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    The economic and time constraints in the repair or upgrading of existing infrastructure have become a major issue, particularly
    extending the service lifespan of bridges. Fibre reinforced polymer (FRP) has shown great promise as a state-of-the-art material in flex-
    ural and shear strengthening as external reinforcement. However, little attention has been paid to torsional strengthening in terms of
    both experimental and numerical research. This paper focuses on the bond-behaviour of externally bonded CFRP in an overall inves-
    tigation of torsional strengthening of solid and box-section reinforced concrete beams. Significant levels of debonding prior to failure by
    CFRP rupture were measured in experiments with photogrammetry. Numerical work was carried out using non-linear finite element
    (FE) modelling. Good agreement in terms of torque-twist behaviour, steel and CFRP reinforcement responses, and crack patterns
    was achieved. The addition of a bond-slip model between the CFRP reinforcement and concrete meant that the debonding mechanisms
    prior to and unique failure modes of all the specimens were modelled correctly as well. 6883
     2006 Elsevier Ltd. All rights reserved.1. Introduction
    The bridging infrastructure in developed countries has
    matured considerably over the last century. However,
    increased traffic volumes and loads, diminished capacity
    through environmental degradation and age, more strin-
    gent updates to design code regulations, and the need for
    seismic retrofit in some parts of the world have necessitated
    the need for repair and rehabilitation of existing infrastruc-
    ture. Many building and bridge elements are subjected to
    significant equilibrium torsional moments that may require
    strengthening. The use and research of externally bonded
    FRP is relatively advanced in flexural and shear strength-
    ening. In comparison, relatively little is known about tor-
    sional strengthening. A good example of this is theWestgate Bridge in Melbourne, Australia [1]. Without suf-
    ficient information, shear strengthening guidelines had to
    be adapted for torsional design. In a literature survey con-
    ducted by the authors [2], no experimental data was found
    before 2001. Several torsional strengthening investigations
    of exploratory nature have been conducted since then [3,4].
    This paper attempts to address the gap in knowledge in this
    area.
    In this study, six CFRP strengthened medium-scale thin-
    walled box-section and solid reinforced concrete (RC)
    beams were investigated. The focus in this paper is on the
    bond-behaviour of the externally bonded CFRP, which is
    the first known study for torsional strengthening. A more
    detailed discussion on the other aspects of the experiments
    can be found in Hii and Al-Mahaidi [2]. The bond behav-
    iour was quantified using photogrammetry measurements
    in experiments. A bond-slip model was introduced into
    non-linear finite element modelling to further investigate
    this aspect in detail.2. Experimental program
    2.1. Specimen details and experimental setup
    Six RC beams of 500 · 350 mm deep cross-section and
    2500 mm long were constructed for this test (Fig. 1). Two
    specimens were solid sections (CS1, FS050D2) while the
    rest were box-sections. A summary of the specimen details
    can be found in Table 1, along with the average concrete
    compressive strength of cores extracted. The reinforcement
    layout was designed for minimum torsional capacity [5] to
    simulate a box-girder that was now torsionally deficient.
    The stirrups were spaced at 125 mm centres in the test zone
    for all the beams.
    For this investigation, MBrace CF130 CFRP sheets and
    epoxy resins [6] were externally bonded using the wet lay-
    up method. The fibre modulus and thickness of the sheet
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