EXPERIMENTAL INVESTIGATION OF FRP RELAXATION AND ITS EFFECT ON PRE-STRESSING TECHNIQUES

INTRODUCTION

Strengthening is often a necessary measure used to overcome an unsatisfactory deficient situation or where a new code requires the structure, or a member of it, to be modified to achieve new requirements 

Since the emergence of Fiber Reinforced Polymer (FRP) as a potential repair material during the last two decades, on-going research has been conducted on retrofitting of various structural components and confining compressive members using FRP. FRPs can be used for both flexural strengthening of members as well as for confinement of compression members. In each case, the materials can be used as passive reinforcement or active reinforcement; the latter requires pretensioning of the FRP prior to application of external loads. Because the relaxation takes place when pre-tensioning is used, the applications where pre-stressing is used in beams or columns are discussed in more details below. Rectangular strips of FRP may be used for strengthening of reinforced concrete beams and slabs. The strips can be pre-stressed and connected to the member through deviators and end-anchorages Among the advantages of this technique are a reduction of service load stresses in steel reinforcement, decreased crack width and mean crack distance, and increased resistance corresponding to the first yielding of the longitudinal steel With respect to concrete columns, retrofit of these elements often requires providing additional confining pressure. Confinement, defined as restraint to lateral dilation, has been traditionally provided for compression members through steel transverse reinforcement in the form of spirals, circular hoops or rectangular ties. Steel and concrete jackets are other techniques for providing additional confinement for compression members. When FRP is used as the confining material, a problem arises in that the strength of the FRP material is usually not utilized until the lateral strain in the confined concrete is very high. In some cases, the failure of concrete will occur before the FRP material is fully utilized. Overcoming this problem can be achieved by using active confinement, which necessitates the pre-stressing of the FRP jacket.