Infrastructure-Oriented Efficient Materials Implemented with Fibers

Abstract

Fiber-reinforced polymers (FRPs) and fiber-reinforced cementitious composites (FRCCs) have been widely applied in infrastructure projects. In this Editorial, the research status of FRPs is first reviewed in terms of fibers, resin matrix, and application technology, and the research progress of FRCCs is then reviewed in terms of mechanical property and durability, as well as application technology. Based on the frontiers and challenges of FRPs and FRCCs, the prospects for future studies are discussed.

1. Introduction

Since the 1980s, safety and durability issues have emerged in engineering structures in developed countries such as the United States and Japan. Seizing this opportunity, countries have applied fiber-reinforced polymer (FRPs) to strengthen their engineering structures. An FRP is composed of continuous fibers and resin matrix. It has lightweight, high-strength, durable, and fatigue-resistant properties. The commonly used FRPs in civil engineering include carbon FRP (CFRP), glass FRP (GFRP), aramid FRP (AFRP), and basalt FRP (BFRP). After the Hanshin earthquake in Japan, the external bonded CFRP [1,2] has played an important role in post-earthquake strengthening and repair. Moreover, FRP has gradually been promoted and applied in civil engineering, receiving widespread attention from researchers and engineers. Nowadays, FRP has been widely applied both in the strengthening of existing structures and in new constructions, which demonstrates its advantages in improving the serviceability, bearing capacity, durability, and fatigue life of engineering structures [3,4].

Cement has become one of the most widely used structural materials in civil engineering. However, it faces challenges such as low tensile strength and poor crack resistance. At the beginning of the 20th century, researchers proposed the idea of the addition of short fiber to improve the crack resistance and tensile strength of cement. 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 cement modification. Since the 1970s, steel fiber-reinforced concrete has been used in countries such as the United Kingdom, the United States, and Japan. With over 50 years of development, fiber-reinforced cementitious composites (FRCCs) have been extensively studied and applied [5,6,7].

2. Frontiers and Challenges of FRPs and FRCCs

2.1. FRPs

2.1.1. Fibers

Carbon, glass, and basalt fibers have been widely used in civil engineering. Future development aims to develop fibers with higher strength, greater elastic modulus, and improved 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

There are two main problems with the resin matrix [9]. One is the durability problem; therefore, the development of high-durability resin products is of great significance for improving the durability of FRPs. 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 are crucial for improving the fatigue resistance of FRPs. The other problem of the resin matrix concerns fire resistance, which requires the development of resin that is resistant to elevated temperatures, such as phenolic resin. Furthermore, the majority of FRP products used in engineering are manufactured using thermosetting resins, which cannot be reprocessed after curing. Thus, the development and research of reusable thermoplastic resins [10] are also an important direction for future development.

2.1.3. FRPs and Their Application in Construction

Civil engineering is advancing towards the direction of large spans, lightweight structures, long life, high durability, and damage controllability [11]. Thus, to maximize the excellent performance of fibers, the fracture mechanism of FRPs under complex service environments and stress should be understood, and corresponding optimization methods should be proposed. Additionally, FRP application technologies need to be developed to enhance the longevity, damage control, and lightweight nature 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 improvement of the specification system for the design, production, testing, and construction of FRPs and structures using FRPs as reinforcement is necessary.

2.2. FRCCs

2.2.1. Mechanical Property and Durability

At present, the mechanism for enhancing the strength and toughness of cementitious composites via short fibers is well known [12], but numerous issues still require further investigation. First, the state of fibers under load in FRCCs needs further exploration. Second, the corrosion problems of alkaline cement on glass fibers, basalt fibers, and polyester fibers are noteworthy. Third, the morphology of fibers and fabrics in FRCCs under long-term loading remains uncertain. Fourth, the impact of chemicals in the service environment on the performance of FRCCs [13] needs to be studied. Finally, there is a lack of research on the fatigue behavior of FRCCs, and the studies on long fibers and their fabrics in FRCCs are rarely reported.

2.2.2. Application Technology

Nowadays, various types of FRCCs such as fiber-reinforced concrete (FRC) [14], engineered cementitious composite (ECC) [15], and ultra-high-performance concrete (UHPC) [16] have been developed. They have been studied and applied by numerous researchers and engineers. These types of materials can not only be used as efficient materials for structural strengthening, but they also provide new methodologies for the revolution of infrastructures.

3. Purposes of the Special Issue

Nineteen papers have been published in the first edition of the Special Issue entitled “Fiber-Reinforced Polymers and Fiber-Reinforced Concrete in Civil Engineering”. Based on the success of the first volume, the second edition is to be launched to further promote the exploration of the studies on FRPs and FRCCs. The topics include the following: new materials/systems/techniques; durability and long-term performance; bond behavior; the strengthening of concrete, metallic, timber, and masonry structures; concrete structures reinforced/prestressed with FRPs; hybrid structures; all FRP structures; structural health monitoring and intelligent sensing; codes, standards, and guidelines; field applications and case studies; and the high performance, longevity, and sustainability of structures.