Fiber-Reinforced Polymers and Fiber-Reinforced Concrete in Civil Engineering

1. Introduction

Concrete has become one of the most widely used structural materials in civil engineering. However, the bottlenecks of low tensile strength and poor crack resistance are inevitable in concrete. At the beginning of the 20th century, researchers proposed the idea of mixing short fiber to improve the crack resistance of concrete, which was inspired by the method of delaying wall cracking by mixing straw or wheat straw into soil in ancient times. From the 1910s to the 1930s, patents were applied in countries such as the United Kingdom, the United States, and France to mix steel fibers for concrete modification. The “fiber crack resistance mechanism” was first proposed in the 1960s, promoting the development of steel fiber reinforced concrete. Since the 1970s, steel fiber reinforced concrete has been used in countries such as the United Kingdom, the United States, and Japan. After a development period of more than 50 years, fiber reinforced concrete (FRC) has been widely studied and applied [1,2,3].

Unlike short fiber reinforced composites, fiber reinforced polymers (FRPs), which are composed of continuous fibers and resin matrix, are lightweight and improve the strength/mass ratio significantly. Additionally, they possess advantageous durability and fatigue resistance. The commonly used FRPs in civil engineering include carbon FRP (CFRP), glass FRP (GFRP), aramid FRP (AFRP), and basalt FRP (BFRP). The development of FRPs began in the 1940s, when glass fiber was applied in the aviation industry. Subsequently, carbon fiber, aramid fiber, and basalt fiber were successively developed. Since the 1980s, safety and durability issues have emerged in engineering structures in developed countries such as the United States and Japan. Noting the opportunity, strengthening using FRPs was applied in those countries owing to the advantages of FRPs mentioned above. After the Hanshin earthquake in Japan, rapid and effective strengthening/repair methods were urgently necessary for the bridges and buildings damaged in the earthquake. The externally bonded carbon fiber sheet [4,5] has played an important role in post-earthquake strengthening and repair. Since then, FRPs have gradually been promoted and applied in civil engineering, receiving widespread attention from engineers. Nowadays, FRPs are widely applied both in the strengthening of existing structures and in new constructions, which demonstrates their advantages in improving the serviceability, bearing capacity, durability, and fatigue life of engineering structures [6,7].

2. Frontiers and Challenges of FRP and FRC

2.1. FRP

2.1.1. Fibers

With their rapid development, the types of fiber materials tend to be diverse. Carbon fiber, glass fiber, and basalt fiber have been widely used in civil engineering. The future development is aimed at developing fibers with higher strength, higher elastic modulus, and high-temperature resistance. Furthermore, with the increasing awareness of environmental protection, studies on green and recyclable types of fiber [8] are also of great significance in practical engineering.

2.1.2. Resin Matrix

Aging problems exist in resin matrix [9]. Therefore, the development of high-durability resin products is of great significance in improving the durability of FRP materials. In addition, FRPs rely mainly on the deformation of the resin matrix to dissipate energy under fatigue loads. Therefore, the research and development of resins with high toughness and fatigue resistance is crucial to improving the fatigue resistance of FRPs. On the other hand, the majority of FRP products used in engineering are manufactured using thermosetting resins, which cannot be reprocessed after curing. Thus, the research and development of reusable thermoplastic resins [10] is also an important direction for future development.

2.1.3. FRPs and Their Application in Construction

At present, civil engineering is developing in the direction of large span, light weight, long life, high durability, and damage controllability. With the complexity of structural forms and the harsh service environment, demands have also been put forward for FRPs. The development trends and challenges are listed as follows: first, the material system of FRPs needs to be optimized, and the fracture mechanism of FRPs under complex service environments and stress should be revealed in order to maximize the excellent performance of the fibers; second, manufacturing technology and equipment for FRPs should be improved, as should the quality and stability of FRPs; third, FRP application technologies need to be developed for the longevity, damage control, and light weight of engineering structures; furthermore, the concept of green energy conservation of FRPs should be implemented, and research on the full life cycle assessment of FRPs should be promoted; finally, further improvements to the standard system with respect to the design, production, testing, and construction of FRPs and structures using FRPs as reinforcement are necessary.

2.2. FRC

2.2.1. Mechanical Property and Durability

The mechanism of the strength and toughness enhancement of concrete via short fibers has been revealed [11], but there remain numerous issues that require further study; first, the state of fibers under load in FRC needs further exploration; second, the corrosion problems of alkaline cement on glass fibers, basalt fibers, and polyester fibers are noteworthy; in addition, the morphology of fibers and fabrics in FRC under long-term loading remains uncertain; moreover, the impacts of chemicals in the service environment on the performance of FRC [12] need to be studied; finally, there is a lack of research on the fatigue behavior of FRC, and studies on long fibers and their fabrics in concrete are rarely reported.

2.2.2. Manufacturing Technology

In terms of the manufacturing of FRC, suitable products should be designed and manufactured based on the special demands of FRC performance, e.g., impermeability, durability, stability. The flowability and operability during the production process should also be considered, and the contradiction between the final performance of FRC and the characteristics and manufacturing process of fibers is yet to be resolved; in addition, contradictions also exist between the reinforcing effects of length-diameter ratio and fiber content of fibers on FRC, and the difficulty of mixing and operation of the mixture during the manufacturing process. On the other hand, due to the high specific surface area and susceptibility to damage, it is difficult to mix short fibers in concrete, and effective solutions have not been proposed.

3. Purposes of the Special Issue

Building on the above perspectives, this Special Issue “Fiber-Reinforced Polymers and Fiber-Reinforced Concrete in Civil Engineering”, seeks to contribute to the latest advancements in the research and application of FRPs and FRC. Potential topics include but, are not limited to, new materials/systems/techniques, durability/long-term performance, bond behavior, strengthening of concrete/metallic/timber/masonry structures, concrete structures reinforced/prestressed with FRPs, hybrid structures, all FRP structures, structural health monitoring and intelligent sensing, codes/standards/guidelines, field applications/case studies, and the high performance/longevity/sustainability of structures.