Bamboo is still considered a material for poor people in rural areas. Still, it also has the potential to be fully developed engineering material reinforced structural beams with concrete infill to increase the structural strength of the structures, which is natural aesthetic, and more durable than bamboo alone as structural parts of buildings. This study aims to determine the flexural strength of bamboo with concrete infill as a structural beam subjected to a concentrated load at midspan. The materials used were whole bamboo culms, specifically Dendrocalamus Asper Schultes, Dendrocalamus Merrillianos Elmer, Bambusa Vulgaris Schrad, and Bambusa Blumeana Schultes cut into 600 mm lengths and treated with seawater and seawater with mango polyphenol infilled with concrete. The concrete was cured for 7, 14, 28, and 56 days, respectively. The results showed that the whole bamboo culm treated with seawater and treated with seawater plus a 10 percent solution of mango polyphenol extract remarkably increased its flexural strength. The concrete infill to the whole bamboo culm as the beam is effective, increasing flexural strength when treated with mango polyphenol. The curing age of concrete and treatment of bamboo culm with seawater and mango polyphenol affects the flexural strength of reinforced bamboo with concrete infill subjected to a concentrated load at midspan. Reinforced bamboo concrete infill-based construction materials can reduce pressure on resources and the environment. These materials have environmental and mechanical advantages over conventional construction materials.