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Article

Short Jute Fiber Reinforced Cement Mortar for Out-of-Plane Strengthening of Masonry Prisms

by
Lidianne do Nascimento Farias
1,
Paulo Roberto Lopes Lima
1,2 and
Romildo Dias Toledo Filho
1,*
1
NUMATS/Civil Engineering Department/COPPE, Federal University of Rio de Janeiro, P.O. Box 68506, Rio de Janeiro 21945-970, RJ, Brazil
2
Post-Graduate Program of Civil e Environmental Engineering, State University of Feira de Santana, Av. Transnordestina, SN, Novo Horizonte, Feira de Santana 44036-900, BA, Brazil
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(13), 5675; https://doi.org/10.3390/su16135675
Submission received: 24 April 2024 / Revised: 24 June 2024 / Accepted: 28 June 2024 / Published: 3 July 2024
(This article belongs to the Special Issue Recent Developments on the Use of Sustainable Retrofitting and Construction Materials)
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Abstract

:
The retrofitting process contributes to the sustainability of the construction sector, since adopting measures to increase the lifespan of buildings reduces the need for new constructions. However, many of the materials used in this process come from nonrenewable sources and require significant water and energy consumption for production. The aim of this study is to assess the viability of using a more environmentally friendly mortar coating reinforced with short jute fibers (SJFRM) to reinforce ceramic brick masonry walls. Both coated and uncoated prisms were subjected to compression and flexural tests under two-point (line) out-of-plane loading. The reinforcement layer comprised mortar without fibers and mortars reinforced with jute fibers at levels of 2% and 4%, with lengths of 20 mm and 40 mm. Physical and mechanical tests were conducted to evaluate the properties of SJFRM in both fresh and hardened states. Results indicate that the compressive and flexural strengths were enhanced with SJFRM reinforcement due to alterations in the failure mode of the prisms. The fibers impede crack propagation in the reinforcement layer, enabling better redistribution of internal stresses in the prisms. This results in an increase of 6 to 9 times in stiffness under direct compression and up to 42 times in toughness under flexion in the prisms reinforced with SJFRM when compared to uncoated prisms.

