2-span reinforced concrete beam strengthened with fibre reinforced polymer

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Flexural and shear strengthening of RC beams with composites materials: the influence of cutting steel stirrups to install CFRP strips

[Excerpt] Introduction: Experimental [1], numerical and analytical [2] research reveals that as larger is the height of the cross section of Carbon Fiber Reinforced Polymer (CFRP) strip, as more effective is the Near Surface Mounted (NSM) technique for flexural strengthening of reinforced concrete (RC) beams. However, this height is, in general, limited to the concrete cover, since the application of strips of cross section height larger than the concrete cover requires that the bottom arm of the steel stirrups be cut [3]. This work aims to assess the influence, in terms of shear resistance, of cutting the bottom arm of steel stirrups to install NSM strips for the flexural strengthening of RC beams. In the present paper the experimental program is described, and the relevant obtained results are presented and discussed. [...

Flexural strengthening of masonry members using advanced cementitious materials

Two different cement based fiber reinforced composites for the flexural strengthening of masonry beams under monotonic loading are studied. Steel Fiber Reinforced Self- Compacting Concrete (SFRSCC) with tensile strain-softening behavior, and PVA fiber reinforced cement based mortar (SHCC) with tensile Strain-Hardening were the developed composites. Both composites were applied on the tensile surface of masonry beams and the effectiveness of this technique for the flexural strengthening of these quasi-brittle structural elements was assessed by performing four point beam bending tests. Both materials contributed effectively to increase the load carrying capacity and ultimate deflection ductility of the tested masonry beams, but, higher average values were obtained for these two indicators of the strengthening effectiveness when using a layer thickness of SHCC that is 2/3 of the thickness of SFRSCC. Furthermore, much more homogenous results, in terms of forcedeflection relationship, were obtained with masonry beams strengthened with SHCC than with SFRSCC

Near surface mounted CFRP strips for the flexural strengthening of RC columns: experimental and numerical research

In the present work, a strengthening technique based on near surface mounted (NSM) carbon fibre laminate strips bonded into slits opened on the concrete cover is used to improve the flexural capacity of columns subjected to bending and compression. This technique avoids the occurrence of the peeling phenomenon, is able to mobilize the full strengthening capacity of the strips, and provides higher protection against fire and vandalism acts. The present paper describes the adopted strengthening technique and reports the experimental characterization of the materials involved in the strengthening process. The results obtained in two series of reinforced concrete columns, subjected to axial compression and lateral cyclic loading, show that a significant increase on the load carrying capacity can be achieved by using the NSM technique. Cyclic material constitutive laws were implemented in a finite element program and the tests with reinforced concrete columns strengthened with the NSM technique were numerically simulated under cyclic loading. These numerical simulations reproduce the experimental load-displacement diagrams satisfactorily.The study reported in this paper is included in the research program "CUTINSHEAR-Performance assessment of an innovative Structural FRP strengthening technique using an integrated system based oil optical fiber sensors" Supported by FCT, POCTI/ECM/59033/2004. The second author acknowledges the support provided by the grant in the ambit of this research project

Efficiency of different techniques in flexural strengthening of RC beams under monotonic and fatigue loading

In the context of flexural strengthening of concrete structures, fiber reinforced polymers (FRP) have been used mostly by two main techniques: Externally Bonded Reinforcement (EBR) and Near-Surface Mounted (NSM). Both strengthening techniques are applied on the cover concrete, which is normally the weakest region of the element to be strengthened. Consequently, the most common problem is the premature failure of the strengthening system that occurs more frequently in the EBR one. In an attempt of overcoming this weakness, another technique has been proposed, called MF-EBR – Mechanically Fastened and Externally Bonded Reinforcement, which uses multi-directional carbon fiber laminates, simultaneously glued and anchored to concrete. To compare the efficiency of NSM, EBR and MF-EBR techniques, four-point bending tests with RC beams were carried out under monotonic and cyclic loading. In this work the tests are described in detail and the obtained results are discussed. Additionally, to assess the performance of a FEM-based computer program for the prediction of the behaviour of RC beams strengthening according to these techniques, the beams submitted to monotonic loading were numerically simulated.FCT; Hilti Portugal - Productos e Serviços Lda.; S&P Clever Reinforcement Ibérica Lda.; SECIL; TSwaterjet, Lda

Analytical and numerical analysis of the behaviour of RC beams flexural strengthened with CFRP

Experimental research regarding the flexural strengthening of reinforced concrete (RC) structures with Carbon Fibre Reinforced (CFRP) systems using the Externally Bonded Reinforcing (EBR) and the Near Surface Mounted (NSM) techniques has been carried out at the University of Minho. Considering the experimental results of this research, the performance of the ACI and fib formulations for the EBR flexural strengthening was appraised in this paper. The same was done in terms of NSM technique adopting for the CFRP debonding strain a value that is 70% of the CFRP ultimate strain, according to the recommendations of the ACI Committee 440. However, the experimental results show that the CFRP debonding strain is dependent on the CFRP percentage and existing longitudinal steel reinforcement ratio. To estimate an equation for the prediction of the CFRP debonding strain, which takes into account these parameters, a numerical model was applied to the simulation of the tests carried out.(undefined

