Innovative methodologies for the enhancement of the flexural strengthening performance of NSM CFRP technique for RC beams

Tese de Doutoramento em Engenharia CivilCarbon fiber reinforced polymer (CFRP) materials have been extensively adopted for the flexural and shear strengthening of RC structures due to their several advantages, like a low mass density, a high strength and stiffness to weight ratios, excellent fatigue behavior, non-susceptible to corrosion, and high durability in typical environment conditions of civil engineering constructions. These CFRP composite materials can be applied to RC structures to be strengthened by using either externally bonded reinforcing (EBR) or near surface mounted (NSM) techniques. The main purpose of the current study is to experimentally evaluate the efficiency of two flexural strengthening methodologies using the NSM CFRP technique for the enhancement not only the load carrying capacity at concrete cracking and steel yielding initiations, serviceability limit state, and ultimate condition, but also the ultimate deflection capacity when compared to the use of non-prestressed NSM CFRP technique. The investigated strengthening methodologies include NSM prestressing and NSM hybrid techniques. In the NSM prestressing technique, an appropriate prestress level is applied on the CFRP reinforcement, while the NSM hybrid technique combines nonprestressed and prestressed CFRP reinforcements in the same application. Moreover, the distribution of tensile strain and bond shear stress along the prestressed NSM CFRP reinforcement is experimentally evaluated after the release of prestress force. On the other hand, the experimental tests are simulated using a nonlinear finite element (FE) model, which considers the nonlinear behavior of the constituent materials, the behavior of CFRP-epoxy adhesive-concrete interfaces, and modeling the prestress process of the CFRP elements adopted in the experimental tests. Besides, simplified analytical approaches, with a design framework, are developed to predict the flexural behavior of RC beams flexurally strengthened with prestressed CFRP reinforcement applied according to either EBR or NSM techniques. Moreover, this study offers an analytical formulation based on a closed form solution for the prediction of the distribution of CFRP tensile strain and bond shear stress and, additionally, the prestress transfer length immediately after the release of the prestress force.Materiais compósitos de matriz polimérica reforçados com fibras de carbono (CFRP) podem ser aplicados no reforço de estruturas de BA por intermédio de colagem exterior de sistemas prefabricados (laminados) ou curados in sito (manta ou tecido) na superfície dos elementos a reforçar (EBR -Externally Bonded Reinforcement) ou por utilização da técnica NSM (Near Surface Mounted). O principal objetivo do presente estudo é avaliar, experimentalmente, a eficiência de duas novas metodologias de reforço à flexão utilizando a técnica NSM com laminados de CFRP, em que numa das metodologias todos os laminados de são aplicados sob determinada pré-tensão, e na outra combinam-se laminados passivos (sem pré-tensão) com laminados pré-tensionados. Quando aplicadas a elementos de BA que desenvolvem rotura por flexão, estas novas técnicas pretendem aumentar a sua capacidade de carga para início da fendilhação do betão, estados limites último e de serviço, bem como melhorar a sua capacidade de deformação última, quando se toma por base de comparação os resultados possíveis de obter com a utilização da técnica NSM com laminados passivos de CFRP. Além disso, a distribuição da deformação por tração e da tensão de corte ao longo do laminado pré-tensionado é experimentalmente avaliada após a libertação da força de pré-esforço. Os ensaios experimentais foram simulados numericamente utilizando modelos constitutivos capazes de modelar o comportamento não-linear dos materiais constituintes, das interfaces CFRP-adesivo e adesivo-betão, bem como o processo de aplicação do préesforço nos laminados de CFRP e transferência deste para o betão do substrato das vigas a reforçado, em similitude com o realizado experimentalmente. Além disso, foram desenvolvidas formulações analíticas simplificadas, para prever o comportamento à flexão de vigas de BA reforçadas à flexão com laminados de CFRP pré-tensionados (e/ou passivos), os quais podem ser aplicados segundo as técnicas EBR ou NSM. A formulação analítica integrada permite prever a relação momento-curvatura de dada secção, e forçaflecha de vigas reforçadas segundo as técnicas de reforço desenvolvidas, bem como o campo de extensões e tensões de corte nas zonas críticas dos laminados de CFRP, e o comprimento de transmissão do pré-esforço, recorrendo-se para tal a metodologias aplicáveis no quadro do projeto de estruturas de BA