1. Introduction

The preservation of ancient buildings represents a great challenge for society due to the significant historical and cultural value of many of them and the technical need to ensure comfort and safety for their users. Built with stone or ceramic bricks, before the advent of reinforced concrete structures, ancient buildings rely on masonry to resist acting forces but are highly susceptible to cracking due to the action of dynamic forces, drying shrinkage, or deformation of the supporting foundations. Additionally, masonry experiences a reduction in material strength due to gradual loss of durability caused by weathering over the years. As a result, the retrofitting of historic buildings has increased in the last decade to extend their lifespan [1]. In this aspect, the retrofitting process contributes to the sustainability of the construction sector, since adopting measures to increase the lifespan of buildings reduces the need for new constructions and results in a decrease in the consumption of nonrenewable materials, water, and energy associated with the construction process. In addition, increasing the service life can also reduce the amount of construction and demolition waste generated.
In a building rehabilitation process, the mechanical knowledge of masonry plays an important role in the maintenance and preservation of old buildings [2]. Through the analysis of the structure, the restoration of load-bearing capacity can be achieved by applying a layer of textile-reinforced mortar (TRM), which is a technique that has been widely adopted due to its good physical compatibility and adhesion with different masonry substrates. This property reduces the risk of delamination, allowing TRM to overcome one of the main limitations of using fiber reinforced polymer (FRP), which is another technique commonly applied in masonry reinforcement [3]. TRM is composed of a cement mortar chemically modified to achieve high ductility, reinforced by a fiber mesh, which can be made of glass [4], steel [5], carbon [6], or basalt [7]. Due to its greater compatibility with the system to be retrofitted and the excellent tensile strength of the fabric, TRM application can provide greater load-bearing capacity for masonry both for in-plane and out-of-plane loads, in static or dynamic situations [8].
The procedure for applying TRM systems to masonry, on the other hand, requires additional care to ensure that the fabric is perfectly positioned over the entire surface, without wrinkles or folds, which requires the use of screws and anchoring pins or the use of additional labor for manual positioning of the fabric during the application of the covering mortar. An alternative process is the use of short fiber-reinforced mortar (SFRM) for masonry retrofitting, where the reinforcement material is applied directly to the wall using traditional plastering techniques. In addition to ease of application, the distribution of short fibers in various directions allows for control of micro-cracks in multiple directions. Sevil [9] demonstrated the efficiency of using short steel-fiber-reinforced mortars to increase strength and hardness in hollow brick walls. The use of engineering composites reinforced with short PVA fibers (ECC) as reinforcement for masonry was evaluated by [10], who found that the ECC layer increases lateral resistance and prevents severe local damage while partially maintaining masonry integrity. A comparative study among the different solutions was conducted by Cheng et al. [11], who identified that textile-reinforced concrete (TRC) and ECC layers have good co-ordination with masonry walls, while FRP had poor synergistic performance. As a result, the shear improvement effect of ECC and TRC strengthening was comparable, with TRC slightly smaller than ECC. However, when the specimen was finally destroyed, the crack width of the ECC layer was larger than that of the TRC layer.
In addition to the pursuit of more optimized solutions for retrofitting, a need in the construction industry is the implementation of more sustainable solutions for masonry reinforcement. Within this context, several researchers have evaluated the replacement of manufactured fiber reinforcement with vegetable fibers in TRC. Sisal [12], flax [13], jute [14], and hemp [15] yarns and fabrics have been assessed. The results indicate that the use of vegetable fabrics can improve strength, deformability, and diagonal compression behavior when compared to unreinforced walls. In addition to providing suitable reinforcement for masonry, carbon footprint assessments indicate that vegetable fibers, such as jute fiber, are more environmentally friendly than manufactured fibers. While steel, glass, and polypropylene fibers have greenhouse gas emissions on the order of 2206 kg CO2eq/t, 2380 kg CO2eq/t, and 3430 kg CO2eq/t, respectively [16,17], vegetable fibers have emission values ranging from 182 to 580 kg CO2eq/kg [16,18,19], resulting in more sustainable retrofitting systems.
The potential use of composites reinforced with short vegetable fibers as reinforcement for masonry was identified by [20] based on mechanical results of composites reinforced with sisal fibers. Indeed, the introduction of short vegetable fibers in cement-based composites promotes an increase in flexural strength and toughness, enabling their application in structural construction elements [21,22]. However, the mechanical behavior of SFRM varies according to the type of fiber used, fiber content, and fiber length [23,24], since the presence of fibers affects the workability of the material in the fresh state and the internal distribution of mechanical stresses. As a result, studies need to be conducted to evaluate the performance of cementitious composites reinforced with short vegetable fibers as a retrofit element for historic masonry. Additionally, the natural chemical incompatibility between vegetable fiber and cementitious matrix requires treatments to be carried out on the fibers and matrix to ensure the durability of the retrofitting system.
The treatment of vegetable fibers has been carried out with the aim of reducing water absorption and dimensional variation due to moisture fluctuations, which, during composite use, can lead to loss of fiber–matrix adhesion and reduction in mechanical performance. The modification of jute fibers through hornification or alkaline treatment has resulted in reduced water absorption and dimensional variation, increased mechanical strength, and improved adhesion with cementitious matrices [25,26,27]. The modification of the cementitious matrix for use in composites with vegetable fibers aims to reduce fiber mineralization [28], which can be achieved by using pozzolanic mineral additives, replacing part of the cement. This results in a cementitious matrix free of calcium hydroxide, with higher mechanical strength and greater durability [29].
The objective of this work is to evaluate the mechanical properties of ceramic brick masonry reinforced with SJFRM. To ensure the durability of the reinforcement system, the jute fibers underwent alkaline treatment with calcium hydroxide, and the cement matrix was produced with the addition of fly ash and metakaolin in replacement of part of the cement. Mortars reinforced with jute fiber contents of 2%, 3%, and 4%, with lengths of 20 mm and 40 mm, were produced and evaluated for physical and mechanical properties before application as masonry reinforcement. Solid ceramic brick wall specimens were reinforced with SJFRM and subjected to compression and flexural tests.

2. Materials and Methods

2.1. Jute Fibers

Jute fibers are extracted from the outer bark of the Corchorus plant stem, found near rivers and in hot and humid regions, and undergo a process of cutting, retting, defibration, and drying before their use. The jute fibers have an irregularly shaped cross-section and are composed of several individual fibrocells linked by a middle lamella and containing a vascular lumen inside, as shown in Figure 1. Due to their high cellulose content (ranging from 61% to 71%), jute fibers exhibit tensile strength of around 250–350 MPa and modulus of elasticity between 25–40 GPa [26,30]. The jute fibers used in the study were collected in the City of Coari, State of Amazonas, Brazil.
Natural fibers have water absorption on the order of 200% of their mass, resulting in dimensional variation due to drying and loss of adhesion with the hardened cement matrix. To reduce the water absorption of jute fiber and improve the fiber–matrix interface, an alkaline treatment was performed on the fibers following the method of [26]; the fibers were immersed in a 0.73% Ca(OH)2 solution at a controlled temperature of 23 °C for 50 min. Subsequently, the dried fibers were exposed to air in a forced air flow chamber at a temperature of 40 °C for 72 h. The water absorption result of natural jute fibers after 72 h of immersion was 272.74%, while, for treated fibers, it was 224.11%. There was a 17% decrease in water absorption with the alkaline treatment of the fibers. The use of treatments to improve the characteristics of natural fibers is carried out by [26,27]. Since the aim of this work is to use reinforcement dispersed randomly in the matrix, the fibers were cut with a metal blade into lengths of 20 mm and 40 mm (Figure 2).

