Flexural and shear strengthening of reinforced concrete elements

This work focuses on research carried out on the use of the near surface mounted (NSM) technique to increase the flexural capacity of statically indeterminate reinforced concrete (RC) slabs or shallow beams, and the use of embedded through-section (ETS) bars for shear strengthening of these structures

NSM technique to increase the load carrying capacity of continuous RC slabs

This work presents and analysis the results of an ongoing research project on the use of the near surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) laminates for the increase of the load carrying capacity of statically indeterminate (two spans) reinforced concrete (RC) slabs. The test program consisted of seventeen slab strips, grouped in two series that are different on the adopted strengthening scheme: H series, where H is the notation to identify the slabs strengthened with NSM CFRP laminates exclusively applied in the hogging region; HS series, where HS is the notation to identify the slabs strengthened with NSM CFRP laminates applied in both hogging and sagging regions. The program includes six unstrengthened reference RC slab strips, and eleven strengthened with CFRP NSM configurations designed to increase in 25% or 50% the load carrying capacity of the corresponding reference RC slabs. An extensive monitoring system was applied in the constituent materials, in the critical regions of the tested slabs, in order to collect information for the discussion about the effectiveness of NSM technique in terms of load carrying and moment redistribution capacity. The strengthening procedures adopted in the laboratory tests followed, as much as possible, the real strengthening practice for this type of interventions. The obtained results show that the proposed technique is able to increase significantly the load carrying capacity of statically indeterminate RC slabs, even for those with relatively high steel reinforcement ratios. The load carrying capacity of the strengthened slabs was limited by its shear capacity or by the detachment of the strengthened concrete cover layer. At failure of the strengthened slabs, the longitudinal steel bars at intermediate support (hogging region) and at loaded sections (sagging regions) have already yielded, and the deflection was quite large. However, for some strengthening configurations, the CFRP laminates led to a decrease of the moment redistribution capacity of the slabs.Fundação para a Ciência e a Tecnologia (FCT

On the behaviour of continuous RC slab strips flexurally strengthened by the NSM technique

This work explores the influence of the amount of CFRP in terms of load carrying capacity, ductility and moment redistribution capacity of continuous RC slab strips. In this way, for assessing the predictive performance of a FEM-based computer program, the experimental results obtained by Bonaldo (2008) and Dalfré (2013) were compared with values predicted by this software. Finally, a parametric study is carried out to investigate the influence of the strengthening arrangement and CFRP percentage in terms of load carrying capacity and moment redistribution capacity of continuous RC slab strips flexurally strengthened by the NSM technique

Steel bar - concrete bond behaviour in the context of the ETS shear strengthening technique for RC beams

The use of near-surface mounted (NSM) technique has been proved to be very effective to increase the flexural and shear resistance of reinforced concrete (RC) beams. However, the NSM is not applicable for the shear strengthening of RC slab, thus a new technique was developed (ETS – Embedded Through-Section), consisting in opening holes across the slab thickness in the shear zone, where bars are inserted and bonded to concrete by an adhesive material. To assess the contribution of the bond mechanism for the shear strengthening effectiveness of this technique, a comprehensive pullout test program was carried out, where the influences of the type of adhesive, a thickness of the adhesive layer, diameter of the strengthening bar and bond length on the bond phenomena were assessed. The experimental program is described and the obtained results are presented and analysed in this paper.Fundação para a Ciência e a Tecnologia (FCT

