Issue 38

I. N. Shardakov et alii, Frattura ed Integrità Strutturale, 38 (2016) 331-338; DOI: 10.3221/IGF-ESIS.38.43 332 08, [3]) regulations, this approach is applied to bending elements reinforced by a strengthening sheet with anchorage and without it. In works [4-6], a series of tests have been performed to study the deformation behavior of a beam initially subjected to loading up to crack generation and then strengthened with CFRP. Besides, the dependence of the strength and stiffness of such beams on preliminary loading was shown. However in the structural practice the reinforcement of beams is usually performed directly during loading and followed by grouting cracks before gluing CFRP sheet. The question of how such a restoring procedure affects the strength properties of beams has not received enough attention yet. The influence of the degree of debonding on the carrying capacity of beams deserves further studies as well. This work studies the debonding of CFRP sheet from the surface of reinforced concrete beams subjected to bending loading. During the experiment carried out in the Laboratory at Perm National Research Polytechnic University, we have investigated the strain behavior of beams strengthened until loading and beams strengthened during loading after the appearance of first cracks and their grouting. Infrared thermography techniques [7, 8] were applied to identify the first signs of debonding. Heat transfer processes develop differently in a multi-layer systems with and without air gaps. The analysis of surface temperature of the beam at its heating and cooling yielded information about the existence and distribution of debonding on the beam surface. P ROGRAM AND METHODS OF TESTING n our experiments we used concrete beams made of concrete B20 (Group B1) and concrete B35 (Group B2). Totally 22 sample-beams were prepared and tested. The schematic representation of a sample strengthened with steel reinforcement rods and a composite layer is shown in Fig. 1. The choice of such reinforcement was mainly caused by the condition of equal strength for beam elements in bending. Each group of beams (B1 and B2) was divided into 3 series with 3–5 samples in each of them: series A – reference samples (ordinary concrete beams with steel reinforcement); series B – preliminary strengthened beams, i.e beams strengthened by the CFRP before load application; series C – beams strengthened at a certain stage of loading after the appearance of first visible cracks and their grouting. During the strengthening procedure CFRP sheet SikaWrap-230 40 mm width and 0.13 mm thickness was glued to the beam bottom surface using epoxy resin Sikadur-330. Carbon fiber sheet was also fixed with transverse wrapping anchorage by CFRP straps in two support sections of the beam. For the beams of cerise C strengthening procedure additionally included widening and grouting of cracks with a repair compound and crack injection with a low-viscosity epoxy resin before gluing of CFRP. Strain gauges were installed on steel reinforcement rods, the carbon-fiber sheet and the surface of all beams to control deformations along the beam axes. Figure 1 : Schematic representation of the concrete beam with steel reinforcement and the carbon fiber strengthening layer: 1 – carbon fiber sheet, 2 – carbon fiber strip, 3 – carbon fiber wrapping anchorage, 4, 5 – steel reinforcement with a diameter of 6 mm and 12 mm, respectively. The tests were performed on a specially designed four-point bending test set-up (Fig. 2a). The loading of the beams was performed by a successive increasing quasistatic load with а step of 2 kN representing 4–6% of the fracture load. At each I