2.2. Short Jute-Fiber-Reinforced Mortars (SJFRM)

To address the chemical incompatibility between the cementitious matrix and jute fiber while improving the workability of the fresh mixture, a composite blend was formulated. This blend consisted of Portland cement from the LafargeHolcim Brasil industry (Barroso, Brazil) composed of filler (type CPII-F-32) (specific mass: 3.05 g/cm3), metakaolin from Brazilian industry—Metacaulim do Brasil Indústria e Comércio Ltda, Jundiaí, Brazil (specific mass: 2.71 g/cm3), and fly ash (FA) from the Brazilian industry Pozo Fly®, Capivari de Baixo, Brazil (specific mass: 2.15 g/cm3). It was used in the production of both the reference mortars (REF) and SJFRM, with proportions of 50%, 30%, and 20%, respectively. The mortar matrix used was based on the mix design of Fidelis et al. [30], for a matrix with 50% cement and changes in the percentages of mineral additions. Chemical additives were incorporated to ensure the homogeneity of the mixture, even with a higher fiber content: (i) superplasticizer additives (SP) based on polycarboxylate polymers, specifically MasterGlenium 51, an ether-type polycarboxylic ether from BASF Brasil (São Paulo, Brazil), with a specific mass of 1.1 g/cm3; (ii) viscosity-modifying agent (VMA), Rheomac UW 410 from Master Builders Solutions (produced by Sika®, São Paulo, Brazil), with a specific mass of 0.31 g/cm3.
The mortar matrix was produced with a blend-to-sand ratio of 1:2 by mass and a water-blend ratio of 0.6. River sand with a particle size passing through a sieve #600 microns was used, with a specific mass of 2.67 g/cm3 and a fineness modulus equal to 1. Reference mortar (REF) and mortars reinforced with jute contents of 2%, 3%, and 4%, by mass, were produced, with the proportions between components shown in Table 1. Each composite was produced with two different fiber lengths, 20 and 40 mm, resulting in a total of 7 mixtures. The nomenclature of the mixtures is presented in Table 2.
For the preparation of the mortars, a mixer with a capacity of 20 L and equipped with 3 speeds (125, 220, and 450 rpm) was used. The mortar was produced by homogenizing all the binding materials of the matrix (cement, metakaolin, and fly ash), followed by the addition of all the water together with the superplasticizer. Subsequently, the total amount of sand in the mixture was added. In the case of mortar reinforced with jute fibers, the fibers were added sequentially and slowly to avoid the formation of lumps in the mixture, which could lead to nonhomogeneous dispersion of the fibers in the matrix; the viscosity-modifying agent was added at the end to improve the fiber–matrix cohesion.
The characterization of the mortars in their fresh state included the standard consistency test, conducted according to NBR 13276 [31], as well as the determination of bulk density and air content, carried out in accordance with NBR 13278 [32].
The physical properties of the mortars, including water absorption, void index, and bulk density, were determined according to the regulatory standard NBR 9778 [33] after 28 days of hydration. Four cylindrical specimens (50 mm × 100 mm) were cast for each mortar mixture.
The mechanical behavior under compression was determined using cylindrical specimens with a diameter of 50 mm and a height of 100 mm for each mixture, after 28 days, using a Shimadzu testing machine (Kyoto, Japan) with a capacity of 1000 kN, equipped with a 100 kN load cell, and a test speed of 0.3 mm/min. Displacements were measured using two LVDTs attached to the central region of the specimen. The compressive strength was obtained with maximum load and the modulus of elasticity was calculated as the slope of the line connecting the stress points of compressive strength at 0.5 MPa and 40% of the ultimate strength of the mortar.
A four-point bending test was conducted on specimens measuring 400 mm × 80 mm × 14 mm, with a free span of 300 mm and a distance between loads of 100 mm. A Shimadzu machine (Kyoto, Japan) with a 100 kN capacity and a test speed of 0.3 mm/min was used. From the test, the flexural strength and toughness of the mixtures were determined. Toughness corresponds to the area under the load–displacement curve up to displacements corresponding to the onset of cracking and, for the SJFRM, up to displacements of 1 mm, 2 mm, 4 mm, 6 mm, and 10 mm.
The evaluation of the fracture surface of the samples broken under flexure was conducted using images at 100× magnification obtained with a Nikon Instruments NI-150 stereoscopic microscope (Tokyo, Japan) equipped with C-W10xB/22 lenses.
Figure 3 presents the flowchart of the study characteristics.

