Use of NSM FRP for torsional strengthening on thin walled tubular RC structures

Tese de Doutoramento em Engenharia CivilThe use of near surface mounted (NSM) FRP reinforcement has been proven to be a very promising technique for the strengthening of reinforced concrete (RC) structures in both flexure and shear. The application of NSM FRP to improve the torsional performance of RC structures is limited, however, and despite the many potential advantages this technique has never been applied to thin walled tubular RC members. This research study focuses on the development of strengthening strategies for torsional deficient elements using NSM straight and L-shaped CFRP laminates. The proposed strengthening solutions offer substantial advantages over other available conventional and innovative (externally bonded techniques) strengthening methods. The research includes an extensive experimental programme followed by the development of a design model, complemented by a numerical study. A nonlinear analysis based on the finite element method is performed to assist in the preparation of the experimental work and the development of the test setup. A parametric study is carried out to assess the influence of longitudinal and transverse steel reinforcement ratios, concrete strength, strengthening configurations and longitudinal and transverse NSM FRP reinforcement ratios. The main objective of the experimental work is to derive reliable data to assess the performance of the NSM technique for the strengthening of thin walled RC elements, and for the development of design formulations. For this purpose, three series of tests with RC tubular prototypes are experimentally tested by determining the torsional moment versus torsional angle of rotation, strains in the reinforcements and in the CFRP laminates, and the crack patterns and failure modes. Digital image correlation is also used in an attempt of enriching the knowledge provided by conventional sensors. The strengthening configurations are categorized into three series (i) four face strengthening with straight CFRP laminates (ii) three face strengthening with straight CFRP laminates and (ii) four face strengthening with LCFRP laminates. All the strengthening proposals show improved performance in torsional moment carrying capacity (18% - 46%), torsional angle of rotation (20% - 76%) and decrease in crack spacing (16% - 56%). Based on the obtained results, analytical equations are developed using space truss analogy for thin walled tubular reinforced concrete members strengthened with NSM CFRP laminates. The equations are presented for ultimate torsional moment carrying capacity, effective strain of CFRP laminates and diagonal compressive strut angle. The proposed analytical equations predict the experimental results well with a 7% error.A técnica de reforço baseada na introdução de finos laminados de fibra de carbono (CFRP) em entalhes executados no betão de recobrimento do elemento a reforçar, designada pelo acrónimo NSM, tem sido aplicada no reforço à flexão e corte de estruturas de betão armado (RC) ao longo das últimas duas décadas. No entanto, a aplicação da técnica NSM no reforço à torção das estruturas de betão armado é praticamente inexistente, em especial em elementos de secção tubular de paredes finas. Assim, a presente tese foca-se no reforço desses elementos com comportamento deficiente à torção, usando a técnica de NSM com utilização de laminados de CFRP retos e em forma de L. A técnica oferece vantagens substanciais em relação às técnicas tradicionais suportadas na utilização de materiais de reforço convencionais, e mesmo em relação à técnica baseada na colagem externa de CFRP (EBR). A investigação envolve trabalho experimental e analítico, sendo complementado com simulações numéricas. A análise não linear material baseada no método dos elementos finitos é realizada para auxiliar a preparação do programa experimental e no projeto do sistema de ensaio. Um estudo paramétrico é efetuado para avaliar a influência que os seguintes parâmetros têm na eficácia da técnica NSM com laminados de CFRP: variação longitudinal e transversal da armadura de aço; variação da classe de resistência do betão; percentagem do reforço longitudinal e transversal em CFRP. O principal objetivo do trabalho experimental é determinar resultados confiáveis para avaliar as potencialidades e as debilidades da técnica NSM no reforço de elementos tubulares de paredes finas em betão armado, e contribuir para o desenvolvimento de formulações que permitam o dimensionamento destes sistemas de reforço. Para o efeito, três séries de testes com protótipos tubulares de betão armado são testadas experimentalmente, determinando-se: a relação entre o momento torsional e o ângulo de rotação por torsão; deformações nas armaduras e nos laminados CFRP; e os padrões de fendilhação e modos de rotura. A técnica baseada na correlação de imagens digitais (DIC) também é usada na tentativa de enriquecer o conhecimento fornecido por esses testes, em especial o relativo à formação e propagação de fissuras, e ao campo de extensões na superfície dos protótipos. As configurações de reforço são categorizadas em três séries: (i) reforço com laminados retilíneos de CFRP nas quatro faces dos protótipos (ii) reforço com laminados retilíneos de CFRP em três das quatro faces dos protótipos e (iii) reforço das quatro faces dos protótipos com laminados de CFRP de geometria L. Todas as propostas de reforço aumentaram o máximo momento de torção (18% - 46%), o correspondente ângulo de rotação torsional (20% - 76%), e promoveram uma diminuição no espaçamento entre fendas (16% - 56%). Com base nos resultados obtidos, equações analíticas são desenvolvidas usando o método de escoras e tirantes para elementos tubular de paredes finas de betão armado reforçados com laminados de CFRP aplicados segundo a técnica NSM. As equações apresentadas servem para determinar o momento de torção último, a extensão efetiva em laminados CFRP e o ângulo diagonal de compressão. As equações analíticas propostas preveem os resultados experimentais com um erro médio de 7%

