1 Introduction

Fabric-reinforced cementitious matrix (FRCM) composites comprising an inorganic matrix of fabrics and cement-based mortar are extensively used in the retrofitting of existing reinforced concrete (RC) buildings. This system is generally known as textile-reinforced mortar (TRM) and has been applied to various substrates for the structural reinforcement of RC structures. The enhanced components can also be referred to as textile-reinforced concrete (TRC) and TRC-strengthened masonry.

Seismic retrofitting techniques for RC structures can be generally classified into local and global methods. Local methods focus on improving the performance of specific structural components and characteristically involve strengthening column–beam connections, sheathing of columns and beams, or strengthening using advanced materials such as fibre-reinforced polymers (FRP) and TRM, in addition to traditional RC jacketing. These methods are used for the rehabilitation of structures, as they may occasionally require the demolition and reconstruction of the structural members, making them expensive. Meanwhile, global methods focus on the structural level, with all measures aimed at improving the overall behaviour of the structure. The most conventional methods in this category include constructing shear walls, using steel cross-bracing, isolating the base, and strengthening infill masonry.

Among these techniques, strengthening the infill walls may be the least invasive alternative. The most commonly used techniques include sheathing with steel straps, applying a thin layer of concrete to the bricks, attaching pre-fabricated concrete panels to walls with dowels, and gluing steel plates or FRP sheets to walls. An improved extension of gluing can be considered as an application of the TRM technique, as it overcomes the disadvantages of similar methods, such as fire resistance, extra weight, and durability, and enhances material bonding and compatibility (Abu Obaida et al., 2021; Alrshoudi, 2021; Al-Salloum et al., 2011; 2009; Cerniauskas et al., 2020; Papanicolaou et al., 2006, 2007; Raoof & Bournas, 2017a, 2017b; Raoof et al., 2017; Tetta et al., 2015). The technique is based on combining textiles with mortar, which is then applied to brick walls in layers. The textile comprises a yarn-made grid of filament fibres, primarily glass, carbon, aramid, basalt, and p-phenylene benzobisoxazole (PBO).

Currently, a large part of the ageing building stock worldwide needs to be significantly modernised because it has exceeded its service life and/or no longer meets the current mandatory safety and energy standards. Therefore, there is a need to develop technologies and processes to retrofit this building stock in terms of safety and energy, which is challenging. From the perspective of sustainability, the focus should be on develo** an integrated structural and energy design methodology for new buildings, which would be preferable over individual measures.

However, for existing buildings, particularly those that have reached a certain age, the problem of seismic and energy inefficiency is paramount, and a similar conceptual approach is required to achieve improvements on both fronts. Recently, it has been shown that such independent retrofit measures should be integrated to enhance overall performance. Attempts to combine seismic efficiency with the environmental benefits of mitigating damage and/or demolition caused by earthquakes have been reported. Subsequently, a multidisciplinary approach has been used to improve building performance, with equal attention paid to seismic and energy efficiencies. To this end, the latest development has proposed a specific novel FRCM system that functions as a single unit and has been used to strengthen masonry walls that were subsequently subjected to out-of-plane cyclic loading under different building configurations (Karlos et al., 2020; Papanicolaou et al., 2007; Triantafillou et al., 2017).

Meanwhile, a local retrofit method involves using FRCM composites on the diagonal bands to target reinforcement against diagonal extrusion, which can damage the infill wall due to frame deformation. Therefore, a total of nine single-storey, one-bay test frames were subjected to quasi-static cyclic in-plane loading history (Ismail et al., 2018). The results of the force–displacement hysteresis curves showed that the RC frame filled with masonry required a larger load than the bare frame to deform to the same lateral displacement, while the specimen reinforced with FRCM was able to displace less under the same load. The hysteretic behaviour of the whole wall reinforcement method was found to be better than the local method. This also shows that FRCM retrofitting techniques can be tailored to specific needs, and local and global methods offer different approaches to improving the performance of the structure.

