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IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 1
L. A. SPAGNOLO JR | E. S. SÁNCHEZ FILHO |
M. S. L. VELASCO
5. Experimental results
5.1 Cracking
The data collected by the strain gauges and LVTDs was recorded
using a special digital data acquisition system. During the test, the
cracks developed were marked without interruption of the load pro-
cess. Flexural cracks at the midspan started from the bottom face
and propagated vertically through the height of the beams. As the
applied load increased and the internal stress was redistributed,
several other secondary flexural cracks were observed, starting
from the main bending cracks. The load cell under the jack fur-
nished the cracking load
P
CR
. Table [3] shows that crack loads for
VR1 and VR2 were of the same level, and that the crack loads for
Series I were greater than those for Series II.
5.2 Ductility
All longitudinal reinforcement yielded, but not simultaneously with
the steel stirrups. Table [3] shows that the steel stirrups of Se-
ries II did not yield. Nevertheless, it is possible to conclude that the
beams tested in this research program presented good ductility.
5.3 Failure
For the beam with no descending branch, the authors considered
the ultimate load as equal to the maximum load.
The reference beams in the experimental program failed by diago-
nal tension, and the strengthened beams failed by diagonal ten-
sion with simultaneous debonding of the CFC stirrups (Table [3]).
Failure of the beams occurred after the steel stirrup yielded and
started to present an inclined shear crack that grew in the direction
of the beam flange. Bending or anchorage failure did not occur.
Figure [11] shows the pattern of failure of beams VR2 and VII‑1.
6. Discussion of results
6.1 Analysis of strength
The variables of the experimental program were concrete strength
and mechanical ratio of CFC stirrups. The tests were conducted
in reinforced concrete beams made of normal strength concrete.
The concrete effectiveness factor varied only with concrete
strength since all beams had the same height and longitudinal re-
inforcement.
After the cracking tension, the strengthened beams experienced a
sudden failure with the debonding and rupture of CFC
U
stirrups.
The theoretical values of ultimate shear force
theor
u
V
,
were cal-
culated by considering the bond between the CFC
U
stirrups and
concrete to be perfect, which was verified by the theoretical ap-
proach shown in this paper.
The load of the strengthened beams was 36 % to 54 % greater
than that of the reference beams. As presented in the penultimate
column of Table [3], the mean strength ratio is
43.1
,
. ref u
exp u,
V V
with a coefficient of variation of 4.41 %. This shows that the shear
strength of all the beams increased substantially, and that all the
arrangements of the CFC
U
stirrups studied are effective for shear
reinforcement.
6.2 Angles
Using all the relevant images of the cracking of the beams and with
the aid of a computer program, digital procedures were used to
obtain the crack angle
q
CR
(Figure [12]).
Figure 10 – General view of strain gauge locations on CFC U stirrups of all strengthened beam, distance
from support: section 12 = 0.33 cm; section 13 = 0.55 cm; the other sections are symmetric
Table 2 – Concrete compressive strength
Beam
Cylindrical
Specimens
Concrete
age (days)
f
c,average
(MPa)
–
4
28
44.90
VR1
3
48
48.44
VR2
3
57
49.92
VII-1
3
153
50.94
VII-2, VI-1, VI-2
3
216
51.73
VII-3, VI-3
3
335
52.30