity environments [17]. Therefore, fiber-reinforced mortar and fiber mesh reinforcement have become the primary methods for strengthening masonry walls.

The addition of short fibers into mortar can significantly improve crack resistance. Fiber mortars, such as Engineered Cementitious Composites (ECCs) [18,19], exhibit enhanced strain-hardening behavior under tension and tough characteristics under compression, making them particularly effective for the seismic strengthening of masonry walls, which can improve various seismic performance parameters of UMWs, with specific results detailed in Table 1.

Multiple studies have demonstrated that fiber mesh-reinforced mortar technology can significantly enhance the seismic performance of masonry structures. Papanicolaou [20] reinforced porous brick masonry with carbon fiber meshes, resulting in strength increases of 350% and 135% for beams and columns, respectively. Nadege [21] compared carbon fiber and glass fiber mesh reinforcements, revealing that the latter achieved a 246% increase in shear capacity, 400% in cumulative energy dissipation, and more pronounced ductility improvements. Deng ZC [22] employed double-sided carbon fiber mesh reinforcement, boosting walls’ ultimate bearing capacity and ductility coefficients by 87% and 52.2%, respectively, while transforming failure modes from shear to flexural–shear composite types and effectively suppressing crack propagation.

Augenti [23] utilized glass fiber meshes to achieve more uniform crack distribution and enhanced ductility; Gattesco [24] observed a 17–39% shear capacity improvement and a 90–155% ductility increase in irregular stone walls with glass fiber meshes.

Table 1. Physical properties of basalt fibers.

Researcher	Fiber Type	Fiber Content	Cracking Load	Peak Load	Ductility	Others
Zhou TG [25]	PVA fiber	2%	+133.3%	+4.5%	+27.0%	Energy dissipation +133.3%
Deng MK [26]	PVA fiber	2%	+20%	+25.5%	+44.2%	/
Wang ZL [27]	PVA fiber	2%	+0.8%	+88.8%	/	/
Zhang W [28]	PVA fiber	1.7%	+4.5%	+26.9%	/	Energy dissipation +343%
Facconi, L [29]	High-carbon steel fiber	0.82%	/	+39%	/	From diagonal shear failure to rocking failure
	Stainless-steel fiber	0.82%	/	+49%	/
Liu GA [30]	Steel fiber PP fiber	1.5% 0.5%	+49%	+85%	/	Initial stiffness +57.9% Energy dissipation +171.8%
Arslan, ME [31]	Basalt fiber	1%	/	+155.1%	+43.6%	Initial stiffness +42.6%
		2%	/	+172.9%	+42.4%	+74.8%,
		3%	/	+108.1%	+42.4%	+129.5%,
	Glass fiber	1%	/	+175.9%	+63.6%	+60.3%,
		2%	/	+181.4%	+75.8%	+74.8%,
		3%	/	+144.9%	+81.8%	+9.4%,
Du YF [32]	PP fiber	/	+30.8%	−12.5%	/	/

Table 1. Physical properties of basalt fibers.

Researcher	Fiber Type	Fiber Content	Cracking Load	Peak Load	Ductility	Others
Zhou TG [25]	PVA fiber	2%	+133.3%	+4.5%	+27.0%	Energy dissipation +133.3%
Deng MK [26]	PVA fiber	2%	+20%	+25.5%	+44.2%	/
Wang ZL [27]	PVA fiber	2%	+0.8%	+88.8%	/	/
Zhang W [28]	PVA fiber	1.7%	+4.5%	+26.9%	/	Energy dissipation +343%
Facconi, L [29]	High-carbon steel fiber	0.82%	/	+39%	/	From diagonal shear failure to rocking failure
	Stainless-steel fiber	0.82%	/	+49%	/
Liu GA [30]	Steel fiber PP fiber	1.5% 0.5%	+49%	+85%	/	Initial stiffness +57.9% Energy dissipation +171.8%
Arslan, ME [31]	Basalt fiber	1%	/	+155.1%	+43.6%	Initial stiffness +42.6%
		2%	/	+172.9%	+42.4%	+74.8%,
		3%	/	+108.1%	+42.4%	+129.5%,
	Glass fiber	1%	/	+175.9%	+63.6%	+60.3%,
		2%	/	+181.4%	+75.8%	+74.8%,
		3%	/	+144.9%	+81.8%	+9.4%,
Du YF [32]	PP fiber	/	+30.8%	−12.5%	/	/

