In recent years, researchers have conducted extensive studies on the factors affecting the chloride corrosion resistance and long-term durability of SFRC, evolving from single-factor analysis to considering multi-factor coupling effects [75]. As shown in Figure 4, the influencing factors are categorized based on steel fiber, concrete matrix, and on-site construction processes. First, the influencing factors related to steel fibers include material, size, shape, distribution, coating, and dosage. Second, various factors of the concrete matrix also affect the chloride corrosion resistance of SFRC. For example, the impact of crack width in the concrete matrix on chloride resistance has been discussed in Section 2.1. In addition, the cover layer, which is the outermost part of the concrete from the surface to the steel fibers, directly affects the penetration path of chloride ions. As the cover layer thickness increases, chloride ions require more time to reach the steel fibers, thereby delaying corrosion. Furthermore, a thicker cover layer helps slow down the ingress of oxygen and moisture, further reducing the corrosion rate of steel fibers. However, an excessively thick cover layer may increase the risk of concrete cracking, affecting long-term durability. The water-to-cement ratio (w/c) determines the porosity and permeability of concrete [39]. A lower w/c typically reduces porosity and increases the density of concrete, thereby decreasing the chloride ion diffusion rate and enhancing chloride corrosion resistance. Conversely, a higher w/c results in more interconnected pores, facilitating chloride ion penetration and making steel fibers more susceptible to corrosion. Therefore, to improve durability, it is essential to optimize the w/c, ensuring sufficient workability while maintaining good compactness. Finally, the construction techniques of SFRC on-site are also crucial factors that require careful consideration. For instance, high-quality formwork can reduce bleeding and shrinkage cracks during concrete casting, ensuring surface density and thereby reducing chloride ion penetration risks. Smooth and dense formwork enhances the quality of the concrete surface, minimizing surface microcracks and further improving chloride corrosion resistance [41]. Moreover, appropriate curing temperature and humidity facilitate complete cement hydration, increasing concrete density and reducing porosity, thus lowering chloride ion penetration rates. Although high curing temperatures can accelerate early strength development, they may lead to uneven hydration product distribution, forming large pore structures that reduce long-term durability.
This section discusses in detail the mechanisms by which steel fibers influence the chloride corrosion resistance of SFRC, considering factors such as the material, coating, shape, and dosage of the steel fibers.
3.1. Types of Steel Fibers
There is no doubt that the chloride corrosion resistance of SFRC is influenced by the material and alloy composition of the steel fibers [76]. Different countries have established clear classifications for steel fibers based on their raw materials. For example, the standards for steel fibers for concrete released in China categorize them into carbon steel, alloy steel, stainless steel, and other types [77]. Similarly, the classifications advocated by the European Union include cold-drawn carbon steel (including low-, medium-, and high-carbon steel), surface coatings, and stainless steel [78].
Different alloy components in steel fibers, such as nickel, chromium, and manganese, can significantly enhance their corrosion resistance [79]. For instance, a study exposing SFRC to a beach environment and subjecting it to 2000 wet–dry cycles indicated that stainless-steel fibers (with nickel and chromium contents of 0.58% and 17.6%, respectively) showed no signs of corrosion when exposed on the concrete surface. However, under the same exposure conditions, low-carbon steel fibers (with nickel and chromium contents of 0.06% and 0.04%, respectively) experienced corrosion after 150 wet–dry cycles, which gradually progressed to widespread corrosion [35]. Therefore, steel fibers with higher carbon content are generally more susceptible to corrosion, while micro-alloying elements such as vanadium and titanium can enhance their corrosion resistance and strength. Furthermore, studies have shown that the addition of manganese effectively improves the corrosion resistance of the alloy. The depth of corrosion products is reduced with manganese addition. The primary reason for the enhanced corrosion resistance is the formation of an oxide film that inhibits chloride ion penetration [80]. However, due to the strong affinity between manganese and sulfur, the formation of manganese sulfide often leads to a decline in the pitting and crevice corrosion resistance of austenitic stainless steel in chloride environments. The crystalline structure of the material (such as martensite or austenite) also affects the corrosion resistance of steel fibers [81]. Austenitic stainless steels typically have higher chromium and nickel content, which allows for the formation of more stable passive films, enhancing their resistance to pitting corrosion. In contrast, martensitic stainless steels have lower chromium content and generally contain little or no nickel, making their passive films more prone to breakdown.
On the other hand, the surface condition of the steel fibers is crucial, as processes such as heat treatment and cold working can influence their corrosion resistance. For example, the cold drawing process creates a more uniform surface on the steel fibers, thereby suppressing the occurrence of pitting. In summary, the material and compositional factors of steel fibers jointly determine their performance in marine corrosion environments, which in turn affects the overall chloride corrosion resistance of SFRC.
3.2. Surface Coatings for Steel Fibers
The use of anti-corrosion coatings can effectively enhance the corrosion resistance of steel fibers to a certain extent [82,83]. For example, a zinc phosphate coating [84] improves the corrosion potential and reduces the corrosion rate of steel by consuming the zinc coating during the anodic process. The coating exhibits strong resistance to various chemicals (such as chloride ions), thereby providing more stable protection in marine corrosion environments. As shown in Figure 5a,b, the surface of ordinary steel fibers is relatively flat with minor undulations, whereas the surface of zinc phosphate-coated steel fibers exhibits noticeable undulations and a distinct conical structure, with particles arranged more closely. After undergoing 30 wet–dry cycles of corrosion, the zinc phosphate-coated steel fibers, while avoiding the occurrence of microcracks, showed little overall change in surface morphology, with the surface becoming rougher [48]. This also provides evidence that zinc phosphate-coated steel fibers can protect the fiber matrix from corrosion and maintain good bonding performance with concrete. Similar studies have shown that after 15 corrosion cycles, the loss of bonding strength in zinc phosphate-coated fibers is negligible compared to that of ordinary steel fibers [85].
