Structural Efficacy and Economic Viability of RC Beams Retrofitted with Hybrid Glass-Jute FRP Composites: An Experimental Study

Structural Efficacy and Economic Viability of RC Beams Retrofitted with Hybrid Glass-Jute FRP Composites: An Experimental Study

Mir Mujtaba Ali

P.G Student, Department of Civil Engineering Muffakham Jah College of Engineering & Technology, Hyderabad, India

Toufeeq Anwar

Faculty, Department of Civil Engineering Muffakham Jah College of Engineering & Technology, Hyderabad, India

Abstract – The progressive deterioration of reinforced concrete (RC) structures due to aging, overloading, and environmental exposure necessitates efficient retrofitting strategies. While conventional synthetic Fiber Reinforced Polymer (FRP) systems provide high strength, they are often limited by high material costs and environmental impact. This study investigates the flexural response of RC beams strengthened with a sustainable Glass-Jute Hybrid Fiber Reinforced Polymer (HFRP) composite, aimed at balancing structural performance, cost-efficiency, and environmental sustainability.

The experimental program involved twelve (12) RC beams (100 mm x 150 mm x 1200 mm) cast using M25 grade concrete and Fe550 steel. A hybrid laminate with a [Glass/Jute/Glass] stacking sequence was developed, yielding a tensile strength of 510 MPa. Three wrapping configurationsBottom Wrap (Type B), Side Wrap (Type C), and U-Wrap (Type D)were tested under four-point bending and compared against unstrengthened control specimens.

The results demonstrate a substantial improvement in structural capacity across all retrofitted specimens. The U-Wrap configuration (Type D) exhibited the highest performance, increasing the moment of resistance by 62.8% and significantly improving crack control. The Side (Type C) and Bottom (Type B) configurations provided increases of 52.7% and 30.1%, respectively. Beyond strength gains, the HFRP shifted the failure mode from brittle shear failure in control beams to a more ductile flexural-compression failure. A comprehensive cost- benefit analysis revealed that while the U-Wrap offered maximum strength, the Bottom Wrap was the most economically efficient, providing a 12.0% strength gain per 100 spent. These findings provide evidence that Glass-Jute HFRP laminates serve as a technically viable, cost-effective, and sustainable alternative for structural retrofitting, offering a foundation for the development of hybrid composite design guidelines.

Keywords: Hybrid FRP, Glass-Jute Composite, Flexural Strengthening, RC Beams, Cost-Benefit Analysis, Sustainable Retrofitting.

1. INTRODUCTION

   The progressive deterioration of reinforced concrete (RC) structures due to aging, overloading, and environmental exposure necessitates efficient retrofitting strategies. While conventional synthetic Fiber Reinforced Polymer (FRP) systems provide high strength, they are often limited by high material costs and a significant environmental footprint [3].

   Recent advancements in material science have led to the exploration of sustainable alternatives, such as natural fiber reinforced polymers (NFRP). Natural fibers, particularly Jute, are gaining attention because of their biodegradability, low cost, and acceptable specific strength [5, 6]. However, natural fibers often lack the stiffness and durability required for heavy structural applications when used alone [4].

   To overcome these limitations, “Hybridization” has emerged as a key strategy. By combining high-strength synthetic fibers (like Glass) with sustainable natural fibers (like Jute), researchers can develop Hybrid Fiber Reinforced Polymers (HFRP) that offer improved mechanical performance and cost-effectiveness [1, 2]. This “sandwich” stacking sequence protects the natural fiber core from environmental degradation while ensuring structural integrity [7].

   Previous studies have explored various configurations for beam rehabilitation, noting that the effectiveness of the strengthening is highly dependent on the wrapping technique such as Bottom, Side, or U-wrap configurations [8, 13]. This study aims to evaluate the flexural efficacy and economic viability of a specific [Glass/Jute/Glass] HFRP system for RC beams.

2. LITERATURE REVIEW

   A. Overview of Literature

   s. The concept of hybridization involves combining two or more fiber types in a single matrix to achieve a “hybrid effect” that compensates for the weaknesses of individual fibers. Mujahid and Hussain [1] highlighted that hybrid laminates can fill critical knowledge gaps in seismic resistance by offering lightweight and cost-effective alternatives to pure synthetic systems. Ribeiro et al. [2] further noted that hybridization allows for pseudo-ductile behavior, which is essential in civil engineering to prevent sudden, brittle failures in reinforced concrete (RC) structures. Recent experimental work by Maciel et al. [3] specifically validated that Jute-Glass hybrid systems, when applied via the externally bonded reinforcement (EBR) technique, provide a balanced increase in shear and flexural capacity.

