Why Use Carbon Fiber Reinforced Concrete (CFRC)?

 

Concrete is the most widely used construction material in the world due to its cost-effectiveness, abundance of raw materials, facile manufacturing, and adequate physical and mechanical properties.


Concrete has limitations, however, which can hinder its durability and useable lifetime. Mainly, its lower flexural and tensile strength and brittleness can cause structures to fail without warning under tensile and shear conditions.

For this reason, structural concrete is commonly reinforced to improve the structural integrity of buildings. Whilst steel fiber is the main reinforcing element used globally, there are some issues with its use, mainly its long-term performance and sustainability.

This article will explore carbon fiber reinforced concrete, an alternative to conventional steel fiber reinforced concrete.
Why Use Carbon Fiber?

Carbon fiber has seen growing use in recent years in the construction industry, mostly in the framework of curtain walls, partition panels, and walls. First explored as a potential construction material in the 1970s, it was demonstrated that carbon fiber reinforced concrete (CFRC) had some suitable properties.

Early studies on CFRC made from PAN fiber significantly improved the impact strength, tensile strength, and elastic modulus of concrete. In 1980, a Japanese study using pitch carbon fiber reported noteworthy improvements using just 0.2% carbon fiber by volume.

Further studies confirmed that even a minuscule fraction of carbon fiber significantly improves the mechanical properties of concrete, and carbon fiber outperforms both steel or glass fiber. This is due to superior durability in adverse conditions, thermal resistance, weatherability, and its finish ability.

One drawback of carbon fiber is its cost, however. This has thus far hindered its large-scale use in structural elements, but researchers have argued that the initial expense is worth it compared to the cost of retrofitting failing structures using conventional steel fiber-reinforced concrete.

Currently, there is a lack of literature on the use of CFRP in construction projects due to its perceived expense, even though its benefits are well-recognized. Furthermore, many studies have concentrated on the use of carbon fibers in mortar composites rather than producing CFRC.
Chemical Composition of Carbon Fiber and Characteristics

Carbon fiber contains over 92 wt% carbon, arranged in parallel crystal structures to the fiber’s long axis. The structure can be either amorphous, partly crystalline, or fully crystalline, depending on the manufacturing process, and can exist as either short or long fibers.

Carbon fibers are classed as high-performance due to their essential characteristics. These include low specific weight, ability to withstand stress during dynamic and quasi-static loading conditions, high thermal conductivity, superior creep resistance, high elastic modulus, non-corrosiveness, and chemical stability.


Furthermore, carbon fibers have excellent aesthetic characteristics, making them attractive for projects that require a good finished product. However, there are some limitations with carbon fiber which can hinder its use.

The fibers tend to oxidize when heated or under alkaline conditions. Also, fibers possess drawbacks such as axial vs. transverse anisotropy and low strain to failure. The main issue that hinders the use of CFRC is carbon fiber’s poor ductility, which is lower than alternatives such as glass, Kevlar, and SiO2.

Selecting carbon fiber for a project requires knowledge of the benefits and drawbacks associated with its use.
Carbon Fiber Grades

Different grades of carbon fiber are available commercially. These include low modulus, intermediate modulus, high modulus, ultra-high modulus, super high tensile, and high tensile grades.
Different Types of Carbon Fiber Used in Construction

There are three main types of carbon fiber used in the construction industry: PAN-based, pitch-based, and rayon-based. PAN stands for polyacrylonitrile, a semicrystalline, synthetic organic polymer resin with the formula CH2CHCN.
Overcoming Cost Limitations

As mentioned previously, the high cost of carbon fiber can be prohibitive. However, the unit cost has gradually been coming down in recent years due to the increasing use of carbon fiber in the automotive, aerospace, construction, and medical industries, to name a few.

The cost of carbon fiber is dependent on the grade needed, as well: the higher the tensile strength, the more expensive the fiber. However, with increasing consumption, the advent of relatively cheap, abundant CFRC in construction projects in the future could be more likely.
Forces Responsible for the Mechanical Properties of CFRC.

Fiber-matrix adhesion is responsible for the mechanical properties of fiber-reinforced concrete, consisting of elements such as chemical bonding, mechanical interlocking, and Van der Waals interactions. Chemical bonding is the strongest interaction and can be further improved using fiber sizing or chemical treatment.

These interactions govern the compressive strength, flexural strength, splitting tensile strength, and workability of CFRC. Several studies over the past few decades have explored the optimal carbon fiber percentage in CFRC, with research demonstrating outperformance compared to conventional fiber-reinforced concrete.
Uses in Construction and Outlook

As mentioned previously, the decreasing cost of carbon fiber has increased the attractiveness of CFRC in the construction industry, piquing the interest of experts. Whereas current applications are largely limited to non-structural elements, this is slowly changing. The use of CFRC is still in its infancy.

Currently, the construction industry uses the cheapest fibers, which are isotropic short fibers with poor mechanical properties. However, increasing the use of high-performance carbon fiber should further reduce its cost for the sector.

Recent studies have shown that the benefits of CFRC could potentially outweigh its cost limitations for use in new builds and as a retrofitting and strengthening material. One of the benefits for new structures and repairs which has been highlighted is the lower structural dead load that can be achieved using carbon fibers due to their low density.

A further benefit that could increase the use of CFRC in building projects is its corrosion and chemical resistance. This would provide significant improvements in terms of durability, decreasing ongoing maintenance costs and extending the useable lifetime of structures.

CFRC has already seen use in structural column and beam repair, smart concrete structures, and in bridge construction. Whilst still a fairly niche material compared to conventional steel-reinforced concrete, CFRC is slowly becoming more commonplace. Furthermore, its green credentials align with current sustainability goals.
Final Thoughts

CFRC is an innovative construction material that provides vast benefits for the construction industry, improving the sustainability of the sector and reducing ongoing maintenance costs for existing structures. However, its use in construction is currently limited.

Due to its superior mechanical properties (albeit with some drawbacks) and the fact that carbon fiber itself is becoming cheaper, CFRC could be one of the answers to key current environmental and economic issues the construction industry is facing. Indeed, already some forward-thinking architects and companies are using it.

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