2.3. Retrofitting of Walls

To assess the effect of applying SJFRM reinforcement on solid brick walls, two types of prismatic specimens were produced: for compression testing, the prisms consisted of five bricks with final dimensions of 190 × 300 × 130 mm, as shown in Figure 4a, and, for flexural testing, the prisms were made with six bricks with final dimensions of 190 × 400 × 130 mm, as shown in Figure 4b. A 20 mm layer of SJFRM was applied to each face of the sample. Samples without the application of a coating layer and samples with a reference mortar coating (without fibers) were also produced.
To produce the walls, solid ceramic bricks with dimensions of 190 mm (length) × 50 mm (height) × 90 mm (width) were used, with a mean compressive strength evaluated at 8.12 MPa ± 1.31 MPa, according to NBR 7170 [34]. The joints between the bricks were filled with mortar composed of cement: lime/sand/water in a ratio of 1:0.5:2, by mass, which exhibited an average compressive strength of 6.40 ± 0.4 MPa at 28 days.
The application of the reinforcement layer on the prisms was carried out 14 days after production, using a manual application method. The mortar was applied to the prism’s surface using a trowel, as shown in Figure 5. The application was performed from top to bottom and, after covering the entire surface, a wooden ruler was used to smooth the surface. This retrofitting procedure is easily applied as it replicates the system already used in various countries for coating ceramic brick walls. At the end of the coating application, the retrofitted masonry prisms were kept curing until they reached 28 days of age before being tested.
Based on the evaluation of the mechanical behavior of SJFRM, the reference mixtures and the mixtures 2J20, 2J40, 4J20, and 4J40 were selected for reinforcing the prisms, with 2% and 4% jute fibers, at lengths of 20 and 40 mm.
Mechanical tests on retrofitted masonry walls were conducted to assess the effect of SJFRM on the load–displacement behavior under compression and flexural loads. The monotonic uniaxial compression test was conducted in accordance with ASTM C1314-16 [35], as shown in Figure 6a. A Shimadzu testing machine (Kyoto, Japan) with a capacity of 1000 kN, equipped with a 200 kN load cell, was used, and the test speed was 0.3 mm/min. Vertical displacement was measured by two LVDTs positioned at the center of the sample, with a gauge length of 100 mm. Upper and lower steel plates were installed to ensure uniform stress distribution on the top surface of the prism.
The prism bending test was carried out according to ASTM E518/E518M-15 [36], using a 4-point configuration, as depicted in Figure 6b. The span between supports was 360 mm and the distance between loads was 120 mm. A Shimadzu testing machine (Kyoto, Japan) with a load capacity of 100 kN was used, with a test speed of 0.3 mm/min. An LVDT positioned under the samples was used to determine transverse displacements.
For each test, 12 masonry prisms were tested, with two samples for each type of applied coating.

3. Results

3.1. SJFRM Assessement

3.1.1. Properties in the Fresh State

Figure 7 presents the consistency results of the reference mortar and SJFRM, as well as the variation in this property concerning the reference mortar. A modification in the spread with the increase in fiber content and fiber length can be observed, with a reduction of up to 24%. For mortars in the fresh state, the presence of fibers alters the rheology of the fluid–fiber systems; an increase in fiber volume tends to create a locking effect in the mixture, resulting in increased flow disruption and, consequently, viscosity. Conversely, fibers with larger dimensions cause greater flow disturbances in the matrix and therefore exert a greater influence on the viscosity of the system. The decrease in workability has been attributed to the surface area, size, and shape of the fibers in relation to the other constituent particles of the mortars [37]. However, the results obtained indicate that all mortars showed a spread greater than 200 mm, which is sufficient for them to be applied to the walls using the manual method.
As a result of the modification of the rheological flow, there is a greater air incorporation during the mixing and casting process of the SJFRM, as can be observed in Figure 8a. The increase in fiber content and fiber length used results in an increase of 86% to 264% in the air incorporation content of the SJFRM in the fresh state, which reduces the density by up to 19% (Figure 8b) when compared to the mortar without fibers.

3.1.2. Physical Properties

The porosity levels of the mortars in their hardened state, ranging from 15% to 30%, as depicted in Figure 9, confirm the impact of air incorporation in the mixtures due to increased fiber content and length. However, a direct correlation is not evident, as the rise in fiber content from 2% to 3% showed no substantial change, maintaining levels between 51% and 55% compared to the reference mortar. Conversely, a significant porosity increase of up to 107% was observed for a 4% fiber content, particularly when 40 mm fibers were included.
The higher occurrence of voids in cementitious materials is undesirable for durability since these pores can act as pathways for the migration of water and harmful ions into the mixture. As shown in Figure 10a, there is a proportional increase in water absorption with the increase in fiber content and length. Besides contributing to void incorporation in the cementitious matrix, jute fibers may enhance water absorption due to the presence of vascular lumens (depicted in Figure 1), which serve as capillary conduits for internal water movement.
The inverse relationship between porosity and density was confirmed in SJFRM, with higher fiber contents and length resulting in up to an 8% reduction in density compared to fiber-free mortar, as shown in Figure 10b. In terms of retrofitting, low-density mortar typically exhibits better thermal insulation properties compared to traditional mortar. This can contribute to energy savings in buildings by reducing heat transfer through walls.