The modern method of masonry reinforcement has benefited greatly from the application and development of Engineered Cementitious Composite (ECC). This cement-based composite material contains discontinuous short polymeric fibres such as polyethylene (PE), polyvinyl alcohol (PVA), and polyester fibres, which exhibit strain-hardening behaviour and high ductility based on its micromechanics (Singh & Munjal, 2020). ECC is also referred to as ductile fibres reinforced composite (DFRCC) due to its high ductile properties after the first crack. Adding polymeric fibres into the mortar of FRCM can improve the performance of retrofitting masonry walls as well. By incorporating these fibres, the FRCM overlay can be expected that a similar effect to ECC can be achieved, resulting in enhanced ductility and strain-hardening behaviour. This approach offers a promising solution for improving the seismic performance and durability of masonry structures.

Overall, this research focused on the development of a new FRCM system to achieve adequate physical, mechanical, and thermal properties for reinforced concrete and masonry buildings. The previous studies on material properties of FRCM has been completed. The research on suitable PCMs for the FRCM system have been carried out by the University of Cyprus (Illampas et al., 2021), which has determined the selection of PCM materials for energy upgrading. Additionally, the author (Wang et al., 2021) concluded that adding XPS plates for thermal insulation performance also can further enhance the stiffness of FRCM overlays by bonding function. The in-plane performance tests are presented in this paper, in order to provide valuable insights into the physical and mechanical properties of this novel FRCM system. And the results can inform further development and refinement of this innovative retrofitting method.

2 Test Setup

2.1 Specimen Design

The seismic behaviour of five specimens of hollow brick masonry walls infilled with RC frames (strengthened by FRCM) was investigated to determine their in-plane behaviour. These specimens were composed of the same materials used in previous studies by the author and were tested at the Large Structures Laboratory of Cyprus University of Technology, as depicted in Fig. 1.

Fig. 1
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Details of the test setup (all dimensions are in mm)

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The RC frames were constructed in accordance with the European code, using C30 cement. All rebars were securely bound by fine iron wires. The base composed of three parts, each of which was reinforced with encrypted stirrups to ensure its rigidity and stability. Each column had a cross-section of 400 mm × 200 mm and was rigidly connected to its independent foundation. The middle part of the foundation was fixed to the laboratory floor using steel tendons, which served as a permanent base for all five tests. Subsequently, the other two tendons were horizontally connected to the two column foundations to achieve stability of the overall foundation, equivalent to a size of 4000 × 1300 × 400 mm. This design saved materials and replacement time, without compromising the experiment. The rebars reserved on the top of the columns were then bound to the beam and loading slab, and they were poured to form a single-unit. This realised rigid connection of the beam–column joints and allowed for the transfer of load to the entire frame, using the slab as the medium.

Afterwards, the two fixtures were lifted onto both sides of the loading slab, and two tendons were passed through them and the slab, and then connected to the corresponding actuators. Each tendon that went through the slab, fixtures, and the foundation was tightened with a prestress of 350 kN. For these experiments, only the laboratory floor and a reaction wall made of high-strength concrete were used as supports. No additional reaction frame was required due to the symmetrical design of the specimen and the output of 930-mm-long steel fixtures clamped on both sides of the loading slab.

The specimens were loaded simultaneously using two hydraulic actuators to prevent horizontal twisting and out-of-plane displacement. Meanwhile, the weight of the slab, the tensile strength of the columns, and connection stiffness of the joints acted as constraints in the vertical direction. This design was intended to simulate a real scenario, as no vertical reaction frame was added to completely restrict the degrees of freedom in the vertical direction.

The concrete covers of all RC components were designed to be 25 mm thick, based on the distance from the outermost side of the reinforcements to the edge of the cross-section. After the completion of the RC frame, each masonry infill wall was constructed using 300 × 200 × 100 mm hollow bricks on the 7th day. The bricks were bonded with 20 mm-thick bed joints and 15-mm-thick head joints using staggered joint construction. Enhancement layers were then added, as shown in Fig. 2.

