Study on the axial compression behavior of masonry columns strengthened with engineered cementitious composite splints and fiber-reinforced polymer strips

Abstract

This paper presents experimental and analytical studies on the axial compressive properties and failure mechanics of 19 strengthened masonry columns. The masonry column was strengthened singly and compositely by applying engineered cementitious composite (ECC) splints and wrapping discontinuous fiber-reinforced polymer (FRP) strips around the sides of the masonry column. The effects of the type of strengthening material (ECC splints or FRP strips), the thickness of the ECC splint, and the FRP strip strengthening area ratio on the failure mode, peak load, strain behavior, and energy dissipation of the specimens were analyzed and discussed in this experimental study. Under the appropriate parameters, the best strengthening effect was observed for the compositely strengthened masonry columns. In particular, the ductility, bearing capacity, and energy dissipation of masonry columns can be significantly improved by utilizing FRP with lower tensile strength, a higher FRP strip strengthening area ratio and a thicker ECC splint. Based on the existing computational models and code calculation methods for masonry columns strengthened with ECC splints and FRP strips, a formula for calculating the compressive bearing capacity of strengthened masonry columns was derived. The error between all the calculated and the measured results was less than 7.1%.

Keywords

engineered cementitious composite splints fiber-reinforced polymer strips ductility strengthened masonry column compressive bearing capacity

Highlights

• A novel strengthening method for masonry columns by combining ECC splints and FRP strips is proposed.

• Effects of the strengthening material, strip strengthening area ratio and ECC splint thickness are discussed.

• A method for calculating the compressive bearing capacity of strengthened masonry columns is proposed.

Introduction

Masonry structures are among the most commonly used constructional structures in civil engineering due to their excellent durability, fire resistance, thermal insulation, and relatively low cost (GB 50003-2011, 2011). Among these, masonry columns are extensively utilized to provide additional support for structures such as large-span industrial plants, warehouses, and corridors within many masonry buildings. However, masonry structures exhibit low strength and poor integrity, leading to premature deterioration of these existing structures over prolonged use. The conventional strengthening methods for masonry columns, such as the increased cross-section method and encased steel strengthening method (Maras and Kilinc, 2016), have inherent limitations in terms of construction technology or strengthening materials. An engineered cementitious composite (ECC) is a type of high-performance short fiber-reinforced cementitious composite that exhibits high tensile ductility and noticeable strain hardening (Jing et al., 2020; Li and Leung, 1992). Owing to their attractive mechanical properties and excellent durability, ECC have been widely adopted for structural strengthening to improve their load-bearing capacity, ductility, and seismic performance (Li et al., 2019; Yuan et al., 2013; Zhang et al., 2019). Fiber-reinforced polymers (FRPs) are composed of fiber and matrix materials. FRP has been widely used to strengthen columns due to its excellent corrosion resistance, high strength-to-weight ratio, and fatigue resistance (Wang et al., 2020; Zeng et al., 2022).

There have been many studies on the performance of masonry structures strengthened by new materials. Deng et al. (2020, 2021) investigated the effect of different masonry mortars on the axial compressive properties of masonry columns strengthened with a highly ductile fiber reinforced concrete (HDC) splint and discovered that the HDC splint had a greater bearing capacity when used with lower masonry mortar. Additionally, the HDC splint played a stronger role in the lassoing effect, thereby improving the bearing and deformation capacity of the masonry columns. However, the tensile strain of the HDC splint weakened, leading to a reduced fiber bridging effect and susceptibility to face peeling during damage. Dehghani et al. (2015) and Kyriakides and Billington (2014) separately examined the effect of the ECC splint thickness and number of splints on the compressive properties of strengthened brick columns. The results indicated that the use of ECC strengthening significantly increased the load carrying capacity and deformation properties of brick columns. Furthermore, they proposed an optimal thickness for the ECC splint. However, combined strengthening methods, such as strengthening meshes, pins, and ECC splints, are unsatisfactory due to splint peeling and brittle damage. Fossetti and Minafo (2017) conducted a comparative study on the effect of confinement masonry columns using textile reinforced concrete (TRC) and FRP. The results indicated that FRP confinement had a greater effect on the load carrying capacity and deformation capacity than did TRC. However, the FRP-strengthened masonry columns exhibited more abrupt failure mode and displayed brittle characteristics. Conversely, columns strengthened with TRC may experience local debonding between the fiber woven mesh and the substrate due to incomplete impregnation. Under the ultimate load, cracks in TRC-strengthened columns continued to expand and extend, leading to failure caused by varying degrees of fiber woven mesh fracture near the corners and loss of TRC confinement (Cascardi et al., 2020; Jing et al., 2021; Mezrea et al., 2017).

Researchers have extensively investigated the performance of FRP-strengthened concrete columns. The findings indicated that the strength and deformation capacity of concrete columns can be improved by FRP strengthening (Guler and Ashour, 2016; Matthys et al., 2005; Ozbakkaloglu et al., 2013). Despite the confinement efficiency of FRP for square columns was low, and the most effective method involved the rounding of the sharp corners of the square section (De Luca et al., 2011; Wang et al., 2016). However, this approach was unable to achieve satisfactory strengthening effects. To address this issue, researchers have proposed modifying the shape of the square column to a cylinder for strengthening (Pham et al., 2013; Yan and Pantelides, 2011). The test results demonstrated that the strength and deformation capacity of FRP-strengthened columns surpassed those of columns without shape modification (Zeng et al., 2021). Zeng et al. (2017) conducted axial compression tests on FRP-strengthened circular square columns, which demonstrated the effectiveness of FRP-strengthened shape-modified columns and proposed a corresponding design model. However, the strengthening process was relatively complex.

To address this challenge, it was proposed to change the strengthening method of masonry structures to greatly improve the strengthening effect of masonry structures. Cascardi et al. (2020), Faella et al. (2011), Di Ludovico et al. (2010) and Witzany et al. (2014) conducted studies on the effects of full FRP sheet strengthening, FRP strip strengthening and the type of FRP sheet on the compressive properties of masonry columns. Their findings indicated that the full-face confinement of FRP sheets significantly improved the load carrying and deformation capacity of brick columns. The utilization of FRP strip strengthening resulted in a smaller reduction in the load carrying capacity and a better ductility effect, as mentioned in the literature (Liu et al., 2009). The arrangement of the strips had little effect on the load carrying and deformation. Using fewer strips with the same amount of strengthening achieved better results. The lateral deformation of masonry activated the FRP confinement effect, and high-strength FRP prevented crack development and expansion, further enhancing the compressive bearing capacity of masonry. However, complete fracture of the FRP strips exhibited a certain brittle pattern, resulting in severe fragmentation of the masonry column bricks and loss of integrity.

