# Resonance Fatigue Behaviour of Concretes with Recycled Cement and Aggregate
###### Abstract
[25; 26] carried out an extensive experimental programme (24 mixes) to study fatigue behaviour of concretes made with RCA (20%, 50% and 100%) and different effective water-cement (w/c)eff ratios (from 0.45 to 0.65). In their results, these authors reported that, for same (w/c)eff, RCA concretes showed a greater reduction in fatigue life than NA concretes. However, concretes with up to 20% RCA and (w/c)eff showed fatigue behaviour similar to that of NA concrete. In general, the reduced fatigue life of RCA concretes can be attributed to: (i) greater numbers of micro-cracks in the RCA produced in the crushing process of concrete waste, which are more easily interconnected as a result of cyclic loading [27] and (ii) larger interfacial transition zones between the cement paste and the RCA, which act as additional weak bonds under cyclic loading [28]. [PERSON] et al. [29] observed through microstructural analysis with computer microtomography of RCA concretes subjected to compressive fatigue loading that cracks are mostly generated at the aggregate interfacial transition zone (ITZ) and the edges of the flakier aggregates, propagating through the weaker planes until concrete collapse occurs. [PERSON] et al. [21] researched the experimental results of [PERSON] et al. [23] and [PERSON] et al. [26] in order to improve the understanding of the fatigue behaviour of concretes made with RCA. These authors were able to develop a predictive algorithm to estimate the fatigue behaviour of RCA concrete, knowing the compressive strength, the percentage of NA substitution by RCA and the type of concrete mix, obtaining good agreement between the experimental results and the theoretical prediction. Furthermore, these authors verified that the concretes with RCA complied with the fatigue life predictions calculated according to the Model Code 2010 [30]. Recently, [PERSON] et al. [31] combined the accelerated Locati fatigue method with \(2\times 10^{5}\) cycles per step with resonant fatigue, minimising the time and cost of fatigue testing. These authors observed more conservative results in the fatigue behaviour of concretestested at moderate frequency (10 Hz) than in the case of those tested at high frequency (\(\approx\)90 Hz), this effect being greater in concretes manufactured with RCA. Moreover, they observed an increase in temperature with increasing frequency, which could reduce the fatigue life of the concrete by increasing creep damage. From all the research carried out to date, great advances have been made in the knowledge of the fatigue behaviour of concrete with RCA. However, this type of recycled aggregate, in Spain only represents between 30% and 40% of CDW [32]. The remainder is represented by mixed recycled aggregates (MRA), the result of jointly crushing used concrete, masonry materials (brick, roof and ceramic tile) and some minor components, including plaster, wood, plastic and glass. MRA are more heterogeneous than RCA, which makes it more difficult to reincorporate them into the concrete production chain. Nevertheless, new scientific contributions are emerging every day that encourage further research in this field, motivated by the possibility of producing concretes with MRA with acceptable mechanical [6; 32; 33; 34] and durability [10; 35; 36; 37] results. Furthermore, according to the latest report of the European Commission [38], MRA are the biggest problem within CDW due to their accumulation in recycling plants. The recent review by [PERSON] et al. [39] shows that the partial substitution of up to 30% by weight of recycled clay waste from CDW as SCM in the manufacture of concrete can improve, if the fineness of the waste is adequate, the properties of new concretes incorporating these new SCMs in the cement with respect to conventional concrete, as well as being an effective solution to mitigate the pollutant emissions associated with cement production, with a reduction of up to 8.1% of cement produced [40].