Drying shrinkage of concrete made from recycled concrete aggregate

Recycled concrete aggregates (RCA) are used in new concrete mixes, termed recycled aggregate concrete (RAC). Among losses in most fresh and hardened properties, literature reports that RAC suffers increased levels of drying shrinkage compared to equivalent mixes incorporating conventional aggregates. This paper describes the procedure and results of a range of experiments conducted on a commercial RCA and its use in new concrete mixes. In particular, this work quantifies the effect 100% RCA has on the drying shrinkage of the resulting concrete. A commercially available RCA was characterised by analysing the particle shape and texture, percentage of solid contaminants, particle size distribution, water absorption and particle densities. The types of rocks present were analysed through the preparation of thin sections. Three concrete mixes were prepared and tested for workability, 7 and 28 day compressive strengths and 112 days of drying shrinkage. The mixes included a 40 MPa control mix incorporating natural crushed aggregates and two equivalent RAC mixes, one with 30% fly ash as a partial cement replacement and one without. The results indicated that the commercial RCA has been produced quite efficiently with negligible solid contaminants. However, due to adhered mortar found on the aggregate particles, the RCA was found to have rougher particle textures, increased water absorptions and lower densities than conventional crushed aggregates. Due to these properties and the subsequent increased water demand the RAC mixes suffered lower compressive strengths and higher drying shrinkages than the control mix. The RAC mix incorporating no fly ash was found to have a 25% average increase in the 112 day drying shrinkage from the control mix while the fly ash mix only had a 7% increase.