Experimental investigation of RC slabs strengthened with NSM CFRP system subjected to elevated temperatures up to 80 ºC

The application of carbon fibre-reinforced polymers (CFRP) according to the near-surface mounted (NSM) technique has proved to be one of the most effective systems to strengthen existing reinforced concrete (RC) members in flexure. In spite of that, there are many open issues that deserve investigation such as the effects of exposure to elevated temperatures on the flexural behaviour of RC slabs strengthened with NSM-CFRP systems. The present work aims to experimentally evaluate the mechanical performance of RC slabs strengthened with NSM-CFRP systems under elevated temperatures by using steady-state and transient heating situations combined with applied loads. The temperatures studied were: 20, 40, 50, 70 and 80 °C for the steady-state tests, and 20 and 80 °C for the case of transient tests. Deflections, strains, temperatures and loads were registered in all phases of the tests, in order to thoroughly analyse the response of the system in terms of the load-deflection curves, evolution of the strains of concrete, CFRP and bond stresses between epoxy adhesive and CFRP. The experimental results have shown that the RC slabs strengthened with NSM CFRP systems presented a slight decrease in the ultimate strength and a change on failure mode at the temperature of 80 °C only.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/ECM-EST/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 material tested in ambit of the present work. The first author wishes also to acknowledge the grant SFRH/BD/89768/2012 provided by FCT

Experimental investigation on creep behaviour of an epoxy adhesive

Epoxy adhesives have been extensively used in structural strengthening. This leads to a great concern in assessing their long-term performance since epoxy resins present viscoelastic behaviour. This work aims to better understand the long-term behaviour of a specific epoxy adhesive due to the creep effects. Therefore, an experimental program comprising tensile creep tests was carried out, divided in two series: (i) series S1 - epoxy specimens subjected to different stress levels for a predefined initial age; and (ii) series S2 - epoxy specimens subjected to equal stress levels but loaded at different ages. The paper presents the main results obtained so far, highlights relevant and corresponding conclusions

An alternative system for measuring displacements in bridges by using displacement transducers

One of the most important controlling parameters in bridges are the vertical displacements. Measuring deflections is not only valuable at the early stages of the construction, but during the service lifetime of the structure also. Furthermore, vertical displacements are one of the most important indicators of the healthy state of the bridges among other structural health monitoring parameters used in the assessment of the structural response of the bridges and for foreseeing eventual damage or deterioration issues. Bridges are often placed over watercourses or over accidentally terrain and normally load tests are carried out before starting its use with the purpose of evaluating the structural response. Measuring vertical displacements are always a challenge and many technics have been developed during the past few years. The present paper introduces a new system based on the use of LVDTs that enables the measurement of vertical displacements in structures when the access to reference points are limited. Also, this system offers a simple application and can be easily placed on site for measuring vertical displacements with both high sampling frequency rates and resolution

Investigação experimental de lajes reforçadas com sistemas NSM CFRP submetidas a temperaturas até 80 °C

A aplicação de polímeros reforçados com fibras de carbono (CFRP) de acordo com a técnica Near-Surface Mounted (NSM) tem provado ser um dos sistemas de reforço mais eficientes no reforço de estruturas de betão armado (BA) à flexão. Apesar disso, existem ainda diversos tópicos que merecem ser investigados, tais como, o comportamento à flexão de lajes de BA reforçadas com sistemas NSM CFRP expostas a temperaturas elevadas. O presente trabalho tem como objetivo avaliar experimentalmente o desempenho mecânico de lajes de BA reforçadas com sistemas NSM-CFRP sujeitos a temperaturas elevadas, através de ensaios térmicos em regime estacionário e transiente, combinados com um carregamento imposto. As temperaturas estudadas foram: 20, 40, 50, 70 e 80 °C para os ensaios em regime estacionário, e 20 °C e 80 °C para o caso dos ensaios em regime transiente. Durante os ensaios registaram-se nas lajes deslocamentos, extensões, temperaturas e cargas em todas as fases, com o objetivo de analisar a resposta do sistema em termos de força-deslocamento, evolução de extensões no betão, no CFRP e tensões de aderência entre o adesivo epoxídico e o CFRP. Os resultados experimentais mostraram que as lajes de BA reforçadas com sistemas de NSM-CFRP apresentaram um ligeiro decréscimo na capacidade última de carga e alteração do modo de rotura para apenas os casos de em que as lajes foram submetidas a temperaturas de 80 °C.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 agradecem também as bolsas SFRH/BD/89768/2012, financiadas pela FCT. Por fim, os autores ainda agradecem também o apoio e a contribuição das seguintes empresas: S&P Clever Reinforcement, Hilti, Casais, Artecanter, Tecnipor, Vialam, Novo Modelo Europa, Sika and Cachapu

