Keywords: CFRP, rehabilitation, composite materials, wrapping concrete members.

Recent tests carried out on the benefit due to the use of the innovative materials applied as confinement on the compressed elements (Carbon fibers, glass or kevlar), encouraged the development of the wrapping technique for the reinforcement and the retrofitting of important structures located in high seismic risk zone. Recently, the composite materials have been employed for the confinement of concrete elements in order to improve their mechanical features such as ductility and resistance even when the elements are damaged, as in the case of seismc events. The study reports the comparison between results obtained from an experimental analysis carried out on a series of concrete cylindric samples confined by C-FRP. Two series of specimens, each constituted by three groups differing for dimensions, were prepared in order to evaluate the influence of the sand used; as well as of the fibers orientation. Samples belonging to series G were characterized by a ratio between Height and Diameter equal to 2:1; Samples belonging to series H were characterized by a ratio between Height and Diameter equal to 3:1. Besides, different orientation of the Carbon fiber layers were tested, namely (90^o 45^o 60^o)

In the paper the experimental data was compared to the theoretical model reported in literature [1] for the compressed element reinforced by the wrapping technique and to a LUSAS F.E.M. analysis [2]. The gain in terms of resistance as well as of ductility was determined by a function of different orientation, in respect to the axis sample, of the applied CFRP layers The ultimate longitudinal strains, as well as the transversal ones are much greater in respect to the unconfined sample, thus, the strain energy increases with the ductility of the specimen. The load- displacement behavior of the confinement specimens is described by a bilinear trend curve except from the confined sample with fibers oriented at a 45^o. The modulus of elasticity for the confined samples decreases in function of the fiber's orientation. In other words a sample becomes more rigid when the fiber angle increases, the trend remains equal even when the variation of the shape ratio is considered.

Generally, the unconfined samples tested show a first linear trend where a decrease of volume is recorded due to the propagation of the micro-fractures [1], after that two possible behaviours have been observed: unstable collapse in the positive strain domain when the sample fails due to a increase of unstable compressibility; an increment of volume and thus a trend inversion due to the propagation of the micro fractures propagated. From the comparison between the series of the tested samples, it can be noted that the curves regarding the H series samples are in the domain of positive volumetric in respect to the ones of the G series; anyway each sample group shows a very similar trend even when the fibres angle changes. The data obtained shows that the real behavior of the confined columns with CFRP are influenced by several parameters as: concrete quality, shape ratio between the height (H) and the diameter (D), fiber thickness etc. this results in changes in failure mode, the ultimate stress, the stiffness and the ductility. The confined samples show an increase of the ultimate stress in respect to the unconfined case. This increment is greater when the fiber inclination is close to 90^o while the ultimate strains are always higher, as well as the strain energy, this produces an increment of the sample ductility. For fibres oriented at 90^o in respect to the longitudinal axes of the sample the resistance is three times greater than the unconfined sample while the fibers oriented at 45^o produce a low increase of compression resistance but an great increment of ductility due to the fibers sliding during the test.The stress-volumetric strain graphs show firstly, a reduction of the lateral expandation of the concrete. up to a value very close to the ultimate stress of the unconfined concrete but then a very great volumetric dilatation can be noted in function of the stiffness of the CFRP confinement. The unconfined samples show a fragile collapse if compared to the confined ones.Obviously in the case of fibres oriented at 90^o the action of the confinement in terms of ductility is lower and then the trend inversion of the volumetric strain is less visible; while in the case of fibres oriented at 45^o and 60^o a great increment of ductility is recorded because the transversal strains control both the volumetric deformation and the modality of collapse. Anyway, the collapse of the cylinder samples was, always, characterized by the crushing of the concrete with the cones formation. This failure mode was also visible by the finite elements investigations carried out both on the confined and the unconfined samples where the cones formation, derived by the analysis of the longitudinal strain, are clearly visible. The experimental investigation carried out by the authors seems to be very useful for the formulation of technical recommendations for the Wrapping use as a system of reinforcement of the columns undergoing uniaxial compression stress.

1

A. La Tegola, O. Manni: "Modellazione Del Comportamento Ciclico Di Colonne In Conglomerato Cementizio Confinate Con Nastri Frp", Dipartimento Di Scienza Dei Materiali, Università Di Lecce.

2

LUSAS software, Finite elements analysis, FEA Ltd. Forge House, 66 high street, Kingston Surrey, KT1 1HN, UK.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £123 +P&P)