Adverse effect of the mass substitution of natural aggregates by air-dried recycled concrete aggregates on the self-compacting ability of concrete: evidence and analysis through an example

This paper is part of a study aiming to define the best way to incorporate recycled concrete aggregates (RCA) in the production of self-compacting concrete (SCC), with a view to reduce the excessive use of natural aggregates, which are a non-renewable resource, and to remove the large quantities of concrete waste generated by demolition. RCA differ from natural aggregates essentially by their lower density, higher water absorption capacity and systematic angularity. Here, the effect on self-compacting ability of replacing natural coarse aggregates (NCA) with coarse RCA in a dry state at the time of mixing is assessed. Starting from a reference SCC incorporating rounded NCA only, three replacement rates were studied: 40%, 60% and 100% by weight. The water amount was adjusted so that the standardized 24-h absorption of RCA was satisfied. The key properties of SCC recommended by European specifications and guidelines (EFNARC), i.e. filling ability, passing ability and segregation resistance, were measured in the fresh state, immediately after mixing (t(0)). Shear-dependent properties were also quantified by means of rheological measurements at t(0) and t(0) + 25 min. Through this study are shown and interpreted the difficulties in achieving the self-compacting ability under certain conditions of use of RCA. The experimental results showed that the self-compacting ability criteria were not satisfied, irrespective of the replacement rate of NCA with RCA. This replacement implied an increase in the rheological properties (yield torque and torque in steady-state flow) of concrete. The increased volume of coarse aggregates together with a constant volume of interstitial paste, and the angular shape and rough surface texture of RCA, are the main causes of the alteration of the flow of concrete. Also, the impairment with time of the flow properties of concrete was noted and is due to the movement of water from the interstitial paste towards dry RCA. Such results were supported by the fact that rheological data on RCA concretes were not in agreement with multi-scale models describing the flow of reference concrete as a suspension of spherical occlusions in a viscoplastic phase. Through this study, it is evidenced what to avoid regarding the technical approach envisaged to the RCA incorporation in SCC. (C) 2014 Elsevier Ltd. All rights reserved.