Bond and flexural behavior of concrete elements strengthened with NSM CFRP laminate strips under fatigue loading

This paper presents the results of an experimental research on bond and flexural behavior of concrete elements strengthened with carbon fiber reinforced polymer (CFRP) laminate strips under fatigue loading conditions, applied according to the near-surface mounted (NSM) strengthening technique. Long-term performance of the NSM system could be compromised by fatigue loading, which may result in deterioration and weakening of individual components (steel, FRP, concrete), or loss of bond performance and composite action. Hence, an experimental program composed of direct pullout tests and slab specimens was carried out to evaluate the influence of the stress level and amplitude of cycles imposed to the specimens. In bond fatigue tests, debonding failure occurred for a maximum stress level corresponding to 60% of the ultimate bond load obtained from monotonic loading. The fatigue loading imposed on the slabs did not yield to its failure. Consequently, the load carrying capacity obtained from post-fatigue tests of strengthened slabs was not significantly affected by this action.This work is supported by FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project CutInDur PTDC/ECM/112396/2009. The authors also like to thank all the companies that have been involved supporting and contributing for the development of this study, mainly: S&P Clever Reinforcement Iberica Lda., Casais - Engenharia & Construcao S.A., Artecanter - Industria de Transformacao de Granitos, Lda., Tecnipor - Gomes & Taveira Lda., Vialam - Industrias Metalurgicas e Metalomecanicas, Lda., Hilti Portugal-Produtos e Servicos, Lda., Novo Modelo Europa, S.A., Sika Portugal - Produtos Construcao e Industria, and S.A., Cachapuz - Equipamentos para Pesagem, Lda. The first and second authors wishes also to acknowledge the Grants SFRH/BD/80338/2011 and SFRH/BD/89768/2012, respectively, provided by FCT

Effect of wet-dry cycles on the bond behaviour of concrete elements strengthened with NSM CFRP laminate strips

The near-surface mounted (NSM) strengthening technique is capable of effectively increase the bearing capacity of structural concrete elements. This technique which basically consists of placing FRP reinforcements inside small grooves cut in the concrete cover, has been widely investigated in terms of structural performance and ability to improve the flexural and shear behaviour of reinforced concrete beams and columns. However, little research has been carried out concerning to the NSM long-term performance and durability. Motivated by the need of increasing the knowledge on the expected durability of the NSM technique using CFRP laminates, this paper presents an experimental program in which direct pull-out tests are carried out for evaluating the bond behaviour of specimens aged through wet-dry cycles. A total of 30 specimens are tested, analysing the effect of the bond length, the groove width, the groove depth and the aging effect on the bond behaviour. Digital image correlation method is also used to identify the bond resistant mechanism developed in an element strengthened using NSM technique. Finally, using the experimental results, an analytical–numerical strategy is applied to establish the local bond stress–slip relationship.This work was supported by FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project CutInDur PTDC/ECM/112396/2009. The third author acknowledge the grant SFRH/BD/80338/2011 provided by FCT. Finally, the authors also like to thank all the companies that have been involved supporting and contributing for the development of this study, mainly S&P Clever Reinforcement Iberica Lda. and SECIL

Regeneração urbana de centros históricos : o caso do Alentejo Litoral

As problemáticas do espaço urbano têm suscitado uma maior atenção nos últimos anos. Após uma época de crescimento que levou à expansão das cidades para a periferia, a crise financeira afectou o sector imobiliário e permitiu um novo despertar para as questões de regeneração e reabilitação das áreas centrais das cidades. É aqui realizada uma reflexão à Regeneração Urbana e seus conceitos associados, particularmente no que concerne à sua aplicação em Centros Históricos. É feita uma retrospectiva às políticas de intervenção urbana nos Centros Históricos portugueses e são analisadas três intervenções diferentes no âmbito do programa Parcerias para a Regeneração Urbana que tiveram lugar no Alentejo Litoral, mais concretamente nas cidades de Alcácer do Sal, Santiago do Cacém e Sines.The problems of urban space have attracted increased attention in recent years. After a growing season that led to the expansion of cities to the periphery, the financial crisis has affected the real estate sector and allowed a new awakening to the regeneration and rehabilitation of the central areas of cities. It is here executed a reflection to urban regeneration and its associated concepts, particularly with regard to its application in historic centers. It’s made a retrospective on urban policy interventions on the Portuguese Historical Centers and three different interventions are examined under the Parcerias para a Regeneração Urbana Programme that took place in Alentejo Litoral, specifically in the cities of Alcacer do Sal, Santiago do Cacém and Sines

