International Concrete Abstracts Portal

This paper presents an analytical investigation conducted to study theflexural behavior of reinforced concrete beams strengthened with various Near-SurfaceMounted (NSM) Fiber-Reinforced Polymers (FRP) reinforcements. The materials used inthis investigation included carbon-fiber-reinforced-polymer (CFRP) rebars and strips,and glass fiber-reinforced-polymer (GFRP) rebars and strips. The analysis included theeffects of strengthening on the serviceability and ultimate limit states as well the effectof tension stiffening. The effectiveness of NSM FRP rebars and strips was examined andcompared to externally bonded (EB) FRP strips and sheets using the same material typeand axial stiffness. Results from the analytical models were compared with thoseobtained from experimental studies. The analytical results agree very well with thoseobtained from the experimental results. It was found that the analytical model couldeffectively simulate the behaviour of the reinforced concrete beams strengthened withvarious NSM FRP and EB FRP reinforcements. Using the same axial stiffness of FRP tostrengthen reinforced concrete beams, the beams strengthened with NSM FRPreinforcement achieved higher ultimate load than beams strengthened with EB FRPreinforcement. This result is due to the high utilization of the tensile strength of the FRPreinforcement.

In strengthened RC members the distribution of cross sectional tensile forcesis affected by the significant differences in bond behavior of the reinforcement layers.As the tensile forces in the externally bonded reinforcement is the essential input valuefor bond verification for example at the end anchorage, the detailed knowledge of thedistribution of forces in cracked sections is of fundamental importance. In this paperthe common models to describe the interactions in tensile members are summarizedand an advanced numerical model based on nonlinear bond stress-slip relationships forstrengthened flexural members is presented. On the basis of experimental resultscombined with parametric studies, the effects of various parameters – for example theaxial stiffness of CFRP, the diameter of internal rebars or the concrete compressivestrength – on the interactions between the different reinforcement layers areexamined. For practical design bond coefficients for a simplified calculation of crosssectional tensile forces are proposed.

A regulatory document was issued by the National Research Council (CNR) ofItaly on the use of FRP for strengthening structures: ‘Instructions for Design, Executionand Control of Strengthening Interventions by Means of Fibre-reinforced Composites’(2004). Emphasis is also given to specific requirements for seismic applications.This document, described in more details in the paper, sets for the first time in Italysome standards for production, design and application of FRP for reinforced concreteand masonry constructions. It is also conceived with an informative and educationalspirit, which is crucial for the dissemination, in the professional sphere, of themechanical and technological knowledge needed for an aware and competent use ofsuch materials.The document is the result of a remarkable joint effort of almost all professors andresearchers involved in this emerging and promising field, from 15 universities, of thetechnical managers of major production and application companies, and of therepresentatives of public and private companies that use FRP for strengtheningartifacts. Thus, the resulting FRP code naturally incorporates the experience andknowledge gained in ten years of studies, researches and applications of FRP in Italy.

In this paper, non-linear finite element modelling of debonding failure ofrectangular reinforced-concrete beams strengthened with externally-bonded CarbonFiber Reinforced Polymer (CFRP) fabrics under bending is presented. Smeared crackmodels were used to simulate concrete cracking. Interface elements were used tomodel the bond between concrete and reinforcement. The model proved to be able tosimulate the beams’ behaviour, predicting the failure modes, the failure loads and thereinforcement strain distributions relatively well. The parametric study using the FEAclarified the effect of several important factors on the capacity of the beams.

When strengthening concrete structures using externally bonded compositematerial plates, the bond between the concrete and plate typically fails well before theplate’s tensile capacity is achieved. To enhance the bond performance, usingcomposite fabric between the plate and the concrete was explored. Six tensiondevelopment specimens were tested to determine the maximum tensile load that couldbe applied to a CFRP plate before it debonded. Three specimens had a Sika CarboDurstrip bonded directly to the concrete, while three others had the CarboDur strip bondedto two plies of Sikawrap Hex 230C fabric material. The fabric was used to distribute theload over a larger bond area. The specimens without fabric failed at an average load of60.5 kN while the specimens with the CFRP fabric failed at an average load of 92.6 kN.These failure loads represent 18 % and 27.5% of the plate capacity respectively.