Characteristics and uses of carbon fiber in construction, the structural reinforcement system that has revolutionized construction
The use of carbon fiber in construction is a relatively recent technique but represents an extraordinary solution in the field of structural consolidation. This article explores the innovative world of carbon fiber reinforcement, shedding light on its revolutionary applications. Together, let’s discover how this cutting-edge material is changing the face of modern construction, offering versatile and durable solutions for future structural challenges.
If you’re interested in these new design solutions but worry about managing them fully and complying with technical standards, you can utilize structural analysis software that helps in safe designing.
What are carbon fibers in construction?
Carbon fibers (or CFRP – Fiber Reinforced Polymers), supplied in tapes or nets, are increasingly used in construction for the structural reinforcement of existing elements. In other cases, in the form of reinforcement bars (the so-called C-Bars), they replace the traditional steel reinforcement.
Carbon fiber tapes and nets are composed of:
- Fibers that represent the structural material;
- Resin which acts as a matrix and “binds” the fibers together.
Carbon fibers used for structural reinforcement boast remarkable mechanical properties and high resistance while maintaining an extremely low weight. This characteristic allows their use in reinforcing existing buildings without significantly compromising the weight of structural elements, thus avoiding excessive increases in operational loads.
Carbon fibers also exhibit low water absorption, approximately around 0.1%, making them suitable for use in very humid environments. They also meet fire requirements due to their resistance to high temperatures.
However, despite the advantages, carbon fibers entail some disadvantages, particularly related to design and implementation. It’s essential for them to be carefully designed by specialized technicians and installed by competent and adequately trained personnel.
In recent years, the most evident use of carbon fibers in the construction sector concerns the structural reinforcement of beam-column joints through the application of fiber tapes. These interventions aim to increase the shear strength of the beam and column at the convergence point.
Beam-column joints are indeed weak points in framed reinforced concrete structures primarily designed considering vertical loads. This vulnerability is often caused by the lack of shear reinforcement in these areas. During reinforcement, the goal is to restore the correct hierarchy of resistances between elements, in compliance with the Technical Standards for constructions, by using specific carbon fiber tapes. This provides the structure with greater robustness and resistance against horizontal loads, such as those generated by a seismic event.
Carbon fibers can be classified based on their orientation. The main categories include unidirectional, bidirectional, and quadraxial fibers.
Here are the main differences between these:
- Unidirectional fibers – aligned in a single direction
- Mechanical properties: Offer high performance in terms of strength and stiffness along the orientation axis but have lower properties in different directions
- Applications: Ideal for applications where strength is required in a specific direction, such as in reinforcement beams.
- Bidirectional fibers – aligned in two orthogonal directions (usually along the 0° and 90° directions).
- Mechanical properties: Offer resistance along both the x and y axes, providing a compromise between strength and flexibility.
- Applications: Used in situations where resistance in more than one direction is required, such as in reinforcing slabs or walls.
- Quadraxial fibers – oriented along four main directions (usually 0°, 90°, ±45°).
- Mechanical properties: Offer resistance in all four directions, providing greater flexibility and adaptability compared to unidirectional or bidirectional fibers.
- Applications: Suitable for complex applications where omnidirectional resistance is required, such as in complex structural components or under variable load conditions.
In summary, the choice between unidirectional, bidirectional, and quadraxial fibers depends on the specific application requirements.
Designing reinforcement with FRP
Interventions with carbon reinforced fibers are relatively recent experiments. Therefore, designing reinforcement interventions with this technique that comply with construction standards may concern professionals. To work safely, the use of structural analysis software that fully manages this type of intervention for both load-bearing masonry structures and reinforced concrete frame structures can be very helpful.
Below are videos that demonstrate how to proceed smoothly.
FRP consolidation of load-bearing masonry
Here’s how to apply reinforcements to masonry structures.
FRP consolidation of reinforced concrete structures
Here’s how to design different types of reinforcements in reinforced concrete.
Creating reinforcement material
With BIM software for structural analysis, you can customize reinforcement material by entering the data and properties from technical data sheets. Here’s how:
Installation of Carbon Fibers for structural reinforcement of beam-column joints
Carbon fibers are a key element in structural consolidation interventions, used in combination with epoxy resins. The synergy between fibers and resins constitutes a complex system that requires the accurate use of the right “ingredients” and adherence to specific procedures. To correctly apply carbon fiber fabrics, the following steps need to be followed:
- Application phases
- Preparation of the reinforced support: for masonry surfaces, remove plaster and friable materials, obtaining a healthy and compact support. For reinforced concrete structural elements, completely remove deteriorated concrete and treat exposed rebars, restoring the section of the structural element to obtain the smoothest and most uniform support possible. For both masonry and reinforced concrete works, it’s important to chamfer the edges of the element to be reinforced (achieving a minimum radius of 20 mm) before applying carbon fabrics, to prevent damage and possible breakage of carbon fiber. After this operation, reconstruction can be carried out using lime-based mortars or cementitious repair mortars
- Application of primer: use an epoxy primer on dry and dust-free surfaces. The primer eliminates residual dust, increases surface resistance, promotes subsequent resin adhesion, and prevents water passage
- Application of structural bonding adhesive: wait between 6 and 36 hours after primer application to proceed with applying the thixotropic adhesive. After the waiting period, apply a two-component epoxy putty on the support with a metal spatula to level irregularities;
- Adhesion of carbon fabric: it’s a simple but careful operation. Firstly, the fabric needs to be pressed into the resin using a metal roller to avoid air bubbles, preventing damage to the fibers
- Impregnation of carbon fabric: after the adhesion phase, proceed to impregnate the fabric with a fluid two-component epoxy resin, using a brush or roller. The operation should be performed slowly and in multiple layers. Once the application cycle is completed, the reinforcement system can be covered with finishing layers.
Pros and Cons of using carbon fibers in construction
Compared to traditional structural reinforcement methods, carbon fibers stand out for their high mechanical strength and minimal intervention impact. Let’s delve into the advantages and disadvantages.
- High mechanical performance: Carbon fibers offer remarkable strength and rigidity, enhancing load-bearing capacity and structure stability
- Reduced weight: Despite advanced mechanical properties, carbon fibers are lightweight, avoiding significant increases in the overall weight of reinforced structures
- Resistance to environmental conditions: Carbon fibers are moisture-resistant and have low water absorption, suitable for use in humid environments without compromising their properties
- Resistance to high temperatures: Carbon maintains its strength at high temperatures, effectively meeting fire design requirements
- Application flexibility: Carbon fibers can be used in various forms like bars, tapes, and nets, offering design and application flexibility
- Intervention selectivity: They can be applied in selected areas like individual beam-column joints, arches, or deteriorated arches requiring urgent consolidation interventions
- Intervention invisibility: Structural reinforcements in carbon fiber have minimal visual impact as they conform to every shape
- Durability: Unlike steel, they resist oxidation, acid rain, saline environments, and have virtually unlimited lifespan
- Reversibility: Interventions with carbon fiber nets are reversible in a short time and with minimal resources.
- High costs: The cost of carbon fibers can be significant compared to other construction materials, influencing the overall project budget
- Specialized design: Designing and applying carbon fibers require specialized skills, increasing the complexity of the process
- UV light sensitivity: Carbon fibers may degrade in the long term due to exposure to ultraviolet light, sometimes requiring protective coatings
- Sensitivity to existing cracks: If carbon fibers are applied to surfaces with existing cracks, they might not effectively prevent crack propagation
- Complex installation process: Applying carbon fibers requires specific procedures and attention to details, including support preparation and the use of epoxy resins, increasing installation complexity.