Transfer zone of prestressed CFRP reinforcement applied according to NSM technique for strengthening of RC structures

This study presents an experimental program to assess the tensile strain distribution along prestressed carbon fiber reinforced polymer (CFRP) reinforcement flexurally applied on the tensile surface of RC beams according to near surface mounted (NSM) technique. Moreover, the current study aims to propose an analytical formulation, with a design framework, for the prediction of distribution of CFRP tensile strain and bond shear stress and, additionally, the prestress transfer length. After demonstration the good predictive performance of the proposed analytical approach, parametric studies were carried out to analytically evaluate the influence of the main material properties, and CFRP and groove cross section on the distribution of the CFRP tensile strain and bond shear stress, and on the prestress transfer length. The proposed analytical approach can also predict the evolution of the prestress transfer length during the curing time of the adhesive by considering the variation of its elasticity modulus during this period.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. 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

Flexural and shear response predictions of statically determinate and indeterminate RC structures strengthened with fiber reinforced polymer

Marie Curie Initial Training Network under the project “ENDURE” with reference number 607851, funded by the EU programme: FP7-people. The third author wish to acknowledge the grant SFRH/BSAB/114302/2016 provided by FC

Shear strengthening of damaged reinforced concrete beams with hybrid composite plates

This paper aims to evaluate experimentally the potentialities of Hybrid Composite Plates (HCPs) technique for the shear strengthening and repairing of reinforced concrete (RC) beams. HCP is a thin plate of Strain Hardening Cementitious Composite (SHCC) reinforced with Carbon Fiber Reinforced Polymer (CFRP) laminates. For this purpose, an experimental program composed of four Rectangular and five T-cross section beams was executed to assess the strengthening efficiency of HCPs technique. There were two control beams without any type of shear reinforcement, and seven beams strengthened with HCPs. The HCPs were applied to the lateral faces of the beams by using a combination of epoxy adhesive and mechanical anchors. The bolts were applied with a certain torque in order to increase the concrete confinement. To have a better understanding of the shear behavior of SHCC material, Iosipescu shear tests were carried out, and the results were used to derive their fracture mode II parameters. To further explore the potentialities of the HCPs technique for the shear strengthening, the experimental tests were simulated numerically by a FEM-based computer program considering the material properties obtained experimentally. After demonstration of the good predictive performance of the numerical model, a parametric study was executed to investigate the influence of some parameters on the load carrying capacity of the strengthened beams, namely: i) Use a mortar instead of the SHCC; ii) application of different applied torque level to the mechanical anchors; iii) different bond condition between HCPs and concrete substrate.Clever Reinforcement Iberica for providing the CFRP laminates and epoxy, Sika for the sand and adhesive, Grace for the superplasticizers, Dow Chemical Co. for viscous modifying agents, ENDESA Compostilla power station for the fly ash, and Casais for assisting in the execution of the beams. FCTinfo:eu-repo/semantics/publishedVersio

Analytical approach for the flexural analysis of RC beams strengthened with prestressed CFRP

