## 2. Scope of Analysis  
Five series of two beams were tested, differing in the level of the load at which the strengthening with the CFRP strip was implemented [3; 4]. All beams of span length 3.0 m were subjected to four-point bending (Figures 1 and 2). The load program was assumed to reflect the nature of work of beams in bridge structures. The series of tested beams differed from each other by the level of applied loads before the CFRP strip application. The load levels have been established in relation to the design capacity of an RC beam without strengthening. The following series of beams were tested:  
* Beam type Br--reference RC beams with no CFRP strip; used as control specimens;
* Beam type Bz--RC beams \"reinforced\" with CFRP strips before the beginning of load application;
* Beam type Boow--RC beams with CFRP strips, examined in two stages (stage 1--RC beam loaded to P = 60 kN, equal to the level of the design load-carrying capacity of reinforced concrete beam without strengthening (which corresponds to \(\sim\)62% of the load-carrying capacity of such beam), then totally relieved; stage 2--beam strengthened by means of CFRP strips and then loaded up to failure);
* Beam type Bow--RC beams with CFRP strips, examined in two stages (stage 1--RC beam loaded to P = 60 kN, then relieved to P = 45 kN, equal to \(\sim\)75% of the design load-carrying capacity of the beam without strengthening (it corresponded to the dead weight of a bridge structure); stage 2--beam strengthened using CFRP strips at a load level of P = 45 kN and then loaded up to failure;
* Beam type Bopw--RC beams with CFRP strips, examined in two stages (stage 1--RC beams loaded to P = 75 kN, equal to a value of 25% above the design load-carrying capacity of the RC beam without strengthening, then relieved to P = 45 kN, equal to \(\sim\)75% of the design load-carrying capacity of the beam without strengthening; stage 2--beam strengthened using CFRP strips at a load level of P = 45 kN and then loaded up to failure). The side view and cross-section of all tested beams together with the layout of reinforcing bars are shown in Figure 1. On the bottom surfaces of all strengthened beams, CFRP strips were applied due to flexure (S&P Lamelle CFK type 150/2000). Each composite strip had a length of 2.80 m and cross-section of 0.05 m \(\times\) 1.2 mm and was bonded to concrete surfaces by means of a two-component epoxy resin adhesive [3]. A detailed description of the test stand, used materials, test procedure, and main results were presented in [3,4]. At the beginning, comprehensive measurements of all used materials have been conducted. The mean measured yield strength and modulus of elasticity of the 12 mm reinforcing bars were 550.80 MPa and 210.50 GPa, respectively. The 4.5 mm reinforcing bars, without explicit yield plateau, had a 0.2% yield proof stress of 554.70 MPa and a modulus of elasticity of 193.30 GPa. Poisson's ratio for both bar types was assumed as 0.3. The modulus of elasticity and Poisson's ratio of the CFRP strips were measured as 158.95 GPa and 0.2, respectively. Figure 1: Construction drawing of tested beams: (a) side view and (b) cross-section [2,3]. All analyzed beams have been made of concrete of strength class C40/50, according to CEB-FIP Model Code 1990 [29]. The maximum aggregate size (gravel) was specified as 16 mm. The concrete cube, cylinder, and small concrete beam (cuboid) specimens were made at the time of casting and were kept in the same condition as the beams during curing. The modulus of elasticity and Poisson's ratio before cracking determined on cylinder specimens were measured as 29.98 GPa and 0.164, respectively. The concrete compressive cube strength was 54.00 MPa, and the concrete compressive cylinder strength was 44.40 MPa. The compressive stress-strain relationship was adopted as a parabola diagram according to [PERSON] [30]. The tensile strength of concrete was determined from the indirect tests (in flexural and splitting cylinder tests) and was equal to 3.467 MPa.