Strengthening of Damaged Masonry Walls Using Engineered Cementitious Composites: Experimental and Numerical Analysis

Engineered cementitious composites (ECC) are special types of high-tensile and high-ductility concrete that are designed using a micromechanics approach, with a tensile strain capability of more than 3%. Due to their higher strain hardening capacity, ECC can be applied as a strengthening material on structural walls, which improves the structural strength and inelastic deformation capacity. ­is study presents an experimental and numerical analysis of brick masonry wall strengthened by traditional mortar, ECC, and ECC with 40% y ash (FAECC) subjected to uniaxial compression. ­e tests, such as compressive strength, indirect tensile strength, and bond strength, were conducted. Based on the experimental results, a numerical model is developed, and a failure prediction for the existing masonry structure is made. ­e compressive strength of ECC is observed to be higher than normal mortar and FAECCwhereas the indirect tensile strength of both ECC and FAECCwas almost similar, which is higher than that of normal mortar. ­e bond strength of ECC and FAECC is found to be 70% higher than that of normal mortar. It is evident that brick masonry units strengthened by ECC have a higher compressive strength than masonry units strengthened by conventional mortar and FAECC. It also controls crack development and spalling of masonry units. ­en, a micromodelling along with CDP model is made in Abaqus/CAE software and an excellent correlation between experimental and numerical results was noted. ­e suggested models were shown to be capable of predicting the common behaviour of masonry units.


