Physical and Mechanical Evaluation of Concrete with Coffee Husk Ash and Coconut Fiber as Sustainable Additives

This study evaluates the combined addition of coffee husk ash (CHA) and coconut fiber (CF) to conventional concrete to enhance its physical and mechanical properties for structural applications. Motivated by the need for sustainable construction materials and utilizing agricultural waste, the study employs an applied experimental design with 200 test specimens made under controlled conditions using local materials. The CHA was calcined to meet ASTM C618 standards, and CF was alkali-treated to improve durability by increasing cellulose content and reducing hemicellulose and lignin. Three experimental mix designs with varying CHA and CF proportions were evaluated against a control mix, focusing on workability, compressive strength, tensile strength, flexural strength, modulus of elasticity, air content, and unit weight. Results revealed that the optimal mix—1.5% CHA combined with 0.65% CF—significantly improved compressive strength (+8.1%), flexural strength (+6.1%), and elasticity (+5.8%) without compromising workability or unit weight. Statistical analyses (ANOVA, Tukey, and Dunnett tests) confirmed the significance and synergy of this combination. Higher dosages resulted in diminished mechanical performance, indicating the importance of dosage control. The study also demonstrated that the physical properties of fresh concrete remained within standard specifications, ensuring the practical feasibility of these additives. This work contributes novel evidence supporting the sustainable reuse of agroindustrial residues to produce mechanically enhanced concrete, aligning with environmental goals and promoting circular economy models. By validating the synergy between CHA and CF through rigorous chemical characterization, mix design, and statistical validation, this research advances knowledge of bio-based concrete additives and offers practical guidelines for their optimized incorporation in structural concrete.