Issue 38

I. N. Shardakov et alii, Frattura ed Integrità Strutturale, 38 (2016) 331-338; DOI: 10.3221/IGF-ESIS.38.43 336 appearance of the first cracks in the concrete causes a sharp increase in deformation. The subsequent grouting of cracks in the beam under the load leads to the restoration of its rigidity. At this stage, the deformation curve has the "step", on which the slope of the curve is almost restored to its initial value. With a further increase in the bending moment beam displays the same behavior pattern, as the preliminarily strengthened one. The beams strengthened under the load showed an increase in their carrying capacity by 38–49% compared to the non- strengthened samples, and the appearance of the second generation cracks started when the bending moment increased by 45–71%. In the graphs the loading intervals are marked (circled zones), corresponding to the beginning of cohesive delamination of CFRP. In the beams strengthened under loading delamination begins when the bending moment is on average 75% of the maximum value. Thus, our tests show that the loss of the bearing capacity of the beam cannot be correlated with the beginning of delamination of CFRP layer. As shown in the graphs of Fig. 4, from the beginning of delamination the beam continues to perceive the load and reduction of rigidity does not take place. This means that from the appearance of the first cracks to the total loss of the bearing capacity the beam has some strength reserve which is about 25% of the ultimate load. (a) (b) Figure 4 : Bending moment–Deflection relationships for series A, B and C: Group B1 (a) , Group B2 (b) . The algorithm of thermogram transformation allows one to estimate the relative area of delamination, accumulated in the beams at each loading step. The data presented in Tab. 3 show that the relative area of delamination in the beams reinforced under loading is on average 2.1–2.3 times greater than in the preliminary strengthened ones. The comparison of the experimental strain values, corresponding to the onset of cohesive delamination of CFRP, with the data theoretically obtained from 5 different methods used in practice, demonstrates a low reliability of these calculation methods (Fig. 5). For instance, the values calculated by the regulatory method used in Russia exceeds the experimentally registered by 15–75% for different classes of concrete. C ONCLUSION . It is shown that for the bearing capacity of CFRP beams destroyed due to the rupture of CFRP does not depend on the point when the strengthening is performed. The beams strengthened before loading and under the load after the appearance of first cracks and their grouting demonstrate the bearing capacity that is higher by 37–39% and 38- 49%, respectively, than the ordinary CR beams. Grouting of cracks in the beams under loading allows one to increase the limiting value of the bending moment by 45–71% compared to the unstrengthened beams. 2. It is established that the process of CFRP delamination in the beams strengthened under loading begins at strain which is 4-65% lower and the relative area of delamination is 2.1-2.3 times higher compared to the beams strengthened before loading. 1 series B series C series A series C series B series A