Mixing recycled aggregates concrete – old methods for new concrete

There have been a great number of laboratory studies on mixing approach for recycled aggregate concrete (RAC) even if in practice, the simplest procedures are generally employed. Some of these methods give good results comparing with the traditional mixing method. The present paper resume some main mixing methods proposed in the literature, specifically for RAC. The two-stage mixing approach (TSMA) splitting the required water into two parts in divers proportions, is a development of the previously Japanese proposed sand enveloped with cement (SEC) mixing method. Other methods were proposed, like for instance the sand enveloped mixing approach (SEMA) or the loading of (recycled) aggregates into a preprocessed mortar or paste (cement paste encapsulating aggregate method). This short literature review also discusses the effect of the initial moisture of the RCA and the concept of effective water which seems still valid for RAC. It is not clarified today the extent to which the improvements resulted from alternative mixing methods are specific to the RAC mixing, or are more general phenomena. In the light of recent works, two points are underlined here. First, the quantity of water absorbed by the aggregates depends of factors not sufficiently elucidated yet. For instance, the pre-wetting method potentially could conduct to different RCA water absorption after mixing. Second, the mechanical benefits of the hardened RAC produced by TSMA it is classically explained by an increase of the ITZ strength. We consider that a complementary explanation should be looked in the higher attrition of the RCA produced by TSMA. This phenomenon is supposed to induce better compacity of the mixture and higher quality of the RCA.