Retrofitting of Reinforced Concrete Beam-Column Joints with Externally Bonded Fiber Laminates

Abstract

Reinforced concrete (RC) is one of the most widely used construction materials globally, but many RC structures deteriorate or become unsafe before reaching their design life due to factors such as earthquakes, corrosion, overloading, poor design, or faulty construction. In framed structures, beam-column joints are critical for transferring loads and moments; however, they are often inadequately designed, making them the most vulnerable components during extreme loading. Retrofitting is the process of strengthening a structure to achieve a predefined performance level, whether damaged or not. It differs from repair, which only restores partial strength, and rehabilitation, which aims to regain original strength. Retrofitting existing structures poses greater challenges than new construction and requires simple, durable, and economical techniques. Among various local retrofitting methods—such as injection, shotcreting, and plate bonding—the plate bonding technique is the most efficient. Common materials include ferrocement, FRP, PMC, and steel plates. Although FRP is effective, it is costly and requires skilled labor. Ferrocement jacketing, by contrast, offers advantages such as ease of application, low weight, durability, and cost-effectiveness. This study investigates the effect of ferrocement jacketing on the strength of RC beam-column joints, considering parameters like mesh layers, orientation, and initial stress levels. For comparison, some joints were retrofitted with two layers of CFRP at 45° orientation. Experimental results were validated using a 3D nonlinear finite element model developed in ATENA 3D, which showed good agreement (within ±10%). Retrofitting with ferrocement or CFRP significantly increased the load-carrying capacity and stiffness of stressed joints. Both methods restored full strength even at 85% of ultimate load, and failure shifted from the joint region to the beam ends, resulting in a safer weak-beam–strong-joint behavior. The findings confirm the effectiveness of ferrocement and CFRP jacketing for strengthening RC beam-column joints.

Introduction

Reinforced Concrete (RC) is widely used in construction due to its strength, durability, and versatility. However, beam-column joints, which connect structural members and transfer loads and moments, are often the weakest and most vulnerable zones during earthquakes. Poor design, substandard construction, corrosion, and inadequate ductility frequently cause premature deterioration and collapse in seismic events, particularly in developing countries where design codes and detailing are often neglected.

Under seismic loading, these joints experience high shear and bond stresses, and their behavior largely dictates the overall structural performance. Ductile joint behavior ensures safety by following the “strong-column, weak-beam” principle—where beams yield before columns or joints—enhancing energy dissipation and preventing collapse.

Aim and Objectives

Aim:
To evaluate the effectiveness of ferrocement and CFRP jacketing in enhancing the strength, stiffness, and failure behavior of RC beam-column joints through experimental modeling.

Objectives:

1. Study the effect of initial damage and loading type on retrofitted joint performance.

2. Examine how wire mesh layers and their orientation affect retrofitted joints.

3. Develop an experimental model to predict the behavior of strengthened joints.

Literature Review

Research shows that Fiber Reinforced Polymers (FRPs)—such as CFRP and GFRP—effectively increase joint strength, stiffness, and ductility. Studies (Lee et al., Bousselham et al., Sharma et al., Li & Chua, Mukherjee et al., 2022–2025) confirm that properly anchored FRP jacketing enhances shear resistance, load-carrying capacity, and seismic resilience, while preventing brittle failure. However, improper anchorage or surface debonding can limit effectiveness.

Methodology

1. Study of Beam-Column Joints:
   Beam-column joints are classified as interior, exterior, and corner joints, depending on beam connections. Their performance depends on concrete strength, joint geometry, and reinforcement anchorage. Adequate confinement and detailing are essential for ductility and to prevent brittle shear failure.

2. Behavior of Beam-Column Joints:
   Under seismic loads, diagonal cracks form in the joint core due to tension and compression. Proper transverse reinforcement and bar anchorage prevent joint degradation. Exterior joints face higher shear stresses and require more confinement.

3. Retrofitting of Beam-Column Joints:
   With many older buildings not designed for seismic safety, retrofitting is essential to enhance capacity and ductility. The process includes assessment, design of retrofit schemes, selection of appropriate techniques, and post-construction monitoring.

   • Global retrofitting: Improves overall structure (e.g., adding shear walls or dampers).

   • Local retrofitting: Strengthens specific members (e.g., joints, beams, columns).

4. Retrofitting Techniques:
   Traditional methods include epoxy injection, mortar filling, shotcreting, concrete replacement, and steel-fiber-reinforced concrete.
   Advanced methods use plate bonding with materials like:

   • Ferrocement plates: Low-cost, durable, corrosion-resistant, ideal for developing regions.

   • FRP plates (CFRP/GFRP): Lightweight, strong, and corrosion-resistant but costly.

   • Steel plates and polymer-modified concretes for additional strength.
     Among these, FRP and ferrocement jacketing are the most effective for beam-column joints.

Results and Discussion

An experimental study was conducted on 27 beam-column joints, retrofitted using ferrocement jackets with 2, 4, and 6 layers of galvanized iron (GI) woven wire mesh. Joints were preloaded at three stress levels—50%, 85%, and 100% of their ultimate capacity—before retrofitting.

Key Findings:

• Ferrocement jacketing significantly enhanced load capacity, stiffness, and crack resistance.

• Strength gains were due to the confinement effect of wire mesh, which delayed crack propagation.

• Crack patterns: Retrofitted joints had finer, uniformly distributed cracks, while control specimens showed wider cracks at lower loads.

• Optimal performance was observed at Stress Level-2 (85%), where moderate pre-damage allowed better bonding and confinement.

Conclusion

The experimental investigation on reinforced concrete (RC) beam-column joints retrofitted with ferrocement jackets using four and six layers of GI woven wire mesh has led to the following conclusions: • Retrofitting considerably enhanced the load-carrying capacity of all beam-column joints. The ultimate load increased by 25–41%, with higher gains observed in specimens retrofitted with six layers compared to those with four layers of wire mesh. • The yield load increased by 35–50%, indicating that the retrofitted joints could sustain higher loads before yielding. This confirms the efficiency of ferrocement jackets in restoring and strengthening damaged joint zones. • The deflection at ultimate load decreased by 17–55%, while rotation reduced by nearly 40–60% in all retrofitted specimens. This demonstrates improved stiffness and better confinement due to the ferrocement layers. • The stiffness of the beam-column joints increased significantly — by 110–180% for four-layer and 70–120% for six-layer ferrocement jackets. This improvement reflects higher rigidity and improved load transfer capacity after retrofitting. • Despite the strength gain, the deflection ductility ratio and energy absorption decreased for all retrofitted specimens. This reduction is attributed to the load-controlled testing conditions and the increased confinement, which led to more brittle post-yield behavior. • Comparing the two retrofitting configurations, six-layer jackets showed higher load capacity and stiffness, whereas four-layer jackets exhibited slightly better ductility. Therefore, the selection of mesh layers should balance between strength enhancement and ductility retention based on performance needs. Retrofitting RC beam-column joints with ferrocement jackets reinforced with GI woven wire mesh is an effective and economical technique to enhance the strength, stiffness, and loadcarrying capacity of damaged or underperforming joints. However, care should be taken to control the brittleness induced by excessive confinement, particularly when increasing the number of wire mesh layers.

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Copyright

Copyright © 2025 Dr. S. S. Angalekar, Miss. Madhura S. Ghorpade. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.