Flexural performance of wire mesh and geotextile-strengthened reinforced concrete beam

Reinforced concrete (RC) is the most used construction material. Improper design and critical loading and environmental conditions cause the lack of load carrying capacity of RC structures, which can be solved by strengthening techniques; to face the challenges from the occupant characteristics, degradation in materials; or needing additional safety precautions [1, 2]. Most of the RC structural members undergo bending action in their service life due to different types of sustained load. Among them, beam elements may frequently fail due to their low flexural performance, which sometimes needs to be strengthened. To ensure the flexural failure of Reinforced Concrete Beam (RCB), generally it needed to design as a ductile member in order to resist bending under sustained loads. The ductile failure mode is more accepted than the brittle failure mode of RCBs; high reinforcement ratio in design may be favorable to enhance the flexural performance as well as ductility of RCBs [3]. The flexural performance of RCBs can be enhanced by providing additional tensile strength along their tension face (generally soffit), which may enhance the ductile deformation under bending action. Jacketing is the most used strengthening technique applied to enhance flexural strength of the RCB [1]. Most of the metallic construction materials have high tensile strength. However, when selecting strengthening materials, the material must have bonding properties with the existing structure and sufficient sustainability should exist in terms of strength, durability, and cost. The currently available flexural strengthening materials are steel sheet/plate, ferrocement, fiber-reinforced polymer sheets/strips, epoxy polymers, and textile fabrics/fibers [2, 4, 5, 6]. Most of them are high cost materials.

Nonwoven polypropylene geotextile fabric and woven wire mesh are selected for the present work. Generally, ferrocement have properties of high performance under load, cracking resistance, availability, high workability, and cost-effectiveness [3]. The performance of strengthened structures depends on the wire mesh used in ferrocement composite, which elaborately depends on the wire mesh orientation, wire direction, and thickness and layers of wire mesh used. Wire mesh is proven as a ductile material, which possesses full flexural strength when attached along the tension face of the beam with mortar with low tensile strength and sufficiently improves the ductile behavior of the beam [7]. Such strengthened beams are expected to have high flexural strength, high ductility ratio, enhanced energy absorption capacity, and reduced crack width with large deflection resistance up to failure [8]. Saadi et al. [2] found 18.3% increase in load carrying capacity of RCB when strengthened by attaching steel wire mesh using mortar along the soffit of the beam. Additionally, increasing the reinforcement ratio after using a steel wire mesh layer along tension face can improve the flexural performance and ductility of RCBs [9]. Xing et al. [5] tested RCBs strengthened in flexure by wire mesh of different areas and found that the flexural strength of strengthened beams increased with the number of wires. The authors observed 6–15% increase in load carrying capacity and decrease in the strain, which could increase the serviceability of the strengthened beams up to 200%. Failure of these specimens was generally controlled by the sudden debonding of wire mesh from the concrete, which should be controlled to obtain the full advantage of strengthening. Increasing the number of wire mesh not only improves the flexural strength but also the overall energy absorption capacity of the beam [7]. The improvement of flexural strength and moment carrying capacity considerably enhances in over-reinforced beams compared with beams with balanced reinforcement ratio and only slightly improves under-reinforced beams [8]. Therefore, selection of strengthening system should conforms all these criteria.

Meanwhile, textile-reinforced concrete layer is used in strengthening of existing load bearing structure because it efficiently increases the tensile strength and deformability of RC structures and reduces cracks width [10]. Geotextiles consist of synthetic fibers, such as polypropylene (75%), polyester (20%), polythene, or in other combination [11]. Nonwoven polypropylene fibers have high tensile strength, which can influence the flexural performance along with additional resistance to adverse environmental effect [12]. Because, nonwoven geotextile (polypropylene fabric)-reinforced concrete reduces water penetration into the RC structure when used as a protective layer [10]. Textiles are needed along the high bending moment zone in RCBs with additional anchorage length to be strengthened in flexure [13]. These materials are cost-effective and locally available in rural areas worldwide, thereby lowering the construction equipment cost and are thus genuinely sustainable. This system of additional protection layer can be used in retrofitting bridge pillars and in waterproof and outdoor structures [10]. The improvement of energy absorption capacity, toughness, and ductility are also observed after using a textile-reinforced layer in concrete structures [12]. Quality and orientation of fibers are the most important parameters that affect the overall performance of reinforcing layer. Most of the studies have used textile mesh with fibers arranged in two or more directions to compose textile mortar or concrete cover along the tension face of RCBs for flexural strengthening [13]. In the present work, nonwoven geotextile fabrics are selected for RCB strengthening, because this type of geotextiles have large fibers and strongly bond with concrete, given a sufficient flow capability [14]. This bond can be easily formed by curing freshly mixed concrete over a geotextile sheet. The pore size of nonwoven geotextiles is between 0.1 mm and 0.2 mm [15]. Thus, grain size below 0.1 mm is recommended for cementitious matrix for attaching the nonwoven geotextile to the concrete structure. According to Zak et al. [10], who found the maximum infiltration of cementitious matrix when its maximum grain size was limited to 0.25 mm. If the mix is compacted through mechanical vibration, then further anchorage would be produced through the infiltration of the cementitious mix, with the fibers in geotextile and concrete, thereby producing an average of 20% improvement in bonding strength [10, 14]. Given the low elastic modulus of polypropylene fiber, geotextile fabric reinforcing cannot improve the post cracking characteristics of brittle structures, which results in strain softening in cement composites [12]. This protective layer can control 25% of strain without collapsing and make the RC structure ductile in nature [10]. Meanwhile, low tensile strength of these nonwoven geotextile fabrics (approximately 50 kN/m) can be enhanced by producing composites from woven and nonwoven geotextiles [15].

