# Deformations of FRP-Concrete Composite Beam: Experiment and Numerical Analysis
## 1 Introduction  
Advanced materials were created for structural application. The use of concrete and fibre-reinforced polymers (FRP) composites has emerged as one of the most exciting and promising techniques in structural engineering. A synergy of the high performance of FRP materials in tension and concrete in compression enables the development of efficient structural systems [1; 2]. However, engineers faced severe problems due to the absence of a reliable technique for ensuring the required structural properties minimising the amount of material used [3]. The first attempts of application of FRP materials are related to the flexural strengthening of reinforced concrete (RC) bridges [4]. At that moment, adhesively bonded external steel reinforcement was the commonly acceptable option of achieving structural rehabilitation. This method was originally invented in France in the mid-sxieties; in the early seventies, it was further developed in Switzerland, Germany and England and is nowadays state-of-art in Western Europe [5]. However, the drawbacks of the steel application overrule the efficiency of the bonding technique. The heaviness of steel plates causes inconvenience of the handling on construction site. The need for the extension of element lengthsresults in the welded connections, which corrupt the adhesive bond. The long-term creep tests reveal progressively growing spots of corrosion at the joint between the steel and adhesive [5]. The mentioned problematic points have made it necessary to look for more appropriate materials and strengthening technologies. Nowadays, there is a wide range of potential applications of FRP reinforcement that cover both initial design as well as strengthening and rehabilitation [6; 7; 8]. However, the brittleness of the FRP systems should be pointed out as the main drawback: FRP materials experience linear elastic behaviour up to failure that causes unfavourable brittleness of the composite [9; 10; 11]. Commonly, FRP strips are adhesively bonded to the most tensioned surface of a deficient structural member [12], and the mechanical parameters of the bond often determine the failure of the strengthening system [12; 13; 14; 15]. Therefore, a comprehensive understanding of the bond behaviour plays an essential role in determining the performance of FRP-concrete elements. Glass fibre-reinforced polymers (GFRP) have great potential as construction material and enable structural engineers to consider them as a feasible alternative to steel for various structural applications [6; 9; 16; 17]. Tests of GFRP-concrete composite elements were initially carried out to estimate the efficiency of the rehabilitation systems [10; 18; 19]. Compared with the use of steel plates, this solution presents several advantages, such as the ease of application, the lower self-weight, and resistance to corrosive environments. The GFRP materials are less expensive than carbon fibre-reinforced polymers (CFRP). Therefore, GFRP is often used for engineering practice. However, due to the low value of the modulus of elasticity, the GFRP-concrete elements exhibit large deformations. Thus, the serviceability criterion governs the design of such structures. Numerical simulations offer a powerful tool for the solution of the serviceability problems [20], but adequate constitutive models are necessary for that purpose. Residual stiffness of flexural concrete elements subjected to short-term load is the focus of this research. This parameter is characteristic of the structural integrity of cracked concrete. In numerical simulations, tension-stiffening models can represent the stiffness decay in an average manner. Numerous studies investigated the tension-stiffening issue. However, only several works addressed flexural effects. Fundamental studies by [PERSON] [21] and [PERSON] et al. [22] should be mentioned in this context. [PERSON] et al. [23; 24] proposed an experimental procedure to estimate the flexural response of concrete elements with different types of composite reinforcement.