International Concrete Abstracts Portal

Wrapping of columns by means of FRP (fiber reinforced polymer) reinforcement enhances the structural behaviour of concrete columns considerably. At the Magnel Laboratory for Concrete Research a test programme is set-up to evaluate some specific problems in the modelling of FRP confined concrete, i.e. effective circumferential FRP failure strain and effect of increasing confining action. Parameters studied are FRP type, bonded or unbonded wrapping application, column shape and strengthening lay-out. Both wrapped cylinders and wrapped columns are investigated. An analytical verification of the test results is performed according to different models. From the test results obtained so far, the efficiency of this strengthening technique has been demonstrated, both in terms of structural performance and ease-of-application. Quality of the application (voids, protrusions, etc.) and the wrapping concept (bonded or unbonded, partial wrapping, etc.) influence the strengthening effect.

In the aftermath of the 1995 Hyogo-Ken Nanbu Earthquake in which many severe failures of bridge piers occurred, numerous studies have been conducted on ways to retrofit existing RC columns and piers. As a result of this research, it has been found that continuous fiber sheet offers a feasible means of retrofitting. Consequently, several design guidelines on the use of FRP sheets for retrofitting highway, railway and subway structures have been proposed in recent years. The JSCE (Japan Society for Civil Engineers) Concrete Committee on FRP Sheet, which was setup in 1998, has been commissioned to establish a new design method for seismic retrofit of bridge columns and piers. It seeks to unify all the existing guidelines on a performance-based design concept. This paper describes mainly the new design method of shear strength and ductility of retrofitted RC structures using FRP sheet. Design equations for shear strength and ductility are also presented.

Prevalently compressed concrete columns can be transversely confined in order to obtain increase in strength and ductility. Generally, two principal methodologies of advanced confinement are used: the “Wrapping” technique which consists of wrapped concrete columns using thin carbon or glass flexible straps epoxy-bonded concrete surface; and concrete filled FRP tubes (CFFT) in which the tube is the pour form, protective jacket, confining mechanism, and shear and flexural reinforcement. The purpose of this paper is to theoretically model the stress-strain behavior of concrete confined by FRP straps or tubes, evidencing the straight dependence of s - e curve on the hoop mechanical properties of the transverse composite reinforcement. The proposed model consists of a generalization of the theory of elasticity, i.e. a step by step application of Navier classical relations which define the stress-strain laws in states of triaxial stress. The theoretical results are compared with experimental compression tests carried out on ten cylindrical concrete specimens confined with two different typologies of fiber composite tubes.

This research aims on understanding the influence of the strengthening method using carbon fiber reinforced plastic (CFRP) sheets on the performance of 13 reinforced concrete columns with waist-high walls. Main parameters of the specimens include the thickness and height of the attached walls, axial load, and the amount of CFRP sheets. The strengthening method is done by wrapping the upper portion of the column with sheets in a closed form. Also, the waist-high walls are equipped with holes where the carbon fiber strands (CF-anchor) are inserted and anchored to the ends of the sheets attached only on the surface of the lower portion of the column. Test result show that there is a great improvement in the ductility of the shear-failure type specimens when strengthened with CFRP. Also with an increase in the amount of the sheets, the ductility is further enhanced. The specimens strengthened using CFRP with CF-anchor also show higher ductility than the specimen where only the upper column is strengthened. The bending and shear strengths of the columns with waist-high walls can be calculated by using effective column height he considering the presence of the wall portions.

In order to improve seismic performance of existing R/C buildings which were designed on the basis of the old seismic design code before 1981, the various retrofitting methods have been proposed, especially after the Hyogo-ken Nanbu earthquake happened in 1995 in Japan. In this study, an experimental study was conducted to examine the seismic performance of existing R/C columns retrofitted with composite-materials such as carbon fiber or aramid fiber sheets. Quantity, retrofitting types and kinds of continuous fiber sheets were adopted as the experimental variables. As a result, by retrofitting columns with continuous fiber sheets, the failure mode of the columns changed from shear failure to flexural one. However, if the ply of continuous fiber sheets is not enough, the columns collapse because of hinge region concrete crashing just after longitudinal rebar yielding and its ductility is poor. Therefore, to make R/C column retrofitted with fiber sheets have good ductility, the fiber sheets should have a certain of membrane stiffness.