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    was reported as ±0.015 mm. The measured twists are illus-
    trated in Fig. 3, which correspond well to conventional
    measurements. The main focus in this paper is on the slip
    behaviour and strain development of the CFRP reinforce-
    ment. More details about the setup and other aspects of the
    photogrammetry investigation can be found in Hii and Al-
    Mahaidi [8].3.1. Slip behaviour between CFRP reinforcement
    and concrete
    Photogrammetry was used to determine slip between the
    CFRP laminates and concrete surface at various points
    along the CFRP strip. It is reasonable to assume that the
    relative movement of the concrete surface adjacent to the
    CFRP strip was representative of the concrete layer under-
    neath. Since load in the CFRP was mainly carried in the
    principal fibre direction, only slip in this direction is consid-
    ered. The scope was narrowed to the CFRP strips which
    ruptured first as being representative for analysis. Two sep-
    arate slip profiles on each side of the CFRP strip along the
    beam depth were obtained. The reference point of the beam
    depth was taken to be the edge of the north-top face in
    Fig. 6 (FH050D2).
    At low load levels, i.e. before the formation of cracks in
    the vicinity, the amount of slip is negligible. Subsequently,
    localized areas of slip would form in the vicinity of the tor-
    sional cracks. This is first observed at torque levels of
    20.1 kN m, 25.7 kN m, 23.8 kN m, and 77.7 kN m for
    beams FH075D1, FH050D1, FH050D2, and FS050D2,
    respectively. This is expected so to satisfy the deformation
    compatibility requirement between the CFRP laminates
    and concrete; otherwise infinite CFRP strains would have
    to develop at the crack which is not physically feasible.
    As the applied torque was increased, the slip values
    increased and gradually propagated from the crack loca-
    tion. The torque levels which showed marked increases in
    slip values correspond closely to the loads where the devel-
    opment of crack widths was significant [8]. The slip
    between CFRP and concrete would decrease as measure-
    ments were taken further from the cracks. Generally, the
    shape of the slip profiles along the beam depth is
    exponential.
    As observed by Pham and Al-Mahaidi [9], debonding
    between the CFRP laminate and concrete generally initi-
    ates when slip exceeds 0.05 mm. However, it is difficult to
    define the exact torque level where this occurs, as the local-
    ized debonded portions were extremely sensitive to the for-
    mation of cracks in the vicinity. As the torque level
    increased, large portions of the CFRP strips clearly deb-onded. In comparing Fig. 6a and b, the locations on the left
    and right sides of the CFRP strip where debonding initi-
    ated for beam FH050D2 are at different depths. The same
    observation could be made for the rest of the strengthened
    beams. This is due to the inclined torsional cracks crossing
    the CFRP reinforcement at an angle. The non-uniform
    width along the length of the crack also explains the
    uneven debonded zone across the width of the CFRP strip.
    A plot of the average bond-slip development of the crit-
    ical CFRP strip with torque is shown in Fig. 7 for all the
    strengthened beams. The average slip was obtained by
    averaging the slip of all the points along and on both sides
    of the critical CFRP strip. At low load levels, the average
    slip is small. At higher torque levels, the average slip
    exceeds 0.05 mm, which is defined as the initiation of mac-
    rodebonding. The exact torque values where this occurs are
    44.0, 38.2, 39.3, and 71.7 kN m for beams FH075D1,
    FH050D1, FH050D2, and FS050D2, respectively. No
    trends between the amount of CFRP reinforcement and
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