Quality control and monitoring of NSM CFRP systems: E-modulus evolution of epoxy adhesive and its relation to the pull-out force

The present paper describes the application of an innovative technique (termed EMM-ARM: Elasticity Modulus Monitoring through Ambient Response Method) for continuous monitoring of the stiffening process of an epoxy adhesive used in near-surface mounted (NSM) fibre reinforced polymer (FRP) reinforcements. A simultaneous study of direct pull-out tests with concrete specimens strengthened with NSM carbon FRP laminate strips was carried out to compare the evolution of bond performance with the E-modulus of epoxy since early ages. A relationship between the evolution of epoxy E-modulus and the maximum pull-out force is assessed, highlighting the potential of applying EMM-ARM for quality control and decision-making assistance of NSM systems.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 projects CutInDur PTDC/ECM/112396/2009 and VisCoDyn EXPL/ECM-EST/1323/2013. 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., Artecanter - Industria de Transformacao de Granitos, Lda., Vialam - Industrias Metalurgicas e Metalomecanicas, Lda. The first and second authors also acknowledge the grants SFRH/BD/80338/2011 and SFRH/BD/80682/2011, respectively, provided by FCT

Effects of curing temperature on pull-out behavior and stiffness evolution of epoxy adhesives for NSM-FRP applications

The efficiency of the FRP-concrete strengthening system is strongly influenced by the mechanical properties of the epoxy adhesive, which depend on the curing temperature.In the present work, the influence of temperature on the curing process of the epoxy was examined. Two different temperatures were studied: 20 and 30 °C. The elastic modulus of the adhesive was continuously monitored by using a variant of the classical resonant frequency methods, called EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method). A simultaneous study of direct pull-out tests with concrete specimens strengthened with NSM carbon FRP laminate strips was carried out at the same two different temperatures to compare the evolution of bond performance with the E-modulus of epoxy since early ages. The results showed that increasing the curing temperature significantly accelerated both the curing process of the epoxy adhesive and the evolution of bond performance. Moreover, the EMM-ARM technique revealed its ability in clearly identifying the hardening kinetics of epoxy adhesives, allowing measurements since very early ages and in different environmental conditions

Monitoring the early stiffness development in epoxy adhesives for structural strengthening

The present work aimed to assess the early-age evolution of E-modulus of epoxy adhesives used for Fibre-Reinforced Polymer (FRP) strengthening applications. The study involved adapting an existing technique devised for continuous monitoring of concrete stiffness since casting, called EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method) for evaluation of epoxy stiffness. Furthermore, monotonic tensile tests according to ISO standards and cyclic tensile tests were carried out at several ages. A comparison between the obtained results was performed in order to better understand the performance of the several techniques in the assessment of stiffness of epoxy resins. When compared to the other methodologies, the method for calculation of E-modulus recommended by ISO standard led to lower values, since in the considered strain interval, the adhesive had a non-linear stress-strain relationship. The EMM-ARM technique revealed its capability in clearly identifying the hardening kinetics of epoxy adhesives, measuring the material stiffness growth during the entire curing period. At very early ages the values of Young's modulus obtained with quasi-static tests were lower than the values collected by EMM-ARM, due to the fact that epoxy resin exhibited a significant visco-elastic behaviour.FEDER funds through the Operational Program for Competitiveness Factors – COMPETE and National Funds through FCT – Foundation for Science and Technology under the projects CutInDur PTDC/ECM/112396/2009 and VisCoDyn EXPL/ECM-EST/1323/2013. 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 Ibérica Lda. and Vialam – Indústrias Metalúrgicas e Metalomecânicas, Lda

Influence of temperature on the curing of an epoxy adhesive and its influence on bond behaviour of NSM-CFRP systems

In NSM-CFRP installations, the mechanical behaviour of the strengthening system is strongly influenced by the epoxy adhesive, particularly at early ages. In the present work, the influence of temperature on the curing process of the epoxy was investigated. Three distinct temperatures were studied: 20, 30 and 40 °C. The elastic modulus of the adhesive was monitored through EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method). Direct pull-out tests with concrete specimens strengthened with NSM CFRP strips were carried out at the same three distinct temperatures to compare the evolution of bond performance with the E-modulus of epoxy since early ages. The results showed that increasing the curing temperature significantly accelerated both the curing process of the epoxy adhesive and the evolution of bond performance. The EMM-ARM technique has revealed its ability in clearly identifying the hardening kinetics of epoxy adhesives, allowing also thermal activation analysis. Finally, existing models for predicting temperature-dependent mechanical properties were extended to also describe the bond behaviour of NSM-CFRP applications.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 projects FRPreDur FCOMP-01-0124-FEDER-028865 (FCT no. PTDC/ECM-EST/2424/2012) and VisCoDyn FCOMP-01-0124-FEDER-041751 (FCT no. EXPL/ECM- EST/1323/2013). 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 Ibérica Lda., Artecanter - Indústria de Transformação de Granitos, Lda., Vialam – Indústrias Metalúrgicas e Metalomecânicas, Lda. The second and third authors also acknowledge the grants SFRH/BD/80338/2011 and SFRH/BD/80682/2011, respectively, provided by FCT

