# Fatigue Behaviour of Concrete Using Siderurgical Aggregates
## 1 Introduction  
We are gradually becoming aware of the need to take care of the environment and impose more sustainable types of construction in which, fundamentally, materials with reduced environmental impact are proposed. One of the options contemplated to achieve more sustainable construction is the use of concrete manufactured with value-added electric arc furnace slags (EAFS) as aggregate. These siderurgical aggregates (value-added EAFS) are usually deposited in inert landfills, while their use in concrete can save 35% in CO\({}_{2}\) emissions without taking into account transport or the manufacturing of materials [1]. The use of siderurgical aggregates in concrete provides general improvement in the mechanical properties of concrete, mainly due to the high quality of the interfacial transition zone (ITZ) between aggregates and paste [2] and to their irregular shape and high tenacity [3]. Compared with concretes that use conventional aggregates, siderurgical aggregate concrete exhibits considerably higher compressive strengths [4; 5; 6; 7; 8] that enable high strength concrete [9] to be obtained easily. However, the property that increases the most is the modulus of elasticity, largely due to the high modulus of elasticity of siderurgical aggregates [6; 10], which increases proportionally to replacement ratio [11]. Although it has been demonstrated that siderurgical aggregate concrete can have high quality mechanical and durability [12; 13] properties, at present, there are no previous studies that evaluate the fatigue behaviour of this type of concrete. Even though concrete elements are not usually exposed to cyclic loads, there are some typical concrete components which are subjected to this type of stress such as railway superstructure elements, bridges, wind turbine foundations or offshore structures [14; 15; 16; 17]. It is worth noting that in some of these applications a higher density is beneficial, as it increases the stability of the components, for example, in the case of railway superstructure elements or foundations. The presence of EAFS increases the density [2], so the use of siderurgical aggregate concrete can be of particular interest in such applications. However, before being able to use this type of concrete for these applications, it should be verified that these concretes have a good response to cyclic loading. Fatigue is a process of progressive and permanent material damage under repeated loading, in such a way that in the end, the component breaks under loads lower than those it was able to resist initially. This reduction in the mechanical properties is due to the growth of defects inside the component [14; 15; 16]. In general, fatigue has three parts, a first one of crack initiation, a second one of crack propagation and finally, a third one when the specimen is about to break [17; 18]. In the case of concrete, where micro-cracks already exist intrinsically, the initial state of these micro-cracks or some phenomenon that accelerates the growth of these cracks can be critical in the fatigue life of concrete, more so given that concrete is a highly heterogeneous material. For these reasons, damage micromechanisms must be studied explicitly. Several authors have concluded that in the case of fatigue in concrete, the dominant cracks generally arise either from the paste-aggregate interface or from pores, indicating that flaky geometry of the aggregates is of particular importance in the fatigue behaviour of concrete [19; 20; 21]. There are two main types of fatigue tests for concrete: bending fatigue and compressive fatigue tests. In the case of bending fatigue tests, they are mainly used to study reinforced or fibre-reinforced concrete [22; 23; 24]. In the case of compressive fatigue, they are normally used to study the behaviour of plane concrete [14; 16]. In this research, when seeking to analyse the influence of the presence of EAFS, it was decided to use plane concrete and consequently, compressive fatigue tests were performed.