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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
Post-strengthening of reinforced concrete beams with prestressed CFRP strips
Part 2: Analysis under cyclic loading
of the yielding stress observed at beam VFC_PE_01, tested under
static loading. Maximum and minimum applied loads were, respec-
tively, 50kN and 40kN, which, added to the self weight of 28 kN,
resulted in applied loads of 78kN and 68kN (48% and 42% of the
ultimate capacity of VFC_PE_01).
Aiming to produce the first cracks, beam VFC_PC_02 was pre-
loaded up to78kN. Then, cyclic loading was applied at a frequency
of 4Hz.
Figure [12(a)] shows that vertical displacements at mid-span, for
beam VFC_PC_02, measured with the beam subjected to the
maximum load (78kN) varied 12,70mm from preloading up to
1.000.000 cycles. From this point, up to the end of the test, vertical
displacements increased just 1,49mm. Data of beam VFC_PC_01
showed a large increase in the vertical displacements at mid-span
after 100.000 cycles, probably due to the fatigue failure of the steel
rebars.
Strains in the concrete and in the CFRP strips (Figure [12(b)]) be-
have similarly to the displacements at mid-span, where most of
the variations occurred before 1.000.000 cycles, and, after that,
showed stability up to 5.000.000 cycles. Beam VFV_PC_01 also
showed a similar behavior between strains and vertical displace-
ments at mid-span, however, with a significant increasing after
100.000 cycles, probably due to the fatigue failure of the steel re-
bars.
Figures [03] to [15] show the strains in concrete and in CFRP strips,
obtained by deformeters, which gauge points were placed along
the bottom of the beam. Measurements were made during pre-
loading, after 30.000 cycles, after 100.000 cycles, after 1.000.000
cycles and after 5.000.000 cycles, with the beam subjected to the
maximum load (78kN).
It can be noticed that the strains obtained by deformeters,
placed along the bottom of the beam VFC_PC_02, varied from
the pre-loading up to 1.000.000, tending to stabilize after 5 mil-
lion cycles. Portions located between the loading points (1.2
m to 4.8 m from the beam end) clearly show the presence of
cracks in the concrete.
From the pre-loading up to 30.000 cycles, it was not observed
any significant variation in the strains along the gradual anchor-
age zone (1,2m from the both beam ends). Strains increased after
100.000 cycles, and, after 5.000.000 cycles, the level of strains of
the beginning of the test could be observed just along the first 0,6m
from both beam ends. Figures [13] to [15] show that strains in the
anchorage zones of beam VFC_PC_01 were higher than the ones
of beam VFC_PC_02.
Data of Figures [13] to [15] show that strains in the CFRP strips,
between the two central loading points, placed at 2,4m and 3,6m
from the beam ends, measured at pre-loading, varied from 1,00 º/
oo
up to 1,50º/
oo
.
After 30.000 cycles strains increased up to 2,00 º/
oo
and no sig-
nificant variation were observed from 30.000 cycles up to 100.000
Figure 11 – 2,2mm width crack at mid-span of
beam VFC_PC_01 after post-strengthening failure
Figure 12 – Beams VFC_PC_01 and VFC_PC_02: (a) Displacement at
mid-span vs number of cycles (b) Concrete and CFRP strains vs number of cycle
A
B