Fig. 2
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Details of the overlays (all dimensions are in mm)

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The design of the five test specimens is as follows: Specimen No. 1 consisted of only the hollow brick masonry wall-infilled RC frame. Specimen No. 2 included a thin mortar layer for bonding the XPS layer, as well as one overlayer of FRCM applied to the outside for comparison with the control sample. Specimen No. 3 has two layers of FRCM composite added to the inside and outside of the XPS layer. The inner layer of FRCM was directly bonded to the wall, which was expected to provide better reinforcement than Specimen No. 2. Specimen No. 4 included only one layer of FRCM mixed with PCMs into the mortar matrix, intended to replace the XPS layer for thermal purposes. Finally, Specimen No. 5 was similar to No. 3, but with the application of PCMs to the outer FRCM layer to achieve better thermal insulation efficiency.

The thermal insulation layer comprised 80-mm-thick XPS plates, measuring 600 mm in height, with two different widths of 800 mm and 1600 mm, staggered from bottom to top. Plastic masonry fasteners (PMF) were used to secure each plate, spaced 400 mm apart along the horizontal joints between each level. Additionally, Tsirco thermostatic adhesive material was applied using a notched trowel on both sides of the XPS plates for bonding.

The FRCM layer was developed using SikaWrap-350 alkali-resistant (AR) styrene-butadiene rubber (SBR)-coated glass fibre fabric, buried inside a 10-mm-thick matrix made of Tsirco-Poly-122 cement-based fibre-reinforced polymeric repair mortar (with/without 20% PCM). The SBR coating provided excellent alkaline resistance of the fibres and improved abrasion resistance and heat-ageing properties. The SikaWrap®-350-G-Grid had two orthogonal glass fibre bunches of unequal quantity and spacing. The vertical bundles (weft yarns) weighed 145 g/m2, spaced 14.2 mm apart and woven using horizontal cords (warp yarns) with a weight of 135 g/m2, spaced 18.1 mm apart. This commercial mesh was sold in rolls, and the width of each roll was 1000 mm. Thus, each layer of FRCM was arranged horizontally by three tiers of the mesh from the bottom to the top, and the overlap between tiers was 17.5 mm.

The Tsirco-Poly-122 commercial mortar, which was previously tested, was produced by Tsircon® Co., Ltd. In addition to cement, fine aggregates (sand), and polymeric admixtures, this mortar also contains a viscosity modifying agent and short polypropylene fibres. Its durable abrasion and excellent resistance to water, oil, and non-aggressive chemicals improved the protection of internal materials, which enhanced the durability of structures. The most obvious advantages are that fast hardening could be helpful for vertical wall construction, and the strong permeability and high adhesion lead to improved bond properties.

The microencapsulated paraffin Nextek-37D PCM with very high-temperature stability, commercialised by Microtek Laboratories® Inc., was in the form of a white dry powder with a mean particle size of 15–30 μm. It was added to the matrix mortar at 20% by weight for Specimen No. 4 and the outer FRCM layer of Specimen No. 5.

All enhancement overlays were performed on the 9th day after the completion of the frame. Except for Specimen No. 1, which was the control group, all the other specimens were manufactured in 10 days. The installation of the measurement devices and experimental tests were carried out the following day.

2.2 Test Arrangement and Loading Procedures

Fig. 3 shows the details of the instrumentation in the front view of the specimens. Two groups of crossed drawn wires were fixed on the core area of the masonry wall and on the frame to measure diagonal deformation. In addition, two sets of potentiometers connected end-to-end using steel wires were installed vertically at a distance of 100 mm from the outside of the columns to measure elongation. Moreover, pieces of equipment were used to measure the gap between the masonry and the frame after debonding. Near the inner sides of the two columns, two more groups of potentiometers were installed horizontally between the columns and the wall at heights of 300 mm and 2100 mm from the foundation, respectively. Three additional groups of linear variable differential transducers (LVDTs) were installed at heights of 300 mm, 1200 mm, and 2100 mm on the backside of each specimen. Seven horizontal lines were marked within 300 mm with 50-mm gaps in the middle of the junctions between the masonry wall and columns.