Though there is several investigations on wire mesh strengthened beam, but in case of geotextile strengthening, there is no significant researches observed. Therefore, this study attempts to investigate the flexural performance of RCBs strengthened using wire mesh and nonwoven geotextile strengthening layer. In order to investigate the appropriate wrapping system, three different configuration of strengthening system were applied for each material.

Readily available materials are suitable for the strengthening and construction of structures because they speed up the system and are easy to work with. Therefore, locally available and easily manufactured wire mesh for strengthening purposes is advantageous in characteristics, which are mentioned in the literature [2, 9, 16]. Compatibility between steel wire mesh and concrete is perfectly coordinated under loadings. Strength-to-weight ratio is extremely high from steel plate-strengthened structures. The application of wire mesh in strengthening concrete structure found cost-effective; because it does not requires special experts. As well, it possesses high bending strength and in-plane shear resistance capacity. Wire mesh possesses ductile nature; hence, the ductility of RC structures may increase up to the benchmark. After covering of mortar, the wire mesh thickness may be 1.5–3 cm [9]. Wire mesh provides extremely low self-weight compared with other steel plate materials for strengthening and is thus considered cost-effective. Wire mesh with 0° orientation with the longitudinal axis of the beam shows the highest strength-to-cost ratio, whereas the 45° orientation possesses higher load carrying and energy absorption capacities while strengthening RCBs in flexure [8]. Textile-reinforced concrete or mortar can be used as a cover for RCB strengthening to improve the flexural strength, ductility, and durability of the beam [12, 13, 17]. Textile mesh is the most used material for textile-reinforced mortar or concrete composites, which consist of fibers in two or more directions [13]. In view of nonwoven polypropylene fabrics, textile mesh possesses high tensile strength because of internal polypropylene fibers, and these fabrics reduce the effect of high concentration of fibers after combining with the cementitious mix [12]. These nonwoven geotextiles may be of 0.25–9 mm thickness and 50–1700 g/m² mass per unit area with 0.075–0.85 mm apparent opening size [11]. Nonwoven geotextiles possess high bonding strength with fine cementitious mix through infiltration and anchorage with the concrete structure. These protective layers can reduce water penetration and enhance flexural properties [10]. Having a low weight per unit area is also an advantage of textile reinforcement layers. Because of the above mentioned reasons, in the present study, wire mesh and nonwoven geotextiles were used for RCB strengthening works.

Wire mesh and nonwoven geotextiles are locally available materials that can be used as RC beam strengthening material to enhance the capacity and improve performance of structures by applying according to any compatible approach. From this experimental study, a single layer of wire mesh applied using mortar may enhance the flexural strength of RCBs more than 90%. The debonding of the wire mesh layer is a considerable hindrance to reach the peak limit of strength, which begins along the highly compressive stress zone. Deflection resisting capacity of these beams is also high. To produce the same deflection in wire mesh-strengthened beam and control beam, the former requires at least 90% more load than the latter. However, geotextile-strengthened beams are flexible in nature and deflects more than three times compared with the control beam when peak loads are comparatively lower than the wire mesh-strengthened beam. Crack width in geotextile-strengthened beams is less than that of the wire mesh-strengthened and control beams. This result ensures good bonding between geotextile and concrete, which provides a grip of concrete up to failure and extends up to its peak limit. The experimental results indicate 8–91% increase in flexural strength of RCBs, which can be enhanced by further treatments. Therefore, these two materials are reliable and compatible in terms of flexural strengthening of RCBs in light to medium structures under moderate loadings and environmental condition.

Furthermore, the authors recommend further research on the characteristics of wire mesh and geotextile to be applied in structures for performance improvement. Therefore, proper guidelines should be studied thoroughly to apply geotextile for flexural strengthening purposes with all possible difficulties. Adhesive requirement and surface treatment may need to be analyzed experimentally with economic feasibility analysis. Wire mesh strengthening techniques have been extensively studied previously; however, geotextile strengthening is still not available in the literature. Therefore, the use of protective measures for strengthening via wire mesh and geotextile should be investigated. These layers have thin layer of cement mortar, which may lead to decreasing the durability and thus warrant further investigation for different adverse environments because ultraviolet radiation has been reported to affect nonwoven geotextiles negatively.