The jute fibers are formed by an irregular aggregation of fibrocells containing a central channel (lumen) inside them. Some modifications were observed in the morphology of jute fiber after alkaline treatment. Images obtained by SEM, shown in Figures 6 and 7, revealed a reduction in the cross-section with a change in fiber diameter and closure of some lumens. Additionally, the lateral surface was damaged by the fiber treatment, resulting in the appearance of lateral cracks due to delamination between the fibrocells, as previously observed by Jo et al. (35) after alkaline treatment. Moreover, deposits of calcium hydroxide crystals of varying dimensions were found on the surface of the treated fiber, which, coupled with increased roughness, could contribute to increased fiber-matrix adhesion (31, 58, 59).

The effect of alkaline treatment on the morphology of fibers, with the closure of lumens (33), directly results in the reduction of the fibers’ water absorption capacity. Vegetable fibers typically display substantial water absorption, particularly in the initial immersion hours due to a significant volumetric presence of permeable voids (60). Following 3 hours of immersion, the untreated fiber exhibited a water absorption rate of 158.77%, while the treated fiber absorbed 149.90%. After 72 hours, natural jute fiber demonstrated an absorption rate of 272.74%, whereas the treated fiber absorbed 224.11%. Cottrell et al. (61) illustrated that their study’s untreated jute fibers absorbed 205% within the first 50 minutes, maintaining a similar range throughout the total observation time of 90 minutes. Comparable trends were observed in other fibers, with average absorptions surpassing 150%, varying according to the observation period, such as sisal fibers (62), pineapple leaf, sisal, and jute fibers (33).

In addition to modifying the lumens, alkaline treatment leads to the partial removal of lignin and surface impurities (63), which are hydrophilic components of the fiber (hydroxyl groups - OH) with greater water absorption capacity. Indeed, the crystallinity index (CI) obtained from X-ray diffraction analysis, shown in Figure 8, indicates that the natural fiber had a CI value of 60.75%, consistent with findings in other studies (64, 65), while the fiber subjected to alkaline treatment showed a CI value of 66.22%. This increase, corresponding to a 9% rise in crystallinity, aligns with the reduction of impurities and lignin in the fiber, while preserving cellulose (66).

Figure 9 illustrates stress-strain curves depicting the tension testing of both natural and treated jute fibers. Their typical behavior demonstrates linear elasticity until a sudden, brittle rupture occurs. The values for tensile strength, rupture deformation, and modulus of elasticity presented in Table 4 were obtained from these curves.

The tensile strength and elastic modulus were improved in 25% and 23%, respectively, with alkaline treatment. According to Sinha and Rout (67), the improvement in mechanical strength and elasticity modulus is attributed to a structural transformation induced by alkaline treatment. In natural jute fibers, hemicelluloses and lignin persist in the inter-fibrillar region, imposing constraints on the cellulose chains, thereby keeping them separate.

The removal of hemicelluloses and lignin during alkaline treatment alleviates this internal constraint, enabling the fibrils to undergo reorganization. This densification of the cellulose chains contributes significantly to the enhancement of overall mechanical properties. The increase in the crystallinity index following treatment supports this hypothesis. Comparable outcomes, demonstrating increased jute fiber tensile strength, were observed by others researches (5, 30, 33, 68), following alkaline treatment of jute.

Mortars reinforced with 2% and 3% of treated jute fibers, denoted as J2 and J3, respectively, underwent a consistency test (flow table test) to assess workability compared to the fiber-free mortar (J0). Figure 10a presents the values obtained in the test and the consistency reduction observed upon fiber introduction. There is a reduction of 2.04% and 12.24% in consistency, concerning J0, for the J2 and J3 mortars, respectively.

This reduction in workability upon fiber addition, also observed by other authors (39), is associated with water loss from the mixture that is rapidly absorbed by the jute fiber due to its high water absorption capacity. Additionally, it involves disturbance in the flow of the mixture due to the presence of fibers, which create friction with the aggregate and tend to entangle, forming clusters within the mixture and increasing the content of entrapped air. As a consequence, there is a reduction in density in the fresh state, as shown in Figure 10b. The addition of 2% and 3% of fibers resulted in reductions of 8.85% and 13.02%, respectively, compared to J0.

The assessment of water absorption, void ratio, and density of mortars in the hardened state confirms the effect of fiber addition on increasing internal porosity and water penetration capacity within the mortars, as depicted in Table 5. With the addition of 2% jute fibers, there is an approximately 49% increase in water absorption and a 50% rise in porosity. Conversely, for the J3 mortar with 3% fibers, the increase in porosity (around 56%) was higher than the observed water absorption (around 83%), indicating that the added fibers create internal voids that are not accessible to water.

The elevation in porosity in fiber-reinforced composites is linked to the entrapment of voids during the mortar production process, given the reduced workability, resulting in greater difficulty in homogenizing the components and in casting into molds. Even in small volumes compared to the matrix, the fibers still contribute to reducing porosity and increasing water absorption in the hardened mortar. This is because the fibers introduce preferred pathways for water movement within the structure. The introduction of fibers led to a decrease in the density of the mortars, although the maximum reduction of 5%, concerning J0, is not proportional to the reduction in porosity.