The objective of this paper is to propose a simplified analytical approach to predict the flexural behavior of simply supported reinforced-concrete (RC) beams flexurally strengthened with prestressed carbon fiber reinforced polymer (CFRP) reinforcements using either externally bonded reinforcing (EBR) or near surface mounted (NSM) techniques. This design methodology also considers the ultimate flexural capacity of NSM CFRP strengthened beams when concrete cover delamination is the governing failure mode. A moment–curvature (M–χ) relationship formed by three linear branches corresponding to the precracking, postcracking, and postyielding stages is established by considering the four critical M–χ points that characterize the flexural behavior of CFRP strengthened beams. Two additional M–χ points, namely, concrete decompression and steel decompression, are also defined to assess the initial effects of the prestress force applied by the FRP reinforcement. The mid-span deflection of the beams is predicted based on the curvature approach, assuming a linear curvature variation between the critical points along the beam length. The good predictive performance of the analytical model is appraised by simulating the force–deflection response registered in experimental programs composed of RC beams strengthened with prestressed NSM CFRP reinforcements.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. The third author also wishes to acknowledge the scholarship granted by FT (SFRH/BD/61756/2009). The authors would also like to acknowledge the support provided by S&P, for supplying the adhesives and the laminates, and Casais and CiviTest for the preparation of the beams

Analytical model to predict dilation behavior of FRP confined circular concrete columns subjected to axial compressive loading

Experimental research and real-case applications are demonstrating that the use of fiber–reinforced polymer (FRP) composite materials can be a solution to substantially improve circular cross section concrete columns in terms of strength, ductility, and energy dissipation. The present study is dedicated to developing a new model for estimating the dilation behavior of fully and partially FRP-based confined concrete columns under axial compressive loading. By considering experimental observations and results, a new relation between secant Poisson's ratio and axial strain is proposed. In order for the model to be applicable to partial confinement configurations, a confinement stiffness index is proposed based on the concept of confinement efficiency factor. A new methodology is also developed to predict the ultimate condition of partially FRP confined concrete taking into account the possibility of concrete crushing and FRP rupture failure modes. By comparing the results from experimental tests available in the literature with those determined with the model, the reliability and the good predictive performance of the developed model are demonstrated.project ‘‘StreColesf_Innovative technique using effectively composite materials for the strengthening of rectangular cross section reinforced concrete columns exposed to seismic loadings and fire’’, with the reference POCI-01-0145-FEDER-029485

Shear strengthening of RC beams with thin panels of mortar reinforced with recycled steel fibres

The use of thin cement based panels reinforced with relatively high content of Recycled Steel Fibres (RSF) for the shear strengthening of Reinforced Concrete (RC) beams is investigated in the present work. The mechanical properties of this Recycled Steel Fibre Reinforced Mortar (RSFRM) are characterised. The panels are produced by using a mixing technique similar to the one used in the slurry infiltrated fibre concrete technology. Then, their potentialities as a shear strengthening solution for RC beams deficiently reinforced in shear are investigated by performing three-point bending tests with RC beams of "I" shape cross section. The RSFRM panels are applied in the lateral faces of the beams by using the two following strategies: (1) bonded to the concrete substrate by applying exclusively epoxy adhesive; and (2) besides epoxy adhesive, mechanical fasteners are applied. The applicability of an analytical approach for estimating the contribution of RSFRM panels for the shear resistance of RC beams is assessed, and a design example is presented.The authors would acknowledge the contribution of CiviTest Company on the production of the specimens and RC beams, and on the execution of the experimental program of the RC beams. The authors wish also to acknowledge the support provided by BioSafe company on providing gratuity the RSF for the experimental program. The third author wish to acknowledge the grant SFRH/BSAB/114302/2016 provided by FCT. The support provided by the PTDC/ECM-EST/2635/2014 FCT project is also acknowledged.info:eu-repo/semantics/publishedVersio

A new dilation model for FRP fully/partially confined concrete column under axial loading

