PSI - Issue 10

N. Nikoloutsopoulos et al. / Procedia Structural Integrity 10 (2018) 141–147 N. Nikoloutsopoulos et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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polymers (FRPs) is a promising and extensively studied technique. As a new building material, Carbon Fibre Rein forced Polymer (CFRP) has many superior material properties, such as high tensile strength, low weight, good resis tance to corrosion, creep and fatigue, and low linear expansion coefficient and relaxation rate (Qinghua et al. (2017)). The most common method of strengthening is the in situ wet application of carbon fibres fabrics or strips, impreg nated with resin and wrapped around the beams in the direction of the perpendicular axis of the beam (Chalioris et al. (2017)). However, premature debonding failure governs in flanged beams with U-strengthening technique since the slab usually prevents the wrapping around the cross section or/and the proper end anchorage of the externally applied FRP (Chen et al. (2017)). For more efficient shear strengthening of beams with carbon fibre strips, each of them is anchored with carbon fibre rope. In this way, the fewer perforations in the existing carrier are assured, compared with the reinforcement of continuous fabrics and quadrilateral strips (Yang et al. (2011)). Experimental research has been carried out with FRP stripes and jackets in U-strengthening technique perpendicular to the axis of the specimen, as well as in two types of anchoring of the above cases without drilling operation. The main conclusion was that the anchoring techniques failed in a higher applied load (Chen et al. (2016)). In another related study, the U-shape wrapping and the full wrapping beam strengthening with CFRP strips, perpendicular to the axis of the specimen, in comparison with strips placed at an angle of 45° was examined. The results showed that the full wrapping technique led to a load carrying capacity enhancement and moreover as a significant result the failure mech anism was observed to change from a brittle shear failure to the desirable flexural failure (Mostofinejad et al. (2016)). In addition, a research has been carried out for two cases of beam shear strengthening with CFRP strips in U strengthening technique perpendicular to the axis of the specimen. In each case a different height of attachment of the composite to the specimen was applied, with corresponding single rope beam anchoring. The experimental results revealed that application of CFRP strips increased overall shear capacity and the use of CFRP anchoring improved further the shear capacity and ductility of failure (Baggio et al. (2014)). In the present research the efficiency of shear strengthening of beams with Fibre Reinforced Polymers (FRP) with various strengthening techniques was studied. From them, the optimized techniques were suggested, taking into account the use of the less possible drilling operations to the existing load bearing element and the less cost. The Some of the techniques were not previously studied (Table 1, Code F).

Nomenclature σ fed

design value of means stress in fibre reinforced polymer (FRP) modulus of elasticity in fibre reinforced polymer (FRP)

E f

2. Experimental procedure

In the context of the research, strength class C30/37 reinforced concrete beams were produced to be strengthened in shear. The beams’ dimensions were 15x15x70cm with only bending reinforcement 3Ø10 and without shear rein forcement. The specimens were strengthened in different schemes and tested in four-point bending test for studying the shear failure. It should be noted that during the production of the beams a curvature was given to their edges in order to avoid stress concentration and failure the strengthening material. The investigated strengthening schemes are presented in Table 1. The beams were strengthened following two different mechanisms, in order to compare the different grade of efficiency of strengthening placed perpendicular to the longitudinal axis of the beam (as it typically applied in con structions) and at an angle of 45 o (according to the truss of Morsch-Ritter). Beams were strengthened with Carbon Fibre Reinforced Polymers (CFRPs) and epoxy based resin. SikaWrap® - 230C was used for strips and jackets and SikaWrap® FX – 50C for rope and for anchoring the strips and the jackets. Sikadur® - 330 was used for boxing the rope in a notch and bonding the FRP with concrete beam surface. Sikadur® - 300 was used for impregnating the rope for stiffening it and to increase its tensile strength. As shown in Fig.1a, which presents the reference beam (Ref) before and after the two-point bending test, the failure was due to shearing. Fig.1b shows the strengthening technique C.2 before and after the two-point bending test.