ReviewLong-term mechanical properties of FRP tendon–anchor systems — A review
Introduction
For decades, the construction and maintenance of civil infrastructure have been of particular concern worldwide. However, the development of construction materials remains far from reaching the expectation of designers and engineers. For example, the specific strength of conventional steel materials is considered insufficiently high to encourage the construction of super long-span bridges. Moreover, given the increasingly serious steel corrosion problem that may lead to the premature failure of the entire structure, developing advanced materials with superior corrosion-resistant property is highly desirable. These issues are expected to be addressed with the development of fiber-reinforced polymer (FRP). Nonmetallic FRP, a composite material that is composed of load-carrying fiber and resin matrix, has been regarded as a competitive substitute for steel due to its light weight (approximately 0.2 times the weight of steel), high strength (more than twice the tensile strength of high-strength steel), and excellent corrosion resistance [1]. These superior properties allow FRP products to be widely utilized in external strengthening, seismic retrofitting, prestressed concrete, and cable structures. Two typical CFRP bridges, constructed in the USA and China respectively, are shown in Fig. 1 [2], [3]. The fundamental information regarding the short-term performance of FRP tendons, including tensile strength, rupture strain, elastic modulus, transverse performance, fracture behavior and failure mechanism, has already been summarized comprehensively in several studies [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. In terms of long-term behaviors, the typical time-dependent behaviors (including creep, stress relaxation, and fatigue) of FRP tendons under actual field conditions remain unclear. This study focuses on the long-term mechanical properties of fiber-reinforced polymer (FRP) tendon–anchor systems that are commonly available in the form of rods, cables, and multiwire strands.
FRP tendon–anchor systems currently use four types of FRP tendons: aramid FRP (AFRP), carbon FRP (CFRP), glass FRP (GFRP), and the latest is basalt FRP (BFRP). Table 1 presents a consolidated view of various commercially available FRP tendon products with different types of fibers and resins. The conventional anchorage systems developed for steel strands are not recommended for anchoring FRP tendons due to the orthotropic characteristic of FRP. At present, three types of anchorage systems, namely, mechanical-, bonded-, and composite-type anchorages, are specially developed for FRP tendons [20], [21]. A typical schematic of these anchorage systems is shown in Fig. 2. The performance of anchorage is of increasing interest because it plays an important role in the practical applications of FRP tendon–anchor systems. A number of experimental investigations have focused on testing the anchorage efficiency and short-term behavior of these anchorage systems [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], whereas their long-term performance requires more in-depth research. Notably, the service life of FRP tendon–anchor systems strongly depends on the long-term behaviors of the two aforementioned integral components. At this point, a review of their long-term mechanical properties will be beneficial to identify critical issues and provide guidance for the future development of FRP tendon–anchor systems.
In the present study, the three typical time-dependent behaviors, namely, creep, stress relaxation, and fatigue, of the four aforementioned types of FRP tendons are first studied, followed by a literature review of the long-term mechanical properties of mechanical-, bonded-, and composite-type anchorage systems. Subsequently, the application of numerical models for simulating and predicting the time-dependent behaviors of FRP tendon–anchor systems is briefly introduced. Lastly, suggestions for future research on the long-term behaviors of FRP tendon–anchor systems and the development of long-span CFRP cable-supported bridges are discussed and proposed.
Section snippets
Time-dependent behaviors of FRP tendons
When subjected to load or deformation, viscoelastic FRPs exhibit three time-dependent behaviors that manifest in several forms: (1) creep, a progressive increase in strain under constant stress; (2) stress relaxation, a gradual reduction in stress under constant strain; and (3) fatigue, progressive damage under cyclic loading. In general, the time-dependent behavior of FRP tendons significantly influences the application of FRP tendon–anchor systems. Thus, a comprehensive review of the
Long-term mechanical properties of anchorage systems
Premature failure at the anchorage is widely known as a common failure mode of FRP tendons due to their orthotropic characteristic. Thus, a reliable anchorage system is essential for high-performance FRP tendons to fully take advantage of their excellent properties. Several novel anchorage systems have been specially developed for FRP tendons, and the detailed information of the design concept, anchoring mechanism, geometric configuration, and static performance of these anchorages was
Numerical modeling
An experimental study is generally regarded as an effective and reliable approach to directly examine the static and long-term performance of FRP tendons and anchorage systems. However, performing high-cost and time-consuming experiments is difficult for researchers because sufficient testing resources, such as materials, apparatuses, sites, and technical personnel, must be provided. Hence, numerical analysis methods, which include the finite element method, are used to simulate and predict the
Discussion and future research
In this section, three aspects of research issues concerned with further studies and applications of FRP tendon–anchor systems are discussed as follows.
First, despite the differences in time-dependent behaviors among various FRP tendons, all tendons share the common goal of enhancing their long-term mechanical properties. To date, three enhancement methods, namely, (1) pretension treatment [4], [5], [12], [13], [15], (2) matrix modification [65], and (3) hybrid technique of composites, have
Conflicts of interest
The authors wish to confirm that there are no known conflicts of interest associated with this publication.
Acknowledgments
The authors of this paper would like to express their gratitude for the financial support provided by National Natural Science Foundation of China (No. 51778059), China, China Postdoctoral Science Foundation Funded Project (No. 2015T80996), China, and Fundamental Research Funds for the Central Universities (No. CHD300102219220), China.
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