3.1.3. Mechanical Properties

Table 3 presents the results of mechanical resistance and modulus of elasticity obtained in the direct compression test, along with the results of first crack stress (σf) and maximum post-crack stress (σu) obtained in the flexural test of the mortar.
The addition of vegetable fibers to the mixtures leads to a decrease in compressive strength, which is attributed to the increased porosity of the mixtures, as depicted in Figure 11. There exists a well-established inverse correlation between mechanical strength and the presence of voids in solid materials. This is because pores tend to concentrate stresses and act as initiators of cracks that propagate within the specimen. For the addition of 2% and 4% of jute fibers, with lengths of 20 mm and 40 mm, there were reductions of 25–27% and 30–33% in compressive strength, respectively, compared to the reference mortars. As for the modulus of elasticity, reductions of 22–40% and 19–38% were observed for the same mixtures. A similar trend has been observed by [38,39].
The compressive strength values of SJFRM ranged from 12.27 MPa to 14.45 MPa. Based on the BS EN 998-1 [40] standard, these mortars can be classified as CS IV because they exhibit compressive strengths greater than 6 MPa. This qualifies the use of all SJFRM mixtures as masonry coatings.
Figure 12 presents the mechanical behavior of SJFRM under flexural loading. The load–displacement curves initially exhibit a linear elastic region mainly governed by the mortar matrix until the appearance of the first crack. For the reference mortar, the first crack stress (σf) represents sample failure but, in SJFRM, the presence of fibers inhibits rapid crack propagation and prevents sudden sample failure. However, due to the fiber’s effect on increasing porosity, the addition of fibers results in a maximum reduction of 25% in the flexural strength of SJFRM compared to the reference mortar.
With the increase in displacement rate, stresses are transferred between the faces of the crack through the fiber and, after an initial drop in strength, there is an increase in post-crack tension up to a limit value (σfu) beyond which the fibers are gradually pulled out from the matrix in a pull-out process. Figure 13 shows the distribution of fibers on the sample surface and the “fiber bridging” process of the crack. As a result, there is an increase in the composite’s rupture deformations and its energy absorption capacity.
Figure 14 depicts the variation in toughness of SJFRM for different vertical displacement values. An increase in toughness proportional to the fiber content is observed, with the effect of fiber length on this property being significant only for a fiber content of 2%.

3.2. Retrofitting of Ceramic Brick Prisms

3.2.1. Compressive Behavior

The load–displacement curves of masonry prisms under compression are presented in Figure 15. Prisms were evaluated without a coating layer (Figure 15a), with a layer of reference mortar (Figure 15b), and with layers of SJFRM containing 20 mm fibers (Figure 15c,d) and 40 mm fibers (Figure 15e,f).
After an initial accommodation, the mechanical behavior is characterized by an approximately linear load–displacement relationship until the emergence of the first vertical cracks for stress values of approximately 50–85% of the total compressive strength. Similar behavior was observed by [41]. After the appearance of cracks, the behavior becomes nonlinear, with an increase in load up to a maximum peak value followed by a gradual loss of stiffness until failure.
Table 4 presents the experimental results of the compression test on the prisms obtained from the load–displacement curves. The compressive strength of the samples with a reinforcement layer, using either reference mortar or SJFRM, was determined using the cross-sectional area of the bare masonry prism following the indications of [42,43] for a more consistent comparison between the samples. The stiffness of the prisms was calculated considering the initial slope of the stress–strain curve up to a stress corresponding to 40% of the maximum stress. The sketches of the mode of failure of the samples, according to ASTM C1314-16 [35], are also presented in Table 4.
It is observed that the application of the coating resulted in an increase in the load-bearing capacity of the prisms, with increments in the maximum stress ranging from 6.93% for prisms with the 2J20 layer to 53.39% for those with the 4J40 layer compared to the uncoated prism. Increasing the fiber content and length led to an increment in the compression of the prisms, resulting in a change in the failure mode, as depicted in Figure 16. However, only the prism with the 4J40 layer exhibited greater strength than the prism reinforced with reference mortar.
For the uncoated prism, two types of failure were observed: cracking between the brick and the horizontal mortar joints, followed by global brick rupture (Figure 16a). These failure modes are classified by ASTM C1314 [35] as semi-conical break and shear break. Uncoated ceramic brick prisms typically exhibit vertical cracking failure for both solid brick masonry [41] and hollow brick masonry [44]. In prisms reinforced with reference mortar, failure occurs due to the detachment of the coating, resulting in a brittle fracture (Figure 16b). The observed failure modes include semi-conical break and face shell separation. After the first crack appears in the coating, loads are transferred to the bricks until the maximum stress is reached, leading to brick rupture.
The rupture of prisms with SJFRM occurs gradually due to the appearance of various mechanisms. Since the coating is ductile due to the action of the fibers, there is the maintenance of residual strength in the reinforcement layer after significant deformations following the emergence of the first crack (Figure 16c), and stresses become concentrated in the brick. There is detachment of the coating at the top of the sample and transfer of stresses to the bricks, which begin to exhibit vertical cracks.
In some samples reinforced with SJFRM, horizontal cracking occurs in the middle of the prism, caused by localized tensile stress [45], which could be characterized as the rupture mode known as tension break. However, due to the presence of fibers stitching the cracks, the coating does not rupture but instead undergoes lateral expansion of the bricks and reinforcement mortar (Figure 17). Due to excessive deformation of the prism, failure occurs through the shearing of the bricks and horizontal joints.
An important effect of the change in the failure mode of reinforced prisms is the modification of stiffness, with an increase of 6 to 9 times compared to the uncoated prism, as shown in Table 4. The presence of the reinforcement layer causes confinement of the brick masonry and modification of internal deformations. An increase in axial stiffness of masonry with the application of reinforcement coated with short steel fibers was observed by [41,43], resulting in improved resistance to seismic forces or lateral forces acting on building walls.
There is a strong correlation between stiffness (Ep) and compressive strength (fp) of prisms, which is assumed to be linear by some standards, with the Ep/fp ratio ranging between 700 and 1000 [46]. Figure 18 illustrates that the Ep/fp ratio for the coated prisms was calculated as 817.62, thereby confirming the applicability of normative guidelines for prisms with SJFRM coatings.

