# Study of Basalt Fibers and Graphene Enriched Polymers on Bond Behavior of FRP Bars in Concrete
## 1 Introduction  
Fiber Reinforced Polymer (FRP) bars were introduced in reinforced concrete structures replacing steel bars primarily to mitigate problems related to corrosion [1, 2, 3, 4, 5, 6]. Although the initial cost of the FRP use is higher than steel, the total life cycle cost of structural components reinforced with the former may be lower due to reduced maintenance expenses. In addition, [PERSON] et al. [7] highlight as advantages the high tensile strength and lower density compared to steel. However, unlike steel reinforcements, there is still no standardization for the characteristics of these bars. In addition to the existence of different types of fibers and polymers, there is also a wide variety of surface finishes (smooth, sand coated, grooved, ribbed, helically wrapped, helically wounded, among others) that directly impact bond mechanisms [2, 8]. Concrete compressive strength and bar diameter are other usual factors that influence bond. FRP bar diameter influence is in general controversial. For instance in [PERSON] et al. [9], [PERSON] et al. [2] strength decreases with larger diameters and [PERSON] et al. [10] observed an inverse effect. This happens because in FRP bars is difficult to isolate the diameter effect due to many variables involved. It should be noticed that rebar diameter influence on bond behavior in general is not well established, even in the case of steel rebars, see for instance discussion in [PERSON] et al. [11]. The same can be said regarding concrete compressive strength effect on bond, see for instance [PERSON] et al. [12]and [PERSON] et al. [13]. However, as pointed out by [PERSON] and [PERSON] [1], the general trend is that for high strength concretes (compressive strength \(>30\) MPa) the rupture of the bond tends to occur on the surface of the FRP bar, so bond strength would not benefit much from an increase in concrete strength beyond this point. FRP is a composite material formed by a polymeric matrix reinforced with fibers. The matrix holds fibers together and distributes forces among them, in addition to protect fibers against abrasion, impact and other environmental hazards. Thermosetting resins such as epoxi, polyester and vinyl-ester are the most common material that compose the matrix [14]. Epoxi has higher mechanical strength, thermal stability and lower thermal contraction/expansion when compared to the others, but the cost is higher. Polyester is the cheapest in term of costs, but it is also the poorest in terms of mechanical properties [14; 15]. Thermal properties also tend to be poorer in the latter case. For instance its glass transition temperature, where mechanical properties degrade substantially, is in general \(10\,^{\circ}\mathrm{C}\) lower than corresponding vinyl-ester polymer [16]. Thermo-mechanical behavior of polymeric resins, in general, can be improved by the addition of fillers. In this regard, studies have been done considering Graphene Nano Platelets (GNPs), see e.g., [PERSON] et al. [17] and [PERSON] et al. [18]. GNPs present exceptional mechanical strength, low weight and can be easily incorporated into polymeric resins through solvents. The cost is lower when compared to other nanostructured materials such as carbon nanotubes with an equivalent performance [19]. GNPs have been also used as fire retardant in polymers due to its high temperature resistance, as reported in [PERSON] et al. [20] and [PERSON] et al. [21]. However, the present authors are unaware of bond studies at different temperatures considering FRP bar polymers enriched by GNPs, which is a motivational aspect for the present work. Fibers give the mechanical strength and the stiffness of the bars. The more traditional fibers are glass (GFRP), carbon (CFRP) and aramid (AFRP). More recently, basalt (BFRP) was considered and it is becoming a very competitive alternative to other types of fibers [3; 22; 23; 24; 25; 26]. BFRP bars present excellent physical and mechanical properties and good cost-benefit ratio.