For the use of a single material in masonry structure strengthening problems, a proposed solution to these problems is to introduce composite materials to the surface of masonry structures (Sui et al., 2018). Zeng et al. (2024) strengthened concrete columns with UHP-ECC and FRP. The experimental results showed that the strengthening method can markedly enhance the compressive performance of concrete columns and validate the effectiveness of FRP confinement and UHP-ECC section strengthening RC columns. Zhu et al. (2022) and Zheng et al. (2016) conducted axial compression tests on short concrete columns strengthened with FRP grid and ECC and found that the combination of the two materials can significantly improve the axial compression capacity of the columns. Mechtcherine (2013) experimentally investigated the seismic performance of FRP grid-strengthened concrete columns under ECC constraints. The test results showed that this composite material can significantly improve the ductility and seismic performance of the specimens and facilitate the strengthening and repair of the pier columns. Jing et al. (2020) used FRP to restrain the ends of TRC-strengthened masonry columns subjected to compressive properties and found that the use of FRP end confinement on masonry columns had the potential to enhance the localized susceptibility and stiffness of brick masonry columns. Additionally, the application of end confinement with an FRP sheet has been demonstrated to effectively restrict the propagation of detrimental cracks and increase the peak load-bearing capacity of the strengthened columns compared to those strengthened solely with the TRC splint. Therefore, it was speculated that the composite strengthened with FRP strips can achieve a better strengthening effect. Although the fully FRP-strengthened concrete columns exhibited improved compressive bearing capacity, deformation capacity and ductility (Bai et al., 2017; Castillo et al., 2018), the high cost of FRP has led to limitations in its application in construction. Furthermore, studies have demonstrated that FRP partially wrapped concrete columns exhibit enhanced economic viability and notable improvements in strength and ductility (Pham et al., 2015). However, the confinement effect of discontinuous FRP strips on concrete columns extends the uneven distribution of column height, impacting the effectiveness of strengthened concrete columns (Mander et al., 1988). Consequently, further research is required to evaluate the strengthening effects of FRP strips and other materials on masonry.

Based on these studies, it can be hypothesized that utilizing multisegmented FRP strips may yield an even more effective composite confinement effect. Through the combination of FRP strips and ECC splints, the high tensile ductility of ECC can be combined with the good confinement effect of FRP strips, effectively resolving the issues of material strengthening material falling off, insufficient bearing capacity of strengthened structures and poor composite strengthening effects. This paper proposed a composite strengthening method for ECC splints and FRP strips. A total of 19 specimens were prepared and tested under axial compression. The design variables included the type of strengthening materials (ECC splints or FRP strips), the thickness of the ECC splints and the strengthening area ratio of the FRP strips. The failure modes, ductility, load–displacement curves, compressive bearing capacity, deformation capacity, and energy dissipation capacity of the specimens were investigated and compared. The effects of different design parameters on the compressive behavior of strengthened masonry columns were further examined and discussed. The existing computational models were employed to derive theoretical formulas for calculating the compressive bearing capacities of masonry columns with both single and composite strengthening.

Experimental program

Specimen design

As tabulated in Table 1, a total of 19 masonry columns were tested under axial compression, including one group of unstrengthened masonry columns, four groups of singly strengthened masonry columns, and four groups of compositely strengthened masonry columns. In order to reduce errors, three replicated specimens were included in the unstrengthened group, while duplicate specimens were included in the strengthened groups (Di Ludovico et al., 2010). The experimental variables included the strengthening scheme (singly strengthened and compositely strengthened), the strengthening material (ECC, GFRP, and CFRP), the thickness of ECC splint (15 mm and 20 mm), and the strip strengthening area ratio (ratio of the strip strengthening area to the masonry column lateral area: 40% and 70%).

Table 1.

Test matrix.

Specimen	Section size	Strengthening
N-a/b/c	240 × 370	None
E-20-a/b	280 × 410	20 mm-ECC splints
G-0.4-a/b	240 × 370	40%-GFRP strips
G-0.7-a/b	240 × 370	70%-GFRP strips
C-0.7-a/b	240 × 370	70%-CFRP strips
E-20-G-0.7-a/b	280 × 410	20 mm-ECC splints and 70%-FRP strips
E-20-G-0.4-a/b	280 × 410	20 mm-ECC splints and 40%-GFRP strips
E-20-C-0.7-a/b	280 × 410	20 mm-ECC splints and 70%-CFRP strips
E-15-G-0.7-a/b	270 × 400	15 mm-ECC splints and 70%-GFRP strips

Note. a/b/c represent replicated specimens in one group.

As shown in Figure 1, the unstrengthened masonry column was constructed using ordinary sintered bricks with dimensions of 240 mm (length) × 115 mm (width) × 53 mm (thickness). The dimensions of the masonry column were 240 mm (thickness) × 370 mm (width) × 720 mm (height), with a height-to-thickness ratio (β) of 3. To ensure proper construction, the masonry column was built on a rigid mat with a thickness of 10 mm. Additionally, the top of the specimen was levelled using a 10 mm thick cement mortar layer. A significant impact of a corner in a masonry column on the confinement effectiveness of the FRP strip has been identified by numerous researchers (Li, 2012; Yeh and Chang, 2012). These researchers have extensively investigated the influence of the corner radius of the column on the confinement effect of the concrete column. In this study, for the masonry column that requires strengthening depicted in Figure 2, the masonry column corner was rounded at a radius of 20 mm to prevent stress concentration (Deng et al., 2023). The masonry columns that were strengthened with FRP strips were equipped with four double-layer strips that were evenly spaced. The length of the overlap between the strips was set at 50 mm.

Figure 1.

Details of the unstrengthened masonry column (Unit: mm).

Figure 2.

Details of strengthened masonry columns (unit: mm): (a) 70%-FRP strips. (b) 40%-FRP strips. (c) cross section. (d) 20 mm-ECC splints and 70%-FRP strips. (e) 20 mm-ECC splints and 40%-FRP strips. (f) cross section.

The test specimens are labelled as follows: The letter N represents the unstrengthened masonry column; The letters E-15 and E-20 represent the 15 mm and 20 mm thick ECC splints, respectively. The letters G and C denote the GFRP and CFRP, respectively. The numbers 0.4 and 0.7 represent the strip strengthening area ratios of 40% and 70%, respectively. For example, E-20-G-0.7 indicates that the masonry column is compositely strengthened with a 20 mm thick ECC splint and GFRP strips, with a strip strengthening area ratio of 70%.

