Strengthening of concrete members with advanced composite materials

This research involves two experimental investigations into the behaviour of prestressed concrete members strengthened using two typical techniques: the first is by application of external post-tensioning using carbon fiber reinforced plastic (CFRP) cables, whereas the second is by bonding composite straps on the tension side of the members. In the first investigation, a total of twelve partially prestressed concrete beams were tested. Nine of the beams were first subjected to loads large enough to cause considerable cracking and deformations and then strengthened with the external CFRP cables and loaded up to failure. The CFRP cables used were CFCC 1 $\times$ 7-5 mm and 7.5 mm diameter, manufactured by Tokyo Rope, Japan. The remaining three beams were loaded monotonically from zero up to failure without strengthening and were used as control beams for comparison purposes. The twelve beams had the same concrete cross-section dimensions and were divided into three groups; each group consisted of four beams of the same length but with different levels of internal prestressing. The purpose was to study the effects of the span-to-depth ratio and the partial prestressing ratio or the reinforcing index on the performance of reinforced or prestressed concrete beams after being strengthened with the external CFCC cables

Flexural performance of reinforced concrete beams strengthened with prestressed near-surface-mounted FRP reinforcements

YesA numerical method for estimating the curvature, deflection and moment capacity of reinforced concrete beams strengthened with prestressed near-surface-mounted (NSM) FRP bars/strips is presented. A sectional analysis is carried out to predict the moment–curvature relationship from which beam deflections and moment capacity are then calculated. Based on the amount of FRP bars, different failure modes were identified, namely tensile rupture of prestressed FRP bars and concrete crushing before or after yielding of steel reinforcement. Comparisons between experimental results available in the literature and predicted curvature, moment capacity and deflection of reinforced concrete beams with prestressed NSM FRP reinforcements show good agreement. A parametric study concluded that higher prestressing levels improved the cracking and yielding loads, but decreased the beam ductility compared with beams strengthened with nonprestressed NSM FRP bars/strips

Experimental behaviour of RC beams shear strengthened with NSM CFRP laminates

The near-surface mounted (NSM) is one of the most recent techniques applied for the increase of the shear resistance of reinforced concrete (RC) beams. This technique involves the installation of carbon fibre reinforcement polymers (CFRP) laminates into thin slits open on the concrete cover of the elements to strengthen. The effectiveness of this technique for the shear strengthening of T crosssection RC beams was assessed by experimental research. For this purpose, three inclinations of laminates were tested (45º, 60º and 90º) and, for each inclination, three percentages of CFRP were applied in RC beams with a percentage of steel stirrups of 0.10% (qsw). The highest percentage of laminates was designed to provide a maximum load similar to the reference RC beam, which was reinforced with a reinforcement ratio of steel stirrups of 0.28% (qsw = 0.28%). For each percentage of laminates, a homologous RC beam strengthened with unidirectional U-shaped CFRP wet lay-up sheets (discrete strips) applied according to the externally bonded reinforcement technique was also tested, with the purpose of comparing the effectiveness of these two CFRP-strengthening techniques. To evaluate the influence of the percentage of steel stirrups in the effectiveness of the NSM technique, some of the abovementioned CFRP configurations were also applied in beams with qsw = 0.17%The authors wish to acknowledge the support provided by the 'Empreiteiros Casais', Degussa, S&P (R) and Secil (Unibetao, Braga). The study reported in this paper forms a part of the research program supported by FCT, PTDC/ECM/73099/2006

Exploring new NSM reinforcements for the flexural strengthening of RC beams: experimental and numerical research

Carbon-fiber-reinforced-polymer (CFRP) composite materials applied according to near-surface-mounted (NSM) technique constitute an effective technique for the flexural and shear strengthening of reinforced-concrete (RC) structures. However, the NSM CFRP reinforcement ratio is limited by the thickness of concrete cover of the longitudinal tensile steel bars, and the minimum distance between consecutive CFRPs, below which premature fracture of surrounding concrete occurs due to group effect. Hence, the current study aims to experimentally and numerically evaluate the strengthening potentialities of a novel NSM system (with high CFRP ratio capability) for the flexural strengthening of RC beams. This new system combines externally-bonded-reinforcement (EBR) and NSM techniques in the same application using T-shaped CFRP profiles. The obtained experimental results of the RC beams strengthened with CFRP profiles are presented and discussed with the aim of evaluating the influence of CFRP profile reinforcement ratio on the strengthening efficiency of this technique. A developed 3D finite-element (FE) approach is used to simulate the experimental tests. After demonstrating its good predictive performance, a series of parametric studies is performed to assess the influence of the main material properties, and ratio of bond area to cross sectional area of the CFRP profiles on the efficiency of the proposed system.The first and the last authors acknowledge the support provided by Cutinov QREN project n. 38780 supported by ADI, co-financed by the European Regional Development Fund (FEDER) through the Operational Program COMPETE. The second and the third authors would like to acknowledge the support provided by Mostostal Warszawa S.A. for providing the CFRP T-shaped profiles and for co-funding the research progra

