۱٫ Introduction

and objectives In the near future, using recycled materials in conventional and high performance applications should be a priority area [1]. At this stage, it is fundamental to analyse the characteristics of recycled materials, recycling procedures and manufacturing processes. The main difference between natural aggregate and the recycled concrete aggregate is the adhered mortar [2,3]. The presence of this material decreases with the number of crushing processes, the size fraction and the original waste quality [4,5]. In general terms, the quality of vibrated recycled concrete is lower than that of conventional concrete with the same mix proportions [6,7]. Many of the current studies in vibrated recycled concrete field deal with short-term analysis related to basic properties and structural performance, and a few of them have studied the long-term behaviour [8,9]. The compressive and splitting tensile strengths and modulus of elasticity decrease when the percentage of recycled aggregate increases, and the shrinkage and creep increase deformations [10,11]. These variations are mostly due to the adhered mortar. On the other hand, self-compacting concrete is a highly flowable concrete that spreads rapidly into place and fills formwork without vibrating compaction in order to ease casting and to achieve durable concrete structures [12,13]. At the construction site, it has increasingly been used over the past two decades and it is empirically described according to its filling ability, passing ability and segregation resistance [14]. Most of studies state that, if a SCC is well designed, it can provide similar mechanical properties to its equivalent vibrated concrete [15]. However, the SCC flow properties and its fresh rheological behaviour diverge from what is expected from vibrated concrete of normal consistency [16]. One of the major obstacles to a more widespread use of selfcompacting concrete is to obtain a robust material [17,18]. Robustness is the capacity of a concrete to maintain its properties when changes in materials, mixing parameters or environmental variables take place [19,20]. Self-compacting concrete has shown to be more sensitive to variations in its design process than vibrated concrete [21,22]. The mix design is a critical step to obtain high quality selfcompacting concrete. A large number of variables must be considered in the mix design process and its interactions are difficult to predict [23]. Different studies have been developed to analyse selfcompacting concrete robustness. In general, aggregate density and size, paste density, type of mixer, mixing protocol, mixing time and total mixing energy are factors that have to be taken into account to analyse robustness [24]. Some works conclude that robustness can be influenced by the water to powder volume ratio, the superplasticiser to powder weight ratio and the solid volume [25–۲۷]. Others state that errors in weighing water and fines content [19] or those affecting aggregate moisture [28] are of capital importance.