Assessment of the effectiveness of prestressed NSM CFRP laminates for the flexural strengthening of RC beams

The flexural behavior of RC beams strengthened with prestressed near-surface-mounted (NSM) carbon-fiber-reinforced-polymer (CFRP) laminate was investigated in this paper. For this purpose, four RC beams were tested under monotonic four-point loading. One beam was kept un-strengthened, as a control beam, and another one was strengthened with a non-prestressed NSM CFRP laminate. The remaining beams were strengthened with NSM CFRP laminates prestressed at 20% and 40% of its ultimate tensile strength. The prestressed NSM CFRP laminate technique provided a significant increment of the load carrying capacity for deflection levels corresponding to serviceability and ultimate limit states. A numerical strategy was also employed to simulate the flexural behavior of the tested RC beams. The experimental and numerical researches are described and the relevant results are presented and discussed.Fundação para a Ciência e a Tecnologia (FCT

SIGNIFICANCE OF U-WRAP FRP SHEAR STRENGTHENING ON FLEXURAL BEHAVIOR OF RC BEAMS STRENGTHENED USING NEAR SURFACE MOUNTED FRP BARS

The aim of this study is to investigate the significance of U-wrap shear strengthening on the flexural behavior of Near Surface Mounted (NSM) Fiber Reinforced Polymers (FRP) strengthened Reinforced Concrete (RC) beams. It is well-known that the performance of NSM-FRP technique is strongly dependent on bond performance between adhesive-concrete and adhesive-FRP interface. Although a full development length is provided for the NSM FRP bar, rupture of the FRP bar is highly unlikely. This is attributed to the fact that the NSM FRP bar typically observes a stress level lower than 60% of its ultimate capacity at RC beam failure by debonding of NSM-FRP from the surrounding adhesive. Here, a typical three-side FRP U-wrap using wet layup was employed to improve the shear strength of the RC beam. A test matrix of 25 beams was tested under static load to failure. Four sets were considered including conventional RC beams, RC-beams with U-wrap only, RC-beams with NSM-FRP strengthening only without U-wrap FRP shear strengthening, RC beams with NSM-FRP flexural strengthening and U-wrap FRP shear strengthening, RC beams with NSM-FRP flexural strengthening and U-wrap FRP shear strengthening in shear zone only. The experimental results showed that incorporating FRP U-wrap has a significant effect of the performance of NSM-FRP strengthened RC beams. While a limited improvement of flexural strength of 20% was observed, NSM-FRP strengthened beams with FRP U-wrap experienced a significant reduction in ductility causing sudden failure. The change in the NSM-FRP strengthening system behavior might be attributed to the confinement provided by the U-wrap FRP which resulted in improving the bond strength of the NSM-FRP to the adhesive. This in its turn lead to NSM-FRP bar picking significantly high load level up to rupture with abrupt RC beam failure. The experimental results shed light on the need to consider design limitations when NSM-FRP flexural strengthening is combined with U-wrap FRP shear strengthening in RC beams

A New Hybrid Methodology According to NSM CFRP Technique for the Flexural Strengthening of RC Beams

The objective of this paper is to propose a new hybrid methodology according to near surface mounted (NSM) technique, using carbon fiber reinforced polymer (CFRP) reinforcement for the flexural strengthening of reinforced concrete (RC) beams. This NSM hybrid flexural strengthening technique combines non-prestressed and prestressed CFRP laminates in the same application in order to provide a good balance in terms of load carrying and ultimate displacement capacity to the strengthened elements. An experimental program composed of six RC beams was carried out to assess the benefits of this NSM hybrid technique when compared to the use of non-prestressed or prestressed NSM CFRP laminates (NSM prestressing technique). For this purpose, the performance of both techniques in terms of crack width, prevailing failure mode, ultimate displacement capacity, energy absorption, and load carrying capacity of the strengthened beams was assessed. The experimental tests were also simulated by executing advanced 3D nonlinear finite element analysis. Moreover, the potentialities of other configurations for the NSM hybrid technique by adopting different non-prestressed CFRP reinforcement ratios were numerically assessed executing a parametric study, and the relevant results are presented and discussed.The study reported in this paper is part of the project "PreLami - Performance of reinforced concrete structures strengthened in flexural with an innovative system using prestressed NSM CFRP laminates", with the reference PTDC/ECM/114945/2009 supported by FCT. The authors would also like to acknowledge the support provided by CLEVER Reinforcement Iberica Company, for supplying the adhesives and the laminates, and Casais and CiviTest for the preparation of the beams