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This paper presents the main results of the research carried out to obtain the fatigue limit of concrete incorporating recycled aggregate from concrete. In this research, recycled aggregate concretes with partial and total coarse aggregate replacement and control concretes with different increasing water/cement ratios have been cast to study the material's behavior in response to repeated compressive loads. The results show that, for the same water/cement ratio, the recycled aggregate concretes present a loss of stiffness higher than the control concrete. Furthermore, the use of recycled aggregate in concrete implies a reduction of the fatigue life. These differences are increased for low water/cement ratios, as in this case the main factor is the lower strength capacity of the aggregate. From the results obtained, recommendable mixtures for recycled aggregates exposed to fatigue loads are proposed.

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The increasing production of construction and demolition waste (C&DW) and the ever-greater consumption of natural resources is forcing society to search for alternatives in order to reduce both. Fortunately, many studies have analysed the possibility of producing recycled aggregates (RA) using old concrete from C&DW [1-6], precast industries [7-10] and industry wastes [11-13]. However, the use of RA against the use of natural aggregate (NA) for structural concrete on material performance, environmental benefits and financial viability of the studies conducted so far do not fully demonstrate the choice of production of recycled aggregate concrete (RAC) with a significant advantage [14]. RA influences the physical and mechanical properties of RAC. The direct influence of the quality of RA on the durability is analysed in [7,15] showing that RA coming from precast-structural concretes is one of the most adequate in order to produce RAC. In terms of durability, the incorporation of recycled aggregate was responsible for worse results but did not compromise their use in structural concrete [16,17]. The properties of the interfacial transition zone (ITZ) have a significant impact on the macro mechanical properties of concrete [18]. X-ray computed axial tomography (CT) provides cross-sectional views of materials, components, and assemblies for non-destructive evaluation [19]. It can be used to examine concrete [20] and the high-resolution X-ray micro-CT allows modelling the permeability of cementitious materials [21]. On the one hand, the irregular surface of the old adhered mortar of the RA contributes to the improvement of the physical bond between the old and new cement matrix [22]. On the other hand, the lower mechanical resistance due to the adhered mortar contributes to reduce the compressive strength [15] and significantly so in the case of dynamic rather than static loading [3,23-25] but no significant influence of the recycled aggregate content on the durability performance of concrete exposed to aggressive environments is detected after years [26]. Multiple recycled aggregate properties analysed by X-ray microtomography C. Thomas a, ⁎

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