## 1 Introduction  
Concrete is the most important structural material in construction, from which durability, stability, resistance and a good response when exposed to fire are expected, preventing the appearance of deformations and loss of rigidity [1; 2; 3]. The incorporation of fibers in concrete modifies its behavior, principally in tension, preventing the opening and spreading of cracks, and increasing its ductility [4; 5]. When concrete is affected by a fire, it experiences a loss of compressive strength in the order of 15% when the said fire reaches temperatures of 400 \({}^{\circ}\)C, whilst its mechanical capacity marginally reduces in temperatures of up to 280 \({}^{\circ}\)C [6]. Furthermore, its longitudinal modulus of elasticity decreases by 75% when temperatures close to 400 \({}^{\circ}\)C are reached, provoking a high number of cracks [7; 8; 9; 10; 11]. Numerous research studies in this field have proven that the incorporation of metallic or polypropylene fibers results in increased resistance and a reduction in cracking when the concrete is exposed to high temperatures [12; 13; 14; 15; 16; 17]. On the other hand, progress in the field of nanomaterials is an inestimable opportunity to develop new materials, existing among others, nanofibers (CNFs), carbon nanotubes (CNTs) and graphene oxide (GO), which offer a further dimension for interacting with cement and with concrete. The effect of incorporating low dosages of nanomaterials delivers improvements in the hydration, microstructure and mechanical properties of cement-based materials, due to their electrical and chemical properties [18; 19]. The main disadvantages of the nanoscale use of these new materials are the lack of binding between the nanoparticles and the cement mixture and the high cost of producing them, although there are more economical options, such as carbon nanoplatelets, which improve watertightness and prevent chloride intrusion [20]. Studies carried out using graphene oxide (GO) show that the material possesses good chemical stability and high heat and electrical conductivity, making it ideal for manufacturing and use in different applications, particularly when good thermal management is required. Its use enables improvements to be achieved in concrete properties, increasing the tensile and flexural strength of the associated cement compounds and the transport phenomena which improve their durability. These phenomena are due to the fact that the incorporation of graphene oxide provokes hydration by adsorption of both water molecules and cement components, which improves the material's performance [21; 22; 23; 24; 25; 26; 27]. Meanwhile, it has been proven that improvements in the mechanical behavior of strain-hardening cementitious composites reinforced with graphene oxide are due to the increased resistance of the matrix, as well as the chemical bond between polyvinyl alcohol and the cement matrix due to the addition of graphene oxide, the amounts added being very important, since the addition of too much graphene oxide would have a detrimental effect [28]. Where carbon nanotubes (CNTs) are concerned, it has been observed that an adequate dispersion of carbon nanotubes increases compressive strength by up to 40% and moderately increases tensile strength but with a high level of deformations, taking into account that low concentrations of long CNTs are more effective than a high volume of short CNTs [29; 30; 31; 32]. When it comes to the use of carbon nanotubes (CNTs) in concrete structures which are exposed to high temperatures, there is barely any research. Their use in the recovery of flexural strength in heat-damaged of reinforced concrete (RC) beams repaired using carbon fiber reinforced polymer compounds (CFRP) is recognized, it having been demonstrated that the addition of CNTs increased the capacity of the compound materials to recover the bending load capacity and rigidity of heat-damaged beams [33].