Experimental investigation on torsional strengthening of box RC structures using NSM FRP

The near surface mounted (NSM) technique is a strengthening method that provides additional reinforcement by means of strips or bars embedded into grooves made in the concrete cover of reinforced concrete (RC) elements. The effectiveness of using NSM fibre reinforced polymer (FRP) bars or strips to enhance the shear and flexural capacity of RC elements has been demonstrated over the past decade. However, the idea of using NSM FRP reinforcement to address issues related to deficient torsional performance is yet to be explored. Torsional strengthening of RC elements (e.g. bridge girders, transfer beams) may be necessary due to degradation of materials, changes in the design codes, deficiencies in the initial design, changes in building usage etc. This paper investigates the torsional strengthening of thin walled tubular RC beams using NSM CFRP laminates. The experimental program involved testing of six box sectioned RC beams, including two reference beams (with and without shear reinforcement) and four beams strengthened with different arrangements of NSM CFRP reinforcement, providing varying longitudinal and transverse reinforcement ratios. All the strengthening proposals resulted in significant increase in torsional moment capacity, ductility, stiffness in the elasto-plastic range and were very efficient in arresting crack propagation, proving the effectiveness of NSM strengthening technique for torsional strengthening. The proposed experimental program is described in detail and the main results are presented and discussed.CASA -Center for Arabic Study Abroad, University of Texas, Austin(undefined

Numerical analyses of the connections between representative SFRC prestressed rings of off-shore wind towers

Off-shore wind towers are the wind farms used to harvest wind energy to generate electricity on water bodies. With the growing need of sustainable production for electricity, off shore wind towers are finding a rapid growth in application. In fact, 4% of European electricity demands will be generated by offshore wind towers by 2020 in European waters. The current project concentrates on development of an innovative structural system using advanced materials for lightweight and durable offshore towers. The present paper discusses the nonlinear finite element modelling of the connections between representative prefabricated rings of off-shore wind towers made by steel fibre reinforced concrete (SFRC) and prestressed by a hybrid system formed by carbon fibre reinforced polymers (CFRP) bars and steel strands. The connection between these two rings are assured by post-tension high steel strength cables and concrete-concrete shear friction of treated surfaces. The model takes into account different types of loads and moments originating from rotor, wind and water currents considering the critical loading conditions. The material nonlinear analyses were carried out inFEMIX V4.0 software, considering a 3D constitutive model capable of simulating the relevant nonlinear features of the SFRC, and interface finite elements for modelling the shear friction of the concrete-concrete surfaces in contact. The SFRC rings are modelled by solid elements, and the longitudinal CFRP bars and steel strands by 3D embedded cables. Parametric studies are carried out in order to assess the influence of different fracture parameters of the SFRC and post-tension level in the cables (steel and CFRP) on the performance of the connection between the two rings.The authors acknowledge the support provided by FEDER funds through the Operational Programme for Competitiveness and Internationalization - COMPETE and by national funds through FCT (Portuguese Foundation for Science and Technology) within the scope of the project InOlicTower, POCI-01-0145-FEDER-016905 (PTDC/ECM-EST/2635/2014)

