# High-Frequency Fatigue Testing of Recycled Aggregate Concrete
## 1 Introduction  
Recycled aggregates have an environmental benefit [1; 2] and recycling is a necessity as the rate of waste generation is such that landfills are close to saturation [3]. Additionally, the possibility of obtaining recycled concrete (RAC) with good mechanical [4] and durability [5] properties has been proven. The ability of these recycled aggregates for use in self-compacting concrete [6], even with fine recycled aggregates [7], has also been proven. Even [PERSON] et al. [8] have used RAC for the manufacture of hot-mix asphalt. Concrete fatigue behaviour has not been extensively studied, partly because of the difficulty, cost and time required. Concrete elements subjected to this type of load include railway superstructures, sleepers or slab tracks [9], rail and road bridges [10], offshore structures subject to variable wind and tidal loads [10; 11] and/or wind generators [12]. [PERSON] et al. [13] have defined a procedure for analyzing ultra-high performance concrete's response to impacts. As [PERSON] et al. state, knowledge about the effect of cyclic loads on concrete is currently very limited [12]. Several authors have analysed the responses of concrete. [PERSON] et al. [11] analysed the behavior of RAC to both compression and bending fatigue. [PERSON] et al. [14] analysed the influence of compressive fatigue on a fiber-reinforced cementitious material. [PERSON] et al. analysed the concrete fatigue behavior using two different methods: the staircase method [15] and the Locati method [16]. Innovative techniques, such as micro computed tomography (micro-CT) have also been used to analyze the behavior of concrete at fatigue, both in compressive fatigue [12] and bending fatigue [17]. Moreover, some micro-CT studies, which is a technique able to analyse the pores and cracks on concrete [18; 19], had been developed tounderstand the concrete damage fatigue micromechanism [20; 21]. In general, fatigue is known to lead to microcracks in concrete growing at lower loads than in static tests, which can lead to concrete failure earlier than expected [22]. It is well known that the fatigue limit of concrete depends on different factors. At higher stresses, the fatigue strength decreases with decreasing frequency [12; 23; 24]. The fatigue strength is also affected by the water/cement ratio, cement content, concrete type, rest periods, curing conditions and age during loading [25]. It is assumed that damage linearly increases with the number of cycles applied at a certain stress level [24]. The strain at the concrete failure during fatigue tests approximately corresponds to that at the peak load during quasi-static tests [26]. The failure meso-mechanism in concrete under fatigue compressive tests is almost the same as in monotonic compressive tests [27]. In order to reduce the characterization time as much as possible, the influence of increasing the test frequency up to the resonance frequency of the set of specimens and test machine [28] has been analyzed. In response to this proposal, several authors [23; 29; 30] indicate that the range of frequencies to be tested should be between 1 and 15 Hz, since they state that within this range, the effect of frequency is limited. It is justifiable to set the minimum at 1 Hz in order to avoid an excessive increase in the effect of creep in very long duration tests. However, there is no justification for setting the maximum at 15 Hz. Fatigue tests are proposed for classification according to the frequency of the test, distinguishing between low frequency tests, moderate frequency tests and high frequency tests. Low frequency tests are carried out at less than 1 Hz, moderate frequency tests between 1 and 15 Hz and high frequency tests at more than 15 Hz. Three types of recycled self-compacting concrete were characterized in this investigation of compression fatigue tests. This characterization was developed both at a moderate frequency (10 Hz) and a high frequency (90 Hz).