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was 240 GPa and 0.176 mm, respectively. Fifty millimetres
wide strips were used in all the strengthened specimens.
Two different number of layers and strip spacings of
0.75D and 0.50D were investigated, D being the full depth
of the beam. The spacings were chosen as they were the
minimum spacing required for shear in the Australian Con-
crete Code, and it was expected in future that existing
structures would be strengthened for both actions.
A 150 mm overlapping length of strip was applied in the
longitudinal fibre direction.
The test setup is shown in Fig. 2. One end of the speci-
men was fixed by a steel collar to an anchored base so no
longitudinal, vertical, transverse and rotational movements
were allowed. On the other end, the specimen was sup-
ported by a spherical seat on linear bearings that allowed
the loaded end to twist freely and elongate/shorten. Torque
was applied on the 1.8 m test zone through the lever arm by
a single 250 kN capacity hydraulic actuator.
Load cells were placed under the spherical seat and the
hydraulic actuator to measure applied torque. LVDTs and
inclinometers were used to determine the exact amount of
extension/shortening and twist respectively. Strain gauges
placed on steel reinforcement and CFRP strips at specific
locations were used to determine the strain distribution.
Photogrammetry was also used to capture the deforma-
tions of the beams under torsion.2.2. Experimental results
The torque-twist curves for all six tested beams are
shown in Fig. 3. The pre- and post-cracking stiffness, crack-
ing and ultimate torque capacity of both solid and box-sec-
tion RC beams are closely related to the amount of CFRP
applied (Table 2). Up to a 40% and 78% increase in crack-
ing and ultimate strengths from FH050D2 is observed. For
the solid RC beams, an increase of 8% and 49% of crackingand ultimate strengths were recorded for FS050D2. The
presence of the CFRP strips inhibited these cracks from
propagating and widening compared to the control beams.
The steel reinforcement was found to have yielded at peak
load for all the beams. Further detailed discussion on the
strains measured in the reinforcement can be found in
Hii and Al-Mahaidi [2].
Generally, the failure mechanism observed is similar for
all the strengthened beams. At higher load levels, cracking
propagated along the CFRP strips (Fig. 4b). This indicates
partial debonding. However, immediate failure through
debonding of the CFRP reinforcement did not occur due
to the use of full hoop strips providing the necessary
anchorage and support to the debonded portions. Review
of the high-speed video recordings show that at failure,
rupture of the CFRP strips first occurred at the corners,followed by peeling away with a thin layer of concrete still
bonded to the laminate (Fig. 4a). It is observed in the
strengthened beams that the succession of CFRP ruptures
occurred at the corners of the beams, and not necessarily at
locations intersecting the spiral cracks.
3. Close-range photogrammetry measurements
The investigation was conducted with the aid of photo-
grammetry, which is the process of obtaining precise mea-
surements about physical objects by means of photography
[7]. Reflective targets were placed on the test zone in each
beam, on both the concrete surface and the CFRP strips
as shown in Fig. 5. During testing, loading of specimens
was paused at certain intervals to allow photogrammetry
surveys to be taken. By taking photos from different loca-
tions, the data was automatically processed on a computer
workstation to obtain the precise location of each target.
The statistical accuracy of the photogrammetry survey