Effect of temperature on the flexural behavior of NSM CFRP strengthened RC beams under time-dependent loading and fatigue
- Autores: Younes Jahani
- Directores de la Tesis: Marta Baena Muñoz (dir. tes.)
- Lectura: En la Universitat de Girona ( España ) en 2022
- Idioma: inglés
- Tribunal Calificador de la Tesis: Valter Carvelli (presid.), Laura Carreras Blasco (secret.), Luís Correia (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnología por la Universidad de Girona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
There is a general trend towards rehabilitation and strengthening of existing reinforced concrete (RC) structures due to the costs associated with the replacement of the structural members and the environmental impact associated with the consumption of raw materials for new constructions. In this regard, fiber reinforced polymers (FRP) are presented to be an excellent solution for strengthening purposes, being the near-surface mounted (NSM) one of the main used techniques, because of the several advantages that it provides over the traditional plate bonding (i.e. externally bonded reinforcement (EBR)) technique. However, in spite of the advantages of NSM technique, premature debonding failure is still of concern.
The effectiveness of a strengthening system relies on the capacity of the joint to allow the stress transfer during the loading process, which has been acknowledged to be partially governed by the mechanical properties of the bonding agent. In this sense, epoxy adhesives are usually chosen to be the bonding agent in NSM technique. However, due to their nature, performance of epoxy adhesives can be affected by temperature, as near or beyond their glass transition temperature (Tg) their mechanical properties may change, thus affecting the behavior of the NSM strengthened member.
Besides, although NSM FRP strengthening has attracted an increasing amount of research, studies are still needed to fully understand their long-term performance, which partially depends on the material’s long-term properties. In this sense, creep and shrinkage of concrete and creep of epoxy adhesive, which are dependent on environmental conditions, are of high importance. Similar to long-term loading, attention should also be given to fatigue loading, where the stiffness of the system degrades due to progressive cracking and stress/strain accumulation in the material.
This work aimed to experimentally and analytically study the flexural performance of NSM carbon FRP (CFRP)-strengthened RC beams subjected to short-term, long-term and fatigue loading under different high service temperatures. To this end, the first experimental program with 14 beams has been performed, where the effect of CFRP strengthening ratio, concrete strength and applied temperatures has been evaluated. The aim was to evaluate the effect of different parameters on the short-term behavior of the beams in order to easily design the long-term and fatigue experimental programs. Analytical simulations have also been performed to predict the load-deflection responses of the tested beams, where shrinkage effects have been accounted for.
Based on the observations from the first experimental program, a comprehensive experimental program (more than 113 epoxy adhesive samples) has been designed, where the effect of curing, post-curing and testing temperature on the instantaneous mechanical (tensile and compressive) properties of the adhesive has been studied. The range of temperatures considered in this experimental program was in line with temperatures applied in the instantaneous flexural program (i.e. temperature varied from 20 °C to 85 °C).
Afterwards, a long-term loading experimental program has been performed, where the effect of steel reinforcement ratio, CFRP strengthening ratio and temperature has been considered, thus giving a total of 23 beams. The distribution of the beams was as follows: i) 10 beams have been tested under short-term loading; ii) 10 beams have been initially tested under long-term loading to be thereafter tested until failure (post long-term); and finally, iii) 3 beams have been rested at laboratory conditions in order to examine the aging effect on their instantaneous response. Besides, the long-term behavior of the involved materials (concrete and epoxy adhesive) has been also evaluated. Furthermore, an analytical procedure based on the age-adjusted modulus method (AEMM) has been presented to correctly predict the experimental time-dependent deflection of the tested beams.
Finally, in the last experimental program, the effect of fatigue loading on the flexural response of the NSM CFRP-strengthened RC beams under different temperatures has been evaluated. The experimental program included 11 beams, 3 of them for short-term loading, in order to define the fatigue load levels, and 8 of them for fatigue loading, to evaluate the effect of CFRP strengthening ratio, fatigue load level and temperature. The fatigue load has been applied for 2 million cycles or failure of the beam (whichever occurs first) with a frequency of 2 Hz.
