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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 3
M. R. GARCEZ | G. L. C. P. SILVA FILHO | URS MEIER
stressing force that avoids the need of anchorage systems provide
very low prestressing levels, 15% to 20%, depending on the cross
section of the CFRP strip.
Thus, the addition of anchors at the end of the prestressed FRP
sheets or strips reduces the shear deformation that occurs within
the resin or adhesive layer upon releasing the prestress force and
reducing the shear stresses transferred to the base of the concrete
section. Thereby, anchorage systems minimize the possibility of
premature failures (El-Hacha [5]).
According to El-Hacha et al. [9], prestressing levels of at least 25%
of the FRP tensile strength may be necessary to achieve a sig-
nificant improvement in terms of the structural stiffnes and load
carrying capacity.
Meier [10] suggests that a prestress level as high as 50% of
the CFRP strength might be necessary to increase the ultimate
strength by delaying the premature failure. Experimental results
presented by Deuring [11] showed that increasing the level of pre-
stress in the CFRP from 50% to 75% reduced the strength of the
beam because the highly prestressed laminates had little strain
capacity remaining and the CFRP presented premature failure.
It is important to have in mind that, when post-strengthening is
prestressed the modulus of elasticity of the FRP is of great sig-
nificance, once the FRP element needs to be stiffer to hold up a
significant loading that, before the post-strengtnening, was made
only by the steel reinforcement (El-Hacha, [5]).
2.3 Prestressing method developed by Stoecklin
and Meier [12]
Stoecklin e Meier [12] developped, at EMPA (Swiss Federal Lab-
oratories for Material Testing and Research) a method to apply
prestressed FRP strips to concrete structures. In this method, the
FRP strip is first prestressed then bonded at the beam that will
receive the post-strengthening. Since it is very complicated to grab
and prestress the FRP strip, due to its anisotropic behavior, a pre-
stressing device was designed, as one can see in Figure [02]. The
stressed will experience a loss of prestress due to the shortening
of the beam upon the prestressing of subsequent FRP elements.
In such cases it is necessary to determine the average loss of pre-
stress per FRP element.
Time dependent losses, due to creep and shrinkage of concrete,
according to the Bulletin 14 do fib [6], reach about 10 to 20% and
are similar to the ones of conventional prestressing.
Prestressing losses due to relaxation of FRP depends, according
to ACI 440.4R-04 [7], on the characteristics of the FRP composite.
The document also informs that losses due to relaxation of fibers
may be neglected when CFRP are used, since the relaxation of
carbon fibers is very low. Losses of 0,6 to 1,2% must be consid-
ered due to the relaxation of the polymer and losses of 1 to 2%
must be considered due to the straightening of fibers.
Results of a research program developed by Triantafillou et al. [8]
indicate that, when prefabricated CFRP are used, prestress losses
of 10% must be considered, due to the instantaneous and time
dependent losses at the concrete and adhesive and also due to the
relaxation of the CFRP.
Garden and Mays [4] consider that prestressed FRP also suffer
prestress losses due to the shear transferred through the adhesive
and into the concrete by the FRP tension. This shear action is suf-
ficient to fracture the concrete even at low prestress levels so it is
necessary to install anchorages at the ends of the FRP element to
resist this action.
2.2 Maximun prestressing force
Figure [01] (a), by Triantafillou et al. [8], shows the premature
failure of a concrete beam post-strengthened with a CFRP strip,
without any anchorage system, immediately after the complete
release of the prestressing force. Horizontal shear cracks propa-
gated from both ends of the CFRP strip through the concrete layer
and stopped at a certain length. Figure [01] (b) shows that this
failure mode may be prevented if anchorage systems are used at
the ends of the strips. The authors suggest that the maximum pre-
Figure 1 – (a) Premature failure of a prestressed CFRP strip without anchorage;
(b) Action of an anchorage system (Triantafillou et al. [8])
A
B