Strength and durability of coconut-fiber-reinforced concrete in aggressive environments

Marine structures have suffered from seawater attacks for decades. Thus far, the best approach to minimize the deleterious effects on these structures is to use high-strength, high-performance concrete. However, this approach has its limitations. When a crack starts because of the expansion and shrinkage at splash zones and expansive products are formed because of sulfate attacks, the crack will grow and propagate uncontrollably. Ultimately, the durability of the structure is drastically reduced. The aim of this experiment is to mitigate this limitation by incorporating short, discrete coconut fibers into high-strength concrete. This method is based on the idea that the localized reinforcing effect provided by the discrete fiber can restrain the development of cracks caused by aggressive environments. The structures were exposed to three types of aggressive environments: air environment in a tropical climate (A-series), alternate air and seawater environments in a 14-day cycle (4 days wetting + 10 days drying) (N-series), and continuous immersion in seawater (W-series). Compressive and flexural parameters were used to examine the strength of each structure, while chloride penetration, intrinsic permeability, and carbonation depth were used to examine their durability properties. The mineralogy and microstructure were studied by means of X-ray diffraction and scanning electron microscopy examinations. The experimental results prove that the compressive and flexural strengths of the structures improve up to 13% and 9%, respectively, with the incorporation of coconut fibers. However, in terms of durability, the chloride penetration, intrinsic permeability, and carbonation depth increase with the increase in fiber content. Most importantly, in the intrinsic permeability, the plain specimen in the N-series showed a sudden increase in intrinsic permeability when the exposure period increased from 365 days to 546 days. This result signifies that the fibers play a role in restraining the development of cracks. In general, the deleterious effects brought about by aggressive environments can be suppressed with fiber-reinforced concrete. However, the dosage of coconut fiber should be low, not exceeding 1.2% of the binder volume, due to the drawback of its natural degradation. This study recommends that the coconut fiber undergo treatment prior to its application in concrete to protect it against degradation or that it be replaced with a non-corrosive fiber.