Introduction
Unreinforced masonry construction is a popular construction method preferred all around the world. Most of the ancient and historical buildings constructed were load bearing structures and hence reconstruction is not an appropriate approach due to the vast number of buildings constructed with URM brick walls [1]. is emphasises the need for evaluating relevant structural strengthening measures for these structures. In order to strengthen the masonry structures, the strengthening material should have adequate tensile capacity to withstand heavy loads. Hence, engineered cementitious composites (ECC) would be a better option as a strengthening material on masonry wall as the application of ECC over the wall is similar to mortar plastering and also the ability to withstand high temperature compared to other retro tting materials, like FRP [2].
ECC is a highly ductile cementitious material having a tensile strain hardening capacity of more than 3%.
is comes under the class of high-performance cementitious composites having high ductility and the ability to bend rather than fracture under severe loading conditions [3]. ECC is a combination of cement, sand, bre, water, and some recurrent chemical additives. Coarse aggregates are not used in the mixture as they adversely a ect the ductile property of the composite. ECC does not utilize a large amount of bre unlike other types of bre-reinforced concrete. Generally, the volume of discontinuous bre less than 2% is appropriate, even though the composite is designed for structural applications [3][4][5]. Unlike high-performance fibre-reinforced concrete, ECC reveals selfcontrolled crack width under increased loading conditions. ese microcracks are formed within the material and then begin to spread once the load is applied. Due to the presence of fibres within the mixture, the cement matrix widens to a mean width of about 60 µm [6,7]. Normally the cement content of ECC is more than 1000 kg/m 3 . is will lead to the generation of large amount of carbon dioxide, responsible for 5% emission of greenhouse gas created by human activities [8]. erefore, it is imperative that green ECC be developed by adding mineral admixtures to partially replace cement to comply with global sustainable development. Fly ash has been an obligatory ingredient for ECC to improve its mechanical strength and reduce drying shrinkage [9].
Since ECC possess excellent crack control capability, researchers have been working on ways to reduce the crack widths that occur on structural elements using ECC. Zheng et al. [10] used a combination of ECC and basalt fibres to strengthen RC beams externally at the soffit, which functioned together as a composite reinforcement layer, and thereafter the flexural behaviour of the beam is examined through four-point loading and an increase in strength and stiffness is observed. Some studies were also made on firedamaged RC beams strengthened by steel-reinforced ECC to determine its shear behavior [11]. Al-Gemeel and Zhuge [12] investigated the confinement effectiveness of different confining systems made using ECC and basalt fibre-reinforced ECC on square column through which the compressive strength of column confined using ECC has been proved to be higher compared to other columns. ECC has also been used for strengthening RC beam-column joints to determine its effectiveness under seismic conditions [13].
ough ECC has been widely used on RC structural elements, research on masonry walls strengthened by ECC is very less comparatively. So far, focus has been more on strengthening of RC structures compared to strengthening of masonry structures. Moreover, the development of masonry strengthening techniques has also taken place at a discrete level. Masonry strengthening is often necessary if damage is caused by earthquakes, poor construction, or deterioration of the structure [14]. Chourasia et al. [15] conducted an experimental investigation of seismic strengthening technique for confined masonry buildings in which masonry walls are strengthened using chicken mesh, welded wire mesh, industrial geogrid mesh, polypropylene band mesh, nylon mesh, and plastic cement bag mesh and their response under uniaxial and lateral loading is examined. Shabdin et al. [16] investigated the effectiveness of textile-reinforced mortars (TRM) in strengthening unreinforced masonry (URM) walls through which the strength enhancement potential of TRMs was determined by conducting diagonal tension (shear) tests on ten masonry walls.
e results confirmed that the masonry units strengthened on both the faces show better performance compared to that strengthened on only one face. ough alternate materials are available for strengthening masonry walls, ECC possess better thermal property compared to other materials [2]. Soleimani-Dashtaki et al. [17] performed a shake table test on unreinforced masonry walls strengthened by sprayable eco-friendly ECC thorough which the lateral load carrying capacity of the masonry walls strengthened on single and double faces was examined. e results confirmed that the single-sided retrofitting is sufficient enough for a low-rise building to withstand major earthquake whereas doublesided retrofitting can be preferred for high-rise buildings carrying heavy loads on walls. Pourfalah et al. [18] conducted a flexural road test on masonry walls partially and fully bonded by ECC through which the out-of-plane behaviour was examined. e deflection that occurred on masonry walls fully bonded by ECC was found to be less compared to the walls with partially bonded ECC. e propagation of cracks can also be controlled by providing ECC overlays on masonry walls [19]. Previous research demonstrates that the ductility of the masonry walls can be improved by applying ECC on both the faces of the masonry walls [18,20]. Prior studies have also shown that the tensile characteristics of ECC possess a substantial impact on the in-plane behaviour of the modified walls [21].
Along with masonry strengthening, numerous studies were also made on bond strength of brick and mortar. e capacity of mortar or any binding material used in masonry units to remain bonded during the application of severe axial and lateral loads is referred to as bond strength. is is a critical parameter of masonry unit to produce appropriate tensile strength, as well as the ability to endure wind and seismic stresses, as well as slight displacement. Deficient bond strength will result in cracking and dislocation of bricks in masonry construction [22]. Since cracking is a brittle mode of failure, redistribution of stresses would occur and hence there is a possibility of widespread damage if bond strength is insufficient. e weakness might be conspicuous only when the masonry is subjected to supreme loading condition, such as a high wind or an earthquake, when it might lead to collapse [23]. According to Sarangapani et al. [24], the bond strength of ordinary brick mortar interface coated with epoxy as an enhancing material increases by 4 times. Also, bricks with rough surface possess higher bond strength compared to bricks having plane surface [25]. Curing is another important factor to be considered for the development of bond strength. In comparison to dry curing, wet curing of masonry units is preferred to gain better bond strength and elastic modulus [26]. Overall, the factors that impact the bond strength between brick and mortar include the mortar type, surface properties, water absorptions of the brick, frog dimension, and curing process [27][28][29].
Although previous studies provided necessary information on ECC and various masonry strengthening techniques, some parameters such as effect of bond strength on strengthened masonry walls and amount of fly ash required in ECC to strengthen masonry walls have not been found. More importantly, most of the studies were undertaken on strengthening of undamaged masonry walls rather than weak and damaged walls. With an emphasis on the development of structural behaviour of historic masonry buildings, this study involves the strengthening of damaged masonry units by ECC. It is also concentrated to prepare an eco-friendly ECC by adding fly ash to the mixture. Hence, in this research, weak masonry units were developed, strengthened by ECC and thereafter the strengthening effect of ECC is studied through experimental and numerical analysis.