A numerical finite element study on connections of SFRC offshore wind towers with prestressed CFRP reinforcement and steel connectors

The growing need for sustainable production of electricity highlights the importance and the necessity of having higher number and more effective offshore wind towers. The rapid growth of offshore wind towers is estimated to produce 4% of electricity demands in Europe by the end of 2020. The research described in this paper is part of a project dedicated for the development of innovative structural system using advanced materials for lightweight and durable offshore towers. Specifically, it discusses the nonlinear finite element modelling of the connection between representative prefabricated rings of offshore wind tower made by steel fibre reinforced concrete (SFRC), and prestressed by a hybrid system of carbon fibre reinforced polymers (CFRP) bars and steel strands. This connection is assured by post-tension high steel strength cables and concrete-concrete shear friction width an idealized geometric configuration of the faces in contact. The model takes into account the loads from the rotor, wind and water currents, by considering the critical loading conditions for the safety verifications of serviceability and ultimate limit states. The material nonlinear analyses are carried out with FEMIX V4.0 software, considering a 3D constitutive model capable of simulating the relevant nonlinear features of the SFRC, and interface finite elements for modelling the shear friction of the concrete-concrete surfaces in contact. The parametric analyses involve the influence on the relevant results of the SFRC fracture parameters, pre-stress level of the reinforcements, shape of interlock mechanism, friction angle and interface cohesion.FEDER funds through the Operational 392 Programme for Competitiveness and Internationalization - COMPETE and by national funds 393 through FCT (Portuguese Foundation for Science and Technology) within the scope of the 394 project InOlicTower, POCI-01-0145-FEDER-016905 (PTDC/ECM-EST/2635/2014

Structural assessment of Companhia Aurífícia, a 19th century industrial building located in northern Portugal

Companhia Aurifícia is located in Porto, Portugal, and was founded in 1864. It was a pioneer factory in the industrial production, casting, rolling and stamping of metallic objects and laboured for about 150 years, in areas as jewellery, manufacture of parts in silver and gold or the production and casting of various metals. In 1866, it began labouring in Rua dos Bragas, its present location, and in 2003 ceased all activities. Companhia Aurifícia is an industrial complex including several buildings, all located in the same block. It is a precious example of the industrial architecture in Porto, where the still existent retaining walls, structures, machinery and decorative elements, make it one of the last examples of nineteenth century industrial life of the city. The present work aims to evaluate the safety condition of one of the buildings included in this industrial complex, in order to propose the necessary strengthening interventions

Exploring NSM technique for torsional strengthening of tubular type RC structures

The present paper explores, by advanced numerical simulations, the potentialities of the Near Surface Mounted (NSM) technique for the torsional strengthening of thin walled tubular type reinforced con-crete (RC) elements. According to this technique, fibre reinforced polymer (FRP) laminates are intro-duced into thin grooves opened on the concrete cover of the RC elements. A 3D multidirectional smeared crack model available in FEMIX computer code is used for the numerical simulations, and its predictive performance for capturing the behaviour of this type of elements was assessed by simulating experimental tests available in the literature. This model was then used to determine the influence of the variation of longitudinal reinforcement, transverse reinforcement and the concrete compressive strength on the torsional behaviour of thin walled tubular RC elements, with the focus on torque-angle of rotation response. The effect of the variation of the FRP’s modulus of elasticity is also investigated. The results allow to conclude that the proposed NSM strengthening technique is effective in increasing the stiffness and the load carrying capacity, as well as in arresting crack propagation of the strengthened tubular RC elements.The authors acknowledge the support of Marie Curie Initial Training Network under the project “ENDURE” with reference number 607851, funded by the EU programme: FP7-people. The study reported in the paper is a part of ongoing PhD research for the project ENDURE