Figure 1. Harkers Island Bridge Replacement Project, courtesy of Paolo Casadei, Owens Corning and Peter DiStefano, Balfour Beatty

Nevertheless, like all groundbreaking advancements, this approach comes with its challenges. This article presents a new hybrid reinforcement strategy. This game-changing solution combines the best of both worlds: the strength and ductility of steel with the corrosion resistance and lightness of GFRP. 

Why GFRP? The sustainable solution for Concrete Reinforcement

GFRP will rapidly transform the construction industry. Unlike steel, GFRP does not rust, making it an ideal choice for structures in harsh environments, such as coastal areas where saltwater can quickly corrode traditional materials. Beyond its corrosion resistance, GFRP offers the advantage of being significantly lighter than steel, simplifying handling and installation. This accelerates construction timelines, reduces labor costs, and contributes to sustainability by lowering transportation costs and environmental impact. The lighter weight of GFRP reduces the energy required for transport, further enhancing its appeal as a modern, eco-friendly solution for construction projects.

However, GFRP is not without its drawbacks. Its brittleness and limited ductility can be concerning, especially in seismic zones where buildings must dissipate energy during the seismic event. This limitation has led engineers to hesitate to adopt GFRP fully. However, combining the durability and resilience of steel in critical areas with the exceptional corrosion resistance of GFRP where it is most effective, presents an optimal solution. This is where the hybrid reinforcement strategy comes into play.

The Hybrid Solution: combining ductility, strength and durability

The hybrid reinforcement strategy is an innovative solution that combines the strength of GFRP with the ductility of steel. This solution strategically places steel bars in the structure’s most critical or plastic hinge regions, as shown in Figure 2, ensuring it maintains its ductility and load-bearing capacity.

These critical regions are areas within the structure where higher demand for strength, ductility, and performance occurs due to concentrated stresses or significant deformation under specific loading conditions. Properly reinforcing these regions for their critical length Lcr (as shown in Figure 2) is crucial for ensuring the overall safety and integrity of the structure, especially during extreme events such as earthquakes. Traditional steel rebars are used in these areas and are detailed with international standards such as Eurocode 8 or ASCE07 to ensure the required ductility. Steel bars are placed with a larger concrete cover depth and are fully anchored, providing enhanced corrosion protection and the necessary ductility to withstand dynamic loads. Meanwhile, GFRP bars are not anchored to the foundation and are used outside the critical regions Lcr, where high ductility is not required. 

By placing GFRP externally, beyond the critical zones, the structure benefits from GFRP’s advantages without compromising the overall ductility. This hybrid approach ensures that the structure not only meets but exceeds the performance of traditional steel-reinforced designs, offering a perfect balance between durability, weight, and strength. 

Finite Element Analysis of Hybrid Reinforced Structures

Extensive 3D solid nonlinear Finite Element (FE) analyses were performed on various structural components and entire systems using STKO and OpenSees. The focus of the study included foundation-column joints, external and internal column-beam joints, and a reinforced concrete (RC) frame. These analyses were validated against experimental data from existing literature to ensure their accuracy and applicability. The simulations employed advanced material models, including the ASDConcrete3D constitutive model, to capture concrete’s plasticity and damage characteristics, as illustrated in Figure 3. 

Figure 3. Finite Element model

The hysteretic behavior of steel was also incorporated, along with the elastic-brittle properties of GFRP rebars. Additionally, the models accounted for critical factors such as rebar buckling and bond-slip effects, which are crucial for accurately simulating the complex interactions within the structures under load. Figure 3 presents the finite element (FE) model of the test conducted by Park and Paulay [7], modified to incorporate the GFRP cage. 

The results indicate that the ductility and load-bearing capacity of the proposed hybrid structures, which integrate traditional steel with GFRP reinforcement, are comparable to those of conventional steel-reinforced structures. 

Figure 5  illustrates the maximum crack width and the stresses in the GFRP in two different time steps. The GFRP is not anchored to the foundation, and its stress levels remain well below the characteristic strength of the GFRP. This is a significant finding, especially for regions prone to seismic activity, where maintaining structural integrity during dynamic events is crucial. The research also highlighted how this hybrid approach could significantly extend the lifespan of structures, reducing maintenance needs and enhancing overall performance.

Figure 5. Maximum crack width and GFRP stress

The Future of Construction

The hybrid GFRP-steel reinforcement strategy is more than just an engineering innovation; it is a leap forward for the construction industry. In a world where infrastructure is the backbone of development, innovations like the GFRP-steel hybrid approach are beneficial and essential; combining the best properties of both materials offers a sustainable, durable, and efficient solution for modern construction challenges. Whether it is building bridges that can withstand the test of time or skyscrapers that remain resilient in the face of natural disasters, the hybrid reinforcement strategy is paving the way for a new, more durable, and sustainable solution in construction. 

Author(s): Guido Camata¹, Kaushal Patel¹, Enrico Spacone¹, Massimo Petracca², Antonio Nanni³ 

1. guido.camata@unich.it, University “G. D’Annunzio” at Chieti-Pescara
2. m.petracca@asdea.net, ASDEA software srl
3. nanni@miami.edu, University of Miami 

Sources:

1. https://www.iifc.org/wp-content/uploads/2024/03/IIFC-Newsletter-2024-Vol21-No1.pdf 
2. El-Salakawy, Ehab & Benmokrane, Brahim & Masmoudi, Rafik & Brière, F. & Breaumier, É. (2003). Concrete Bridge Barriers Reinforced with Glass Fiber-Reinforced Polymer Composite Bars. Aci Structural Journal. 100. 815-824.
3. https://www.pci.org/PCI_Docs/Publications/PCI%20Journal/2002/Sept-Oct/Design-Construction%20of%20Bridge%20Street%20Bridge%20-%20First%20CFRP%20Bridge%20in%20the%20United%20States.pdf
4. https://www.tac-atc.ca/wp-content/uploads/benmok.pdf
5. https://www.fdot.gov/docs/default-source/structures/Innovation/CAMX-2017-Halls-River-Bridge-corrosion-free-Design-with-FRP-composites.pdf
6. https://www.ncdot.gov/projects/harkers-island/Pages/default.aspx
7. Park, R.; and Paulay, T. 1990. Use of Interlocking Spirals for Transverse Reinforcement in Bridge Columns, Strength and Ductility of Concrete Substructures of Bridges, RRU (Road Research Unit) Bulletin 84, Vol. 1, pp 77-92  (https://nisee.berkeley.edu/spd/servlet/display?format=html&id=26)