Fig. 3
figure 3

Details of the instrumentation (all dimensions are in mm)

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The horizontal pulling force exerted by the actuators to the left was considered the positive direction in this test. All the specimens were loaded in the horizontal direction at a constant speed of 0.4 mm/s until failure using displacement control. The shift amounts were designed to be 3, 10, 15, 25, 40, 55, 65, and 80 mm in both the positive and negative directions, as shown in Fig. 4.

Fig. 4
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Loading procedure

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Finally, the whitewashed specimens were recorded using a digital camera to observe the details of crack development. A photograph of the test setup is shown in Fig. 5.

Fig. 5
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Photo of the test setup (Specimen No. 5)

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3 Failure Mode Analysis

3.1 Observations of Damage

Three cameras were used to record the complete process of each experiment. During the entire loading process, neither the loading slabs nor the foundations were significantly deformed, and all failures occurred in the test areas. The stiffnesses of the beam and the foundation were sufficiently large due to overall reinforcement and pouring of the beam with the slab, the deformation of all frames was primarily realised by the bending and stretching of the column, and the beam–column joints were oblique shear failures at the end.

In the test of Specimen No. 1, which was not reinforced by FRCM, many fine horizontal cracks distributed over the full height of the sides of the columns were observed. The remaining specimens exhibited tensile and bending failures of the FRCM sheathing overlays at the tops of the columns to varying degrees. Debonding between the enhancement layers and RC frames also partly occurred from the front surfaces to the side surfaces of all the columns. At the beam–column joints, the FRCM layers were also sheared obliquely with the frames, and Specimens No. 3 and No. 5 exhibited fibre rupture of the fabric and fragmentation of the matrix.

The failure of the infill walls represented different forms. All specimens suffered brick crushing at the corners at different levels. Among them, the control group was the most serious, and the level of damage decreased as the FRCM layers increased. However, the development of cracks was the opposite. The less crushed the wall, the more uniform the shear stress distribution, and the cracks were fine, but the distribution range was much wider. Because hollow bricks were used in this experiment, many partially crushed bricks were exposed on the on the wall surfaces. Meanwhile, some completely crushed bricks would also have some fragments remaining on the wall, but they would have lost almost all the bearing capacity.

The complete details of each specimen test area after testing were obtained from the videos. The images were processed with increased sharpening, higher contrast, and lower colour saturation so that the cracks appeared clearly. Photographs and corresponding drawings are shown in Fig. 6.

Fig. 6
figure 6

Failure details of all specimens

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This experimental study revealed varying degrees of shear sliding failure, shear diagonal failure, and corner crushing failure of the masonry as specified. However, none of these specimens was destroyed by a single mode. To further study the causes of different failure types, it is necessary to combine the deformation of the specimens and the specific mechanical mechanism to conduct failure mode analysis.

3.2 Diagonal Extrusion of Masonry-Infilled RC Frame

Masonry infill walls commonly fail due to brick crushing in the corners and step-type cracks between the bricks and mortar joints, which are typical baroclinic failures caused by diagonal extrusion of the RC frames and the insufficient compressive and shear strengths of the masonry. To investigate this factor, Fig. 7 compares the changes enhanced by different FRCM layers using measured data from two groups of draw wires set on all specimens. It should be noted that some test failures occurred during the tests, which were corrected to present the original image. For example, draw wires were hit by broken bricks, causing many huge fluctuations lasting 0.2–0.6 s. These invalid data were precisely eliminated, and parts of the data exceeding the test range or dislocations due to the draw wires being pressed by broken bricks were reset by adding fixed correction values. Therefore, the original data of this figure were preserved to the greatest extent, but there may be a correction error of no more than 1 mm near the peak of the latter half of the tests. The corresponding data of draw wires were assigned different colours for clarity.

Fig. 7
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Measured changing of diagonal distances

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In the line charts, it can be observed that the diagonal deformation displacements of the central masonry and peripheral RC frame are not directly proportional to the measured distances, indicating only a positive correlation. The data tested from the same group of draw wires show substantial symmetry on the horizontal axis.