3.2.2. Flexural Behavior

Figure 19 illustrates the stress–deflection curves for both the unreinforced prism and the prism reinforced with reference mortar without fibers. In both cases, the prisms initially displayed linear elastic behavior until reaching their maximum load, followed by sudden brittle rupture.
The mechanical behavior under bending of the SJFRM-reinforced prisms is depicted in Figure 20. The stress–deflection curves exhibit a linear segment until the first crack appears in the lower reinforcement layer. For prisms reinforced with 2% fibers, the first crack marks the maximum stress, while prisms containing 4% fiber show a nonlinear segment in the curve after the initial crack, with stress increasing until reaching the maximum. Unlike the unreinforced SJFRM prisms, these reinforced ones display ductile behavior after cracking, maintaining residual stress before eventual rupture, which occurs at higher deflections.
Table 5 displays the values for maximum flexural strength based on ASTM E518/E518M-15 [36], ultimate deflection, and toughness, calculated as the area under the curve up to the ultimate deflection, for all samples.
The uncoated prism experienced a peak stress of 0.19 MPa. Upon adding the reference coating, the peak stress rose to 0.71 MPa, indicating an almost fourfold increase in flexural strength. This substantial strength enhancement is attributed to the prism’s failure mode. In the initial scenario, failure occurs due to the adhesion loss between the brick and the mortar joint, as depicted in Figure 21a. Yu and Park [43] investigated the impact of coating addition on the bond strength between the masonry layer and the ASFRM overlay. They observed a strength increase when applying one or two layers of coating, attributed to alterations in internal stress distribution. In prisms lacking a coating layer, flexural failure tends to propagate throughout the entire mortar joint [47]. In reinforced prisms, flexural loads initially lead to an increase in tensile stress within the lower mortar layer. It is only after this stress reaches the tensile strength of the reinforcement that a crack appears and propagates to the coating–brick interface, activating the adhesive stresses. Figure 21b illustrates the failure mode of the mortar-reinforced prism without fibers.
The flexural strength of the prisms reinforced with SJFRM was higher than that of the other prisms, with an increase of 5–7 times compared to the prism without any type of coating. Studies on masonry reinforcement with basalt fiber mesh (BFRCM) conducted by Padalu et al. [48] resulted in a flexural strength increase ranging from 2 to 4 times compared to unreinforced masonry. This indicates that the use of SJFRM, despite using short fibers, provides an effective contribution to flexural reinforcement.
Regarding the prism with reference mortar, the use of SJFRM resulted in a flexural strength increase ranging from 42.25% to 92.96%, depending on the fibrous reinforcement configuration. Increasing the fiber content led to an increase in flexural strength. However, for mortars with the same fiber content, there was a reduction in this strength with increasing fiber length, which is proportional to the behavior of SJFRM under flexure. The results confirm the potential contribution of short fibers in mortars for masonry retrofitting, as already identified by [20].
The inclusion of fibers delays the crack propagation in the coating situated at the bottom of the prism under flexion and prevents its brittle rupture. The ultimate deflection increased from 0.05 mm in the uncoated prisms to approximately 9 mm in the prisms with SJFRM. This increase in deformation results in a higher energy absorption capacity of the prism, as can be observed in Table 5. Compared to the prism without coating, the introduction of the reference coating results in a 4.75-fold increase in toughness. The introduction of short jute fiber, on the other hand, with its ability to transfer stresses in the region of the crack, increases toughness by up to 42 times compared to the uncoated prism and up to 9 times compared to the prism with the reference mortar. Figure 21c illustrates the rupture pattern of prisms reinforced with SJFRM, where fibers are seen stitching the open crack. In addition to the detachment rupture at the brick interface, some prisms experienced ceramic brick rupture, indicating a shift in internal stress distribution due to the presence of fibers in the reinforcement layer.

4. Conclusions

This study evaluates the effect of applying mortar reinforced with short jute fibers as reinforcement for solid ceramic brick prisms subjected to compression and flexion.
The evaluation of SJFRM with jute fiber contents of 2%, 3%, and 4% and lengths of 20 mm and 40 mm allowed us to conclude that the addition of fibers causes a modification in workability in the fresh state. It increases the air incorporation content and reduces the density by up to 19% when compared to mortar without fibers. This implies an increase in the air-incorporated content in the hardened state, notably for mixtures with 4% fiber content and 40 mm length.
The results of the mechanical properties of SJFRM in the hardened state indicate that, for the addition of 2% and 4% of jute fibers with lengths of 20 mm and 40 mm, there were reductions of up to 33% in compressive strength, respectively, compared to the reference mortars. Regarding the modulus of elasticity, reductions of up to 38% were observed for the same mixtures. However, under flexural loading, the presence of fibers allowed an increase in ultimate deformation, modifying the failure mode of the mortar from brittle to ductile. An increase in toughness proportional to the fiber content is observed, with the effect of fiber length on this property being significant only for a fiber content of 2%.
The mechanical behavior of prisms under compression was modified with the application of reinforcement using SJFRM, resulting in an increase in the load-bearing capacity and stiffness of the prisms. While the failure mode of unreinforced prisms is characterized by the emergence of vertical cracks, the addition of SJFRM results in the emergence of new failure modes due to crack control in the coating layer.
Under out-of-plane loading, the base prisms initially exhibited brittle failure characterized by the loss of adhesion between the brick-and-mortar joint. However, the addition of SJFRM reinforcement allows for the maintenance of residual stress after coating cracking, leading to increased flexural strength and toughness.
These modifications in the mechanical behavior of the prisms with the introduction of SJFRM have proven particularly advantageous for walls subjected to dynamic loads, highlighting the potential of SJFRM as a reinforcement material for walls in seismic regions.
Future research on the subject of study can be suggested by carrying out mechanical tests on larger masonry panels and carrying out a thermal performance study of masonry reinforced with jute fibers.

