# Resonance Fatigue Behaviour of Concretes with Recycled Cement and Aggregate
###### Abstract  
The huge increase in production of construction and demolition waste (CDW) worldwide is leading to the valorisation of as recycled aggregates. One of the most promising alternatives is its use as a recycled aggregate in the manufacture of structural concrete, which motivates the study of the dynamic behaviour of these materials in order to ensure their suitability for use in elements subjected to dynamic loads. This work evaluated the resonant compressive fatigue behaviour of structural concretes with 25% or 50% recycled mixed aggregates, either individually or in combination with 25% recycled cement of clay-based materials both from CDW. All mixes were subjected to compressive fatigue tests using the accelerated Locati method. Regarding the fatigue limit, the results showed that for all mixes, it was between 30% and 45% of the compressive strength. In addition, a correlation was also found between the resonance frequency of the test and the deformation suffered by the specimen. This correlation enabled the estimation of the fatigue limit through a more stable parameter than the strain measured by strain gauges, namely, the resonance frequency. In addition, it was found that the resonance frequency of the test changed as the specimen damage increased. This observation enabled the estimation of the fatigue limit through a more stable parameter than the strain measured by strain gauges, namely, the resonance frequency. recycled concrete; resonant fatigue; Locati method; high frequency 2021 27 May 2021 29 May 2021 29 May 2021 20  
durability [9; 10; 11; 12]. In this line, there are other works that attempt to assess CDW as RA together with new recycled supplementary cementitious material (SCM) (ground granulated blast fume [13; 14], ceramics recycled [15; 16], concrete recycled [17; 18] and biomass waste [19; 20]) as a partial substitute for cement, providing a double benefit by reducing the consumption of NA and cement, and achieving promising results in the improvement in mechanical and durability performance of concretes made only with CDW. At present, the fatigue process involving recycled concrete is as yet an inadequately investigated topic [21]. A deeper understanding of this structural behaviour is essential to promote the reincorporation of these new raw materials into the production cycles in the concrete industry. Fatigue in concrete can be defined as a process of mechanical degradation leading to failure. Due to cyclic loads, crack initiation and propagation within the specimen occur until final failure. However, during the application of the variable loads, a modification of the mechanical parameters of the concrete occurs, progressively varying the structural responses of an element [22]. This behaviour is of great importance in the design of structural elements, such as bridges, airport pavements, roads, marine structures, wind generators, foundations of industrial equipment, etc. Compared to permanent loads, these elements are subjected to a higher number of cyclic loads due to variable loads (transient loads, wind, etc. ), and these cyclic loads produce a major reduction in mechanical properties, shortening the service life of this type of structure. Some researchers have focused their efforts on increasing knowledge about the compressive fatigue behaviour of this new generation of recycled concretes. In this area, [PERSON] et al. [23] studied the fatigue behaviour of concretes with 100% recycled coarse aggregates (RCA) (>95% waste concrete). For this purpose, they tested cylindrical specimens in cyclic uniaxial compression tests with a constant minimum fatigue load stress and a frequency of 10 Hz. These authors observed that concretes with 100% RCA showed fatigue behaviour in three different stages: (i) cyclic creep stage, (ii) creep coupling stage and (iii) fatigue stage, similar to the behaviour pattern of conventional concrete with NA [24]. [PERSON] et al.