International Journal of Civil and Structural Engineering Research ISSN 2348-7607 (Online) Vol. 9, Issue 2, pp: (48-55), Month: October 2021 - March 2022, Available at: www.researchpublish.com
Enhancement the efficiency of FRP External strengthening system of RC Flexural Member Azza.Abdelrashed1, T.Aly2, W. Ibrahim3, M. Meged4 1, 2.3, 4
University of Helwan – Egypt.
Abstract: This paper presents an experimental study on twelve reinforced concrete (RC) slabs strengthened with carbon, glass, basalt fiber reinforce polymer (CFRP) laminate/sheet under four-point bending, aiming to reveal the effects of the bond-dependent coefficient, (Km) on the load-carrying capacity of strengthened the slabs at debonding failure (i.e. debonding strength). For the purpose, the test specimens are such designed to explore effects of several important factors, including modulus of elasticity of the fibers, the thickness of the used fibers, and the number of layers. The experimental results showed that the number of plies and thickness of the carbon laminate/sheet are strongly influences the debonding strengths of the strengthened slabs. It was also found that the (Afiber /Asteel) ratio is major parameter that affects the performance of strengthened slabs. Keywords: FRP, debonding, concrete, slabs, strengthening.
1. INTRODUCTION One of the successful structural applications of fiber reinforced polymers (FRP) is the external composite strengthening for repair and upgrade of the structural capacity and ductility of concrete members. The advantages of FRP composites are high strength, low weight, good corrosion resistance and ease of installation. Existing studies have showed that debonding failure is a typical failure for concrete beams strengthened with externally bonded FRP composites. The ACI440 guidelines recommended the required cross-sectional area of FRP fabrics is based on satisfying a limited effective strain in the fibers matching a predetermined value controlled by the bond-dependent coefficient, (Km). The bonddependent coefficient is account only for stiffness of the laminate/sheet and ignores the stiffness of the member to which the laminate is bonded. The objective of this paper is to estimation the bond-dependent coefficient, taking into account the modulus of elasticity of the fibers, the thickness of the used fabrics, compressive strength of concrete, and the number of layers. Failure modes have been observed in the experimental program. These modes can be divided into two categories: "FRP debonding" and "FRP fracture" failures (Buyukozturk, 2004). Since in many cases, the failure of strengthened beams is governed by the FRP debonding failure, the investigation of the stresses at the concrete/strengthening layer interface is an important issue in analysis and design. An analytical model is presented in this paper to calculate the normal interfacial stress. The predicted capacities are compared to the measured values.
2. EXPERIMENTAL PROGRAM 2.1 Test specimens Twelve simply supported RC slabs with a span of 2 m were constructed and tested at the Structural Laboratory of Helwan University. Details of the test specimens are given in Table 1. This paper focuses mainly on the analytical prediction and estimating of the bond-dependent coefficient, Km. The top and bottom reinforcement were 10 mm diameter steel bars spaced at 150mm as shown in Fig. 1. All slabs were constructed with a depth of 150mm. four different types of strengthening schemes were used. Two slabs were strengthened using carbon laminates. Two slabs were strengthened using carbon sheets. Two slabs were strengthened using glass sheets. Twonormal compressive strength of concrete slabs were strengthened using basalt sheets, two high compressive strength of concrete slabs were strengthened using basalt sheets, and one slab was tested as control specimen at normal and high compressive strength. With the maximum moment occurring at the mid-span section of the slab, failure could be due to debonding, concrete crushing and rupture of the externally bonded laminates or sheets. The specimens were designed as per ACI 318-2002 design guideline (ACI 318, 2019).
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