   Natural fibers, particularly Jute, have gained traction due to their low cost and low environmental footprint. Jirawattanasomkul et al. [4] investigated Jute-FRP (JFRP) and found that it significantly enhances the shear capacity of pre- damaged beams, behaving similarly to conventional FRPs.

   The structural performance of RC elements can be further optimized by targeting specific failure modes using volcanic fiber systems like Basalt FRP. Toufeeq Anwar et al. [5] investigated the shear performance of RC slabs strengthened with Basalt FRP, demonstrating that external bonding significantly enhances the punching shear resistance of structural slabs by providing additional confinement.

   In the context of beam applications. M.Rehan Ali et al. [6] explored the influence of externally bonded Basalt FRP wraps of varied densities. Their research found that the fiber density is a critical parameter in determining the shear strength gains, where higher density wraps effectively delayed the formation of diagonal tension cracks. These findings align with the behavior observed in this study, where the stacking sequence and wrap density of the [Glass/Jute/Glass] hybrid laminate play a vital role in preventing brittle shear failure.

   While synthetic systems like Carbon FRP (CFRP) are the industry standard for high-load applications, they are often characterized by brittle debonding failure. Karzad et al. [7] observed that while EB-CFRP can increase capacity by over 60%, the failure remains sudden. To mitigate this, Aravind et al. [8] explored corrugated GFRP laminates, finding that geometry and stacking sequence can delay delamination, a principle that is fundamental to the [Glass/Jute/Glass] sandwich stacking sequence used in this study.

3. EXPERIMENTAL PROGRAM

   A A total of twelve reinforced concrete beams, with nominal dimensions of 100 mm *150 mm *1200 mm were cast using M25 grade concrete. The mix proportioning was carried out in accordance with IS 10262:2019 [9] to achieve the target mean strength and were evaluated under a symmetrical four- point bending configuration with an 800 mm effective span. To ensure a flexure-critical failure mode, specimens were designed as under-reinforced sections using two 12 mm diameter Fe550 HYSD tension bars and two 8 mm diameter compression bars. Shear integrity was maintained via 8 mm diameter vertical stirrups spaced at 230 mm center-to-center.

   Fig. 1: Reinforcement detailing and dimensions of the RC beam specimen

   The experimental matrix categorized the twlve specimens into four distinct groups. Three beams remained unstrengthened as Control Specimen Beams (Type A) to establish a performance baseline. The remaining nine specimens were retrofitted with an inter-ply [Glass/Jute/Glass]

   hybrid fiber-reinforced polymer (HFRP) system and subdivided into three configurations. Type B (Bottom Wrap) specimens featured the HFRP laminate bonded exclusively to the tension soffit. Type C (Side Wrap) specimens utilized continuous HFRP patches on the vertical longitudinal faces to assess side-confinement. Type D (U-Wrap) specimens integrated both bottom and side bonding to maximize transverse confinement and mitigate premature delamination.

   Fig. 2: Schematic diagram of the Four-Point Bending Test Setup

   1. MATERIALS

      The experimental program involved casting twelve reinforced concrete beams using Ordinary Portland Cement (OPC) of 53 grade, conforming to IS 12269. The fine aggregate utilized was clean river sand passing through a 4.75 mm IS sieve, while the coarse aggregate consisted of crushed granite with a maximum nominal size of 20 mm. To determine the mechanical properties of the concrete, a mix proportion of 1:1.64:2.85 was adopted as per IS 10262:2019 [9]. Compressive strength tests were conducted on standard cubes of 150 mm *150 mm *150 mm at 28 days, yielding an average compressive strength of 32.4 MPa. All beams featured a uniform reinforcement layout consisting of two 12 mm HYSD bars of grade Fe550 as longitudinal tension reinforcement and two 8 mm bars as hanger bars. The steel properties were verified through tensile testing as per IS 1786:2008 confirming the high yield strength and ductility required for the experimental baseline.

      The Hybrid Fiber Reinforced Polymer (HFRP) system composed of bi-directional woven roving E-glass fabric (600 gsm) and natural jute fiber mats (300 gsm). The glass fiber provides superior tensile strength and durability, while the jute fiber offers an ecological and cost-effective core. To ensure a high-performance bond, a two-part epoxy resin system (Araldite LY 556 and HY 951) was used as the matrix. The mechanical properties of the [Glass/Jute/Glass] hybrid laminate were determined by conducting tensile tests on coupons as per ASTM D3039 [10].

      Fig 3: Hybrid Fiber Reinforced Polymer (HFRP) system

   2. STRENGTHENING PROCEDURE

      The HFRP preparation and application were executed using the synchronized wet layup technique. The procedure began with surface engineering of the concrete substrate; the tension and side faces were mechanically ground to remove laitance and expose the coarse aggregate, followed by cleaning to ensure a robust mechanical interlock. An initial coat of epoxy primer was applied to the prepared surface to facilitate better adhesion.

