ABSTRACT
There are obstacles to the use of Recycled Aggregates in Ready-Mix Concrete (RMC), one of them being the need of additional silos, often impossible for plants in urban areas, while it is in these areas that recycled aggregates are mostly available. Another main issue is the control of moisture in the aggregates and the impact on fresh concrete rheology. It is generally admitted that the RA high level of water absorption generates difficulties in the correct water proportioning. Also, the water absorbed between end of mixing and casting can change the consistency of the concrete during transport. RECYBÉTON project studies influence of the pre-saturation level on the RAC rheology (see Chapter 7 for details). There is no significant adverse effect of the pre-humidification of RA on the rheological properties of fresh concrete at the end of mixing. The mixing time seems sufficient for under-saturated RA to absorb a large part of the water necessary to their saturation. Another topic was abrasion during mixing, in order to better understand this evolution and improve the recycled aggregate concrete mix-design, the friability of recycled concrete aggregate was studied under different mixing configurations. The reduction of the size of the RA during mixing seems to be caused by a combination of attrition and fragmentation phenomena which are dependent of the MDE of the RA and the content on attached cement pastes and mortars. 100The fresh cement paste protects the RA during the mixing and limits these phenomena. However, for mixing time inferior to 1 min, which is frequently the case in industrial batching, the attrition mechanism is not very intense and does not seem impact the quality of the concrete production. All these results are in good agreement with usual method of batching used with success for the experimental construction sites of RECYBÉTON project (see Chapter 7).