Table 6 displays the results of the mechanical characterization of the mortars. The compressive strength and modulus of elasticity of the mortars decreased with the increase in fiber content. Compared to J0, there was a reduction of 7.3% and 22.3% in compressive strength, and a reduction of 6.5% and 13.9% in modulus of elasticity with the addition of 2% and 3% of fibers, respectively. The result confirms the inversely proportional relationship between porosity and the mechanical strength of solid materials, as the addition of fibers led to greater air incorporation into the mortar structure. However, due to the use of a more fluid mixture with a high content of fines, it is observed that the decrease in compressive strength was less severe than that reported by Majumder et al. (38) for mortar with 2% jute fibers, which showed a drop in compressive strength of up to 41.37%.

The stress-strain curves obtained in the flexural test are shown in Figure 11. While the mortar J0 exhibited sudden failure after the appearance of the first crack, in the J2 and J3 mortars, a change in behavior under flexion is observed, with the maintenance of residual stress after cracking and an increase in rupture deformation. This leads to an increase in energy absorption capacity (material toughness). Before the appearance of the first crack, the mortars exhibit linear elastic behavior with minimal contribution from the fibers to the flexural strength or stiffness. When compared to the flexural strength of mortar J0, it’s observed that the first crack stress remains unchanged with the addition of 2% of fibers, as indicated in Table 6. However, with the addition of 3% of fibers, there is a 14% reduction in flexural strength.

The post-cracking behavior is characterized by two main stages:

Consequently, the stress-displacement behavior becomes governed by the frictional adherence of the fiber, with a gradual reduction in stress and sliding of the fiber in the cracked region. The maximum residual stress σ_u presented in Table 6 indicates that the addition of 2% and 3% of fibers allows achieving post-cracking stresses of approximately 69% and 85% of the flexural strength, respectively, with deformation values 8 to 10 times higher than the peak deformation.

As a result, the mortars demonstrate an increase in material toughness, as shown in Table 7. There is a variation of the toughness index (TI) of the J2 and J3 mortars with increasing displacement. It is observed that for displacements up to 2 mm, mortar J2 exhibits a greater increase in toughness compared to mortar J3. However, this behavior changes for larger displacements, with the higher fiber content being more decisive in stress transfer and the final fiber pull-out process. The fracture energy of fiber-reinforced mortars typically stands out in comparison to ordinary mortars, as their mechanical behavior becomes less brittle after cracking in the flexural test, with the mortars maintaining stability and shape (40). In this regard, jute fiber has positively influenced mechanical results to produce new materials (69).

Figure 12 shows the typical load displacement curves obtained in the diagonal compression test of walls. The mechanical behavior of the walls is characterized by an approximately linear load-displacement relationship until the appearance of the first crack in the coating layer. For the unreinforced wall J0, the maintenance of the cracking load is observed until a small displacement (of the order of 0.4 mm) with a sudden loss of strength. For the walls coated with SJFRM, on the other hand, the post-cracking behavior is followed by an increase in wall strength and higher rupture displacements. Table 8 presents the results of the maximum load for each tested wall sample, as well as the mean value and standard deviation. The shear stresses, calculated according to Equations [2] and [4] established by the ASTM E519/E519M-15 (56) and RILEM-TC-76-LUM (57) standards, are also presented. The area of the specimens was calculated based on Equation [3].

It is observed that the presence of short jute fiber as reinforcement in the coating mortar results in an increase of up to 41% in the shear strength of the walls. Increased shear strength of walls has also been achieved with the use of reinforcement in plant fabrics such as jute (52) or hemp (20). Like fabric reinforcement, the use of short fibers as wall reinforcement inhibits the propagation of the initial crack and, thus, modifies the failure mode (27). Figure 12 shows the load-displacement curves obtained in the diagonal compression test of walls coated with mortar without fibers (WJ0) and with mortar containing 2% (WJ2) and 3% (WJ3) of short jute fibers. The positive side shows the horizontal stretching of the prism, while the negative side shows the vertical shortening based on the prism deformation.

The WJ0 prisms reached maximum applied load forces around 42.1 kN, on average. The unreinforced specimens exhibited typical brittle failure behavior, consisting of an elastic phase until reaching the maximum load, followed by sudden failure in both samples, as shown in the curves in Figure 12a. During the test, detachment of the mortar coating from the units was observed, indicating low adhesion, along with diagonal cracks extending towards the edges of the specimens. Unlike the unreinforced prisms, the curves of the reinforced prisms, with 2% and 3% fibers, were characterized by a post-peak phase with load maintenance up to higher deformations. As the crack opening grows, there is a gradual reduction in load due to the fiber pullout process in the crack region.