The diagonal deformations of the walls located in the central regions of Specimen No. 1 and No. 2 were extremely small. The data of the crossed draw wires were axisymmetric to each other in the first five cycles. However, the subsequent positive correlation occurred because of the horizontal cracks that developed at the middle height of the walls. The same displacements were the gaps caused by cracks. However, the masonry of Specimen No. 4 did not exhibit horizontal cracks after being crushed diagonally, so the diagonal displacements of the central region consistently approached zero. Specimens No. 3 and No. 5, which were strengthened by double-layer FRCM, were different. The diagonal displacements of the central masonry were large, which did not indicate that the stiffness of the walls was low. Instead, this was because the walls protected by two layers of FRCM were not severely crushed, indicating that the synergy between the infill walls and the RC frames was maintained well. This discovery proves that using the global method of reinforcement by the FRCM system can effectively enhance the overall stiffness of the masonry-infilled RC frame and prevent premature failure of the brick wall.

The diagonal displacements of the RC frames changed with cyclic loading. The data of crossed draw wires were negatively correlated with each other. However, the sum of values of two draw wires in the same group increased due to the tensile deformation of the columns during the later stage of loading. This increase was also observed in elongation measurements of the columns.

3.3 Deformation of RC Frame

In this study, hydraulic actuators were used to apply displacements to the masonry-infilled RC frame. The horizontal loads were then transmitted to the beam and beam–column joints through the loading slab. The bottom base provided support through its high rigidity and the connection between the foundations and the floor. Because the beam was cast integrally with the loading slab, its stiffness was sufficiently high to resist the horizontal shear force, resulting in a lateral displacement with a twist of the entire frame, as well as bending deformation occurring at both ends of the columns. Furthermore, the masonry infill wall acted as a diagonal strut to prevent the deformation of the frame. Therefore, the tension side of the frame was slightly upturned due to the absence of a vertical reaction frame. Two sets of potentiometers near the outsides of both columns were used to measure the changes in the length of the columns as they underwent tensile and flexural deformations, as shown in Fig. 8. The potentiometer at the bottom of the right column of Specimen No. 5 had some measurement faults, and its data image was fixed and corrected by adding 0.6 mm upward.

Fig. 8
figure 8

Measured changes in the length of columns

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The graphs illustrate that the top and bottom length changes in each column are almost equal at the beginning. The upper parts change less than the lower parts of both columns and exhibit positive correlations consistently. In other words, the upper parts of the columns are always in tension when the horizontal displacement load is received, and continue moving with the drifted beam. Instead, stretches of the lower parts of the columns on both sides were negatively correlated. When the specimen rotates, the bottom of one column bends by compression, while the bottom of the other column bends by tension. However, all the tested plotlines were essentially above the horizontal axis due to the high compressive strength and stiffness of concrete. Moreover, the peak stretch values in the lower parts of the columns decreased with an increase in the number of reinforced FRCM layers. This is because columns are equivalent to three-sided textile-reinforced concrete members before the bond failure between the FRCM overlays and the RC frame occurs, reducing the failures by shear and bending. These results indirectly prove that the strength and stiffness in compression, tension, shear, and bending of FRCM-strengthened concrete are enhanced, and the FRCM composite can also effectively enhance both the shear strength and ductility of seismically deficient beam–column joints. This is consistent with the TRC research conclusions of (Al-Salloum et al., 2011; Bournas et al., 2009; Colajanni et al., 2014; Faleschini et al., 2019; Ngo et al., 2020; Shi-** et al.,

$$\it \it \it \it {\text{v}}_{{{\text{eq}}}} = \frac{{{\text{E}}_{{\text{i}}} }}{{2\pi \times \Delta_{{\text{i, max}}} \times {\text{V}}_{{\text{i, max}}}}},$$
(1)

where veq is the equivalent viscous dam** coefficient, Ei is the dissipated hysteretic energy, ∆i, max is the maximum displacement, Vi, max is the maximum shear force.

Table 2 Summary of dissipated hysteretic energies and equivalent viscous dam** coefficients
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Fig. 14
figure 14

Total dissipated energy versus each half-cycle

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Except for Specimen No. 4, the total dissipated energy and equivalent coefficients of the in-plane behaviour significantly improved as the number of FRCM layers increased, particularly in the later cycles.