Material properties

According to GB/T 2542-2012 (2012), the compressive strength of the MU15 grade ordinary sintered brick was determined to be 16.11 MPa. The compressive strength of the M7.5 grade cement mortar was measured as 8.3 MPa via cubic coupons with a side length of 70.7 mm. The ECC composition included ordinary silicate cement (P.O 42.5), fly ash (Class I), quartz sand (200 mesh particle size), silica fume, a water reducer, a thickener, a defoamer, PE fiber, and water, as shown in Table 2. The mechanical parameters of the PE fibers are shown in Table 3 (Zhang et al., 2022). The ECC dog-bone tensile coupons (Figure 3) and cubic compressive coupons with dimensions of 100 mm × 100 mm × 100 mm were tested in accordance with JC/T 2461-2018 (2018), and the ultimate tensile strength, compressive strength, and ultimate strain were 6.82 MPa, 50.6 MPa, and 5.82%, respectively. The material properties of the FRP strips and impregnated adhesive were determined in accordance with the standards GB/T 1446-2005 (2005) and GB/T 2567-2021 (2021), respectively, and are presented in Table 4. The test specimens and devices used for the ECC and FRP are shown in Figure 4. The tensile stress‒strain curve of the ECC is shown in Figure 5.

Table 2.

Mix proportions of the ECC matrix (kg/m³).

Binder			Quartz sand	Water	SP (%)	Thickener (%)	Defoamer (%)	Fiber (vol%)
Cement	FA	SF	Quartz sand	Water	SP (%)	Thickener (%)	Defoamer (%)	Fiber (vol%)
0.253	0.668	0.079	0.23	0.30	0.18	0.04	0.15	2

Table 3.

Properties of PE fibers.

Length (mm)	Diameter (μm)	Tensile strength (MPa)	Elastic modulus (GPa)	Elongation at break (%)	Density (g/cm³)	Volume mass fraction (kg/m³)
12	24	3000	115	2.4	0.97	19.4

Figure 3.

The geometry of dog-bone-shaped specimen (mm).

Table 4.

Material properties of the FRP sheet and epoxy resin.

Material	Tensile strength (MPa)	Elastic modulus (MPa)	Ultimate tensile strain (%)
GFRP strip	1925	7.8 × 10⁴	2.46
CFRP strip	3923	2.4 × 10⁵	1.79
Epoxy resin	49.3	2560	2.58

Figure 4.

The test set-up: (a) Axial compression test set-up. (b) The setup of the uniaxial tensile test (ECC). (c) The setup of the uniaxial tensile test (FRP).

Figure 5.

ECC stress‒strain curve.

Strengthening procedures

The strengthening procedures for strengthened masonry columns are summarized as follows:

The strengthening of masonry columns with ECC splints (Deng et al., 2015) involves the following steps: (1) The masonry columns are wetted with water. (2) An ECC slurry is applied to the masonry columns, ensuring that no obvious water stains are present on the surface of the bricks. (3) The masonry columns are covered with cling film and water-absorbing construction sheets. (4) The masonry columns are cured for 28 days.

The masonry columns were strengthened with FRP strips in three steps: (1) The masonry column surface was cleaned and adhesive impregnated. (2) The adhesive was allowed to stand for 30–40 min until it had solidified. (3) The FRP strips were pasted on the designed strengthened areas.

The masonry columns were strengthened using both ECC splints and FRP strips: (1) The masonry columns were wetted with water. (2) The ECC slurry was brushed on the strengthened surface of the masonry columns. (3) The masonry columns were cured for 28 days. (4) The FRP strips were pasted on the designed strengthened areas.

Loading configuration

The loading device used in this study is a 1000 t hydraulic servo testing machine (Figure 6), following a displacement-control loading method with a loading rate of 0.2 mm/min specified in GB/T 50129-2011 (2011). The load value was collected by a 500 t force sensor. The crack development and damage morphology of the specimen were recorded at intervals of 50 kN. Once the load dropped to 80% of the peak load, the loading was terminated.

Figure 6.

Testing machine.

The layout of the measurement points is shown in Figure 7. The axial and lateral displacements of the masonry columns were recorded using displacement meters. The transverse strains of the masonry column were measured by strain gauges at the center of the two sides of the masonry column. Strain gauges were used to measure the strain of the FRP strips at the center and corner of the second FRP strip on the side.

Figure 7.

Measurement layout.

Experimental results and discussion

Failure modes

Figure 8(a)–(e) illustrate the typical failure modes of the unstrengthened and singly strengthened masonry columns. In Figure 8(a), when the unstrengthened masonry column was loaded to approximately 50% of the peak load, the first crack appeared in the vertical mortar joints of the specimen. With increasing load, the cracks rapidly extended from the upper end to the lower end forming a through crack. At the point of failure, the bricks were crushed, indicating a loss of integrity in the masonry column. The failure characteristics of the unstrengthened masonry column were brittle.

Figure 8.

Failure modes of all masonry columns: (a) N. (b) E-20. (c) G-0.7. (d) G-0.4. (e) C-0.7. (f) E-20-G-0.7. (g) E-20-G-0.4. (h) E-15-G-0.7. (i) E-20-C-0.7.

Singly strengthened masonry columns

The failure modes of the masonry columns strengthened with the ECC splints are shown in Figure 8(b). At the initial stage of loading, the ECC splint effectively constrained the expansion of vertical cracks through the ring hoop effect on the masonry columns. With increasing loading, fine cracks emerged on the ECC due to the bridging effect of the fibers. Damage cracks emerged due to the stress concentration at the masonry column corners, and the transverse confinement effect of the ECC splint gradually weakened. Finally, the ECC splint was removed, and the internal blocks were crushed. The failure characteristics of the ECC splint-strengthened masonry columns exhibited ductility.

The failure modes of the GFRP strip-strengthened masonry columns are shown in Figure 8(c) and (d). Upon reaching the load of 60% to 70% of the peak load, initial vertical cracks were observed at the middle and upper mortar joints of the short edges. The crack width increased with decreasing strip strengthening area ratio. With increasing load, the stress concentration at the junction of the strengthened area and the unstrengthened area of the specimen led to an increase in the deformation of the block, and some of the upper and middle FRP strip corners were broken. After the load began to fall, the bricks in the unstrengthened zone gradually crushed and peeled off. The failure mode of the two groups of specimens was pseudoplastic failure.

The failure mode of the CFRP strip-strengthened masonry column is shown in Figure 8(e). The first crack appeared in the specimen at a load of 45% to 50% of the peak load. No damage occurred in the strengthened areas due to the greater tensile strength of the CFRP strips with stronger circumferential confinement. With increasing load, small-scale fractures occurred in the middle and upper CFRP strips, and the bricks between the strips at the upper end experienced rapid crack development, deformation arching, and large-scale spalling. Finally, the bricks in the unstrengthened areas and the fiber fracture areas were destroyed. Ductile failure of the specimens occurred. The ductility characteristics of the specimens exhibited a superior performance compared to those of the G-0.4 group and worse than those of the G-0.7 and E-20 groups.