CFRP flexural and shear strengthening technique for RC beams : experimental and numerical research

Near surface mounted (NSM) technique has proved to be a very effective technique for the flexural strengthening of RC beams. Due to the relatively small thickness of the concrete cover that several beams present, cutting the bottom arm of steel stirrups for the installation of NSM laminates might be a possible strategy, whose implications on the beam’s load carrying capacity need to be assessed. When steel stirrups are cut, however, the shear resistance can be a concern. This also happens when a strengthening intervention is carried out to increase the flexural resistance of a beam, since in certain cases it is also necessary to increase the shear resistance in order to avoid the occurrence of brittle shear failure. The present work assesses the effectiveness of a technique that aims to increase both the flexural and shear resistance of RC beams that have the bottom arm of the steel stirrups cut for the application of NSM laminates. This assessment is performed by experimental and numerical research. The main results of the experimental program are presented and analyzed, and the innovative aspects of a constitutive model implemented in a computer program are described, being their virtues and deficiencies discussed.The study reported in this paper forms a part of the research program "CUTINEMO - Carbon fiber laminates applied according to the near surface mounted technique to increase the flexural resistance to negative moments of continuous reinforced concrete structures" supported by FCT, PTDC/ECM/73099/2006. The authors wish to acknowledge the support also provided by the S&P, Casais and Artecanter Companies. The second Author acknowledges the grant under the aforementioned research project. The third author acknowledges the financial support of FCT, PhD Grant number SFRH/BD/23326/2005

Flexural strengthening of RC continuous slab strips using NSM CFRP laminates

To assess the effectiveness of the near surface mounted (NSM) technique, in terms of load carrying and moment redistribution capacities, for the flexural strengthening of continuous reinforced concrete (RC) slabs, an experimental program was carried out. The experimental program is composed of three series of three slab strips of two equal span length, in order to verify the possibility of increasing the negative (at the intermediate support region) resisting bending moment in 25% and 50% and maintaining moment redistribution levels of 15%, 30% and 45%. Though the flexural resistance of the NSM strengthened sections has exceeded the target values, the moment redistribution was relatively low, and the increase of the load carrying capacity of the strengthened slabs did not exceed 25%. This experimental program is analyzed to highlight the possibilities of NSM technique for statically indeterminate RC slabs in terms of flexural strengthening effectiveness, moment redistribution and ductility performance. Using a FEM-based computer program, which predictive performance was appraised using the obtained experimental results, a high effective NSM flexural strengthening strategy is proposed, capable of enhancing the slab’s load carrying capacity and maintaining high levels of ductility.The study reported in this paper forms a part of the research program "CUTINEMO - Carbon fiber laminates applied according to the near surface mounted technique to increase the flexural resistance to negative moments of continuous reinforced concrete structures" supported by FCT, PTDC/ECM/73099/2006. The authors wish to acknowledge the support also provided by the S&P, Casais and Artecanter Companies. The first Author acknowledges the financial support of National Council for Scientific and Technological Development (CNPq) - Brazil, Ph.D. Grant no. 200953/2007-9. The second Author wishes to acknowledge the support provided by FCT, by means of the SFRH/BSAB/818/2008 and SFRH/BSAB/913/2009 sabbatical grants

Cohesive Fracture Study of a Bonded Coarse Silica Sand Aggregate Bond Interface Subjected to Mixed-Mode Bending Conditions

One of the primary objectives in the design of composite structures is the prevention of premature bond failure. Therefore, the characterization of cohesive behavior is an important field of study in structural engineering. Using fracture mechanics principles, the cohesive behavior of an epoxy bonded coarse silica sand aggregate bond interface is studied in this paper, with a focus on finding a general analytical form of idealizing its behavior when used in a specimen possessing asymmetric and inhomogeneous qualities. Two series of small-scale specimens were experimentally tested under mixed-mode bending (MMB) conditions, where it was found that there was negligible influence exerted on the fracture energy of the interface due to changes in the mixed-mode ratio or initial crack length. Using finite element analysis (FEA) methods, an appropriate bilinear traction-separation model was developed to both validate as well as obtain a set of consistent parameters applicable to all tested specimens. Comparison of the Global Method and the Local Method, used to obtain partitioned Mode I and Mode II fracture energy values from MMB specimens, were made, with the conclusion that both methods are adequate in the calculation of the total fracture energy though the Local Method should be used to obtain accurate partitioned Mode I and Mode II fracture energy values. Idealization of the bond interface using the cohesive parameters derived can be accurately achieved by the use of both contact interactions and cohesive elements in two-dimensional and three-dimensional FE models, though the results obtained using contact interactions would be expected to exhibit greater global stiffness