Additionally, some researchers have attempted to use graphene oxide-coated steel fibers to improve the interface transition zone and enhance the overall performance of SFRC [86]. The results show that the surface roughness and hydrophilicity of the graphene oxide-coated steel fibers treated with a three-step coating method increased by approximately 280.6% and 40.6%, respectively. At the same time, graphene oxide-coated steel fibers can significantly reduce the porosity of SFRC and optimize the pore size distribution, which will improve its resistance to chloride ion corrosion. Recently, researchers have focused on the surface modification of steel fibers using nanomaterials. The latest studies suggest that nano-SiO2 is a promising coating material for steel fibers because it can react with Ca(OH)2 to form calcium silicate hydrate (C-S-H), effectively improving the adhesion between the steel fibers and the cement matrix. The use of these coating materials greatly contributes to enhancing SFRC’s resistance to both load and environmental corrosion [87].
The bond strength between the coating and steel fibers is a key issue. Different types of coating materials may exhibit variations in their adhesion to the surface of the steel fibers. Inadequate adhesion of the coating could lead to delamination or peeling, which in turn affects the mechanical properties and durability of the concrete. Additionally, ensuring that the coating is uniformly applied to all steel fibers without any omissions remains a technical challenge. The uniformity of the coating is crucial for the performance of the steel fibers in the concrete, as uneven coatings may result in instability in the concrete’s strength and corrosion resistance. Reference [86] compared three different steel fiber coating methods and showed that some coating methods negatively affect the microstructure of SFRC. The standardized coating specifications for steel fiber surface modification have not yet been established. Some literature indicates that while the use of coatings prolongs the initiation time of corrosion in the steel matrix, unfortunately, it cannot prevent the development of corrosion over long periods in corrosive environments. Therefore, once the coating is removed due to local damage or large strain in the steel, conditions for electrochemical corrosion reactions are established, particularly in the presence of electrolytes, where localized anodic and cathodic reactions may become more pronounced, leading to an accelerated corrosion rate.
3.3. Shape of Steel Fibers
To enhance anchoring strength and bonding with the concrete matrix, the ends of steel fibers are typically designed to be hook-shaped, corrugated [88], or conical [43]. The importance of using shaped steel fibers in providing superior pull-out resistance has been well recognized compared to straight steel fibers [58]. Especially, the hooked design facilitates the use and placement of steel fibers during manufacturing and handling, allowing for more uniform dispersion in concrete. Therefore, hooked-end fibers are the most widely studied and applied (Table 1).
However, researchers have found that the bends and ends of hook-shaped steel fibers in the concrete matrix are more susceptible to chloride-induced corrosion. As shown in Figure 6, there are three main reasons: On the one hand, the hook structure creates stress concentrations at the bends and ends, where mechanical stress is higher, leading to the formation of micro-defects that allow corrosive media to penetrate more easily, resulting in preferential anodic pitting and accumulation of corrosion products [89]. On the other hand, under load, the contact between the concrete matrix and the steel fibers may fail, creating gaps that facilitate the penetration of moisture, chloride ions, and other corrosive media [42]. Finally, the damaged interfacial transition zone provides a preferential pathway for the transport of chlorides, metal ions, and oxygen, thereby accelerating the corrosion process [90].
3.4. Dosage of Steel Fibers
A substantial body of literature indicates that SFRC exhibits different dosage thresholds for various performance indicators, such as tensile strength, shrinkage strain, and thermal conductivity [91,92,93]. When designing concrete mix proportions, it is essential to consider the balance among different performance characteristics [94]. There is also a dosage threshold for steel fibers concerning the performance indicators for resisting chloride corrosion. Below this threshold, an increase in fiber dosage optimizes the pore structure of SFRC, as the dispersed steel fibers disrupt the continuity of pores and the interconnectivity of porous channels within the concrete matrix. As the steel fiber dosage increases, the proportion of closed pores within the total porosity also increases [91]. Conversely, above the threshold, the porosity increases with fiber dosage, leading to the degradation of the concrete’s pore structure, which negatively impacts its resistance to chloride ion corrosion [95]. Moreover, a high dosage of steel fibers, particularly those with irregular ends, tends to agglomerate, forming “bridging” or “layering” structures that further deteriorate the pore structure. Certainly, the steel fiber dosage threshold is characteristic and dependent on factors such as environmental conditions, concrete components, and other material properties. For instance, results from a case study [40] indicate that the volume fraction threshold for the durability of SFRC is 1.5%. Similarly, a case study [96] analyzed the mapping relationship between the pore structure of SFRC and steel fiber dosage using mercury intrusion porosimetry and scanning electron microscopy, showing that when the steel fiber dosage increased from 1.5% to 2%, the number of large pores in the concrete significantly increased. Research under the coupled conditions of salt freeze–thaw and wet–dry cycles indicates that 2% is the dosage threshold for changes in porosity induced by SFRC [53].
Furthermore, excessive use of steel fibers may lead to a decrease in the workability and constructability of concrete, increasing construction costs. In the design process of subsea tunnel structures, reasonably determining the dosage threshold helps ensure optimal performance, and different thresholds may be required for SFRC components under various uses and environmental conditions. Certainly, the above conclusions are based on the assumption of a uniform distribution of steel fibers; excessive fiber dosage can lead to non-uniform fiber density, which weakens the structure’s resistance to chloride corrosion, with areas of the lowest and highest initial fiber density being most susceptible to surface damage [53].
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