Concrete is one of the most widely used construction materials in the world, mainly due to its favorable features such as durability, versatility, satisfactory compressive strength, cost-effectiveness and availability. Globally, the concrete industry consumes large quantities of natural resources, which are becoming insufficient to meet the increasing demands. At the same time, many old buildings have reached the end of their service life and are being demolished, resulting in waste concrete; some concrete waste is used as backfill material, with much being sent to landfills. Recycling concrete by using it as new aggregate in concrete could reduce concrete waste and conserve natural sources of aggregate. In the last two decades, a variety of recycling methods for construction and demolition wastes (CDW) were explored and developed. It is known as recycled aggregate (RA). BS EN Standards (BS 8500-1 Concrete, 2006; and BS 8500-2 Concrete, 2006) recommends that recycled concrete be used in secondary structural members of relatively low grades, e.g., curbs, paving blocks and ground bearing floor slabs.

Cho and Yeo (2004) found that due to high water absorption of the recycled aggregate, a higher slump loss was observed when compared to that of natural aggregate concrete. Dhir et al. (1999) showed that the compressive strength of concrete prepared with 100% coarse and 50% fine recycled aggregates was between 20 and 30% lower than that of the corresponding natural aggregate concrete. However, the reduction in strength can be minimized if the mixing procedure is modified (Otsuki et al. 2003; and Ravindrarajah et al. 2005). Further, Olorunsogo and Padayachee (2002) found that the water absorptivity of concrete prepared with 100% recycled aggregate was higher than that of the natural aggregate concrete at the curing age of 28 days. Abou-Zeid et al. (2005) reported that recycled aggregate concrete exhibited higher water permeability and lower resistance to chloride ion penetration compared to conventional concrete. Salem et al. (2003) showed that recycled aggregate concrete had a lower resistance to freezing and thawing compared to natural concrete. Otsuki et al. (2003) reported that the carbonation resistance of recycled aggregate concrete was inferior compared to that of the natural aggregate concrete. Finally, the drying shrinkage and creep of recycled aggregate concrete was found to be higher than that of the natural aggregate concrete (Tavakoli and Soroushian, 1996; and Gomez-Soberon, 2003).

Fly ash is known to be a good pozzolanic material and was used to increase the ultimate compressive strength and workability of fresh concrete (Mehta, 1985). Naik and Ramme (1989) produced concrete mixes containing large quantities of fly ash which achieved compressive strengths of 21 and 28 MPa within 28 days.

Structural Engineering Journal, Recycled aggregates, Fly ash, Recycled aggregate concrete.