      The hybrid laminate was then integrated layer-by-layer in a single continuous cycle. The first layer of saturated E-glass fabric was bonded to the concrete, followed by the insertion of the jute fiber mat core, and finally encapsulated by the outer E- glass skin. This “sandwich” configuration ensures the glass layers act as a functional environmental barrier, protecting the moisture-sensitive jute from alkalinity and ingress.

      After each layer was placed, a ribbed roller was utilized to expel entrapped air and consolidate the fibers, resulting in a monolithic structural skin bonded permanently to the RC beam. Reinforced concrete beams strengthened with HFRP were cured for minimum of 24 hours at room temperature before testing.

      1. Primer Coat Application
      2. First Glass Layer Placement
      3. Impregnation of Glass 1 (e) Impregnation of Jute
      4. Jute Layer Insertion

         (f) Final Glass Layer Placement Fig. 4: Inter-ply stacking sequence of the Glass-Jute Hybrid FRP laminate

   3. TEST SETUP

   The structural performance of the RC beams was evaluated under a symmetrical four-point bending configuration, as illustrated in the experimental setup in Fig. 1. The load was applied using a servo-controlled Universal Testing Machine (UTM) with a 1000 kN capacity. To ensure a uniform distribution of the force, the load was transferred from the machine actuator through a stiff steel spreader beam, creating two symmetric point loads on the specimen.

   The beams were simply supported on two steel rollers spaced at an effective span of 800 mm. This arrangement created a central pure-moment zone of 266 mm and two outer shear spans of 267 mm. To monitor the structural response throughout the loading history, mid-span deflections were recorded at regular load intervals using high-precision Linear Variable Differential Transformers (LVDTs).

   The loading was applied at a constant displacement rate to facilitate the observation of crack initiation, propagation, and the ultimate failure morphology of the Glass-Jute HFRP system.

   Fig. 5: Experimental setup for four-point bending test.

4. RESULT AND DISCUSSIONS

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   1. Characteristics of Glass-jute HFRP

      The Before structural testing, the mechanical properties of the fabricated Glass-Jute HFRP were established to understand its contribution to the beam’s performance. Tensile tests conducted as per ASTM D3039 revealed that the inter-ply [Glass/Jute/Glass] sequence successfully compensated for the lower stiffness of the natural fibers. The hybrid laminate achieved an average tensile strength of 510 MPa and an elastic modulus of 24.5 GPa. These results confirm that the synthetic glass outer layers not only protect the jute core but also ensure a high-strength composite capable of withstanding the significant tensile stresses developed during flexural loading.

   2. Crack Pattern and Failure Modes

      The structural response and transition in failure morphology for all tested specimens are detailed below. The primary observation across all strengthened groups was the significant shift from the brittle shear failure observed in control specimens to more ductile flexural failure modes.

      • Type A (Control Specimen Beams): The initial visible flexural cracks developed at an average load of 24.5 kN in the constant moment zone. As the load increased, several inclined cracks developed in the shear span, accompanied by a rapid widening of the primary diagonal crack. These beams finally failed in a brittle shear mode with an average ultimate load-carrying capacity of 80.37 kN.
      • Type B (Bottom Wrap): Strengthening was provided only on the tension face. Initial cracks were recorded at approximately 32 kN. Failure was primarily initiated by the debonding of the HFRP laminate from the concrete soffit, followed by sudden flexure. The average ultimate load capacity reached 104.57 kN, a 30.1% increase over Type A.
      • Type C (Side Wrap): The HFRP laminate covered the vertical longitudinal faces. Initial cracks were difficult to trace due to the fabric coverage, but failure was eventually initiated by the localized tearing of the glass-jute fabric over the shear-flexure cracks. These beams reached an average ultimate load of 122.80 kN, marking a 52.7% increase.
      • Type D (U-Wrap): This group exhibited the highest structural integrity. The U-wrap configuration provided superior transverse confinement, effectively suppressing shear rack widening and preventing premature debonding. Failure was initiated by gradual concrete crushing in the compression zone. Type D beams attained a maximum ultimate load of 130.83 kN, a 62.8% improvement.

        Fig. 6: Observed Diagonal Crack Pattern and Failure in Type A (Control) Specimen

        Fig. 7: Final Failure Mode and Diagonal Cracking Pattern in Type B (Bottom Wrap) Specimen

        Fig. 8: Final Failure Mode and Crack pattern in Type C (Side Wrap) Specimen

        Fig. 9: Ultimate Failure Mode (Concrete Crushing) and Crack Pattern in Type D (U-Wrap) Specimen

   3. Flexural Strength and Moment of Resistance

      The primary objective of the retrofitting was to enhance the moment-carrying capacity of the beams. The experimental data shows that the hybrid system effectively increases the section modulus and delays the yielding of the internal steel reinforcement. The results for the Increase in Moment of Resistance are summarized in Table 1 below.