On the use of embedded bars for shear strengthening of RC beams

A técnica de reforço baseada na utilização de varões embebidos para o reforço ao corte de vigas de betão armado (BA), designada na literatura inglesa por “Embedded Through-Section (ETS)”, é relativamente recente e consiste na instalação de varões em furos abertos na espessura do elemento a reforçar, com inclinações desejadas, sendo fixos ao betão por meio de adesivos. Para avaliar a eficácia desta técnica, um programa experimental composto por 16 vigas de BA foi realizado e os resultados obtidos revelaram um significativo aumento da capacidade máxima de carga e deformação. De forma a obter um melhor entendimento sobre a utilização dos varões embebidos aplicados segundo a técnica ETS na resistência ao corte de vigas de BA, uma análise numérica com o uso de um programa de cálculo automático baseado no método dos elementos finitos foi realizada. De acordo com os resultados, uma boa concordância entre os valores experimentais e numéricos foram obtidos.Embedded Through-Section (ETS) technique is a relatively recent shear strengthening strategy for reinforced concrete (RC) beams, and consists on opening holes across the beam thickness, with the desired inclinations, where bars are introduced and are bonded to the concrete substrate with adhesive materials. To assess the effectiveness of this technique, a comprehensive experimental program composed of 16 RC beams was carried out and the obtained results reveal a significant increase in terms of ultimate load and deflection. To have a better assessment of the contribution of the ETS bars for the shear resistance of RC beams, numerical analysis were performed with a FEM-based computer program. A good agreement between the experimental and numerical values was obtained.Fundação para a Ciência e a Tecnologia (FCT

Assessing the effectiveness of a NSM-CFRP flexural strengthening technique for continuous RC slabs by experimental research

This work reports the results of an ongoing investigation on the use of the Near Surface Mounted (NSM) CFRP laminates for the flexural strengthening of continuous Reinforced Concrete (RC) slabs, in order to verify the possibility of increasing the negative resisting bending moment in 25%, maintaining moment redistribution levels of about 30%. To better understand the behavior of these structures, a comprehensive experimental program was proposed with the aim of evaluating the possibilities of NSM technique for statically indeterminate RC slab strips in terms of flexural strengthening effectiveness, moment redistribution and ductility performance. To assess the behavior of the tested slab strips the strains in CFRP laminates, steel reinforcement and concrete surfaces, as well as the applied load and support reactions were registered up to failure of the slabs. The experimental program is described and the obtained results are presented and analyzed in this paper.Fundação para a Ciência e a Tecnologia (FCT) - 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” (PTDC/ECM/73099/2006) (GDE 200953/2007-9).National Council for Scientific and Technological Development (CNPq) - GDE 200953/2007-

Experimental and numerical analysis of RC two-span slabs strengthened with NSM CFRP laminates

This work reports the results of an ongoing research program on the use of the near surface mounted (NSM) CFRP laminates for the flexural strengthening of continuous reinforced concrete (RC) slabs. The experimental program is formed by two slab strips of two equal span lengths, and has the main purpose of verifying the possibility of increasing the negative resisting bending moment in 25%, maintaining a relatively high level of moment redistribution. To assess the predictive performance of a FEM-base computer program, the experimental results are compared with the values estimated by the numerical analysis carried out using a FEM-based computer program. The results show that the proposed strengthening technique is able to increase significantly the load carrying capacity of RC slabs. However, the load carrying capacity of the strengthened slabs was limited by the detachment of the strengthened concrete cover layer at the intermediate support. The numerical model predicts with high accuracy the behavior of this type of structures.(undefined

Analytical and experimental study of RC slab strips strengthned with CFRP laminates

