by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
Concrete In Australia : September 2013
60 Concrete in Australia Vol 39 No 3 Enhancing the efficiency of FRP strengthening systems Scott T. Smith Foundation Professor of Engineering, Southern Cross University, Lismore NSW Externally bonded fibre-reinforced polymer (FRP) composites have been used to repair and strengthen reinforced concrete (RC) flexural members such as beams and slabs for about two decades. Many design guidelines have been written around the world and Australia has had one since 2008 (1). A common limitation of the FRP strengthening technique though is the premature and sudden debonding failure of the FRP at relatively low strains. In addition, the application of FRP composites considerably reduces the ductility and deformability of RC flexural members. An effective means to minimise and even eliminate such limitations is via the addition of anchorage devices made from FRP (herein FRP anchors). e behaviour of FRP-strengthened concrete anchored with FRP anchors is briefly reviewed in this extended abstract and more thoroughly in the conference paper version. is extended abstract contains key findings of research conducted by the author of this paper and his research group over the past few years on FRP-to-concrete joints. e work presented herein has an emphasis on physical behaviour for the benefit of engineers designing FRP strengthening solutions. Anchorage devices can be installed onto FRP strengthening to increase the debonding resistance of the FRP and also produce ductile FRP-strengthened structures (e.g. 2). Such research is relatively new and therefore still to find its way into design guidelines. Kalfat et al. (3) recently conducted a state-of- the-art review on anchorage devices for application to FRP- strengthened RC structures. Figure 1a is a schematic drawing of an installed FRP anchor. An FRP anchor is made from bundles of fibres or rolled fibre sheets. One end of the anchor may be impregnated with epoxy in order to form a solid dowel for insertion into the concrete member. e anchor fan component is splayed onto the FRP strengthening. e fans may be comprised of two components (i.e. bow-tie anchor as shown in Figure 1a) or one component that is oriented towards the load direction (i.e. single fan anchor). A convenient means with which to characterise and quantify the behaviour and strength of FRP anchors is via FRP-to- concrete joints (e.g. typical single-shear test set-up shown in Figure 1b). Common results obtained include the load (P) applied to the plate and the relative slip (δ) between the FRP plate and concrete at the loaded end of the bonded plate. Figure 2 is the generic load-slip response for single-fan anchors. A recently completed PhD dissertation (5) provides a summary of hundreds of joint tests. In Figure 2, subscripts db, max,1 and max,2 refer to initiation of plate debonding, complete plate debonding, and maximum joint capacity following complete plate debonding, respectively. e following is a physical description of the key stages of Figure 2: • Pre-plate debonding: Relationship between force and slip is largely linear. • Plate debonding: Debonding initiates at the loaded end of the plate at db. In the absence of an anchor, the load largely remains constant while the slip increases and the plate fully debonds. e length of the load plateau is dependent upon the length of the bonded plate. In the presence of an anchor, the load continues to increase until the plate fully debonds at max,1. • Post-plate debonding: A post-peak reserve of strength is achieved due to frictional resistance between the debonded plate and the roughened concrete substrate. Friction is achieved as the anchor clamps the plate onto the concrete. e peak occurs at max,2. • Anchor Failure: Anchors can fail in a relatively sudden manner (i.e. Mode 2a where the anchor ruptures) or a gradual manner (i.e. Mode 2c where the inner anchor fibres pull out). Regardless of the failure mode, for the purpose of design, the coordinates Pmax,1 and δmax,1 are utilised and represent the capacity of the joint. Coordinates Pmax,2 and δmax,2 are generally not to be used in design. Tests have shown joints strengths to increase when (i) the anchor was located closer to the loaded end of the joint, (ii) the length of the plate between the anchor and unloaded plate end was increased, (iii) the angle of the anchor dowel was increased relative to the direction of load, (iv) the width of the plate was increased, (v) the elastic modulus of the plate was increased, (vi) the number of anchors was increased, and (viii) an increase of anchor fan fibre. Overall, the anchored joints achieve greater strength and slip capacity than the unanchored joints. ese are desirable characteristics that translate into greater debonding resistance of the FRP and enhanced member deformability (and ductility).