Experimental research has confirmed that the usage of fiber reinforced polymer (FRP) composite materials can be a reliable solution to substantially improve axial and dilation behavior of confined concrete columns. In this regard, FRP partial confinement system is a good compromise from the cost competitiveness point of the view, while the application of discrete FRP strips provides less confinement efficiency compared to full confinement system. Experimental observations demonstrated that the concrete at the middle distance between the FRP strips experiences more transversal expansion compared to concrete at the strip regions. It can result in a considerable decrease in the confinement performance in curtailing concrete transversal expansion, overwhelming the activation of FRP confining pressure. The present study is dedicated to the development of a new dilation model for both full and partial confinement systems, which takes into account the substantial impact of non-uniform distribution of concrete transversal expansion, a scientific topic not yet addressed comprehensibly in existing formulations. For this purpose, a reduction factor was developed in the determination of the efficiency confinement parameter, by considering available experimental results. Furthermore, based on a database of FRP fully/partially confined concrete, a new analytical relation between secant Poisson’s ratio and axial strain was proposed. To evaluate the reliability and predictive performance of the developed dilation model, it was applied on the simulation of experimental tests available in the literature. The results revealed that the developed model is capable of predicting the experimental counterparts with acceptable accuracy in a design context.The study reported in this paper is part of the project ‘‘StreColesf_Innovative technique using effectively composite materials for the strengthening of rectangular cross section reinforced concrete columns exposed to seismic loadings and fire’’, with the reference POCI-01-0145-FEDER-029485. The forth author also acknowledges the grant provided by PufProtec project with the reference POCI-01-0145-FEDER-028256

End concrete cover separation in RC structures strengthened in flexure with NSM FRP: Analytical design approach

Fiber-reinforced-polymer (FRP) composite materials applied according to the near-surface-mounted (NSM) technique are very effective for the flexural strengthening of reinforced-concrete (RC) structures. However, the flexural strengthening effectiveness of this NSM technique is sometimes compromised by end concrete cover separation (CCS) failure, which is a premature failure before occurring the conventional flexural failure modes. Due to the complexity of this failure mode, no analytical approach, with a design framework for its accurate prediction, was published despite the available experimental results on this premature failure. In the present study, a novel simplified analytical approach is developed based on a closed form solution for an almost accurate prediction of CCS failure in RC structures strengthened in flexure with NSM FRP reinforcement. After demonstrating the good predictive performance of the proposed model, it was used for executing parametric studies in order to evaluate the influence of the material properties and FRP strengthening configuration on the susceptibility of occurring the CCS failure. At the end, regarding to the FRP strengthening configuration, some design recommendations were proposed to maximize the resistance of NSM FRP strengthened structures to the susceptibility of occurring the CCS failure.The authors acknowledge the financial support provided by QREN (through the Operational Program COMPETE) in the scope of the CutInov Project (n. 38780) involving the Clever Reinforcement Company and the Structural Composites Research group of ISISE-Minho University

An analytical approach for evaluating the impact response of steel fiber reinforced concrete beam

In this paper, a new approach is proposed for predicting reaction force in simply supported steel fiber reinforced concrete (SFRC) beams under impact loading (drop weight test) considering the energy conservation approach. If SFRC beams completely fail under impact load, it can be found that the total reaction force is equal to force capacity of SFRC beams. The force-deflection relationship can show the peak force that the SFRC beam can carry under impact load. Since concrete is a material sensitive to loading rates, the strain rate of loading and also the volume fraction of steel fiber will influence the beam´s response. The force-deflection relationship of the SFRC beam under impact loading is obtained using the proposed model. This model considers the effect of volume fraction of steel fiber and also the strain rate on the concrete properties. The model is then verified with the results collected from the literature that include 189 SFRC beams tested under drop-weight impacts and included in a database. The results obtained show that this method can estimate the maximum impact force with acceptable accuracy.The study reported in this paper is part of the project “PufProtec - Prefabricated Urban Furniture Made by Advanced Materials for Protecting Public Built” with the reference of (POCI-01-0145-FEDER-028256) supported by FEDER and FCT funds. The third author also acknowledges the support provided by FEDER and FCT funds within the scope of the project StreColesf (POCI-01-0145-FEDER-029485)