Bond behavior of EBR CFRP systems in concrete: influence of surface preparation

The EBR strengthening technique has been used to improve existing reinforced concrete (RC) structures. In many cases CFRP laminates are used as reinforcing material, whereas epoxy adhesives are used as the bonding agent. In the last decades several investigations have been carried out in order to predict the bond strength of EBR CFRP systems in concrete and, as consequence of that, many analytical expressions can be found out in the literature, including in standards. However, these expressions do not account for the influence of the type of surface preparation, which is a mandatory and critical task in the strengthening application. The present work gives contributions for this lake of knowledge. For this purpose an experimental program composed of single shear lap bond tests was carried out. The main parameters studied were the type of surface preparation and the bond length. The instrumentation included sensors to measure the pullout force and the loaded and free end slips. This paper details the experimental program, presents and analyzes the obtained results. As expected, the results revealed that the bond strength depends on the type of surface preparation. Finally, existing expressions in the literature were upgraded in order to account for the type of surface preparation in the estimation of the bond strength.This work was supported by FEDER funds through the Operational Program for Competitiveness Factors – COMPETE and National Funds through FCT (Portuguese Foundation for Science and Technology) under the project FRPLongDur POCI-01-0145-FEDER-016900 (FCT PTDC/ECMEST/1282/2014) and partly financed by the project POCI-01-0145-FEDER-007633. The authors also like to thank the S&P Clever Reinforcement Ibérica Lda. company for providing the materials.info:eu-repo/semantics/publishedVersio

A new methodology for assisting quality control of NSM-CFRP systems since very early ages

The performance of NSM systems directly depends on the performance of the adhesive which can only be reached if a proper curing conditions and time are assured. In order to assess the curing process of the adhesives, as well as the corresponding bond behavior, adequate non-destructive testing approaches are required. Nonetheless, scarce information could be found in the literature in concern to this relevant topic. To fill such gap, the present paper proposes a new method for continuous quality control of epoxy adhesives, based on adaptations of an existing technique originally devised for continuous monitoring of concrete E-modulus since casting, called EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method). This work reports the simultaneous study of the adhesive through EMM-ARM, together with direct pullout tests at several ages on concrete specimens strengthened with NSM CFRP laminate strips using the same epoxy. The tests are described, and the obtained results are presented and discussed, highlighting the potential of applying EMM-ARM for quality control and decision-making assistance of NSM systems.FCT -Fuel Cycle Technologies(undefined

Influência do tipo de adesivo no comportamento à flexão de faixas de laje reforçadas com sistemas NSM-CFRP

Atualmente verifica-se que muitas das estruturas de betão armado existentes apresentam necessidade de reparação/reforço estrutural devido a diferentes fatores, tais como a degradação dos seus materiais constituintes e a necessidade de uma capacidade resistente superior. O recurso à técnica NSM (Near Surface Mounted na literatura inglesa), baseada na introdução dos laminados de CFRP no betão de recobrimento, é uma possibilidade para o reforço de estruturas de betão armado. Nos últimos anos esta técnica tem sido alvo de vários estudos; no entanto, o conhecimento na temática da influência do tipo de adesivo no comportamento de elementos de betão reforçados na presença de sistemas NSM-CFRP é algo limitado. Assim, o principal objetivo deste estudo consiste em avaliar o comportamento estrutural de faixas de laje sujeitas a esforços de flexão e reforçadas com laminados de CFRP fixos com recurso a adesivos de propriedades mecânicas distintas. O programa experimental desenvolvido é constituído por ensaios à flexão em faixas de laje em que as variáveis alvo de estudo foram: (i) tipo de adesivo utilizado na ligação dos laminados de CFRP ao betão; e, (ii) existência ou não de pré-fendilhação do betão constituinte das lajes. Ao longo dos ensaios foram monitorizadas a força aplicada, o deslocamento a meio vão, a largura de fendas e extensões. Em termos gerais, o comportamento observado revela uma correlação entre as propriedades mecânicas dos adesivos e a capacidade resistente das faixas de laje. Contudo, não existe proporcionalidade entre estas duas variáveis. A pré-fendilhação não provoca alterações significativas na capacidade de carga das faixas de laje.FCT e cofinanciado pelo Fundo Europeu de Desenvolvimento Regional (FEDER) através do Program a Operacional Fatores de Competitividade e internacionalização (POCI) e do Programa Regional d e Lisboa no âmbito dos projetos FRPLongDur POCI-01-0145-FEDER-016900 (PTDC/ECM-EST/1282/2014) e POCI-01-0145-FEDER-007633. Os autores ainda agradecem também o apoio e a contribu ição das seguintes empresas: S&P Clever Reinforcement (oferta do laminado de CFK e adesivo S&P Resin 220 ) e Sika (oferta do adesivo SikaDur 30