3D Thermo-hygro-mechanical approach for simulation of the cracking behaviour of a RC slab under the combined effects of applied loads and restrained shrinkage

The design of reinforced concrete (RC) structures that meet safety, functionality and aesthetic requirements during their lifespan, without unforeseen maintenance costs, depends on adequate design practices that allow engineers to properly control and predict crack widths on concrete. Even though there is a wide body of design codes and recommendations providing methodologies for reinforcement design on elements subjected to applied loads or imposed deformations, they do not provide unambiguous rules for RC structures under the combined effect of these actions, which is a typical situation in RC slabs applied in buildings. This is motivated by the lack of knowledge about the complex interactions that take place between self-imposed deformations, viscoelasticity and the effect of applied loads. This work intends to contribute for deepening the knowledge on this subject by performing a 3D thermo-hygro-mechanical analysis on a highly restrained slab in service load conditions, in which the temperature and moisture fields of the slab are determined in order to take into account the non-uniform distribution of stresses (in space and time) due to hydration and drying shrinkage. This analysis shows that the real restraint forces applied to the slab are in fact just a mere fraction of the those that would be expected in a hypothetical tie subjected to total restraint due to the loss of rigidity caused by crack development induced by a combination of flexural and self-induced stressesPortuguese Foundation for Science and Technology (FCT) to the Research Project IntegraCrete (PTDC/ECM-EST/1056/2014 -POCI-01-0145-FEDER-016841) as well to the Research Units ISISE (POCI-01-0145-FEDER-007633) and CONSTRUCT (POCI-01-0145-FEDER-0074

Bond on NSM CFRP systems: recent contributions of UMinho on durability, quality control and design

In the last years, significant research in the context of bond of near-surface mounted (NSM) fibre reinforced polymer (FRP) systems in concrete has been conducted at the Department of Civil Engineering of the University of Minho. This paper presents a brief summary of the major results obtained in that research, namely in terms of durability, quality control and design topics. Accelerated ageing tests on NSM FRP bond specimens were conducted to simulate different environmental conditions. A new method was developed and applied to investigate the evolution of the adhesive stiffness and the bond behaviour of NSM systems for different curing conditions used for quality control of FRP installations. Regarding the bond design, two existing guidelines’ formulas were adapted to the partial safety factors framework.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 projects CutInDur FCOMP-01-0124-FEDER-014811 (FCT PTDC/ECM/112396/2009), FRPreDur FCOMP-01- 0124-FEDER-028865 (FCT PTDC/ECM-EST/2424/2012), FRPLongDur POCI-01-0145-FEDER-016900 (FCT PTDC/ECM-EST/1282/2014) and partly financed by the project POCI-01-0145-FEDER-007633. The following individual grants are also acknowledged: SFRH/BD/80338/2011 (P. Fernandes), SFRH/BD/87443/2012 (M. Coelho), SFRH/BD/89768/2012 (P. Silva) and SFRH/BD/80682/2011 (J. Granja)

Proposal of a test set up for simultaneous application of axial restraint and vertical loads to slab-like specimens: sizing principles and application

Cracking control in reinforced concrete (RC) is a key factor to ensure proper service life behaviour. However, current design recommendations are unable to provide straightforward methodologies for crack width prediction in RC structures subjected to the combined effects of applied loads and restrained deformations, which is a common situation in civil engineering. This is motivated by the lack of knowledge about the complex interactions that take place between self-imposed deformations, viscoelasticity and the effects of applied loads in the process of crack development. A major challenge in studying these combined effects is the validation of numerical simulations with real scale experimental data. For that purpose, an experimental system for testing real scale RC slabs subjected to the above-mentioned conditions was developed. This system is capable of inducing a prescribed axial restraint to the slab, in correspondence to a high restraint degree that induces cracking in view of expectable shrinkage. At the same time, the setup enables the application of vertical loads. The experimental results obtained in this work allowed for the validation of the test setup, as well as the suitability of the slab geometry and reinforcement.This work was financially supported by: Project POCI-01-0145-FEDER-007457 (CONSTRUCT - Institute of R&D in Structures and Construction) and by project POCI-01- 0145-FEDER-007633 (ISISE), funded by FEDER funds through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI), and by national funds through FCT - Fundação para a Ciência e a Tecnologia. FCT and FEDER (COMPETE2020) are also acknowledged for the funding of the research project IntegraCrete PTDC/ECMEST/1056/2014 (POCI-01-0145-FEDER-016841). The financial support of COST Action TU1404 through its several networking instruments is also gratefully acknowledged.info:eu-repo/semantics/publishedVersio