Evaluating the prisms reinforced with mortars containing 2% fibers, there was a greater difference in the maximum load obtained by each sample, WJ2-1 and WJ2-2, which obtained peak loads of 42 kN and 73 kN, respectively. The lower peak load of sample WJ2-1 resulted from the detachment between the masonry and the reinforcement layer at the point of load application, as shown in Figure 15. Wall WJ2-2, which reached the highest applied load among the studied walls, exhibited an initial crack with a load close to 50 kN, with a slight drop in load, followed by a load increase up to 73 kN. After the peak load, there is an abrupt reduction in load, with continued horizontal deformation and a tendency for cracking, both diagonal and horizontal, near the application of the test load. This occurs mainly in the reinforced specimens, where there is a greater accumulation of stresses (70).

The effect of applying mortar with 3% of fibers can be observed in Figure 12c. Specimens WJ3-1 and WJ3-2 exhibited peak loads of 53 kN and 64 kN, respectively. In prism WJ3-1, delamination was observed between the coating and the ceramic block near the lower load application shoe, along with horizontal cracks that appeared in the blocks after reaching the maximum load. There was also rupture between the block-coating in contact with the crack region. Small cracks at the top and bottom of the prism were caused by tensile forces. Sample WJ3-2 exhibited small diagonal cracks, attributed to diagonal/shear compression forces. Despite the crushing of the blocks near the load application, there was significant maintenance of wall stability even after the end of the test.

It is important to highlight that the dimensions of the prisms used in the tests were smaller than those prescribed by ASTM E519/E519M-15 (56), which may have influenced the mode of failure, especially at the support and load application points. As a result, the obtained results may be more conservative than those obtained in larger samples. Typically, larger samples have a higher probability of defects. In addition, the presence of a larger number of blocks in the sample influences the results because, according to Yu et al (71), the contribution of the wall coating layer to mechanical resistance is influenced by the integrity of each component of the system. Samples smaller than those established by the standard have been frequently used by some researchers due to equipment limitations or for ease of execution and handling (22, 52, 53, 54).

The unreinforced walls, under diagonal compression test, presented vertical cracking from the point of load application, which is common for this type of test, as shown in Figure 13. This shear failure mode, already identified by other authors (72, 13), occurs when the principal tensile stress developed internally in the wall exceeds the tensile strength of the masonry components, including the coating (73).

Figure 14 depicts the failure mode of walls reinforced with short jute fibers, identified as toe crushing, which is a result of wall crushing under compression (toe compression failure). In this case, initial arched cracking is observed below the loading apparatus and roughly transverse to the loading direction. With increasing loading, a vertical crack appears, and the ceramic block is crushed by the support apparatus. Laterally, the crack propagates between the wall and the coating, with the fibers acting to inhibit crack propagation, as shown in Figure 14b. Figure 14c illustrates the crack propagation on the other side of the wall.

After reaching the maximum load, the mechanical behavior of walls reinforced with fibers is characterized by a gradual loss of load-bearing capacity and the simultaneous formation of various failure mechanisms, as shown in Figure 14. In addition to coating cracking, detachment between the coating and the ceramic blocks was observed, as reported by Mezrea et al. (72), and the rupture of the ceramic block. The characterization results demonstrate that the ceramic block has a compressive strength lower than that of the coating mortar and the mortar used in the joints, making it the component most susceptible to cracking and rupture, as shown in Figure 15b and 15d, due to crushing under compression at the load application point.

Four failure mechanisms of walls coated with mortar reinforced with jute fibers, both with 2% and 3% fibers, were more notably encountered in the diagonal compression tests. Based on Figure 15, the following are identified in the samples:

It is observed, therefore, that the presence of fiber reinforcement inhibits crack propagation in the coating, modifying the wall failure mode, which is now controlled by the strength of the ceramic block and the wall-coating adhesion. For this reason, the fiber content used as reinforcement does not affect the maximum shear stress of the walls, as there was control of microcracking, and the coating was not brought to rupture. Overall, there is a significant contribution of the composite material in applications such as rendering mortar, maintaining masonry stability even after cracking. This indicates the potential of this type of mortar for retrofitting, resulting in crack control and failure mode as already observed by Khaleel; Madhavi and Basutkar (74) and Dong et al. (75).

The presence of fibers inhibits the propagation of initial cracks in the coating, resulting in a reduction of axial and transversal deformations. Table 9 presents the deformation coefficient (γ) found and the transversal deformation modulus (G) of each wall, based on Equations [5] and [6]. The values of vertical shortening Δx and horizontal elongation Δy were obtained considering the elastic phase up to the first crack of the samples.

Based on Table 9, it is observed that the deformation modulus was enhanced for walls with jute reinforcement in the composition of the coating mortar. The results presented here confirmed the increased capacity of fiber-reinforced walls to resist elastic deformation throughout the application of load. This also resulted in an increase in the stiffness of these walls, as observed in Figure 12, with the continuity of deformation during the plastic phase of the material still under load. The deformation coefficients showed increases around 2 and 3 times higher for 2% and 3% of fibers, respectively, compared to the unreinforced wall (WJ0).

The positive results obtained with the application of fiber-reinforced mortars, when compared to conventional mortar reinforcement, indicate the great potential of this technique for retrofitting. Compared to the TRM reinforcement technique, the application of fiber-reinforced mortar is faster and easier to apply and requires less labor, as it does not require placement of the mesh over the masonry.