Recycled aggregate concrete

Construction and demolition waste is one of the largest waste streams in growing cities, and much of it is old concrete. Crushed and processed, it yields recycled concrete aggregate: natural stone particles with some old mortar still attached. Using it in new concrete saves quarrying, landfill space and transport, and is part of a circular approach to construction.

The attached mortar changes the properties. Recycled aggregate is more porous, so it absorbs much more water than natural aggregate, has lower density and is weaker, with a higher crushing and abrasion value. Concrete made with it tends to have somewhat lower strength and modulus and higher shrinkage and creep, the more so as the replacement percentage rises. The interface between old mortar, new paste and aggregate creates more weak zones.

Good practice manages these effects. Aggregates are sorted and washed to remove brick, gypsum, wood, glass and other contaminants. Water absorption is measured and the mix water adjusted, or the aggregate is pre-soaked, so that workability and the effective water-cement ratio are controlled. Replacement levels are kept moderate for structural use, and higher levels are used in lean concrete, sub-base, blocks and kerbs. Two-stage mixing, coating the aggregate with a cement slurry first, can strengthen the interface.

IS 383:2016 recognises recycled concrete aggregate and sets limits on its proportion, the grades and the uses for which it may be employed. Research continues on durability, carbonation, and treatments such as accelerated carbonation of recycled aggregate.