Compositely strengthened masonry columns

The failure modes of the ECC splint and GFRP strip-compositely strengthened masonry columns are shown in Figure 8(f) and (g). The initial cracking occurred at the ECC splint at the upper end of the short side in the E-20-G-0.7 and E-20-G-0.4 groups at a load of approximately 700 kN. Under the confinement of the central GFRP strips on the masonry, the development of cracks to the central part of the masonry was limited. When the load reached the peak load, part of the GFRP strips in the E-20-G-0.7 and E-20-G-0.4 groups were broken. The GFRP strip can still have a certain lateral confinement effect on the masonry column, cooperating with the ECC splint to confine the deformation of the masonry columns. With the gradual formation of corner failure cracks in the two groups of specimens, transverse cracks appeared in the ECC splint at the middle and lower parts of the specimens under compression. When the GFRP strips in the middle of the E-20-G-0.7 and E-20-G-0.4 specimens were completely broken, the structure lost its bearing capacity and was destroyed. The lateral confinement of the GFRP strips was fully utilized, and ductile failure occurred in both groups. During the failure process, the number, development speed and size of the cracks in the E-20-G-0.4 group were much greater than those in the E-20-G-0.7 group, and the cracks in the E-20-G-0.4 group were destroyed at an earlier stage.

The failure mode of the E-15-G-0.7 group of specimens is shown in Figure 8(h). Compared with the failure mode of the specimens in the E-20-G-0.7 group, that of the specimens in the E-15-G-0.7 group weakened the confinement effect of the ECC on the masonry column due to the decrease in the thickness of the ECC splint, and the development of ECC surface cracks accelerated. At the subsequent stage of loading, the ECC damage was large, rendering the required binding force unattainable, and the load decreased rapidly. The GFRP strips in the upper part of the specimen broke earlier at the corner. Local failure occurred in the specimen.

The failure modes of masonry columns strengthened with ECC splints and CFRP strips are shown in Figure 8(i). Under the high circumferential confinement provided by the CFRP strips, when the load reached approximately 900 kN, the initial crack appeared at the upper end of the wide edge. As the load increased, cracks gradually appeared around the upper end and continued to develop to the middle. After reaching the peak load, the CFRP strips in the middle and upper parts of the masonry continued to fracture, and the ECC splint on the long side of the specimen exhibited transverse cracks under compression. Compared with the E-20-G-0.7 group, the E-20-C-0.7 group exhibited a more rapid failure following the attainment of the peak load. The continuous fracture of the CFRP strips under a high peak load made the strength of the CFRP not fully utilized, resulting in a poor lateral confinement effect, and finally, end failure occurred. The role of the ECC splint was not fully utilized. Therefore, the compressive performance and ductility of the E-20-C-0.7 group could not be comparable to those of the E-20-G-0.7 group that failed in the middle, showing obvious brittle failure characteristics.

Load‒displacement curves

The load‒displacement curves of each group are shown in Figure 9. The curve can be divided into two stages: rising and falling. In the ascending section, the slopes of the curves of all the specimens before cracking were not very different. With increasing load, the deformation of the masonry column was constrained by the strengthening material, leading to greater stiffness compared to unstrengthened masonry columns. For compositely strengthened masonry columns, the compositely strengthened masonry columns exhibited reduced deformation and increased bearing capacity under the combined confinement effect of FRP strips and ECC splints, thereby maintaining higher stiffness. As the crack width and number of the specimens increased, the bricks of the masonry column specimens strengthened with a single material were gradually crushed. The GFRP and CFRP strips of the compositely strengthened masonry column specimens were fractured to varying degrees, and cracks in the ECC splint continued to develop. The load‒displacement curves of all the specimens began to decline. The test results are shown in Table 5, including the initial crack load, peak load, peak axial displacement, peak lateral displacement, and their increase ratios compared with those of the unstrengthened masonry columns. The values for load and displacement presented in the table represent the mean of the results obtained from a single group.

Figure 9.

Load‒displacement curves: (a) Singly strengthened masonry columns. (b) Compositely strengthened masonry columns. (c) Comparison of typical masonry columns.

Table 5.

Specimen load and displacement statistics.

Specimen	Initial crack load (kN)	Increase ratio (%)	Peak load (kN)	Increase ratio (%)	Peak axial displacement (mm)	Increase ratio (%)	Peak lateral displacement (mm)	Increase ratio (%)
N	205.3	—	430	—	1.71	—	0.58	—
E-20	480	134	941.5	118	3.52	106	2.53	337
G-0.7	575	180	817.7	90	2.52	48	0.74	27
G-0.4	375	82	615.9	43	2.29	33	0.85	47
C-0.7	500	144	984.5	129	2.62	52	1.73	198
E-20-G-0.7	625	204	1610.3	274	3.85	125	4.01	591
E-20-G-0.4	725	253	1322.9	208	2.18	27	3.8	555
E-20-C-0.7	875	326	1984	361	3.5	105	2.71	367
E-15-G-0.7	475	131	1338.5	211	3.58	109	4.08	603

Singly strengthened masonry columns

The load‒displacement curves of the singly strengthened masonry columns are shown in Figure 9(a). Compared with those of the unstrengthened masonry columns, the initial crack load, peak load, peak axial displacement and peak lateral displacement of the specimens in the E-20 group increased by 134%, 118%, 106% and 337%, respectively. This phenomenon demonstrated that the bridging effect of fibers in ECC effectively constrained the deformation and damage of masonry columns, significantly enhancing their bearing capacity and deformation capacity. After the peak load, the load‒displacement curve of the unstrengthened masonry column decreased rapidly, indicating obvious brittleness. However, the load‒displacement curve of the ECC-strengthened masonry columns exhibited good postpeak ductility.

Compared with those of the unstrengthened specimens, the initial crack load, peak load, peak axial displacement, and peak lateral displacement of the G-0.7 group specimens increased by 180%, 90%, 48% and 27%, respectively. The improvement effect decreased with decreasing strip strengthening area ratio. After reaching the peak load, the load‒displacement curve of the G-0.7 group specimens decreased slowly. Compared with those of the unstrengthened specimens, the initial crack load, peak load, peak axial displacement, and peak lateral displacement of the C-0.7 group specimens strengthened with high-strength CFRP strips increased by 144%, 129%, 52% and 198%, respectively. The deformation of the specimens in the C-0.7 group after reaching the peak load was mainly concentrated in the unstrengthened area, and the load decreased slowly. The specimens strengthened with a single material exhibited a notable enhancement in bearing capacity, while the ECC splint-strengthened specimens demonstrated a superior deformation capacity compared to the GFRP or CFRP strip-strengthened specimens.

Compositely strengthened masonry columns

The load‒displacement curves of the compositely strengthened groups are shown in Figure 9(b). Compared with those of the specimens in the E-20-G-0.4 group, the peak load, peak axial displacement and peak lateral displacement of the specimens in the E-20-G-0.7 group increased by 18%, 43.2% and 5.2%, respectively. This showed that the increase in the strip strengthening ratio caused the E-20-G-0.7 group specimens to have a higher bearing capacity and deformation capacity. However, the initial crack load of the E-20-G-0.7 group decreased by 15%, which may have been caused by construction errors and material dispersion during the specimen fabrication process.