Basalt fiber is a type of inorganic, environmentally friendly, and high-performance fiber material. It offers high strength, corrosion resistance, and heat resistance. Moreover, the production process of basalt fiber is mature, cost-effective, and has large-scale production capabilities, with minimal waste pollution during production, making it highly suitable as a reinforcement material for masonry walls.

Arslan et al. [31] applied basalt fiber mortar to strengthen masonry walls placed in a hinged steel frame, finding that the seismic performance of the basalt fiber mortar-strengthened walls was significantly improved compared to the control group. The best improvement in shear capacity and energy dissipation was observed when the basalt fiber content was 2.0%. Wei Chong [33] found that basalt fiber concrete surface reinforcement could effectively improve the brittle failure mode of masonry walls. Compared to unreinforced walls, reinforced walls exhibited fuller hysteresis curves, smoother stiffness degradation curves, and more complete stiffness degradation, with a higher peak load, ultimate bearing capacity, ultimate displacement, energy dissipation capacity, and ductility.

For the basalt fiber mesh reinforcement of masonry walls, Meriggi et al. [34] used basalt fiber mesh lime mortar (BTRM) to strengthen both undamaged and damaged irregular stone masonry walls. The results showed that the shear capacity of the damaged walls increased by 27% after reinforcement with BTRM. BTRM improved the masonry ductility and integrity of walls, changing the brittle failure mode. Larisa et al. [35] applied basalt fiber mesh-enhanced mortar to strengthen both undamaged and damaged masonry walls and studied their seismic performance through quasi-static experiments. The results indicated that the reinforced masonry walls could sustain more damage than the unreinforced walls. The basalt fiber mesh-enhanced mortar caused a redistribution of stresses. The shear capacity, deformation ability, and energy dissipation improved by 31%, 30%, and 406%, respectively. Notably, they found that energy dissipation was mainly caused by crack formation, and the damage mode and damage level of the masonry wall significantly affected energy dissipation. Additionally, Chen Shifeng et al. [36] used 25 mm × 25 mm basalt fiber woven mesh in a C60 fine concrete surface layer to strengthen recycled concrete brick masonry walls, and through quasi-static tests, they studied the seismic performance of walls reinforced with one-sided double layers and four one-sided layers of basalt fiber woven mesh. The results indicated that basalt fiber woven mesh-enhanced mortar could transform the failure mode of masonry walls from diagonal shear to shear sliding, improving the wall’s integrity. The shear capacity of the reinforced walls increased by 34%, and energy dissipation and equivalent viscous damping were both improved to varying extents.

In summary, basalt fiber, as a reinforcing material for masonry walls, can significantly improve seismic performance. However, the differences in performance improvements between various basalt fiber reinforcement methods for masonry walls remain unclear. Therefore, this paper compares the seismic performance of masonry walls with three reinforcement methods: ordinary mortar, basalt fiber mesh mortar, and basalt fiber mortar. Through quasi-static tests, this study analyzes the failure phenomena, hysteresis curves, backbone curves, energy dissipation capacity, and stiffness degradation to provide a reference for the basalt fiber reinforcement of masonry walls.

2. Materials

2.1. Basalt Fiber

Basalt fiber was produced by the Sichuan Glass Fiber Group based in Chengdu, China., the surface was coated with a silane-based impregnation agent, and its physical and mechanical properties are shown in

Table 2. Physical properties of basalt fibers.

Diameter (um)	Length (mm)	Density (g/cm3)	Tensile Strength (MPa)	Modulus of Elasticity (GPa)	Elongation (%)
17	17.78	2.7	2400	86	2.8

.