Experimental Program
is study investigates the efficacy of using ECC in strengthening masonry walls. Initially the compressive strength and indirect tensile strength of mortar, ECC, and ECC with fly ash were tested. Following that, the shear bond strength of these samples on masonry units was determined through triplet test [22][23][24][25][26]. en, the major investigation was performed on three sets of double-layered weak masonry units strengthened by normal mortar, ECC, and ECC with 40% fly ash. All the elastic and plastic properties were collected from the above tests and are further used for numerical analysis.

Materials.
Ordinary Portland Cement (OPC 53 grade) having a specific gravity of 3.15 is utilized. Manufactured or M sand that has been passed through 300-micron sieve is used as fine aggregate [30]. For both mixing and curing purpose, ordinary portable water is used. To ensure acceptable workability, a high-range water-reducing admixture was utilized. Two types of fibres were used in this study; they are polypropylene fibre and steel fibre. e conventional mortar is prepared according to IS:2250 (1981) [31] and the ECC mixture is made using the micromechanics design concept. Class I clay bricks of size (10 × 7.5 × 20.5) were used for masonry purpose. e bricks were brought from NRA Traders, Chennai, and it is confirmed that the properties satisfy IS-1077 [32]. e properties of fibres and superplasticizer were mentioned in Tables 1 and 2.

Mix Design.
e mix design of ECC is completely based on the micromechanics design principle [30,33]. e micromechanics of ECC is a body of knowledge that defines the interaction between fibres and cement matrix synergized to form multiple cracks under tension. e mixing procedures of ECC with and FAECC were similar [34]. e dry cement and fly ash mixtures were thoroughly blended together for few minutes. e polycarboxylate superplasticizer was added with water and mixed effectively before combining it with cement and fine aggregate. While mixing the wet cement and fine aggregate mixture, the steel and polypropylene fibres were included. e polypropylene fibres were dipped in water before combining them with wet mixture as the dry fibre has the ability to absorb water from the wet mixture. Since fibre distribution has a significant impact on mechanical qualities, it is critical to ensure that the fibres are evenly dispersed within the mortar. e mix proportions for normal mortar, ECC, and ECC with fly ash are mentioned in Table 3.

Test Specimen.
e samples were prepared for mortar, ECC, and ECC with 40% fly ash to test the mechanical properties. e compressive strength test was carried out on 50 mm × 50 mm × 50 mm cubes in accordance with ASTM C109/109M-21 [35] whereas the indirect tensile strength was performed on 200 mm × 100 mm concrete cylinder based on ASTM C496/C 496M-04 [36]. ree sets of double-layered masonry units of size 190 mm × 220 mm × 420 mm were prepared for strengthening and are tested for compression based on BS EN772-1-2000 [37]. To determine the shear bond strength of masonry units with normal mortar, ECC, and ECC with 40% fly ash as bed joints, a triplet test was conducted based on EN1052-1 [38].

Strengthening Procedure.
e double-layered masonry units prepared for the compressive strength test are intentionally damaged by applying initial cracking load. Once the cracks are formed, the loading is stopped, and the specimens are shifted for the strengthening process. ree sets of specimens were taken, and the strengthening is made using mortar, ECC, and FAECC for each set of specimens. e strengthening is made by filling the cracks and plastering both faces of the masonry units. e thickness of the strengthening layer is 10 mm. A thickness of about 3 to 30 mm has been proved to be preferable in a study conducted by Arslan and Celebi [14]. Once the strengthening is done, the specimens were tested for compression after 28 and 90 days of curing.

Compressive Strength of Mortar and ECC.
e compressive strength of mortar and ECC was conducted based on ASTM C109/109M-21 [35]. Fresh mortar and ECC were prepared and were poured into 50 mm × 50 mm × 50 mm     ECC without fly ash but comparatively similar to normal mortar. But ECC with fly ash has a greater rate of increase in strength compared to traditional mortar after 90 days. is increase in strength of ECC is due to the high load carrying capacity of the composites and also the bridging of cement matrix by fibres resulting in formation of microcracks. is microcrack behaviour was also observed in ECC with 40% addition of fly ash. e tested samples were shown in Figure 4.

