Concrete is used in the construction industry and exhibits the advantages of high strength, excellent plasticity, and easy access to materials, providing safe places to live for people around the world [1,2,3]. Concrete has been used for more than 200 years, but engineering is more concerned about mechanical properties than durability in the past. In fact, the service life of the engineering structures and the ability to operate effectively are determined by the durability [4]. As the aging problem of buildings becomes more and more serious, its durability is also getting more and more attention. Generally speaking, the degradation of the durability of concrete is related to the formation and development of cracks. When cracks appear, they accelerate the rate of transport of harmful ions such as chloride ions, leading to a decrease in the durability of the concrete structure, which in turn affects the safety of the structure [5,6]. It is well known that most of the engineering structures are in complex environments, resulting in concrete structures that are often subjected to a combination of temperature, high humidity, and high salt erosion [7,8]. This undoubtedly accelerates the formation of cracks.
In order to figure out the deterioration process and principles of cementitious composites (CCs) in different environments, many researchers have done a good deal of research. When CC is exposed to chloride solutions, some of the chlorides enter the material through the pores for free transport [9]. As a result, the compressive, tensile, and flexural strength properties of CC are reduced to varying degrees [10]. It has been demonstrated that the rate of chloride ion transport can be impacted by several elements – for instance, the pore structure properties of concrete itself, the chemical composition inside the pore solution, and the binding capacity of the chloride ions themselves [11]. Exposure to different salt solutions can also result in different chloride binding patterns. When exposed to sodium chloride solution, it is widely believed by domestic and foreign scholars that the Friedel salt (C3A–CaCl2–10H2O) is formed due to the association of 3CaO–Al2O3 (C3A) with chlorine ions through chemical binding. However, more chlorides are adsorbed on calcium silicate hydrate (C–S–H) by physical binding when exposed to CaCl2 solution. The role of physical adsorption is relatively minor compared to chemical binding methods, but it is also particularly significant because of the high number of C–S–H gels. The higher number of bound chloride ions reduces the content of free chloride ions and thus reduces the possibility of erosion of the reinforcement [9,11,12,13,14].
Due to the complex climate around the world, concrete is also susceptible to high temperature and high humidity in addition to the high-salt environment. With the increase in temperature from 200 to 800°C, hydration products decomposed continuously and the appearance of concrete changed from light gray to slightly yellow to grayish brown and finally grayish white, accompanied by the appearance of microcracks [15]. When cracks were formed, they inevitably had a negative effect on the properties of concrete. As the temperature continued to rise, the weight loss of the specimen increased due to the evaporation of water from within the concrete and the spalling of the concrete. The compressive strength and modulus of elasticity of concrete in high-temperature environments were reduced to varying degrees [16,17]. At the same time, high temperatures may lead to brittle failure of CC and reduce strain-hardening properties [18]. The results of a number of studies have already shown that temperature variations are sensitive to the effects of tensile behavior [19,20]. The humid and hot geological environments are distributed all over the world. Both temperature and humidity are high in the construction of underground works such as tunnels. A hygrothermal environment is defined as a construction where the temperature is greater than or equal to 50°C and the humidity is greater than or equal to 80% and where the underground environment is greater than or equal to 30°C and the humidity is greater than or equal to 60°C [21]. The results of numerous studies have shown that concrete erosion is more severe when it is subjected to the action of wet–heat–salt coupling environment (WHSCE).
Compared to natural conditions, salt erosion is more corrosive to concrete materials under the action of dry and wet (D–W) cycles. It is obvious that the strength grows in the early stages owing to the hydration of concrete and the penetration of salts when concrete is subjected to the combined action of both [22]. As the number of D–W cycles continues to increase, the damage to the concrete is also increasing. As a consequence, the relative dynamic modulus of elasticity of concrete shows a trend of decreasing. Zhang et al. studied the resistance of CC to chloride salt erosion by simulating marine tidal environment. It was found that the content of free chloride ions increased with the increased exposure time [23]. A similar study was carried out by Li et al. [24]. The variable of temperature was added to the experiment. It was found that the movement of chloride ions accelerated as the temperature increased, which also led to the formation of cracks, thus hastening the transport of chloride ions in the cementitious material.
Fibers can improve the durability of CC [25]. For instance, the strength loss of concrete can be reduced by using glass beads as well as steel and polypropylene fibers [26], where steel and polypropylene fibers can also reduce porosity and thus improve durability [21]. It has also been demonstrated that the presence of fibers has a beneficial influence on both strength and crack development [27,28,29]. In summary, the inclusion of fibers can be seen to have a good enhancing effect on the durability of concrete. This may be due to the fact that fiber-reinforced concrete allows the CC to have a uniform distribution of reinforcing components in all directions. In general, fibers can be classified as natural and synthetic fibers [30]. Since the fibers themselves exhibit good toughness and high aspect ratio, they can be used as a reinforcing material for CC to diminish the generation of cracks and also to strengthen the performance of concrete after cracking [31]. A great deal of research has been conducted on the incorporation of fibers in CC. These results show that different types of fibers have their own characteristics. Basalt fibers are more inclusive of temperature and have good acid resistance [32]. They can also enhance the toughness and impact resistance [33]. Nevertheless, tensile strength and elasticity modulus are quite at a low level [34]. When 2% steel fibers were added to the engineered CC, the compressive strength and flexural strength increased by approximately 11 and 23.9%, respectively [10]. However, it causes the concrete to become unstable and rusty, accelerating the rate of steel consumption [35]. In view of this, many scholars have performed studies on polyvinyl alcohol (PVA) fibers as additives to enhance the durability.
PVA Fibers have advantages of lower cost, better acid and alkali resistance, higher corrosion resistance, as well as a high modulus of elasticity. PVA fibers can form a mesh structure inside the concrete, making it less likely to crack when subjected to external forces [36,37]. Two of the more prominent advantages of PVA fibers are high tensile strength and good dispersibility [38,39,40]. The investigation results demonstrated that the splitting tensile strength of concrete increased by 4–7% when 0.2% PVA fiber was added [39]. Drying shrinkage performance is also an important performance of concrete. When PVA fibers are added, the cementitious material is more strongly bonded to the PVA fibers, and the fibers inhibit shrinkage by shearing along the fiber–matrix interface [41,42]. Based on this principle, it is believed that the drying shrinkage strain of concrete decreases as the PVA fiber admixture increases [39,43]. As it is known, fibers can effectively limit the development of cracks. Therefore, the resistance to chloride ion penetration can also be improved by adding PVA fibers. However, when the content of PVA fibers is excessive, an agglomeration effect occurs and the bond between matrix and fibers may be weakened, resulting in the formation of cracks, which in turn increases the risk of chloride ion entry and destroys the durability.
At present, there are plenty of studies on the impact of PVA fibers on the durability of CC under the single action of humidity, temperature, and salt or the synergistic action of both. But many engineering is in more complex engineering environments, and there are a few studies on the effects of coupling multiple environmental factors on durability. Furthermore, the research of PVA fibers on durability under the coupling effect of multiple factors is even less. In order to overcome this shortcoming of the existing literature, this study focuses on the effect of PVA fiber content on the durability of CC in complex environments. The purpose of this study is to analyze the mechanism of damage to durability of CC by the WHSCE and the influence of PVA fiber on durability of CC in the WHSCE. When PVA fiber is added, the stress–strain properties of the matrix are improved and self-healing can occur, which greatly improves the durability of CC. By adding PVA fiber, the durability of CC in the WHSCE is improved, which is helpful to the study of the durability of CC under multi-factor coupling environment, and will help to explore an effective method to extend the service life of CC in complex environments.