2.2. Basalt Fiber Mesh

Basalt fiber mesh, due to its mesh form and bidirectional load-bearing capacity, can better anchor with mortar during surface reinforcement, allowing the mesh and mortar to work together synergistically and preventing localized stress issues. The dimensions of the mesh were 25 mm × 25 mm. Its specific material properties are detailed in

Table 3. Basalt fiber mesh properties.

Mesh Density	Weight	Longitudinal Tensile Fracture Strength	Latitudinal Tensile Fracture Strength
6/25 mm	113.9 g/m2	1802 N/(50 × 100 mm)	1385 N/(50 × 100 mm)

.

2.3. Masonry Brick

The experiment utilized ordinary sintered bricks with a strength grade of MU15, each measuring 240 mm × 115 mm × 53 mm. To determine the compressive strength of the masonry bricks, non-formed specimens were prepared, with a total of 10 brick samples tested. The specific testing methods followed the “Test Methods for Wall Bricks” GB/T 2542-2012 [37] standard. The compressive strength values of the masonry bricks are presented in

Table 4. Compressive strength of masonry bricks.

No.	1	2	3	4	5	6	7	8	9	10	Average
Ultimate Load (kN)	575.8	578.7	512.3	523.8	497.3	563.7	528.9	487.3	476.3	502.5	524.7
Compression Strength (MPa)	20.9	21.0	18.6	19.0	18.0	20.4	19.2	17.7	17.3	18.2	19.0

.

2.4. Mortar

The masonry mortar used had a strength grade of M7.5, the ordinary mortar surface layer had a strength grade of M15, and the basalt fiber mortar was prepared using the optimal mix ratio from the previous section. The mortar specimens had dimensions of 70.7 mm × 70.7 mm × 70.7 mm, with six specimens for each type. The specific testing methods followed the “Standard for Test Method of Performance on Building Mortar” JGJ/T 70-2009 [38]. The mortar strengths are presented in

Table 5. Compressive strength of the mortars.

Mortar Types	No.	1	2	3	4	5	6	Average
Masonry Mortar	Ultimate Load (kN)	47.4	50.5	45.1	36.2	33.8	45.9	43.2
	Compression Strength (MPa)	9.5	10.1	9.0	7.2	6.8	9.2	8.6
M15 Mortar	Ultimate Load (kN)	85.3	86.2	92.3	79.5	95.8	75.9	85.8
	Compression Strength (MPa)	17.1	17.2	18.5	15.9	19.2	15.2	17.2
Basalt Rock Fiber Mortar	Ultimate Load (kN)	78.9	85.6	62.3	96.3	78.4	86.9	81.4
	Compression Strength (MPa)	15.8	17.1	12.5	19.3	15.7	17.4	16.3

.

3. Test Process

3.1. Specimen Preparation

A total of three masonry wall specimens were designed for this experiment: ordinary mortar surface-reinforced masonry wall (O-MW), basalt fiber mortar surface-reinforced masonry wall (BF-MW), and basalt fiber mesh mortar surface-reinforced masonry wall (BFM-MW). The reinforcement mortar thickness was 20 mm for all specimens, with reinforcement applied on one side. The specific reinforcement methods are shown in Figure 1.

The wall dimensions were 1500 mm × 1400 mm × 240 mm, using MU15 sintered common bricks for the masonry. The brick dimensions were 240 mm × 115 mm × 53 mm, and the masonry mortar grade was M7.5. Both the bottom and top beams comprised C30 concrete. The reinforcement mortar thickness was 20 mm, and the reinforcement was applied on one side only. To prevent relative sliding between the masonry wall and the top and bottom beams during the experiment, grooves were created in the middle of the top and bottom beams. The masonry wall was placed into these grooves, with the ends of the beams raised 150 mm above the grooves. Before the experiment, steel beams were loaded at both ends of the bottom beam to prevent horizontal and vertical displacement of the bottom beam during the test. The geometric parameters of the specimens are shown in Figure 2.

3.2. Tester

Under actual seismic conditions, multi-story masonry building walls simultaneously bear horizontal and vertical loads, primarily resulting in shear deformation and, to some extent, bending deformation. Therefore, the experimental setup comprised both horizontal and vertical loading systems. To better simulate the scenario where masonry walls bear the loads from the upper structure, the load applied by the vertical hydraulic jack was evenly distributed across the top beam through a distribution steel beam.

