HOW TO PRODUCE HIGH QUALITY CONCRETE MIXES WITH RECYCLED CONCRETE AGGREGATE

The dwindling supplies of fresh natural aggregates for making concrete and the desire for sustainable concrete practices have opened up the possibility for using demolition concrete as recycled concrete aggregate (RCA) in new concrete mixes However researchers in the past have discovered that simple replacement of virgin aggregates by an equivalent volume or weight of RCA in a conventional concrete mix leads to a mix with lower elastic modulus and higher creep and shrinkage and possibly lower durability, than the original mix without RCA In this study we first describe the potential sustainability benefits of RCA concrete and subsequently explain the reasons for the previously reported results Next, we describe a novel method of concrete mix design which can be used to avoid the above problems of RCA-concrete, i.e. concrete made with RCA The results of the most comprehensive experimental program ever mounted on this subject will be subsequently presented The experimental program involved different types of RCA and their detailed material characterization, determination of their mechanical and physical properties and their durability, long-term creep and shrinkage behaviour In addition, the structural performance of RCA-concrete members was investigated by testing large scale reinforced RCA-concrete members under flexure and shear The principal conclusion of this comprehensive study is that the properties of RCA-concrete are mainly dependent on its mix proportions and due to the fact that RCA is a two phase composite material and not a homogeneous material like natural aggregate, the volumetric ratio and properties of each phase must be explicitly considered in the mix design Towards this end, a novel concrete mix proportioning method was developed and tested, which verified the basic assumptions of the method