Um modelo analítico foi desenvolvido para prever a relação força-flecha de estruturas de concreto armado (CA) estaticamente indeterminadas até o seu colapso e o seu desempenho foi avaliado usando os dados obtidos em programas experimentais. Este modelo é baseado no método das forças (flexibilidade), o qual estabelece uma série de equações de compatibilidade de deslocamentos que podem fornecer variáveis desconhecidas. Para se determinar a rigidez à flexão de cada trecho da estrutura, as relações momento-curvatura das seções transversais representativas da estrutura foram obtidas. Este modelo pode ser facilmente implementado e é aplicável a estruturas de CA estaticamente determinadas ou indeterminadas reforçadas segundo a técnica NSM (Near Surface Mounted technique, em língua inglesa) ou pela colagem externa de compósitos segundo a técnica EBR (Externally Bonded Reinforcement technique, em língua externa). Neste trabalho, o desempenho preditivo do modelo foi avaliado por meio da simulação dos ensaios das faixas de laje da série SL30 reforçadas com laminados de carbono aplicados segundo a técnica NSM.To predict the load-deflection response up to the collapse of statically indeterminate reinforced concrete (RC) structures, an analytical model was developed and its predictive performance was appraised by using the data obtained in experimental programs. The proposed approach is based on the force method by establishing a number of displacement compatibility equations that can provide the unknown variables. To determine the tangential flexural stiffness making part of these equations, moment-curvature relationships are determined for the cross sections representative of the structure. This model can be easily implemented and is applicable to statically determinate or indeterminate RC structures strengthened according to the near surface mounted (NSM) or externally bonded reinforcement (EBR) techniques. In this work, the predictive performance of the model was appraised by simulating the SL30 series tests of RC slab strips strengthened with NSM carbon fiber reinforced polymer (CFRP) laminates

Assessing the effectiveness of a NSM-CFRP flexural strengthening arrangements for continuous RC slabs by experimental and numerical research

A program formed by continuous slab strips strengthened in flexure with near surface mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) laminates was carried out, and the obtained results showed the possibility of increasing significantly the load carrying capacity of these elements, maintaining high levels of ductility. The experimental program is formed by slab strips of two equal span lengths, and has the main purpose of exploring the potentialities of distinct NSM flexural strengthening configurations for the increase of the load carrying capacity of this type of structures. In the present work two types of strengthening arrangements were investigated, one with CFRP laminates exclusively applied at the intermediate support, H series (hogging region), and the other with laminates applied at both hogging and sagging regions (HS series). For assessing the predictive performance of a FEM-based computer program (FEMIX V4.0), the experimental results are compared with values predicted by this software. Then, a parametric study with 144 numerical models is carried out to investigate the influence of the strengthening arrangement and CFRP percentage in terms of moment redistribution and rotational capacities of continuous RC slab strips flexurally strengthened by the NSM technique. The obtained results are presented and analyzed in this paper.Fundação para a Ciência e a Tecnologia (FCT)The first author acknowledges the financial support of FCT (PhD Grant number SFRH/BD/69818/2010) and the National Council for Scientific and Technological Development (CNPq) –Brazil (GDE 200953/2007-9

Reforço à flexão de faixas de lajes contínuas utilizando laminados de carbono aplicados segundo a técnica NSM

Um programa experimental foi realizado para avaliar a eficácia da técnica “Near Surface Mounted” (NSM) para reforço à flexão de elementos contínuos de betão armado (BA), em termos de capacidade de carga, redistribuição de momentos e ductilidade. O programa experimental é composto por três séries com três faixas de laje de dois tramos iguais, e tem por objectivo verificar a possibilidade de aumentar a resistência aos momentos negativos (na região do apoio central) em 25% e 50%, mantendo níveis de redistribuição de momentos de 15%, 30% e 45%. Além disso, para fins de validação, os resultados experimentais são comparados com os valores previstos pela análise efectuada com um programa de cálculo automático baseado no método dos elementos finitos. Por fim, é proposta uma estratégia eficaz de reforço à flexão, capaz de aumentar a capacidade resistente das lajes e mantendo elevados níveis de redistribuição de momentos.An experimental program was carried out to assess the effectiveness of the near surface mounted (NSM) technique for the flexural strengthening of continuous reinforced concrete (RC) slabs, in terms of load carrying, moment redistribution and ductility capacities. 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%. Furthermore, for validation purposes, the experimental results are compared with values predicted by the analysis carried out using a FEM-based computer program. Finally, a high effective NSM flexural strengthening strategy is proposed, capable of enhancing the slab’s load carrying capacity and maintaining high levels of moment redistribution