Presented by: Prof. Dr. Mayada Waheed Falah Head of the Civil Engineering Department – Al-Mustaqbal University The disposal of construction and demolition waste is a growing environmental and engineering challenge, especially with urban expansion and the continuous need for building materials. One promising solution is to recycle demolished concrete and turn it into aggregate that can be used as a partial or full substitute for natural aggregate in concrete production. This approach is an important step toward strengthening the concept of the circular economy and reducing the depletion of natural resources, while maintaining the requirements of safety and structural performance. Mechanical Performance of Recycled Concrete Recycled concrete aggregate is usually produced by crushing, sorting, and cleaning concrete waste, and residues of old cement mortar may remain on the surface of its particles. These residues affect porosity, water absorption, and density, which may in turn influence the properties of the new concrete. Studies indicate that the effect of recycled aggregate depends on its quality, the replacement ratio, the treatment method, and the concrete mix design. Compressive, tensile, and flexural strength and the modulus of elasticity may decrease as the replacement ratio increases in some mixes, and workability, shrinkage, and durability may also be affected. Nevertheless, these effects can be limited by controlling the water-cement ratio, correcting the aggregate’s moisture content and absorption, improving the particle size distribution, and using appropriate curing and admixtures when needed. In structural elements such as beams, columns, and slabs, evaluating compressive strength alone is not sufficient; rather, load-bearing capacity, cracking, the bond between concrete and reinforcing steel, deformations, and long-term durability should be studied in accordance with the design requirements and the adopted code. Sustainability and Environmental Impact The use of recycled aggregate helps reduce the quantities of demolition waste sent to landfills and limits the extraction of natural sand and gravel. It may also help lower the environmental impact associated with transport and material extraction, depending on the location of the recycling facility and the efficiency of the crushing and treatment processes. However, achieving sustainability does not depend on recycling alone; the environmental benefits must be balanced against the energy and water consumed in processing and transport, as well as the structure’s service life and maintenance requirements. Therefore, life cycle assessment is an important tool for comparing conventional concrete with concrete containing recycled aggregate. Requirements for Safe Use To ensure effective use in structural elements, it is recommended to carry out tests for particle size distribution, density, water absorption, and impurity content, in addition to tests on fresh and hardened concrete. Trial mixes and appropriate tests should also be carried out to verify compressive, tensile, and flexural strength and durability, and then the replacement ratios should be adopted according to the results and the requirements of the applicable code. Conclusion Recycled aggregate represents a practical opportunity to combine engineering performance with environmental sustainability. However, the success of its use in structural elements requires selecting materials of suitable quality, a well-considered mix design, quality control, and a comprehensive evaluation of performance and durability. Under these conditions, part of construction waste can be turned into a valuable resource that supports the development of a more efficient and sustainable construction sector.