Flexural response of coated steel-reinforced seawater-sea sand concrete beams incorporating black sand from Egypt

The construction industry faces increasing environmental challenges due to freshwater scarcity and river sand depletion, prompting a shift toward sustainable materials and methods. One promising approach is the use of seawater and sea sand in concrete, which conserves freshwater, reduces river sand exploitation, and aligns with global sustainability goals by utilizing abundant local resources. This study investigates the mechanical performance and durability of reinforced concrete (RC) beams made with different combinations of seawater, sea sand, river water, and river sand, subjected to varying durations of exposure to a marine environment. The experimental program includes testing concrete cubes, cylinders, and RC beams to assess parameters such as compressive and tensile strengths, load-deflection behavior, cracking patterns, stiffness, and ultimate load capacity. The use of Sulfate Resistant Cement (SRC) and anti-corrosive coatings on the steel reinforcement was incorporated to mitigate chloride-induced corrosion and sulfate attack. Results show that concrete mixtures incorporating seawater and sea sand consistently outperformed those made with river water and river sand, particularly in terms of early-stage strength development and long-term durability. For instance, the compressive strength of seawater-sea sand mixtures was up to 24.4 % higher than their river water-river sand counterparts, while ultimate load capacities of beams were increased by up to 23.5 % in the same conditions. Beams exposed to seawater demonstrated a gradual reduction in performance with prolonged exposure, but seawater-sea sand beams maintained superior stiffness and load-carrying capacity over time compared to river water-based mixtures. Furthermore, beams using seawater and sea sand exhibited more ductile flexural failure modes, characterized by gradual crack widening and higher ductility, compared to the brittle shear failure observed in river water-river sand beams. The application of SRC and anti-corrosive coatings played a significant role in maintaining the structural integrity of the beams, limiting the detrimental effects of chloride ingress and sulfate expansion. This study highlights the potential benefits of seawater and sea sand in concrete for marine structures, especially when coupled with protective measures such as SRC and anti-corrosive coatings. Future work should explore larger-scale applications and investigate the performance of alternative cementitious materials and coatings in harsher marine environments. • Seawater and sea sand improved early-stage compressive and tensile strengths of concrete. • Extended seawater exposure enhanced capacity of seawater-sea sand beams up to 23.5 %. • Seawater and sea sand beams exhibited superior stiffness and cracking load improvements.