Compared with those of the specimens in the E-20-G-0.7 group, the initial crack and peak load of the specimens in the E-15-G-0.7 group decreased by 24% and 17%, respectively, and the peak vertical displacement decreased by 6.4%. This phenomenon indicated that a decrease in the thickness of the ECC splint could lead to a decrease in the confinement effect of the ECC on the masonry. As a result, the bearing capacity and deformation capacity of the E-15-G-0.7 group of specimens decreased.

Compared with the E-20-G-0.7 group specimens, the E-20-C-0.7 group specimens exhibited initial crack and peak load enhancements of 40% and 23%, respectively, by virtue of the strong circumferential confinement from the CFRP strips. However, due to the reduced elongation ability of the CFRP strips and the damage being concentrated in the upper end of the specimen, the peak axial displacement and peak lateral displacement of group E-20-C-0.7 were reduced by 9.2% and 32.4%, respectively.

The load‒displacement curves of E-20-G-0.7 and E-20-C-0.7 in the compositely strengthened masonry columns are compared with those of the singly strengthened group, as shown in Figure 9(c). In comparison to the unstrengthened group, the peak load of the E-20-G-0.7 group exhibited an in increase by 1.31 times the sum of the peak loads of the E-20 and G-0.7 groups. The E-20-G-0.7 group of specimens demonstrated the potential to exploit the advantages of the two materials, thereby achieving further enhancement of the bearing capacity. The increase in the peak load of the E-20-C-0.7 group was 1.47 times that of the sum of the peak loads of the E-20 and C-0.7 groups. However, the continued fracture of the CFRP strips after the peak load of the specimens in the E-20-C-0.7 group led to the failure of the ECC splint under high stress. Obvious brittle failure occurred, and the strengthening effect of the ECC splint was not fully exerted. Therefore, the composite strengthening effect of high-strength CFRP strips and ECC splints is not ideal.

Load–lateral strain analysis

The load–lateral strain curves for the unstrengthened and singly strengthened groups are shown in Figure 10(a)–(c). As shown in Figure 10(a), before the peak load (430 kN), a wide crack appeared in the unstrengthened specimens, which made the strain gauge measurements fail. From the initial crack load to the peak load, the transverse strain of the specimen in the E-20 group increased faster than that of the specimen in the G-0.7 group. The strain of the E-20 group before reaching the peak load was 1585 με greater than that of the G-0.7 (850 με) group. This result demonstrated that the E-20 group of specimens exhibited superior deformation capabilities. The strain of the G-0.4 group before the peak load was 300 με less than that of the G-0.7 group. For the C-0.7 group of specimens strengthened with CFRP strips, due to the stronger confinement effect of the CFRP strips on the strengthened area of the masonry column, more cracks developed in the unstrengthened area. Therefore, the lateral strain of C-0.7 (2591 με) before the peak load increased. For the composite strengthening group, the transverse strain before the peak load was smaller than that of the E-20 group and larger than that of the corresponding GFRP or CFRP group. The composite effect of FRP and ECC can make the masonry more uniform.

Figure 10.

Load-lateral strain curves: (a) The middle of the masonry. (b) The fiber. (c) The corner fiber.

The transverse strain of the masonry and the fiber strain in the middle of the masonry were compared, as shown in Figure 10(a) and (b). The strain of the ECC splint in the middle of the compositely strengthened specimens was consistently less than the strain of the GFRP and CFRP strips in most cases. As the crack progressed to the middle, the strain of the ECC splint and the FRP (GFRP, CFRP) strips in the middle began to increased rapidly and reached a peak, demonstrating effective synergistic deformation performance.

Compared with that of the E-20-G-0.7 group (7691 με), the peak transverse strain of the GFRP strip in the E-20-G-0.4 group was lower (3795 με). In the E-15-G-0.7 group, the GFRP strips fractured before reaching the peak load, and after reaching 1529 με, the strain gauge failed. A decrease in the thickness of the ECC splint or the strip strengthening area ratio decreased the peak strain of the specimen. The deformation capacity of the specimens decreased, showing different degrees of degradation of the strengthening effect. After the strain of the middle CFRP strip of the E-20-C-0.7 group reached 5076 με before the peak load, the load-transverse strain curve decreased. In comparison to Figure 10(b) and (c), it was observed that cracks frequently manifested in the central mortar joint of the masonry columns. Consequently, the strain values in the middle of the GFRP and CFRP strips were found to be greater than those at the corners.

The E-20-G-0.7 group specimen with the best strengthening effect among the compositely strengthened specimens, the unstrengthened specimens and the corresponding single material strengthening group specimens (E-20, G-0.7) were selected for analysis. After reaching the initial crack load, the strain of the middle strip of the G-0.7 group of specimens increased slightly, indicating that the middle strips of the G-0.7 group specimen did not fully participate in the stress before reaching the peak load. The utilization rate of the GFRP strips in the G-0.7 group was not good. The strain of the middle GFRP strip of the E-20-G-0.7 group began to increase rapidly, and the GFRP strips were fully utilized. Upon reaching the peak load, the transverse strain in the middle of the E-20-G-0.7 group was found to be situated between that of the E-20 group and the G-0.7 group. This showed that ECC can prevent wide cracks in the ECC splint through the confinement of strips. The ECC splint made the stress of the specimen more uniform and prevented local damage to the unstrengthened area when the masonry was strengthened by the GFRP strips alone.

Energy dissipation

Energy dissipation is an important parameter for evaluating the compressive performance of strengthened masonry columns (Jing et al., 2021). In this study, after reaching the peak load, the ultimate point for calculating the energy dissipation is set at 85% of the peak load. As indicated in Table 6, the E-20 group showed the highest energy dissipation capacity among the singly strengthened masonry columns, and its energy dissipation value was 10.2 times that of the unstrengthened group. However, the energy dissipation of the G-0.7 group was 5.7 times greater than that of the unstrengthened group. Compared with the singly strengthened masonry columns, the E-20 group exhibited better compression performance and ductility, enabling it to absorb more energy under load. When the ECC splint and the GFRP strips were combined to strengthen the masonry columns, the energy dissipation capacity of the composite strengthened masonry columns was further improved compared with that of the single strengthened masonry columns. The energy dissipation of the E-20-G-0.7 group was 28.9 times greater than that of the unstrengthened group. The energy dissipation of the E-20-G-0.7 group was 1.82 times greater than the sum of the energy dissipation of the E-20 group and the G-0.7 group. The confinement effect of ECC and FRP on masonry columns was fully realized in the E-20-G-0.7 group. The ECC splint and FRP strips collaborated to resist the failure of the masonry columns and absorbed more energy, indicating that E-20-G-0.7 exhibited better ductility and showed a commendable strengthening effect. This group of specimens combined GFRP and ECC and had a good composite strengthening effect. The energy dissipation values of the E-20-G-0.4 and E-15-G-0.7 groups were 9.7 and 9.3 times that of the unstrengthened group, which were far lower than that of the E-20-G-0.7 group. A decrease in the strip strengthening area ratio or the thickness of the ECC splint will reduce the compression performance and ductility of the masonry column, ultimately diminishing its energy dissipation capacity. The energy dissipation of the E-20-G-0.4 and E-15-G-0.7 groups was lower than that of the E-20 group. The ductility of the specimens in the E-20-G-0.4 and E-15-G-0.7 groups was lower than that in the E-20 group because of the combination of GFRP and ECC. The energy dissipation of the E-20-C-0.7 group was 9.6 times that of the unstrengthened group, which was much lower than that of the E-20-G-0.7 group (28.9). For the E-20-C-0.7 group, the energy dissipation decreased due to the brittle failure of the specimen.