Modelação de microestruturas de material periódico para estudo de concentração de tensões

Esta dissertação tem como principal foco a análise da concentração de tensões em materiais celulares. São considerados dois materiais compósitos de microestrutura periódica neste trabalho: um material compósito com fases sólida e vazia; um material compósito com duas fases sólidas. Os materiais são representados por uma célula unitária. Recorre-se à modelação por elementos finitos para modelar as células unitárias e para resolver os casos de carga trabalhados. É feito um estudo de convergência de malha para duas células unitárias com foco na tensão na fronteira entre sólido e vazio. É realizado um estudo de efeito de escala para perceber como o meio celular afeta a concentração de tensões numa célula unitária. É realizado um estudo de concentração de tensões para uma célula unitária representativa de um arranjo hexagonal. Os resultados mostram bons resultados de convergência e que o efeito de escala tem influência na concentração de tensões

Durabilidade de um adesivo epóxi e um laminado de CFRP expostos a distintas condições ambientais

No contexto das estruturas reforçadas, os polímeros reforçados com fibras de carbono (CFRP) são normalmente colados aos elementos a reforçar através de adesivos epóxidos. Nas duas últimas décadas tem sido desenvolvida significativa investigação, dedicada principalmente ao comportamento estrutural a curto prazo de estruturas reforçadas e raramente foi considerado a avaliação do tempo de vida útil da estrutura. A previsão do tempo de vida útil de uma estrutura requere a avaliação da durabilidade dos materiais envolvidos. O presente trabalho tem como objetivo contribuir para o conhecimento de dois materiais específicos, um laminado de CFRP e um adesivo epoxídico, amplamente utilizados neste contexto. Para esse efeito, foi desenvolvido um programa experimental para avaliar a degradação das propriedades mecânicas em tração destes materiais devido ao efeito de cinco ações ambientais distintas: (i) ciclos térmicos; (ii) ciclos de gelo-degelo; (iii) imersão em água pura; (iv) imersão em água com cloretos; e, (v) ciclos de molhagem-secagem em água com cloretos. Os resultados obtidos indicam que a imersão em água pura foi o ambiente mais crítico para as amostras de epóxi, verificando-se uma redução da resistência máxima de 38% após dois anos de exposição. Os resultados também mostraram que os provetes de CFRP não apresentaram alterações significativas nas suas propriedades à tração, sendo o máximo decréscimo de 7% na tensão máxima

Durability of an epoxy adhesive and a CFRP laminate under different exposure conditions

In many of the retrofitted structures, carbon fibre reinforced polymer (CFRP) laminates bonded with epoxy adhesives are used. In the last decades the developed extensive research was mostly devoted to the short-term structural performance of strengthened structural elements and rarely considered the full structure’s life time. The assessment of the full structure’s life time requires the evaluation of the durability of the involved materials. The present work aims to contribute for the knowledge on the durability of a specific CFRP laminate strip and an epoxy adhesive widely used in this context. For that purpose an experimental program was carried out to assess the degradation on their tensile properties due to five distinct environments: (i) thermal cycles; (ii) freeze-thaw cycles; (iii) immersion in pure water; (iv) immersion in salt water; and, (v) wet-dry cycles with salt water. The obtained results indicate that the immersion in pure water was the most critical environment for the adhesive specimens, with a reduction on the strength of 38% after 2 years of exposure. Results also showed that the CFRP samples had negligible losses on their tensile properties

Efficiency of different techniques in seismic strengthening of RC beam-column joints

In the last years several techniques have been proposed and used for the seismic strengthening of reinforced concrete (RC) beam-column joints using fiber reinforced polymers (FRP). The Near Surface Mounted (NSM) technique uses FRP bars or laminates inserted into grooves opened on the concrete cover and filled with epoxy adhesive. In the Mechanically Fastened and Externally Bonded Reinforcement (MF-EBR) technique, multidirectional laminates of carbon fiber reinforced polymers (MDL-CFRP) are simultaneously bonded with epoxy adhesive and mechanically fixed with anchors to the faces of the elements to be strengthened. With the aim of comparing the seismic efficiency of NSM and MF-EBR techniques, tests with RC joints, representative of the buildings construction practice in Southern European countries until the early 1970s were carried. The experimental campaign comprises cyclic tests on seven full-scale RC joints with distinct configurations when both, NSM and MF-EBR techniques, are used. The tests are described and the main results are presented and analyzed