*1.1. Background*

Reinforced concrete structures are deteriorated by seismic and other loadings, which cause structural damage or failure. Recently, carbon fiber-reinforced plastics (CFRPs) have been widely used in structural repair and seismic retrofit. The merits of carbon fiber are that it does not corrode, degrade, fatigue, and it possesses high specific strength. In addition, CFRP is a composite material with carbon fiber as the stiffener and thermosetting or thermoplastic resins as the matrix. Therefore, it has been widely used in the aerospace industry, automotive industry, sports equipment, and civil engineering. However, CFRP is a kind of material that is difficult to decompose by nature and sometimes can produce toxic gases when burned. The traditional disposal of the waste CFRPs is harmful to the environment; thus, an effective recycling method is urgently needed to turn waste CFRPs into potential recycling products, such as the fiber of fiber-reinforced concrete used in

Ramanathan, G.K.; Chang, S.-M.; Shen, M.-Y.; Tsai, Y.-K.; Huang, C.-H. An Experimental Study on Mechanical Behaviors of Carbon Fiber and Microwave-Assisted Pyrolysis Recycled Carbon Fiber-Reinforced Concrete. *Sustainability* **2021**, *13*, 6829. https://doi.org/10.3390/su13126829

Academic Editor: Constantin Chalioris

**Citation:** Li, Y.-F.; Li, J.-Y.;

Received: 25 May 2021 Accepted: 9 June 2021 Published: 17 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

civil engineering infrastructures. The world's population is increasing gradually, and it is affecting the environment by raw material wastage.

In the linear economy, renewable and nonrenewable raw material sources are collected and transformed into products until it is discarded as waste. For example, the CFRP bicycle frame has been used in linear economic material and it is discarded as waste. Recently, some carbon fiber composite material has been recycled in different methods, such as mechanical, thermal, and chemical recycling approaches. In this paper, microwave-assisted pyrolysis (MAP) technology approaches are used to remove the resin from the CFRP bicycle frame and transform it into recycled carbon fiber, and then it is used in fiber-reinforced concrete (FRC) structures; the waste materials have been recycled and turn the linear economy to a circular economy. The recycled carbon fiber from the CFRP wastes reduces environmental pollution by being applied in civil engineering.

#### *1.2. Literature Review*

Fiber-reinforced cement matrix materials can significantly improve the mechanical properties of fiber-reinforced cement or fiber-reinforced concrete. So far, there have been many studies on fiber-reinforced cement-based materials. Fiber-reinforced concrete can inhibit the formation of large cracks, control the development direction of concrete cracks, and improve the toughness of concrete, thereby improving the tensile and crack resistance of traditional concrete [1,2].

Compared with glass fiber and polypropylene (PP) fiber, steel fiber has better elastic modulus and tensile strength, and the steel fiber-reinforced concrete enhances the effect of shock absorption, compressive and flexural strength of concrete specimens. According to some research test results, adding silica fume can effectively disperse the steel fiber more evenly [3,4]; and adding 1.5% fiber proportion of steel fiber to concrete can increase the impact number in the free-fall impact test [4,5].

There have been many studies on the application of chopped carbon fiber in cement matrix materials over the last few decades. For example, the mechanical properties and microstructures of cement and concrete have been studied by using different types of carbon fibers, carbon fiber lengths, and carbon fiber proportion [6–13]. Furthermore, the level of dispersion of carbon fiber in the cement will greatly affect the strength of the specimen after solidification. Research has shown that different mixing methods will affect the level of dispersion of carbon fiber [14–17]. The carbon fibers were immersed in the hydrolysis method and the furnace heating method to remove silane on the surface of the carbon fiber. The furnace heating method can effectively remove the silane on the surface of the carbon fiber, and combining the furnace heating method with the pneumatic dispersion method can improve the chopped carbon fiber to uniformly disperse inside cement [18].

The amount of industrial waste and commercial waste increases with an increase in demand. Many scholars have begun to study the application of waste in civil engineering to improve the mechanical properties or durability of concrete. Polyethylene terephthalate (PET) waste replacement weight percentage and volume fraction affect the compressive strength. Due to the incorporation of PET waste, the flexural strength and split tensile strength were also deteriorating to the concrete, but the impact resistance had a tiny enhancement effect by weight percentage replacement with aggregates [19,20]. The addition of 1% volume fraction of metalized plastic waste (MPW) fibers into the concrete enhances the compressive strength and impact resistance [21]. The compressive strength was increased by using scrap tire fibers compared with scrap tire fragments. In addition, the fiber-reinforced concrete with a 0.8% weight proportion of recycled tire steel fiber (RTSF) and 0.2% of polypropylene fiber (PPF) had the highest compressive strength. Using PPF instead of RTSF will not greatly affect the flexural strength [22,23]. The glass fiber-reinforced plastics (GFRP) sheets were chopped into short fiber and incorporated into concrete. The recycled GFRP was not affected by alkaline aggregate reaction and shrinkage of concrete. The test results showed that 3 wt % aggregates replacement with GFRP attained maximum compressive and flexural strength. Adding recycled GFRP into

concrete improves the impact strength of concrete, but it will reduce the slump flow [24–26]. The chopped recycled CFRP improves the compressive, flexural, and impact strengths of concrete [27–29]. Adding 3 wt % of CFRP waste into the mortar enhances the stiffness and also the flexural strength [29].

Based on the above research background and literature review, this research will use recycled carbon fiber obtained by microwave-assisted pyrolysis (MAP) technology and apply it to fiber-reinforced concrete. The three different types of carbon fibers (recycled carbon fiber, normal carbon fiber, and carbon fiber without coupling agent) were used with different fiber weight proportions (5‰, 10‰, and 15‰). Additionally, the pneumatic dispersion process was used to disperse the fibers uniformly into cement. The mechanical properties of recycled carbon fiber-reinforced concrete were studied through a compression test, three-point bending test, and impact test.
