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 Measurement 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.

This work is supported by 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/ECMEST/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. The first and second authors acknowledge the grants SFRH/BD/80682/2011 and SFRH/BD/80338/2011, respectively, provided by FCT.