Table 6.

Energy consumption of the specimen.

Specimen	$\bar{E}$ (J)	Increase multiple
N	503	—
E-20	5135	10.2
G-0.7	2871	5.7
G-0.4	1268	2.5
C-0.7	4588	9.1
E-20-G-0.7	14,574	28.9
E-20-G-0.4	4909	9.7
E-20-C-0.7	4849	9.6
E-15-G-0.7	4710	9.3

Theoretical calculations

Calculation methods

Compressive bearing capacity of the unstrengthened masonry column

The axial compressive strength of the masonry column is calculated per GB 50003-2011 (2011), as shown in equation (1).

{f_{c}}_{o} = k_{1} {f_{1}}^{α} (1 + 0.07 f_{2}) k_{2}

(1)

where f_co is the axial compressive strength of the masonry column; f₁ is the compressive strength of the brick; f₂ is the compressive strength of the mortar; and k₁ and k₂ are the correction parameters of the construction method of the masonry column and the influence of the mortar strength, respectively. According to the standard (GB 50003-2011, 2011), the recommended values are 0.78 and 1, respectively, and

α

is the parameter related to the brick height and brick category, which is taken to be 0.5 (GB 50003-2011, 2011).

Compressive bearing capacity of the ECC splint-strengthened masonry column

In an ECC splint-strengthened masonry column, the cross-sectional stress distribution can be divided into a strong confinement area and a weak confinement area provided by the ECC splint. The confined rectangular cross-section of the effective confinement area (Deng et al., 2015) is determined by a standard parabola. The angle between the parabola and the long side of the cross-section is 45. The effective confinement area calculation model is shown in Figure 11, where the black area represents the area of the strong confinement area, designated as A_e. The blank area corresponds to the area of the weak confinement area, designated as A₀. The gray area represents the area of the ECC splint, designated as A_c. The calculation formulas for A₀, A_e, and A_c can be found in equation (2).

{\begin{cases} A_{0} = \frac{{b^{'}}^{2} + {h^{'}}^{2}}{3} \\ A_{e} = b h - 4 {r_{c}}^{2} + π r_{c}^{2} - \frac{{b^{'}}^{2} + {h^{'}}^{2}}{3} \\ A_{c} = 2 t_{e} (b + h) + 4 t_{e}^{2} \end{cases}

(2)

where b and h are the length and width of the cross section of the unstrengthened masonry column, respectively; t_e is the thickness of the ECC splint; r_c is the radius of the rounded corners; and b' and h' are the length and width of the cross section of the unstrengthened masonry column minus twice the radius of the corner, respectively, such as b' = b − 2r_c.

Figure 11.

The effective confinement area.

Calculation of the compressive bearing capacity of the strong confinement area

The force state of the masonry column strengthened by the ECC splint is analogous to three-way circumferential pressure. When lateral expansion of the masonry column occurs under compression, the ECC splint provides hoop confinement on the masonry column and produces lateral confinement stress. A schematic representation of the calculation of the equivalent transverse confining compressive stress of the ECC splint on masonry columns is shown in Figure 12 (Ouyang et al., 2004). According to the force balance in Figure 12, the equivalent transverse confinement compressive stresses f_x and f_y of masonry in the x and y directions can be calculated as follows:

f_{x} = \frac{2 t_{e} f}{h}

(3)

f_{y} = \frac{2 t_{e} f}{b}

(4)

where the tensile stress of the ECC in the uniaxial tensile test is adopted as the effective tensile stress f of the ECC splint. As shown in Figure 13, t_e is the thickness of the ECC splint, which is taken as 20 mm; b and h are the cross-sectional width and height of the masonry column, respectively.

Figure 12.

Equivalent transverse confined stress.

Figure 13.

Effective confinements model: (a) Cross-sectional effective confinement model. (b) FRP strips spacing confinement model.

The average values of f_x and f_y are adopted as the equivalent transverse confining compressive stress $\bar{f}$ , as shown in equation (5).

\bar{f} = \frac{t_{e} f (b + h)}{b h}

(5)

According to the calculation method for the axial compressive bearing capacity of highly ductile concrete splint-strengthened masonry columns (Deng et al., 2019) indicates that the formula for the ultimate compressive strength in the strong confinement area of the ECC splint-strengthened masonry column f_ce can be obtained as shown in equation (6).

f_{c e} = f_{c o} [1 + 1.5 \sqrt{\frac{\bar{f}}{f_{c o}}} + 2 \frac{\bar{f}}{f_{c o}}]

(6)

The calculation of the compressive bearing capacity of the strong confinement area of the ECC splint-strengthened masonry column specimen is shown in equation (7):

N_{1} = f_{c e} A_{e}

(7)

Calculation of the compressive bearing capacity of the weak confinement area

It is assumed that the axial compressive strength of the brick masonry at the outermost edge of the weak confinement area is equivalent to the ultimate compressive strength of the unconfined masonry, f_co. From the point of intersection between the weak and strong confinement areas and the outer edge of the brick column cross-section, the ultimate compressive strength of the brick masonry in the weak confinement area declines in a linear manner (Deng et al., 2015).

The compressive bearing capacity of the weak confinement area of the ECC splint-strengthened masonry column specimen is calculated via equation (8):

N_{2} = \frac{1}{2} (f_{c o} + f_{c e}) A_{0}

(8)

Calculation of the compressive bearing capacity of the ECC splint

Upon reaching the ultimate state, it is assumed that the ECC splint and the masonry deform in a synergistic manner, with the bond slip between the ECC splint and the masonry column being disregarded. The ECC splint reaches the ultimate tensile stress due to the lateral expansion of the masonry column. The force state of the ECC splint can be comparable to that of the biaxial tension-compression model. The circumferential tension of the ECC splint will lead to a decrease in the compressive bearing capacity. Therefore, the strength utilization coefficient $α_{1}$ of the ECC surface layer under the biaxial tension-compression model is introduced in the calculation of the compressive bearing capacity of the ECC splint.