In this experiment, hydraulic jacks were used for loading, with the vertical load fully applied before the horizontal load. Additionally, to prevent wall movement or warping during the experiment, steel beams were used to press and fix the ends of the bottom beam. The bottom beam was equipped with a displacement gauge to monitor any sliding of the masonry wall specimen, allowing for the elimination of initial displacement. The top beam also had a displacement gauge with which to measure the upper displacement of the wall. The loading device is shown in Figure 3.

3.3. Loading System

Vertical Pressure: In the experiment, the vertical load applied to the masonry wall was set at 0.8 MPa (280 kN), based on the compressive stress in actual engineering and other masonry wall seismic tests. To achieve a uniform distribution of the vertical load, a 140 kN load was applied twice, followed by loading to full capacity. During the application of horizontal cyclic loads, the vertical load remained constant.

Horizontal Seismic Action: In the experiment, horizontal displacement was applied to the specimen using a horizontal actuator to simulate horizontal seismic effects. Initially, a 1 mm horizontal displacement was applied, and the process was repeated twice to check the operational status of the testing equipment. For the formal loading, a stepwise displacement increase method was adopted. Before cracking, loading was displacement-controlled with a 1 mm increment per step, and each step was repeated once. After cracking, loading remained displacement-controlled with an increased step size of 2 mm per step, and each step was repeated three times. The test concluded when the masonry wall was completely destroyed or when the load decreased to 85% of the peak load. The loading scheme is shown in Figure 4.

4. Results and Discussion

4.1. Failure Phenomena

During the initial loading phase, all specimens exhibited linear elastic behavior with minimal residual deformation under various load levels. As displacement increased, different specimens demonstrated distinct failure patterns. Taking O-MW (flexural failure type) as an example, when the top horizontal displacement reached ±2 mm (displacement angle θ = 1/700), 225 mm diagonal cracks and 418 mm horizontal cracks initiated at 150 mm and 320 mm heights on the left and right sides of the wall, respectively, corresponding to horizontal thrust and tensile forces of 116.7 kN and 109.0 kN. At ±4 mm displacement, cracks extended 489 mm toward the central axis, forming through-cracks that completely detached the wall base from the concrete foundation beam, causing wall sway during loading. By +26 mm displacement, diagonal compressive cracks appeared at the bottom-right corner accompanied by mortar surface spalling, with the wall base lifting over 20 mm and resulting in complete failure. Damage concentrated in the lower section with vertical compressive cracks (shown in Figure 5).

Although BF-MW also experienced flexural failure, its crack propagation differed. At ±2 mm displacement, 829 mm intersecting cracks formed at 160/248 mm heights on the left wall, while 60° diagonal cracks emerged on the right side. We saw that ±3 mm displacement drove left and right main cracks to extend and interconnect at the central axis, ultimately leading to over 20 mm base uplift failure at 26 mm displacement without vertical compressive cracks (shown in Figure 6). In contrast, BFM-MW (shear failure type) displayed characteristic shear behavior: ±2 mm displacement triggered 829 mm and 332 mm horizontal–diagonal cracks on the left/right sides, followed by bidirectional crack propagation along interfaces during ±5–9 mm displacement stages, forming intersecting networks. Vertical compressive cracks (310 mm/145 mm) developed at ±25 mm displacement, with large horizontal cracks and crossed diagonal cracks in mid-lower regions constituting the typical shear failure morphology (shown in Figure 7).

All specimens ultimately failed due to over 20 mm base uplift from foundation beam detachment, inducing global sway and bearing capacity loss. The failure progression exhibited displacement symmetry, where positive/negative displacements activated left/right crack systems, respectively. Flexural failures concentrated in lower zones, while shear failure manifested as crossed crack networks in mid-lower regions.

