Construction and demolition (C&D) waste is one of the main sources of waste that occupies huge space in landfills and that can be recycled economically, without compromising the essential material properties. With the limited supply of natural materials, concrete recycling has gained much importance.Studies have revealed that concrete is the most commonly used product after drinkable water. Due to the recycling concrete, necessary disposal requirements of large quantities of used concrete, concrete recycling techniques have been developed, that can be utilized in a large number of applications. With the frequent demolition of roadways and buildings that have completed their useful life, large aggregates are available for recycling. The quality of recycled concrete is identical, or superior in a few specifications, compared to virgin aggregates. Compression and constructability characteristics are very good, and the yield of recycled mass is more. Therefore, expenditures on materials, hauling costs, and project costs are reduced. Furthermore, since limestone is not used, carbon dioxide is not released during the recycling of concrete, and harmful effects to the environment are minimized.
Chemical Properties Of Recycled Concrete. There are several issues regarding the use of recycled crushed aggregate (RCA) that should be considered analytically before the decision is taken to replace virgin aggregate. One of the foremost concerns is the chemical reaction that may occur between the alkaline water and the recycled crushed aggregate that causes a volumetric expansion, which may result in internal fractures and premature deterioration. Furthermore, sulphation of the recycled crushed aggregates may occur due to the reaction of plaster and gypsum in a moist environment. The chloride ions in recycled crushed aggregate may react with reinforced steel to form rust that may damage the concrete structure. Since the harmful contaminants cannot be eliminated completely, acceptable levels should be determined to achieve a quality product.
Aggregate Grading Of Recycled Concrete. Aggregate grading is one of the important issues that should be considered while using recycled crushed aggregate. Grading refers to the particle size distribution of the aggregate. Grade limits are significant since they determine the aggregate quantity to be used, durability and workability of the concrete, and the requirements of cement. It has been evaluated that approximately 25% of recycled crushed aggregate may be used in lieu of the conventional aggregate without affecting considerably the mechanical properties of the concrete manufactured, thus making the process more economical.
Quality Control. Recycled crushed aggregate normally produces a good quality aggregate that varies with the source of debris and the impurities present in the rubble. Contamination of the crushed aggregate by detrimental substances from buildings or other harmful materials, should be avoided since they affect the quality of concrete produced. Concrete taken from highways or pavements for recycling is not contaminated, though building concrete may contain a few contaminants like plaster, gypsum and vinyl, and their use should be carefully assessed. The properties of reclaimed concrete may differ from each other since the sources may be extensive, and its performance in a secondary application may not be good unless it performed well originally in the initial application.