Early age measurement of the coefficient of thermal expansion of concrete: a new test setup based on internal heating/cooling

Um dos fatoresque influencia a durabilidade das estruturas de betão armado é o aparecimento de fendas nas primeiras idades devido às tensões internas que se geram por restrição de deformações térmicas durante a cura. A sensibilidade da deformação térmica do betão, representada pelo coeficiente de dilatação térmica (CDT), é uma propriedade que se altera durante as primeiras idades,devidoàs modificações micro-estruturais que ocorrem durante a hidratação do cimento. O conhecimento da evolução do CDTnas primeiras idades é fundamental para uma correta previsão das extensões e tensões internas que se geram logo após a presa do betão. Neste trabalho é apresentado um método integrado para determinar a evolução do CDT de provetes de betão desdeidades anteriores à finalizaçãoda presa, ultrapassando deste modo as principais limitações dos métodos atualmente utilizados para determinação deste coeficienteem materiais cimentícios. O método proposto consiste na exposição de um provete de betão a ciclos de variações térmicas de ±2.5°C,tendo-se desenvolvido um molde de ensaio que permite a leitura da resposta do provete durante as primeiras idades. Asvariações térmicassão impostasatravés da imersão do molde de ensaio em água, a temperatura controlada, e impondo a circulação dessa água no interior do provete através de um tubo em espiral embutido, permitindo desta forma reduzir o gradiente de temperatura entre a superfície e o núcleo e, consequentemente, o tempo de duração dos ciclos térmicos.No presente trabalho são testadas diferentes composições de betão para verificar a aplicabilidade do método proposto para determinação do CDT do betão nas primeiras idades, tendo-se obtido resultados que estão em acordo com o atual estado da arteOne of thefactors influencing the durability of reinforced concrete structures is the occurrence of cracking during the early ages, caused by the internal stresses generated by restrained thermal deformations during cement hydration. Thermal deformation sensitivity, represented by its coefficient of thermal expansion (CTE), is a property that changes during early ages due to, essentially, micro-structural changes that occur during cement hydration.Knowing the evolution of the CTE sinceearly ages is essential for a correct prediction of the internal stresses that occur right after setting of the concrete. In this work, an integrated methodfor determination of concreteCTE evolution duringthe early ages is proposed, in which the main limitationsof the current methods are circumvented. The proposed method consists in exposing a concrete specimen to thermalvariations of ±2.5°C,and a test rig that enables measuringthe specimen’sresponse to those thermal variations since early ages. The thermal variationis imposed by immersing the concrete specimen in a water bath, with controlled temperature, and imposing water circulation inside the specimen through an embedded spiral tube. This allows reducingthe difference between the surface and the core temperatures,andso minimizingthe duration of thermal cycles. In the present work several concrete compositions are tested in order to verify the applicability of the proposed method for determination of concrete CTE in early ages,beingobserved that the results are in accordance with the current state of the artPOCI-01-0145-FEDER-007457 (CONSTRUCT -Institute of R&D in Structures and Construction) e POCI-01-0145-FEDER-007633 (ISISE), financiados por fundos FEDER através do COMPETE2020 -Programa Operacional Competitividade e Internacionalização (POCI), e por fundos nacionais através da FCT –Fundação para a Ciência e Tecnologia. Agradece-se ainda à FCT e FEDER (COMPETE2020) o financiamento do projeto IntegraCrete PTDC/ECM-EST/1056/2014 (POCI-01-0145-FEDER-016841

Monitorização contínua da deformabilidade de solos estabilizados com cimento desde as primeiras idades em amostras recolhidas diretamente de uma camada piloto

A metodologia EMM-ARM (Elasticity Modulus Measurement through Ambient Response Method) permite a avaliação contínua da rigidez de solos estabilizados desde o instante da compactação. Esta técnica inovadora baseia-se na identificação contínua da primeira frequência de ressonância à flexão de uma viga mista simplesmente apoiada, para inferir o módulo de deformabilidade da amostra de solo estabilizado colocada no interior da viga. Apesar do sucesso obtido na aplicação piloto EMM-ARM, foram identificadas limitações associadas ao tipo de molde utilizado (robustez e custo) bem como quanto à representatividade da amostra ensaiada. Neste trabalho são apresentados alguns dos desenvolvimentos mais recentes da metodologia EMM-ARM, aplicada aos solos-estabilizados, que compreenderam alterações ao molde de ensaio ao nível do comprimento e secção transversal, e uma proposta para recolha de amostras intactas. São também comparadas as cinéticas de hidratação do cimento numa mistura areia-cimento e numa pasta de cimento.Fundação para a Ciência e a Tecnologia (FCT