Author Contributions

Conceptualization, P.R.L.L. and R.D.T.F.; methodology, L.d.N.F., P.R.L.L., and R.D.T.F.; formal analysis, P.R.L.L. and L.d.N.F.; investigation, L.d.N.F.; resources, R.D.T.F.; data curation, L.d.N.F.; writing—original draft preparation, L.d.N.F.; writing—review and editing, P.R.L.L. and, L.d.N.F.; supervision, P.R.L.L. and R.D.T.F.; project administration, R.D.T.F.; funding acquisition, R.D.T.F. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brasil (CAPES)—Finance Code 001. The study is also funded by Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brasil (CNPQ)—Postdoctoral Scholarship for second author (Grant number 102164/2022-3).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data supporting these findings are available within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. SEM images of jute fibers (cross-section).
Figure 1. SEM images of jute fibers (cross-section).
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Figure 2. Length of jute fiber: (a) 20 mm, (b) 40 mm.
Figure 2. Length of jute fiber: (a) 20 mm, (b) 40 mm.
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Figure 3. Flowchart of the study characteristics.
Figure 3. Flowchart of the study characteristics.
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Figure 4. Dimensions of samples: (a) compression, (b) flexural testing.
Figure 4. Dimensions of samples: (a) compression, (b) flexural testing.
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Figure 5. Production procedure for masonry prisms.
Figure 5. Production procedure for masonry prisms.
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Figure 6. Set-up for mechanical tests: (a) compression, (b) flexural testing.
Figure 6. Set-up for mechanical tests: (a) compression, (b) flexural testing.
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Figure 7. Effect of fiber content on workability.
Figure 7. Effect of fiber content on workability.
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Figure 8. Effect of fiber content: (a) air incorporation, (b) density mortar in the fresh state.
Figure 8. Effect of fiber content: (a) air incorporation, (b) density mortar in the fresh state.
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Figure 9. Porosity of reference mortar and SJFRM.
Figure 9. Porosity of reference mortar and SJFRM.
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Figure 10. Effect of fiber content: (a) water absorption, (b) density.
Figure 10. Effect of fiber content: (a) water absorption, (b) density.
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Figure 11. Relationship between compressive strength and porosity of mortars.
Figure 11. Relationship between compressive strength and porosity of mortars.
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Figure 12. Stress × displacement curves of mortars with fibers obtained in flexural tests: (a) 2J20, (b) 2J40, (c) 3J20, (d) 3J40, (e) 4J20, (f) 4J40.
Figure 12. Stress × displacement curves of mortars with fibers obtained in flexural tests: (a) 2J20, (b) 2J40, (c) 3J20, (d) 3J40, (e) 4J20, (f) 4J40.
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Figure 13. Effect of fiber on SJFRM fracture surface: (a) presence of pores, (b) fiber bridging of crack, (c) fiber distribution, (d) pull-out of fiber.
Figure 13. Effect of fiber on SJFRM fracture surface: (a) presence of pores, (b) fiber bridging of crack, (c) fiber distribution, (d) pull-out of fiber.
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Figure 14. Effect of fiber on toughness.
Figure 14. Effect of fiber on toughness.
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Figure 15. Axial compression behaviors of masonry prisms: (a) uncoated prism, (b) reference prism, (c) 2J20 prism, (d) 4J20 prism, (e) 2J40 prism, (f) 4J40 prism.
Figure 15. Axial compression behaviors of masonry prisms: (a) uncoated prism, (b) reference prism, (c) 2J20 prism, (d) 4J20 prism, (e) 2J40 prism, (f) 4J40 prism.
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Figure 16. Typical failure modes of masonry prisms under compression: (a) uncoated, (b) REF, (c) SJFRM.
Figure 16. Typical failure modes of masonry prisms under compression: (a) uncoated, (b) REF, (c) SJFRM.
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Figure 17. Compression failure mode of fiber-reinforced masonry prisms.