Influência do efeito de exposições ambientais no comportamento pós-fissurado de um BAC reforçado com fibras de aço

Neste trabalho apresentam-se os principais resultados do estudo da influência de certo tipo de exposições ambientais no comportamento pós-fissurado de betão auto-compactável reforçado com fibras de aço, BACRFA. Para o devido efeito foram produzidos diversos provetes prismáticos de BACRFA. Inicialmente os provetes foram sujeitos a ensaios cíclicos de flexão em três pontos, com o objetivo de se induzir dois níveis de pré-fissuração distintos, respetivamente, com uma abertura de fenda de 0.3 e 1.0 mm. Posteriormente, os provetes pré-fissurados foram submetidos a três tipos de exposição ambiental durante um período de dezoito meses. Por fim caracterizou-se o desempenho mecânico dos provetes após exposição ambiental

Effect of distinct environmental actions on the durability of RC slabs strengthened with prestressed CFRP laminate strips

In the context of concrete structures strengthening, the externally bonded reinforcement (EBR) technique is the most widely used strategy. By prestressing the FRP materials attached to the concrete substrate, the ad-vantages of external prestressing and of the EBR technique are combined. The present work investigates the durability of reinforced concrete (RC) slabs strengthened with prestressed CFRP strips according the EBR technique. Two different anchorage systems were studied: (i) the Mechanical Anchorage (MA) and (ii) Gradi-ent Anchorage (GA). The durability of the specimens was evaluated by exposing them to one out of four envi-ronmental conditions, this combined with a sustained load of 1/3 of the ultimate load for a period of 8 months. Finally, the slabs were monotonically tested up to failure by using a four-point bending test configuration. This work presents the general description of the experimental program, the analysis of the main results, as well as retained conclusions.(undefined

Efeito de distintas condições ambientes na durabilidade de lajes de betão armado reforçadas com laminados de CFRP pré-esforçados

No contexto da reabilitação de estruturas de betão armado (BA) com materiais FRP (Fiber Reinforced Polimers), a técnica EBR (Externally Bonded Reinforcement) é habitualmente utilizada para aumentar a resistência à flexão. A utilização de pré-esforço em sistemas EBR-FRP combina os benefícios da técnica EBR com as vantagens associadas ao pré-esforço externo. O presente trabalho tem como objetivo contribuir para o aumento do conhecimento na durabilidade de lajes de BA reforçadas com laminados de CFRP (Carbon FRP) pré-esforçados segundo a técnica EBR, de modo a colmatar algumas lacunas relevantes que existem sobre este assunto. Neste contexto, dois sistemas de ancoragem foram estudados: (i) a ancoragem mecânica (MA – Mechanical Anchorage) e (ii) o método do gradiente da força (GA – Gradiente Anchorage). Foi estudado o efeito das seguintes condições ambientais na resposta estrutural das lajes: (i) imersão em água; (ii) imersão em água com cloretos; (iii) ciclos de molhagem/secagem em água; para além destas condições ambientais foram ainda utilizadas lajes de referência (iv). O programa experimental de ensaios de envelhecimento acelerado incluiu 16 lajes, nas quais, metade para além da condição ambiental foi também submetida a uma carga gravítica (cerca de 1/3 da capacidade máxima) durante um período de oito meses. No final da aplicação das distintas condições ambientais, as lajes foram ensaiadas monotonicamente até à rotura segundo uma configuração de ensaio de quatro pontos de carga. Este artigo faz uma descrição geral do programa experimental realizado e apresenta uma análise dos correspondentes resultadosEDER através do Programa Operacional Fatores de Competividade – COMPETE e fundos nacionais, através da FCT – Fundação para a Ciência e Tecnologia sob o projeto FRPreDur - FCOMP-01-0124-FEDER-028865 (PTDC/ECM-EST/2424/2012). Este trabalho é também cofinanciado pelo Fundo Europeu de Desenvolvimento Regional (FEDER) através do Programa Operacional Fatores de Competitividade e internacionalização (POCI) e do Programa Regional de Lisboa no âmbito dos projetos FRPLongDur POCI-01-0145-FEDER-016900 (PTDC/ECM- EST/1282/2014) e POCI-01-0145-FEDER-00763. S&P Clever Reinforcement Ibérica Lda., S&P Clever Reinforcement Company (Switzerland), Tecnipor - Gomes & Taveira Lda., Vialam – Indústrias Metalúrgicas e Metalomecânicas, Lda., Hilti Portugal - Produtos e Serviços, Lda