4.2. Load-Bearing Characteristics and Cyclic Response Analysis

4.2.1. Feature Bearing Capacity

The initial cracking load (Pcr) is defined as the load at which the first visible crack appears in the masonry wall during loading. The peak load (Pp) is defined as the maximum load value reached on the load–displacement curve. In this study, specimen O-MW was reinforced with epoxy resin at the base after loading, which may have infiltrated the cracks, thereby enhancing the specimen’s strength. Therefore, specimen BFM-MW, reinforced with a conventional mortar surface layer, was selected as the control group for the comparison of characteristic bearing capacities, as shown in

Table 6. Characteristic bearing capacity of specimens.

	Cracking Load Pcr (kN)			Rise (%)	Peak Load PP (kN)			Rise (%)
	+	−	Average		+	−	Average
O-MW	116.7	109.0	112.9	—	179.2	172.3	175.8	—
BF-MW	135.3	113.7	124.5	10.3	166.9	190.0	178.5	1.5
BFM-MW	151.7	168.1	159.9	41.6	182.3	183.1	182.7	3.9

.

The following can be seen from Table 6:

(1)

The application of basalt fiber mesh mortar or basalt fiber mortar led to an increase in both the cracking load and peak load of masonry walls compared to those reinforced with ordinary mortar. Specifically, the cracking load increased by 41.6% and 10.3%, while the peak load increased by 3.9% and 1.5%, respectively.

(2)

Masonry walls reinforced with a basalt fiber mesh mortar layer exhibited cracking and peak loads that were 28.4% and 2.3% higher, respectively, than those reinforced with basalt fiber mortar.

(3)

The increase in cracking load for masonry walls reinforced with basalt fiber mesh mortar and basalt fiber mortar was significantly greater than that of the peak load.

The underlying reasons for these observations include the following: Incorporating basalt fibers into the mortar enhances its crack resistance. Basalt fibers are randomly distributed within the mortar, whereas basalt fiber mesh provides better integrity and uniform stress distribution in the mortar layer. Post-reinforcement, the wall exhibits improved overall integrity and stiffness. When a through-crack develops at the junction between the masonry wall and the concrete base beam, the wall begins to bend and sway. As the displacement at the top increases, the uplift of the tensile region at the bottom of the wall also increases. Consequently, the peak load-bearing capacity is primarily determined by the compressive strength of the bricks.

4.2.2. Hysteretic Curve

Based on the quasi-static test results, the hysteretic curves of the three masonry wall specimens are shown in Figure 8. From the figure, the following observations can be made.

During the loading process, all walls exhibited similar hysteretic characteristics, progressing through three stages: elastic, elastic–plastic, and failure. Before cracking, the curves had a small slope change, and the residual deformation after unloading was minimal, resulting in incomplete hysteretic loops with a sharp shape. After cracking, plastic deformation gradually increased, the slope of the loading curve decreased with increasing load, and under cyclic loading, the wall stiffness degraded. After reaching the peak load, the unloading stiffness gradually decreased, and the residual deformation slightly increased.

All specimens’ hysteretic curves were S-shaped, with a noticeable slip segment and minimal residual deformation, exhibiting significant “pinching” phenomena. This was primarily because the masonry walls had good integrity and high stiffness. All specimens failed through bending, with cracks concentrated in the lower part of the wall and relatively few cracks, resulting in limited energy dissipation.

During the loading of specimen O-MW, the ground anchor bolts loosened. After timely tightening, the subsequent hysteretic curve shifted to the right. Therefore, the hysteretic curve envelope areas of reinforced specimens O-MW, BFM-MW, and BF-MW were similar and very narrow. This was mainly because the surface layer bonded the masonry wall into a single unit, significantly increasing the wall’s stiffness and integrity. During loading, the wall bent and swayed along the bottom without significant energy dissipation.

These findings indicate that the reinforcement method significantly influences the hysteretic behavior and energy dissipation capacity of masonry walls.

4.2.3. Backbone Curve

The backbone curve refers to the envelope formed by extracting the peak point of each loading stage from the specimen hysteresis curve. The backbone curve can be the basis for determining the characteristic points in the resilience model to reflect the structural strength and ductility of different stages. The seismic performance of the masonry wall can also be qualitatively measured. See Figure 9 for the backbone curve of each specimen.