      TABLE I. Increase in moment of resistance

      Beam Type	Configuration	Avg. Ultimate Load (kN)	Avg. Moment of Resistance (kN-m)	% Increase in Moment (from Control)
      A	Control	80.37	10.73	–
      B	Bottom Wrap	104.57	13.96	+30.1%
      C	Side Wrap	122.80	16.39	+52.7%
      D	U-Wrap	130.83	17.47	+62.8%

      1. Cost-Efficiency Analysis

         To evaluate the practical viability of the Glass-Jute HFRP system, a Structural Efficiency Index was developed, measuring the percentage strength gain per 100 of material cost (comprising fiber mats and epoxy resin).

         • Type B (Bottom Wrap): Achieved the highest economic efficiency with an index of 12.0% gain per

           100 spent. Since it utilized the least material to achieve a significant 30.1% strength increase, it represents the most viable option for low-budget rehabilitation.

         • Type C (Side Wrap): Provided a balanced performance with an efficiency index of 7.0% per

           100 spent. While the strength gain was high, the increased surface area required more resin and fiber, lowering the relative return on investment.

         • Type D (U-Wrap): While providing the maximum absolute structural enhancement, the extensive material consumption resulted in the lowest efficiency index of 6.3% per 100 spent. However, it remains the superior choice for high-load scenarios requiring maximum debonding resistance.
5. CONCLUSIONS
   1. • The Glass-Jute HFRP system, with a tensile strength of

        510 MPa, is a technically viable and sustainable alternative to pure synthetic FRPs.

      • Retrofitting significantly enhanced the flexural capacity, with the U-Wrap (Type D) achieving the maximum moment of resistance increase of 62.8%.
      • The inter-ply strategy successfully protected the natural jute core and shifted the failure morphology from brittle shear to ductile flexural-compression.
      • Type B (Bottom Wrap) is the most cost-effective solution for moderate strengthening, whereas Type D (U-Wrap) is recommended for critical structural applications requiring maximum confinement.
6. REFERENCES

1. Mir Ahmed Ali Mujahid and Mir Manzoor Hussain, Experimental Investigation of Fibre Reinforced Composite Materials for Engineering Applications, IOSR Journal of Engineering (IOSRJEN), vol. 07, no. 12, 2017, pp. 0104.
2. Filipe Ribeiro, Luís Correia, and Joaquim Sena-Cruz, Hybridization in FRP Composites for Construction: State-of-the-Art Review and Trends, Journal of Composites for Construction, 2024.
3. Leticia P. Maciel, Paulo S. B. Leão Júnior, et al., Experimental Analysis of Shear-Strengthened RC Beams with Jute and JuteGlass Hybrid FRPs Using the EBR Technique, Buildings, 2024.
4. Tamonari Jirawattanasomkul, Sahaput Likitlersuang, et al., Structural Behaviour of Pre-Damaged Reinforced Concrete Beams Strengthened with Natural Fibre Reinforced Polymer Composites, Composite Structures, 2020.
5. Toufeeq Anwar, Syed Jawwad Ahmed, Syed Mushtaq Hashmi, and Syed Jafer Hussaini, Shear Performance of RC Slabs Strengthened with Basalt FRP, International Conference on Recent Advances in Civil Engineering Infrastructure (RACEI-2019), 2019.
6. Mohammed Rehan Ali, Toufeeq Anwar, and Syed Jawwad Ahmed, Influence of Externally Bonded Basalt FRP Wraps of Varied Fibre Densities on Shear Strength of RC Beams, International Conference on Recent Advances in Civil Engineering Infrastructure (RACEI-2019), 2019.
7. Ahmad Shayan Karzad, Mohammed Leblouba, Salah Al Toubat, and Mohammed Maalej, Repair and Strengthening of Shear-Deficient Reinforced Concrete Beams Using Carbon Fiber Reinforced Polymer, Composite Structures, 2019.
8. N. Aravind, Amiya K. Samanta, J. V. Thanikal, and D. K. S. Roy, An Experimental Study on the Effectiveness of Externally Bonded Corrugated GFRP Laminates for Flexural Cracks of RC Beams, Construction and Building Materials, 2017.
9. IS: 10262-2019, Concrete Mix Proportioning Guidelines, Bureau

   of Indian Standards, New Delhi, India.

10. ASTM D3039, Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials, ASTM International, West Conshohocken, PA, USA.