Indirect Tensile Strength.
Splitting tensile strength test is an indirect method of determining the tensile strength of concrete or any cementitious material [39]. In this study, the indirect tensile strength tests of mortar and ECC were conducted based on ASTM C496/C496M-04 [36]. is codal practice was chosen since Qudoos et al. [39] clearly state that conducting indirect tensile strength test for mortar using concrete testing methods does not affect the experimental results of mortar specimen.
Fresh mortar and ECC were prepared, and they were poured into 200 mm × 100 mm cylinder. en, each layer was tamped properly to prevent segregation and to improve the distribution of fibres in ECC. Once the specimens were cast, they were kept under room temperature for about 24 hours. en, the specimens were demoulded and they were dipped into water for curing. After curing the cylinders for 28 and 90 days, they were tested for tensile strength. e cylindrical specimen is placed in such a way that the longitudinal axis is perpendicular to the load. e load was progressively increased at a nominal rate without creating any shock. e maximum applied load as reported by the testing equipment and the mode of fracture was recorded. Figure 5 shows the indirect tensile strength of mortar, ECC, and FAECC. At 28 days, ECC attains 88% increase in strength and FAECC attains 56% increase in strength compared to normal mortar. After 90 days of curing, the indirect tensile strength of both ECC and ECC with fly ash were nearly found to be similar. e tested samples were shown in Figure 6.

Bond Strength.
To find out bond strength, the specimens were made in the form of stack bonded triplets by standard bricklaying procedures using masonry mortar and ECC as bed joints. e area of the masonry units where the load is to be applied should be plane and perpendicular to the bearing surface. Once the specimens were cast, it was kept under curing for 28 and 90 days. e specimen after curing was placed into the Universal Testing Machine (UTM) and the load was applied parallel to the mortar joints. To reduce the bending moment, it is better to relocate the point of load application as close to the joint as feasible. e load is applied in a way that creates a peak load within 1 to 2 minutes of completion of the test. Using tangential force and area of application of mortar, the shear bond strength is determined.
e test was carried out in clay bricks with normal mortar, ECC, and ECC with fly ash as bed joints, and the results are shown in Figure 7. e bond strength of brick with conventional mortar, ECC, and FAECC is found to be 0.45 MPa, 0.8 MPa, and 0.81 MPa at 28 days. e bond strength is almost the same for ECC and FAECC when further curing is made. e use of polypropylene fibres and higher cement content in ECC and FAECC improved the shear capacity of bed joints, resulting in higher bond strength. Ronald Lumantarna and Biggs [28] investigated the shear bond strength of masonry walls constructed between 1880s and 1940s in New Zealand using lime mortar through which a bond strength ranging from 0.02 MPa to 0.6 MPa was achieved for distinct loading levels. Also, for general kinds of masonry units, an implicit bond strength of 0.2 MPa was achieved using mortar with mix

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proportions specified in AS3700:2018 [40]. us, in comparison to other mortar mixes used in prior testing, ECC achieved a 20 percent improvement in bonding strength.

Compressive Strength of Strengthened Masonry Prism.
e compressive strength test was conducted on three sets of double-layered stack bond masonry prism. First, the masonry prisms were weakened by applying the initial cracking load and then the strengthening process is done. e strengthening was made by mortar, ECC, and ECC with fly ash on both the faces of the prism respectively. After casting, the specimens were kept under curing for 28 and 90 days. e curing is made using wet sack and is made sure that there is no evaporation of water. After 28 and 90 days of curing, they were tested for compression using UTM. e deformations and the crack patterns were noted and thereby all the elastic and plastic properties were collected from the experimental data. e compressive strength of weak masonry units strengthened by normal mortar, ECC, and ECC with 40% fly ash was tested for 28 and 90 days as per BS EN772-1-2000 [37]. In Table 4, the test results are shown. It is observed that the compressive strength of the masonry units strengthened