(1) Before cracking, the backbone curves of all specimens are approximately linear, indicating an elastic stage. As the load increases, cracks appear at a displacement of 2 mm, causing the curves to deviate from linearity and enter the elastic–plastic stage. After the cracks at the bottom come through and cease to develop, the specimens undergo bending and rocking motion as the displacement increases, resulting in backbone curves without a significant descending segment.

(2) In the positive displacement range, the backbone curves of specimens O-MW, BFM-MW, and BF-MW are nearly identical. In the negative displacement range, when the displacement is less than 10 mm, the curves of W4 and BFM-MW are almost consistent. When the displacement exceeds 10 mm, the load of W4 gradually becomes greater than that of BFM-MW at the same displacement level, with the load difference remaining stable in the later stages. When the displacement of the specimens exceeds 2 mm (initial cracking displacement), the load of BF-MW gradually becomes greater than that of BFM-MW at the same displacement level, with the load difference first increasing and then decreasing during the loading process. This indicates that both basalt fiber mesh mortar reinforcement and basalt fiber mortar reinforcement improve the bearing capacity of masonry walls compared to ordinary mortar reinforcement.

4.3. Energy Dissipation Calculation and Analysis

The area enclosed by the load–displacement hysteresis curve of a masonry wall represents its energy dissipation capacity, and the calculation method is shown in Formula (1). The energy consumed by each hysteresis loop is shown in Figure 10. In summing the areas of each cyclic hysteresis loop, the cumulative energy dissipation for each specimen is obtained, as depicted in Figure 11, and the final cumulative energy consumption of the three is shown in

Table 7. Final cumulative energy of specimens.

	Final Cumulative Energy (kJ)	Rise (%)
O-MW	29.2	—
BF-MW	37.4	28.1
BFM-MW	29.8	2.1

. Since not all walls reached 85% of the peak load, a final displacement of 24 mm was used here.

$$E_i={\displaystyle \oint Fd\delta }$$

(1)

From Figure 9 and Figure 10 and Table 7, the following can be observed.

When the displacement is less than 4 mm, all walls are essentially in the elastic stage, with minimal and nearly identical hysteretic energy dissipation. However, after the masonry walls enter the elastic–plastic stage, the dissipated energy begins to differ and increases with displacement. The cumulative energy dissipation curves for all specimens are very smooth, primarily due to the specimens performing rocking motions after forming through cracks.

Specimen BF-MW exhibits significantly higher single-cycle and cumulative energy dissipation compared to the control group O-MW, with cumulative energy dissipation 28.1% higher than that of BFM-MW. This indicates that short-cut basalt fiber mortar enhances the energy dissipation capacity of masonry walls more effectively than ordinary mortar. This improvement is mainly because the short-cut basalt fiber mortar layer increases the plastic deformation capacity of the masonry wall. When cracks form, the short-cut basalt fibers absorb more energy and convert it into deformation.

The single-cycle energy dissipation capacity and cumulative energy dissipation of specimen BF-MW are almost identical to those of the control group O-MW. This may be due to the better integrity of the basalt fiber mesh, which, despite experiencing higher loads, results in smaller corresponding deformations.

4.4. Degradation Curve of Stiffness

Stiffness degradation refers to the phenomenon where under cyclic loading, the stiffness of a structure decreases as the number of loading cycles increases, even when the displacement amplitude remains constant. According to the “Specification for Seismic Test of Buildings” (JGJ/T 101-2015), [39] the stiffness of a wall can be represented by the first cycle hysteresis loop peak chord stiffness K_i:

$$K_i={\displaystyle \frac{\left|+P_i\right|+\left|-P_i\right|}{\left|+{\Delta }_i\right|+\left|-{\Delta }_i\right|}}$$

(2)

In Equation (2), +P_i and −P_i represent the maximum loads during the i-th cycle of positive and negative loading, respectively, and +∆_i and −∆_i correspond to the horizontal displacements at the peak load.

Figure 12 shows the specimen stiffness degradation curves, and

Table 8. Initial stiffness of specimens.

	Final Cumulative Energy (kN/mm)	Rise (%)
O-MW	100.1	—
BF-MW	96.6	−3.5
BFM-MW	132.8	32.8

lists the specimen initial stiffnesses.

