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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 3
Post-strengthening of reinforced concrete beams with prestressed CFRP strips.
Part 1: Analysis under static loading.
CFRP strips, increased 62,41% the load bearing capacity of
the beam.
Results of beams VFC_NP_01 and VFC_PE_01 can be explained
by the principles of prestressing. When the prestressing force ap-
plied on the CFRP strips is released, compressive stresses are in-
duced on the concrete. Such compressive stresses delay the con-
crete cracking and the yielding of the steel reinforcement. Thus, the
load bearing capacity of the post-strengthened element is increased.
Premature failures (peeling-off) of beams VFC_NP_01 and VFC_
PE_01 occurred due to the high interfacial stresses near to the
ends of strips.
Peeling-off failures are catastrophic and happen without any previ-
ous advice. Figure [9] (b) shows two CFRP strips of beam VFC_
NP_01 after peeling-off. Both strips are completely detached from
the beam; however, strips do not present any damage, once the
failure occurred at the concrete/adhesive interface.
4.2 Displacements at mid-span
Figure [10] shows that all post-strengthened beams present similar
behavior regarding stiffness until concrete cracking. Results indi-
cate that in such cases the action of the post-strengthening begins
just when the structural element is already cracked.
Beam VFC_NP_01 post-strengthened with non-prestressed strips
and the control beam, showed no significant difference regarding stiff-
ness. However, beam VFC_PE_01 showed a stiffer behavior, when
compared to beans control and VFC_NP_01, due to the increase of
the cracking load and the later yielding of the reinforcement steel.
Control beam and beam VFC_NP_01, post-strengthened with
non-prestressed CFRP strips present similar values of loading and
displacement at mid-span at concrete cracking, due to the fact that
the post-strengthening begins to act just after concrete cracking.
Cracking load of beam VFC_PE_01 was 57% higher than the ones
of control beam and beamVFC_NP_01. Differences at the first stage
of the loading versus displacement response at mid-span happen be-
cause prestressing leads to the development of compressive stresses
at the bottom face of the beam. When non-prestressed strips are ap-
plied, the bottom face of the beam is already tensioned at the beginning
of the loading process. Thus, when post-strengthening is prestressed,
the loading needed to crack the concrete is significantly higher.
Differences regarding yielding of the reinforcement steel were sig-
nificant. Increasing of 22% for beam VFC_NP_01 and 45 % for
beam VFC_PE_01, related to control beam, was observed.
4.3 Anchorage system
The gradual anchorage system worked properly and allowed the
use of 83% of the tensile strength of the strips. Advantages of the
gradual anchorage include the fact that the same device is used to
prestress the strip and to gradually reduce the prestress load from
the mid-span to the ends of the strip. The result is a prestressed
CFRP strip without any external anchorage systems.
5. Conclusions
Results obtained with the development of the research program
allowed the investigation of changes on the behavior of post-
strengthened elements due to prestressing.
Figure 10 – Load vs Displacement response of the beams tested under static loading