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interlock between the strengthening material on both faces of the masonry units. When load is applied, the faces of the masonry prism undergo tension, which pulls the strengthened layer out of plane. Since ECC and FAECC possess similar bond strength, the out-of-plane behaviour is also observed to be in a similar manner. e tensile property of the strengthening material also plays a major role in the confinement of masonry [41]. Hence, high tensile strength of ECC and FAECC also contributes to the control of out-ofplane behaviour and spalling of damaged masonry units. e stress-strain graph obtained from the experimental result was used to calculate the modulus of elasticity in addition to compressive strength. Finding the slope of the stress-strain curve determines the modulus of elasticity. Also, the strain values along the transverse and longitudinal direction were recorded and hence the ratio of transverse strain to longitudinal strain gives the Poisson ratio. It is observed that the masonry units strengthened by ECC and FAECC possess almost similar modulus of elasticity, which is higher than that of masonry units strengthened by normal mortar. Also, the Poisson ratio for all the three specimens matched with the previous studies conducted on clay brick masonry walls [42]. e acquired compressive strength of masonry walls strengthened by ECC and FAECC is also noted to be within the range of strengths obtained in previous studies made on masonry retrofitting [14,33].

Numerical Analysis
All the elastic and plastic properties were collected from the experimental test involved and are further used for numerical modelling. A masonry model is made similar to the specimens used for the experimental tests and properties were loaded. Since the material is brittle and possess interactions, a Concrete Damage Plasticity (CDP) model is used along with cohesive surface parameters. Concrete Damage Plasticity (CDP) model, which is typically favoured for brittle materials, may be used to simulate the nonlinear behaviour of masonry units in Abaqus [43]. To characterise the inelastic behaviour of the brittle material, the Concrete Damage Plasticity model combines the ideas of isotropic damaged elasticity with isotropic tensile and compressive plasticity. e failure mode of this model depicts that the crushing in compression and cracks in tension. Figure 8 shows the response of concrete under axial compression and tension, illustrated by concrete damage plasticity. Figure 8(a) states that when a concrete material undergoes tension, it is subjected to a linearly elastic deformation until ultimate stress σ tu . Microcracks begin to originate once the material reaches the failure stress. e softening stress-strain response beyond ultimate stress indicates further dispersion of microcracks, which promotes strain concentration within the concrete material. Figure 8(b) shows the stress-strain graph of a concrete material under uniaxial compression. e material under compression undergoes a linear response until yield point σ co and beyond the ultimate stress σ cu , the strain begins to soften gradually [44].

Model Input.
e materials properties mentioned in Table 5 were used to model the masonry units. e mass density, Young's modulus, Poisson's ratio, and the dilation  angle were collected from the experimental data. e value of eccentricity, ratio of initial biaxial compressive stress to initial uniaxial compressive stress(f bo /f co ), and the ratio of second stress invariant on the tensile meridian to that on the compressive meridian (K) are the default values provided by Duval [44]. e compressive and tensile behaviour of the CDP model is derived from the experimental data of masonry and strengthening materials. As per Abaqus theory manual, the tension properties have to be applied from the ultimate point to the softening point and the compression properties have to be applied from the elastic limit to the softening point [44]. e stress-strain graphs of masonry compression and tension are shown in Figures 9(a) and 9(b). Also, the compressive and tensile stress-strain graph of ECC and FAECC are illustrated in Figures 10(a) and 10(b). Tables 6  and 7 list the values of stress-strain and damage statistics that were used in this model. e brick and the strengthening materials were connected by providing a hard contact and coefficient of friction values for each material. e coefficient of friction was calculated using the normal stress and shear stress of masonry units with mortar, ECC, and FAECC in accordance with Binda et al. [45].