(1)

The stiffness degradation curves of all specimens exhibit a similar shape, comprising a sharp decline followed by a gradual leveling off. Specifically, the initial stiffness is high; as cracks develop and propagate, stiffness degradation becomes more pronounced. With increasing horizontal displacement, the masonry wall’s cracks coalesce into a main crack, and stiffness degradation stabilizes. When the displacement exceeds 13 mm, all specimens transition into bending–swaying walls, and their stiffness degradation curves completely overlap.

(2)

Specimen BFM-MW’s initial stiffness is 32.8% higher than that of the control specimen O-MW. Throughout the loading process, BFM-MW’s stiffness remains significantly greater than O-MW’s until it becomes a sway wall. This indicates that incorporating basalt fiber mesh into ordinary mortar substantially enhances the masonry wall’s stiffness and seismic performance. The primary reason is that basalt fiber mesh can suppress crack development, slow the decline in wall integrity, and thereby improve overall stiffness and delay degradation.

(3)

Specimen BF-MW’s stiffness degradation curve nearly coincides with that of the control specimen O-MW, suggesting that short-cut basalt fiber mortar and ordinary mortar have similar effects in strengthening the masonry wall’s surface layer. This similarity is mainly because both materials have equal strength, M15.

(4)

The stiffness degradation rates of all specimens are generally consistent. This consistency is due to the masonry walls’ inherent integrity and high stiffness, resulting in fewer cracks during loading. Consequently, the surface layer has minimal influence on the stiffness degradation rate.

5. Conclusions

Quasi-static tests were conducted on three masonry wall specimens, O-MW, BFM-MW, and BF-MW, representing masonry reinforced with ordinary mortar plastering, basalt fiber mesh mortar plastering, and basalt fiber mortar plastering, respectively. The following main conclusions can be drawn:

(1)

Specimens O-MW and BF-MW exhibited bending failure, with through-thickness cracks forming at the bottom. However, specimen BF-MW had fewer cracks, indicating that basalt fibers can suppress crack formation. Specimen BFM-MW experienced shear failure, with distinct diagonal shear cracks, demonstrating that basalt fiber mesh can fully utilize the inherent strength of the wall.

(2)

Compared with the cracking load of specimen O-MW, those of specimens BFM-MW and BF-MW were increased by 41.6% and 10.3%, respectively, and peak loads increased by 3.9% and 1.5%, respectively. This indicates that basalt fiber mesh and basalt fibers can enhance the shear load-bearing capacity of masonry walls.

(3)

All the specimens’ hysteresis curves were S-shaped with significant “pinching” phenomena, primarily due to the sway–slip behavior observed in all specimens. In terms of energy dissipation, specimen BF-MW had cumulative energy dissipation and average energy dissipation coefficients 28.1% and 9% higher than those of specimen O-MW, respectively. Regarding stiffness degradation, specimen BF-MW’s initial stiffness was 32.8% higher than that of specimen O-MW, and its stiffness remained significantly higher during loading. This indicates that basalt fiber mortar provides a greater improvement in the energy dissipation capacity of the masonry wall, while basalt fiber mesh mortar is more effective in enhancing the stiffness of the masonry wall and its resistance to stiffness degradation.

(4)

In summary, through quasi-static experiments, it was found that basalt fiber materials can improve the seismic performance of masonry walls and alter the failure mode. Both forms of basalt fiber reinforcement have their respective advantages and disadvantages, and the reinforcement method can be selected according to the specific application scenario.

(5)

The test confirmed the positive effect of basalt fiber reinforcement on UMWs. However, the synergistic effects of basalt fiber-reinforced mortar and basalt fiber mesh-reinforced mortar on unreinforced masonry walls remain to be elucidated. Furthermore, the distinctions between the mechanisms of basalt fibers and other fiber types warrant further systematic comparative investigation.

(6)

The test focused on unreinforced masonry walls, with the specimen configuration limited to a single aspect ratio. However, the influence on walls with different aspect ratios and with openings still warrants further investigation, as well as the combined effects of these factors on UMWs.