Strength of recycled aggregate concrete for structural uses

Abstract

The suitability of using Recycled Concrete Aggregate (RCA) in structural concrete based on a better understanding of its strength and stiffness is reported here. Concrete containing RCA at replacement percentages of 0%, 30% and 100% was investigated experimentally for compressive strength classes in the range 30–40 MPa, using three sources of RCA. It was found that RCA replacement by 30% of Normal Aggregate (NA) does not show any significant difference in strength and stiffness compared to concrete containing 100% NA in concrete. Even though RCA100% replacement does show reduced strength and stiffness, this is not significant and can be compensated in standard ways. Concrete produced with the RCA from the three sources showed that better quality RCA in terms of lower absorption and higher strength produces better concrete, although the strength and stiffness improvement was not significant in this low-moderate strength class of 30–40 MPa. Based on the results, recommendations are made on limits for RCA absorption, flakiness index, density and 10% FACT values for this class of concrete made with RCA30%. for specific applications. Internationally it has been found that RCA30% can be used for structural concrete without no significant difference in workability, strength and stiffness compared with NA (Hansen & Bagh 1995, Clarke et al., 2008, Obla et al., 2007). Some researchers reported that concrete containing RCA is prone to higher shrinkage, creep and reduced durability even at 30% replacement of NA (Gomez 2002, Conclaves et al., 2004), but these results vary from country to country and also from different sources of RCA. Therefore there remains a need for more research in this regard. This paper reports the results of a recent study (Paul 2011) at Stellenbosch University on the feasibility of RCA replacement of NA in the production of concrete manufactured in a conventional ready-mix concrete plant. Aggregate physical properties, and fresh and hardened fresh concrete properties of RCA concrete were studied and compared with the associated properties of Natural Aggregate Concrete (NAC). 2 EXPERIMENTAL DESIGNS 2.1 Experimental outline Four types of coarse aggregates were used in this work namely three RCAs and one NA, as well as natural fine aggregate. The total work was divided into four stages and RCA from three different