Model Output.
e stress-strain graph generated from the experimental tests and numerical analysis is shown in Figure 11. It shows that the maximum stresses obtained in numerical analysis has a difference of 8% for masonry units strengthened by mortar and 10% for masonry units strengthened by ECC and FAECC compared to the experimental results. e maximum stresses for all the three samples occur at the bottom edges, which then further leads to the centre of the masonry prism. It is also observed that the percentage strain for the strengthened masonry prism is highly improved as the stress induced in it was further increased. e strain occurring on the masonry prism strengthened by mortar, ECC, and FAECC is shown in Figures 12(a)-12(c). e maximum strain occurring at the centre of the masonry units strengthened by normal mortar is observed and as a result vertical cracks are formed at the centre of the prism. In case of masonry units strengthened by ECC and FAECC, the strain hardens on the portion where the strengthening is made and hence cracking is controlled. e strain percentage is predominant on the masonry joints and as a result cracks originate from mortar bed, which is comparable to what was discovered experimentally.

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Figures 13(a)-13(c) show the deformed shape of the masonry prism strengthened by mortar, ECC, and FAECC under compression in which the separation of joints and the retrofitting materials were clearly observed. When load is gradually applied, cracks originate from the mortar bed joints and thereafter lead to the separation of the strengthened layers, which is comparable to the failure found in experimental specimen. is clearly highlights the importance of bond strength in masonry prism and hence the increase in bond strength of the bed joint mortar will decrease the rate of deformation of masonry walls.

Conclusions
e primary goal of this research is to look at the feasibility of using ECC to strengthen masonry walls instead of standard retrofitting materials. For this purpose, an experimental and numerical analysis is carried out to determine the effectiveness of ECC on masonry walls. For mortar, ECC, and FAECC, tests including indirect tensile strength and compressive strength were performed. e bond strength of these cementitious materials on brick masonry walls is then determined using a triplet test. Following that, three sets of weak masonry units were chosen and strengthened using mortar, ECC and ECC with 40% fly ash on both the faces of the masonry units and is tested under uniaxial loading. en, the material properties were collected from the experimental tests and a numerical analysis is made using Abaqus software. Based on the experimental and numerical investigation of the masonry units and strengthening material, the following conclusions are made.
(i) After 90 days of curing, ECC has a compressive strength 20% higher than conventional mortar and 17% higher than FAECC. ough ECC with fly ash showed lesser compressive strength after 28 days, the strength improved when the curing time was extended to 90 days. e indirect tensile strengths of ECC and FAECC were almost similar at the end of 90 days, which is 90% higher than the normal mortar. (ii) When compared to conventional mortar, ECC and FAECC achieve higher shear bond strength. At the end of 28 and 90 days, the shear bond strengths of ECC and FAECC were nearly identical. Microcrack behaviour is seen, which is caused by polypropylene and steel fibres holding the cement matrix; as a result, an increase in shear capacity is observed. (iii) e compressive strength of masonry units strengthened by ECC is higher than the masonry units strengthened by normal mortar and FAECC. But the crack patterns of masonry units strengthened by ECC and FAECC were almost similar. e spalling of the damaged masonry unit is controlled by ECC and FACC, which is due to its higher strain hardening capacity. When axial load is applied on masonry walls, the faces of the wall undergo tension, which pulls the strengthening material out of the plane. Hence, bond strength and tensile strength are important parameters to be considered when ECC and FAECC are used as a strengthening material on masonry walls. (iv) e stresses obtained from numerical analysis shows a difference of 8% for masonry units strengthened by normal mortar and 10% for masonry units strengthened by ECC and FAECC compared to the experimental results. Using a simple method, the micromodelling approach was successful in producing accurate results from masonry assemblages. (v) e crack patterns observed on experimental samples matched with those specimens speculated by the Finite Element models quite well. e experimental and computational stress distribution, failure load, and displacement results are also in good agreement. (vi) ough ECC and FAECC exhibit almost similar results in both experimental and numerical analysis, FAECC with 40% fly ash is highly recommended for masonry strengthening purpose due to its less heat of hydration.

Data Availability
e data used to support this study are provided within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper.