The Key to the Design and Production of High Quality Structural-Grade Recycled Aggregate Concrete

This paper will discuss that every year hundreds of millions of tons of demolition waste is generated throughout the world and concrete constitutes a major portion of this waste. In many countries the demolition concrete is sent either to landfills or is recycled as aggregate to be used for base and/or sub-base of roads, or as bedding for underground pipes. On the other hand, millions of tons of fresh aggregates are annually extracted and processed for making concrete. From both the environmental and economic points of view, it may be difficult to sustain this practice, for fresh aggregate extraction causes land denudation and depletion of a non-renewable resource while it is becoming increasingly expensive to transport it from distant sources to mixing plants and construction sites. Consequently, the need for recycling of old concrete as aggregate in new concrete exists. One reason for the limited use of recycled concrete aggregate (RCA) in new concrete is the perception, substantiated by some experimental data, that concrete made with RCA, termed RCA-concrete, is inherently inferior to concrete made with fresh aggregates. It is the purpose of this paper to demonstrate by means of experimental data that RCA-concrete is not intrinsically inferior, and that the reported inferiorities are the consequence of using inappropriate concrete mixture design methods. To rectify this situation, a new mix design method is proposed which can be applied to conventional or RCA-concrete with practically any fresh aggregate replacement ratio. Using this method, it is demonstrated by testing an extensive number of specimens that the proposed method would result in the production of high quality structural grade RCA-concrete, with predictable fresh and hardened properties (i.e., slump, fresh and hardened density, elastic modulus, compressive strength, creep and shrinkage) comparable to similar concrete made with fresh natural aggregates. The concrete mixes thus designed also show outstanding performance at both service and ultimate states in reinforced beams made of RCA-concrete and no major difference is observed between the cracking and ultimate moment capacities and load-deflection responses of these beams made and of those made of similar conventional concrete.