Figure 17. Compression failure mode of fiber-reinforced masonry prisms.
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Figure 18. Relationship between compressive strength and modulus of elasticity of prisms.
Figure 18. Relationship between compressive strength and modulus of elasticity of prisms.
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Figure 19. Bending behavior of prisms: (a) uncoated prism, (b) reference prism.
Figure 19. Bending behavior of prisms: (a) uncoated prism, (b) reference prism.
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Figure 20. Bending behavior of prisms reinforced with SJFRM: (a) 2J20 prism, (b) 4J20 prism, (c) 2J40 prism, (d) 4J40 prism.
Figure 20. Bending behavior of prisms reinforced with SJFRM: (a) 2J20 prism, (b) 4J20 prism, (c) 2J40 prism, (d) 4J40 prism.
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Figure 21. Form of rupture of prisms under bending: (a) uncoated, (b) REF, (c) SJFRM.
Figure 21. Form of rupture of prisms under bending: (a) uncoated, (b) REF, (c) SJFRM.
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Table 1. Mix proportions of the mixtures (kg/m3).
Table 1. Mix proportions of the mixtures (kg/m3).
CementFAMetakaolinSandWaterSPFiberVMA
291.25116.50174.751165.01349.505.24--
288.67115.46173.191154.66346.3915.3911.541.04
287.39114.95172.431149.56344.8622.9917.241.04
285.20114.08171.121140.81342.2428.5222.821.25
Table 2. Nomenclature of mixtures.
Table 2. Nomenclature of mixtures.
MixFiber ContentFiber Length
REFNo fiberNo fiber
2J202% 20 mm
3J203%
4J204%
2J402% 40 mm
3J403%
4J404%
Table 3. Average results of mechanical tests and standard deviation in parentheses.
Table 3. Average results of mechanical tests and standard deviation in parentheses.
MixCompressive Strentgh
(MPa)		Elastic Modulus
(GPa)		σf
(MPa)		σu
(MPa)
REF19.36 (0.23)20.99 (3.37)3.14 (0.06)-
2J2014.41 (0.49)16.36 (2.62)2.46 (0.07)1.28 (0.17)
3J2012.27 (0.12)15.28 (5.47)2.45 (0.02)2.17 (0.50)
4J2013.59 (1.12)17.09 (4.46)2.37 (0.07)2.29 (0.26)
2J4014.01 (0.24)12.58 (0.29)2.97 (0.08)1.83 (0.46)
3J4014.45 (1.72)13.83 (2.71)2.64 (0.07)2.44 (0.03)
4J4012.94 (0.61)13.02 (2.49)2.34 (0.24)2.25 (0.24)
Table 4. Results of the prism compression test.
Table 4. Results of the prism compression test.
Coating TypeSampleCompressive Strength (MPa)Stiffness (MPa)Mode of Failure
Uncoated16.161254.23Sustainability 16 05675 i001
27.62641. 84
Average6.89948.03
SD1.04433.03
REF38.757334.78Sustainability 16 05675 i002
49.147171.07
Average8.94 7252.92
SD0.27 115.76
2J2058.32 5290.54 Sustainability 16 05675 i003
66.42 6548.89
Average
SD		7.37 5919.71
1.35 889.79
4J2077.23 6672.78 Sustainability 16 05675 i004
88.77 5960.01
Average8.00 6316.40
SD1.09 504.01
2J4098.17 6951.50 Sustainability 16 05675 i005
108.14 6692.31
Average8.16 6821.90
SD0.02 183.27
4J401111.73 9218.45 Sustainability 16 05675 i006
129.41 8428.60
Average10.57 8823.52
SD1.64 558.51
Table 5. Prism bending test results.
Table 5. Prism bending test results.
Coating TypeSampleFlexural Strength (MPa)Ultimate Deflection (mm)Toughness (N·mm)
Uncoated10.180.050.04
20.200.020.05
Average0.190.030.04
SD0.010.020.00
REF30.800.150.16
40.610.130.22
Average0.710.140.19
SD0.130.020.04
2J2051.0010.000.87
61.487.921.25
Average
SD		1.248.961.06
0.341.470.26
4J2071.285.971.61
81.458.191.73
Average1.377.081.67
SD0.121.570.09
2J4090.989.991.03
100.017.510.07
Average1.018.751.07
SD1.381.761.16
4J40111.019.181.07
121.388.301.16
Average1.208.741.11
SD0.260.630.06
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Farias, L.d.N.; Lima, P.R.L.; Filho, R.D.T. Short Jute Fiber Reinforced Cement Mortar for Out-of-Plane Strengthening of Masonry Prisms. Sustainability 2024, 16, 5675. https://doi.org/10.3390/su16135675

AMA Style

Farias LdN, Lima PRL, Filho RDT. Short Jute Fiber Reinforced Cement Mortar for Out-of-Plane Strengthening of Masonry Prisms. Sustainability. 2024; 16(13):5675. https://doi.org/10.3390/su16135675

Chicago/Turabian Style

Farias, Lidianne do Nascimento, Paulo Roberto Lopes Lima, and Romildo Dias Toledo Filho. 2024. "Short Jute Fiber Reinforced Cement Mortar for Out-of-Plane Strengthening of Masonry Prisms" Sustainability 16, no. 13: 5675. https://doi.org/10.3390/su16135675

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