In order to improve the brittle characteristics of coral cement-based composites and increase their toughness, an experimental study was carried out on the basic mechanical properties of PVA (polyvinyl alcohol) fiber-reinforced coral cement-based composites, taking into account the fiber content and length-to-diameter ratio (L/D). The results showed that PVA Fibers can effectively improve the mechanical properties of concrete, especially its tensile strength. At the same time, PVA fibers improved the damage characteristics of cement-based composites and had obvious toughening and brittleness reduction effects. The PVA fibers, with a volume content of 1.5% and an L/D of 225, had the best performance in reinforcing the overall performance of the coral cement-based composites. Too many PVA fibers or a large length-to-diameter ratio would make it difficult for the fibers to contribute to toughness and cracking resistance and even cause defects in the matrix, reducing the mechanical properties. The tensile stress-strain curves of PVA fiber-reinforced coral cement-based composites were consistent with the trilinear constitutive model curves and showed the tensile characteristic of strain hardening after the occurrence of the main cracks.
There are nearly 10 million square kilometers of coral reefs and coral islands in various forms in waters between 30 degrees north and south latitude on Earth, especially in the central and western Pacific, such as along the Mediterranean Coast, in the Persian Gulf of Iran, and in the Red Sea of Egypt. Coral reefs are special rock masses formed by the limestone shells of reef-building coral colonies over long periods of compaction and fossilization after they die, in which the colonies secrete calcium carbonate and absorb carbon dioxide from seawater as they grow. Sometimes, under the action of hydrodynamics, coral reefs are broken down into coral sand with calcium carbonate as the main component, which is a typical marine rock and soil with a wide particle size distribution, mainly distributed in sandy and silty soil sequences [1]. Marine engineering constructions far from the mainland are restricted by their geographical location and ocean conditions, so transporting building materials from the mainland on a grand scale is neither economical nor practical. The local use of coral sand as an aggregate to prepare cement-based materials can effectively solve the problem of the lack of traditional building materials, which has outstanding economic benefits and strategic significance [2,3,4].
Coral sand is classified as a natural light aggregate due to its light weight, porosity, and low cylindrical compressive strength. Its particles are irregular in shape and prone to breakage, and it has abundant pores its surface and inside. The physical properties of coral sand are completely different from those of terrestrial sand [5,6]. Therefore, the porosity of the coral concrete prepared by conventional methods is as high as approximately 20%, which is obviously higher than that of ordinary sand concrete. Further improvement of the strength grade of coral concrete when it reaches C30–C50 is difficult [7,8,9]. Under uniaxial compression, coral concrete has a steeper stress-strain curve, which is typical of splitting failure. The higher the strength grade of coral concrete is, the stronger the brittleness character [10,11]. Coral concrete has great discreteness in mechanical characteristics, and its axial compression ratio and tensile compression ratio are higher than those of ordinary sand concrete [12,13]. The elastic modulus of coral concrete is lower than that of ordinary sand concrete and lightweight aggregate concrete with the same strength grade, and Poisson’s ratio is similar to that of lightweight aggregate concrete with the same strength grade [14]. The above studies show that compared with ordinary sand cement-based composites, coral cement-based composites have disadvantages such as low strength, high brittleness, and large deformation, which have hindered the extensive use of coral concrete [15,16], especially for protective engineering.
Since the 1960s, fiber reinforcement and material composites have become important topics in construction materials research. Romaldi et al. [17] suggested improving the brittleness of concrete and limiting the development of cracks by adding fibers, and the tensile strength and toughness of concrete were greatly enhanced. Rao Lan et al. [18] proposed that PVA fiber-reinforced coral concrete has the characteristics of high ductility, high toughness, and multiple cracking, which can effectively address the weaknesses of the brittle failure of concrete. Takashif et al. [19] found that, under a high stress level, cement-based composites have better resistance to fatigue damage and stronger resistance to deformation and multipoint cracking than ordinary fiber-reinforced concrete. Kong Yan et al. [20] showed that the tensile strength and flexural strength of engineering cement-based composites mainly depend on the fiber volume content, and the fiber content controls the strain hardening and softening behavior of the engineering cement-based composites. Izaguirre et al. [21] prepared polypropylene fiber lime-based mortar. Polypropylene fibers can not only improve the tensile properties of lime-based mortar but also reduce its plastic shrinkage and early drying shrinkage deformation. PVA fibers can restrain crack expansion in cement-based composites, improve the mechanical properties of cement-based composites, enhance toughness, and prolong the fatigue fracture life [22,23].
Due to the particularity of coral cement-based composites, the large amount of salt, and the relatively limited application scope, existing studies on fiber-reinforced coral cement-based composites are mainly aimed at those with strength grades below C50 and are still in the initial exploration stage [24,25]. To meet the requirements on coral cement-based composites for blast resistance and impact resistance in offshore reef protection projects and change the current situation that coral cement-based composites can only meet the needs of civil use and some special cases, in this study, based on the low-cost preparation technology of medium- and high-strength coral cement-based composites in the early stage [26], PVA fibers with high toughness, high crack resistance, high deformation, and corrosion resistance, which are used in common cement-based composites, are adopted to reduce the brittleness and improve the toughness of high-strength coral cement-based composites [27]. The basic mechanical properties of PVA fiber-reinforced coral cement-based composites with high strength were tested, and the effects of the PVA fiber content and length-to-diameter ratio (L/D) on the mechanical properties of the coral cement-based composites were analyzed to reveal the strengthening mechanism of its PVA fibers. The research results can lay a theoretical foundation for the preparation of high-strength and high-toughness coral cement-based composites and their application in protective engineering.