The use of genetic algorithms for structural optimization of hybrid sandwich panels

This paper describes the procedures followed to develop an optimization method for the design of a sandwich panel to be used in flooring applications. This sandwich panel is composed of polyurethane foam core, fibre reinforced polymer bottom layer and webs, and a fibre reinforced mortar top layer. The possibility of adopting additional internal ribs to increase the flexural and shear stiffness was also considered. The panel was described using a standard stacking sequence, coded as a string, using continuous variables to describe the geometric, economic and environmental parameters, and discrete variables to describe the laminate stack architecture. The optimization procedure was based on a global approach strategy, divided into two steps: (i) firstly, the features of each individual panel solution were assessed by analytical procedures and a fitness was assigned using a ranking function; (ii) secondly, the multi-objective optimization problem was solved by using a genetic algorithm, which performs a random search from generation to generation and keeps the “best individuals”. Penalty criteria were also considered when any panel solution was not satisfying the restrictions and design requirements. Different solutions were obtained by imposing different restrictions to the design of the sandwich panel, namely considering: (i) the length; (ii) the width; and, (iii) the use of one or two types of fibres (carbon and glass). This paper discusses the results obtained, both regarding the performance of the optimization procedure developed and the optimal solutions obtained for each case studied.The study presented in this paper is a part of the research project “EasyFloor – Development of composite sandwich panels for rehabilitation of floor buildings”, with reference number 3480, supported by ANI, through FEDER. The last author acknowledge the grant SFRH/BSAB/114302/2016 provided by FCT.info:eu-repo/semantics/publishedVersio

Shear behaviour of polyurethane foam

Polymeric based foams are widely used as insulation or structural core materials. Typically, plastic foams such as polyurethane (PUR) are commercially available in a variety of densities and, consequently, with distinct mechanical properties to fulfil a wide range of uses and applications. Usually, in sandwich construction, PUR foams are used as core material enclosed by two external faces of higher stiffness and strength. Since the lightweight PUR foams are relatively inexpensive, they are prone to be used as core material in sandwich construction, but may limit the ultimate strength of the sandwich panels due to core shear failure. Thus, a detailed characterization of PUR mechanical properties, namely shear modulus and shear strength, is required to efficiently use these materials in structural applications. Thereby, the present study deals with the experimental characterisation of the shear behaviour of a specific PUR closed-cell foam, with density of 70 kg/m3, in particular, considering the implications of size effect on the obtained results and on the estimation of the material properties. The experimental procedure was based on the standards ISO 1922 and ASTM C273, as well as, other existing publications, considering that this PUR is intended to be used as structural core material in sandwich panels for flooring purposes. Instrumentation included a load cell, LVDTs and digital image correlation. The present work details the experimental program and analyses the main results obtained.The study presented in this paper is a part of the research project “EasyFloor – Development of composite sandwich panels for rehabilitation of floor buildings”, with reference number 3480, supported by ANI, through FEDER.info:eu-repo/semantics/publishedVersio