α_{1} = σ_{1} / f_{c}

(9)

where

σ_{1}

is the compressive stress of the ECC surface layer under the peak load and

f_{c}

is the compressive strength of the ECC material.

The calculation of the compressive bearing capacity of the ECC splint is shown in equation (10):

N_{2} = α_{1} f_{c} A_{c}

(10)

Calculation of the compressive bearing capacity of the ECC splint-strengthened masonry column

The axial compressive bearing capacity of masonry columns strengthened with an ECC splint is borne by three parts: the weak confinement area, the strong confinement area and the ECC splint compression area.

The calculation of the compressive bearing capacity is shown in equation (11):

N_{u} = φ (N_{1} + N_{2} + N_{3})

(11)

where N_u is the axial compressive bearing capacity of masonry columns strengthened with an ECC splint and

φ

is the ratio of the height of the masonry columns to the thickness of the masonry columns on the axial compressive bearing capacity of the coefficient, taking the value of 1.

Compressive bearing capacity of the FRP strip-strengthened masonry column

According to CNR-DT 200 R1/2013 (2013), the analytical model of confinement compressive strength and the calculation formula of compressive bearing capacity are provided, as shown in equation (12). With regard to the circumferential confinement of the FRP strips, a rectangular cross-section confinement model of the externally wrapped FRP strips is given, as shown in Figure 10.

{\begin{cases} f_{m c d} = f_{c o} [1 + k^{'} {(\frac{f_{l, e f f}}{f_{c o}})}^{α_{1}}] \\ N_{u} = \frac{1}{γ_{R d}} A_{m} f_{m c d} \end{cases}

(12)

where f_cmd is the confinement compressive strength of the FRP strip-strengthened masonry column; f_l,eff is the effective confinement lateral compressive stress;

α_{1}

is the confinement coefficient; if there are no additional experimental data, the coefficient of

α_{1}

is equal to 0.5;

k^{'}

is the nondimensional confinement efficiency coefficient (Bai et al., 2017);

γ_{R d}

is the confinement ultimate load carrying capacity; N_u is the compressive bearing capacity of the FRP strip-strengthened masonry column; and A_m is the cross-sectional area of the FRP strip-strengthened masonry column.

(1) Calculation of the compressive bearing capacity of unstrengthened masonry columns

N_{1} = f_{c o} A_{m}

(13)

(2) Calculation of the compressive bearing capacity of the FRP strips

N_{2} = f_{c o} A_{m} k^{'} {(\frac{f_{l, e f f}}{f_{c o}})}^{α_{1}}

(14)

k^{'} = α_{2} {(\frac{g_{m}}{1000})}^{α_{3}}

(15)

{\begin{cases} f_{l, e f f} = k_{e f f} f_{l} = k_{H} k_{V} f_{l} \\ k_{H} = 1 - \frac{b^{' 2} + h^{' 2}}{3 A_{m}} \\ k_{V} = {(1 - \frac{p_{f}^{'}}{2 \min (b, h)})}^{2} \end{cases}

(16)

{\begin{cases} f_{l} = \frac{1}{2} ρ_{f} E_{f} ε_{f d, r i d} \\ ρ_{f} = \frac{2 t_{f} (b^{'} + h^{'}) b_{f} + π [{(r_{c} + t_{f})}^{2} - {r_{c}}^{2}] b_{f}}{[b h - (4 - π) {r_{c}}^{2}] p_{f}} \\ ε_{f d, r i d} = \min {\frac{η_{a} ε_{t k}}{γ_{f}}; 0.004} \end{cases}

(17)

where g_m is the mass density of the masonry, taken as 1600 kg/m³; the coefficients of

α_{2}

and

α_{3}

are equal to 1.0 if no further experimental data are available; k_eff is a coefficient of the efficiency, defined as the ratio between the volume of the effectively confined concrete and the volume of the concrete member; k_H is the coefficient of efficiency; k_V is the coefficient of horizontal efficiency; and f_l is the confinement lateral pressure (f_l is related to the mass and type of FRP material, as well as to the geometry of the confinement and the confinement scheme); b and h are the width and height of the rectangular cross-section, respectively;

p_{f}^{'}

is the length of the unstrengthened area between adjacent strips;

ρ_{f}

is the geometric strengthening ratio as a function of section shape and FRP configuration; E_f is Young’s modulus of elasticity of the FRP strip;

ε_{f d, r i d}

is a reduced FRP design strain; t_f and b_f are the thickness and width of the FRP strip, respectively; p_f is the center distance between the two neighboring strips of the FRP strip;

η_{a}

is the environmental conversion factor, which is taken to be 0.75 for the GFRP and 0.95 for the CFRP;

ε_{t k}

is the value of the ultimate tensile strain at the end of the measured end; and

γ_{f}

is the partial factor, which is taken as 1.1.

(3) Calculation of the compressive bearing capacity of FRP strip-strengthened masonry columns

N_{u} = \frac{1}{γ_{R d}} (N_{1} + N_{2})

(18)

A number of analytical models have been proposed by domestic and foreign scholars for determining the confinement compressive strength of masonry columns strengthened by FRP strips. These include the CM model (Campione and Miraglia, 2003), the CO model (Corradi et al., 2007), and the DIL model (Di Ludovico et al., 2010). All of these models fit the compressive bearing capacity formula for FRP strip-strengthened masonry columns. In this paper, the fitting curve of the CNR model is shown in Figure 14. $k^{'}$ and $α_{1}$ are 1.3678 and 0.39,684, respectively.

Figure 14.

Fitting curve of the compressive capacity of FRP strengthened specimens.

Calculation of the compressive bearing capacity of compositely strengthened masonry columns

The compressive bearing capacity of compositely strengthened masonry columns is contingent upon the compressive strength of the confined masonry columns. The predictive model for the compressive strength of confined masonry columns is typically established by the principle of superposition, as shown in equation (19). The compressive strength of confined masonry columns is equal to the sum of the contributions of unstrengthened masonry columns and externally confined layers to the compressive strength (Cascardi et al., 2017).

{\begin{cases} f_{m c d} = f_{c o} [α + k^{'} {(\frac{f_{l, e f f}}{f_{c o}})}^{α_{1}}] \\ N_{u} = \frac{1}{γ_{R d}} A_{m} f_{m c d} \end{cases}

(19)

where

α

is the strength correction factor for unstrengthened masonry columns.

