<p>Sustainability in construction can be enhanced by focusing on reducing, reusing, and recycling waste materials. Cement production is a significant contributor to anthropogenic carbon dioxide emissions, and reducing its usage in concrete structures is crucial for making concrete more environmentally friendly. This can be achieved by increasing the use of supplementary cementitious materials in concrete or substituting less stressed or underutilized concrete with more sustainable, low-cost alternatives, such as alternative aggregates or bricks. This study explores the use of infilled reinforced concrete beams as a sustainable construction approach. In traditional reinforced concrete beams, the concrete below the neutral axis experiences less stress and can be replaced with lightweight materials, such as bricks, to reduce the overall weight of the structure and lower costs. The replacement zone is determined using stress–strain curves specified in IS: 456, along with the properties of the substitute materials. Since infilled beams function as composite members, a suitable analytical approach is required. This study employs the Method of Initial Functions (MIF) to analyze the behavior of infilled reinforced concrete composite beams.</p>