Proceedings of the Institution of Civil Engineers Structures and Buildings 162 October 2009 Issue SB5 Pages 311–321 doi: 10.1680/stbu.2009.162.5.311 Paper SB 800079 Received 24/10/2008 Accepted 27/01/2009 Keywords: bridges/concrete structures/resins & plastics Pierfrancesco Valerio Senior Bridge Engineer, Mott MacDonald, Croydon, UK Timothy James Ibell Professor, Department of Architecture and Civil Engineering, University of Bath, UK Antony Peter Darby Senior Lecturer, Department of Architecture and Civil Engineering, University of Bath, UK Deep embedment of FRP for concrete shear strengthening P. Valerio MEng, CEng, MICE, T. J. Ibell PhD, CEng, FIStructE and A. P. Darby PhD, CEng, MIStructE The shear capacity of existing concrete structures is often unable to meet current standards requirements. This may be attributable to increased load requirements, inadequate shear provisions in the original design, deterioration of materials or an increased demand in shear capacity owing to flexural strengthening. There are various approaches to the repair and strengthening of existing concrete structures in shear using fibre- reinforced polymer (FRP), involving the use of plates or fabrics externally bonded to the web, prestressed straps wrapped around the beam or the use of bars mounted near the surfaces of the web. However, when only the top or bottom faces of the concrete member are accessible, as in the case of bridge beams made contiguous within a deck or for corbels, a different approach is proposed, called the deep embedment technique: vertical holes are drilled into concrete upwards from the soffit in the shear zones, high-viscosity epoxy resin is injected and then FRP or steel bars are embedded into place. In this paper, the results of a series of tests on unstrengthened and strengthened small-scale and large-scale reinforced concrete and prestressed concrete specimens with and without stirrups are presented. The proposed technique is shown to be feasible, successful and potentially more effective than other shear strengthening approaches. A simple model derived from the truss analogy is shown to be able to predict the capacity of the strengthened beams and can therefore be used as a design tool for the scheme. 1. INTRODUCTION Owing to increased traffic loads, aggressive environments or poor initial design and construction, the safety margin against flexural and shear failure of many concrete bridges is reducing. An area of concern, identified by Network Rail, is the actual shear strength of prestressed rectangular bridge beams when post-tensioned transversely in the horizontal plane and made contiguous within a deck; this is a common typology of beam bridge used for railway underbridges, with simply supported spans ranging from 6 to 20 m (see Figures 1 and 2). Analytical assessments performed in accordance with the current UK Network Rail code, 1 based on the truss analogy, have demonstrated an apparent shear deficiency in these bridges, and therefore it is necessary to ensure that a practical shear strengthening scheme is available. As part of this research project, an experimental campaign (not presented in this paper) on small-scale bridges that were replica scaled- down models of the actual bridges, has shown that, although the lateral prestressing can substantially increase the load- carrying capacity of these bridges, the collapse always occurs when a single beam, under increasing vertical loads, slips from the contiguous ones and fails in shear while the other beams recover elastically remaining undamaged. 2 Shear strengthening of the individual beams, therefore, can enhance the overall capacity of the bridge leading to more ductile flexural failures. Most newly built concrete structures are reinforced with steel bars. However, steel is less attractive for retrofitting concrete structures owing to its high weight, its accompanying cumbersome strengthening solutions and its susceptibility to corrosion. Thus, fibre-reinforced polymer (FRP) materials have become very popular in recent years for strengthening works owing to their strength, lightness, ease of application and durability. 3,4 There are several approaches to retrofitting existing concrete bridges in shear with FRP, involving the use of plates or fabric externally bonded (EB) to the webs of the bridge beams, prestressed straps wrapped around the beams 5 or bars mounted within grooves in the near-surface mounted (NSM) technique. 3,4 In this particular case, however, where the webs of the beams are inaccessible, a new approach is adopted: vertical holes are drilled into the beams upwards from the soffit in the shear zones, high-viscosity epoxy resin is injected and then FRP bars are embedded into place (see Figure 3). For comparison purposes, the same technique using steel bars is also considered here. In this paper, tests on unstrengthened small-scale and large- scale beams are compared with the equivalent beams strengthened in shear with the deep embedment technique described above. An experimental campaign of pull-out tests on carbon, glass, aramid and steel bars embedded into concrete with varying embedment lengths is also presented, which defines the parameters for the design of the strengthening scheme. Comparisons are made against current code predictions for the capacity of all beams, and a simple model derived from the truss analogy is shown to be able to accurately predict the capacity of the strengthened beams. The main objectives of this research project are therefore to develop a feasible, efficient and cost-effective shear strengthening technique for bridge beams where only access to the soffit is permitted and provide the engineers with a simple but still Structures and Buildings 162 Issue SB5 Deep embedment of FRP for concrete shear strengthening Valerio et al. 311 Downloaded by [] on [13/06/22]. Copyright © ICE Publishing, all rights reserved.