Calculation of the compressive bearing capacity of the strengthened masonry columns

Because the ECC splint is involved in compression and contributes greatly to the compressive bearing capacity of the masonry column, the masonry column is confined by the ECC splint and the FRP strips. Therefore, the confinement model of ECC splint and FRP strip-compositely strengthened masonry columns can be simplified as follows: FRP strip-strengthened “equivalent masonry columns,” and the equivalent masonry columns are ECC splint-strengthened masonry columns. The arching action results in an uneven distribution of the confinement effect of discontinuous FRP strips on concrete columns as well as alterations to the strong and weak confinement areas of masonry columns. In this study, the influence of these factors on the calculation model is not considered, and the calculation model is limited to determining the bearing capacity of the masonry column based on the size of the test specimen. This is shown in Figure 15.

Figure 15.

Confinement model.

The basic assumptions are as follows: (1) ECC splint and masonry column deformation, without considering bond slip; (2) the ECC splint is not considered in the transverse and longitudinal cross-section confinements of the specimen, and its confinement is reflected in the enhancement of the “strengthened masonry column” compressive strength and modulus of elasticity.

The calculation of the compressive bearing capacity is shown in equation (20):

N_{1} = α f_{c o, e} A_{m}

(20)

where f_co,e is the axial compressive strength of the ECC splint-strengthened masonry column.

Calculation of the compressive bearing capacity of the FRP strips

N_{2} = f_{c o, e} k^{'} {(\frac{f_{l, e f f}}{f_{c o, e}})}^{α_{1}} A_{m}

(21)

Evaluation of the modified model

The calculation model of the compressive bearing capacity of FRP strip-strengthened “equivalent masonry columns” is different from that of FRP strip-strengthened ordinary masonry columns. Therefore, equation (12) needs to be adjusted, and the fitting curve is shown in Figure 16(a). $k^{'}$ and $α_{1}$ are 1.32696 and 0.40757, respectively.

Figure 16.

The fitting curves of the compositely strengthened specimens: (a) Equation (12). (b) Equation (19).

At the same time, the calculation model (equation (19)) of the compressive bearing capacity of the compositely strengthened masonry column is modified. The fitting curve is shown in Figure 16(b), and $k^{'}$ , $α$ and $α_{1}$ are 2.95922, −0.64128 and 0.1459, respectively.

According to two calculation methods, the compressive bearing capacity of the compositely strengthened masonry column is calculated:

N_{u} = \frac{1}{γ_{R d}} (N_{1} + N_{2})

(22)

Evaluation of the test results

The results of the compressive bearing capacity calculations for all specimens are shown in Table 7. The difference between the test results of the unstrengthened masonry columns and the calculated results is 2.2%.

Table 7.

Table of calculated results.

Specimen	Test value (kN)	Calculated value (kN)	Error (%)	Fitted value (kN)	Error (%)	Fitted value (kN)	Error (%)
N	430	439.5	2.2%	—	—	—	—
E-20	941.5	932.3	1.0%	—	—	—	—
G-0.7	817.8	829.8	1.5%	797.9	2.4%	—	—
G-0.4	615.9	601.2	2.4%	571.7	7.1%	—	—
C-0.7	1040.4	1155.4	11.0%	1045.4	0.5%	—	—
E-20-G-0.7	1607.6	1636.2	1.8%	1573.9	2.1%	1600.5	0.4%
E-20-G-0.4	1322.9	1340.9	1.4%	1343.3	1.5%	1322.2	0.5%
E-20-C-0.7	1984.0	2224.5	12.1%	1991.1	0.36%	1983.5	0.3%

The error of the calculation results of the compressive bearing capacity of the ECC splint-strengthened masonry columns is within 1%, which is in good agreement with the experimental results.

The error of the calculated compressive bearing capacity of masonry columns strengthened with GFRP strips obtained according to the CNR analytical model is less than 3%, and its strength prediction is accurate. However, for masonry columns strengthened with high-strength CFRP strips, the error in the compressive bearing capacity calculation is 11%. After the modification of the model, the difference in the predicted compressive bearing capacity of the different strengthening methods is within 7.1%, showing good precision.

According to the calculation results of equation (12), the error in the compressive bearing capacity calculations for the E-20-G-0.7 and E-20-G-0.4 groups of specimens is within 2%. In contrast, the error in the compressive bearing capacity calculations for the E-20-C-0.7 group of specimens is 12.1%, which is a larger error. After modifying equation (12), the error of the calculation model of the FRP strip-strengthened “equivalent masonry columns” is within 3%. Meanwhile, the calculation error of the calculation model of the compressive bearing capacity of the compositely strengthened masonry column obtained by the modified equation (19) is less than 1%, and the strength prediction model has high accuracy. Therefore, the confinement calculation model used in equation (19) should be adopted.

Conclusions

This study investigated the compressive properties of ECC splints and FRP strips on single and composite strengthened masonry columns under axial compression, considering the effects of the type of strengthening material (ECC splints or FRP strips), the FRP strip strengthening area ratio and the thickness of the ECC splint. The main conclusions are as follows:

(1) Compared with unstrengthened masonry columns, FRP strips or ECC splints can effectively improve the ductility, cracking load, peak load, lateral deformation capacity and energy absorption capacity of masonry columns under compression and change the failure mode under axial compression from brittle failure to ductile failure by providing effective confinement to the concrete core.

(2) Compared to those of the singly strengthened masonry columns, the composite strengthening of the ECC splints and GFRP strips can greatly increase the compressive strength and ductility of masonry columns. The peak load and energy dissipation of the E-20-G-0.7 group increased by 1.31 times and 1.82 times the sum of those of the E-20 and G-0.7 groups, respectively. The compressive bearing capacity of the masonry column of the E-20-C-0.7 group was greater than that of the other groups, but brittle failure occurred in the masonry columns strengthened with the ECC splints and high-strength CFRP strips.

(3) The masonry column was most effectively strengthened by a 20 mm ECC splint and a 70% GFRP strip strengthening area ratio. Compared with those of the E-20-G-0.4 group, the peak load, peak axial displacement and peak lateral displacement of the E-20-G-0.7 group increased by 18%, 43.2% and 5.2%, respectively. The compressive performance of the E-20-G-0.7 group was found to be superior to that of the E-15-G-0.7 group. This indicates that the compressive capacity of strengthened masonry columns can be enhanced by increasing the thickness of the ECC splint or the GFRP strip strengthening area ratio.

(4) The formula for calculating the compressive bearing capacity of strengthened masonry columns was derived from existing calculation models. The accuracy of the proposed formula for calculating the compressive bearing capacity of strengthened masonry columns was fully validated by comparison with the present test results. The error between the fitting results of the calculation formula and the test results was within 7.1%.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by grants from the National Natural Science Foundation of China (U1904177), the Excellent Youth Foundation of Henan Province of China (212300410079), and the Project of Young Key Teachers in Henan Province of China (2019GGJS01).

ORCID iDs

Weilai Chen

Pu Zhang

Ye Liu

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