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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.
1. Introduction
Reinforced concrete structures are, frequently, submitted to inter-
ventions aiming to restore or increase their original load capacity.
According to Garden & Hollaway [1], the choice between upgrad-
ing and rebuilding is based on factors specific to each individual
case, but certain issues are considered in every case. These are
the length of time during which the structure will be out of service
or providing a reduced service, relative costs upgrading and re-
building in terms of labor, materials and plant, and disruption of
other facilities.
Several post-strengthening techniques were developed in the last
decades. Most of them are based on the addiction of a structural el-
ement to the external face of the element to be post-strengthened.
According to Täljsten [2], the method of post-strengthening exist-
ing structures with steel plates bonded to the structure with epoxy
adhesive was originated in France, in the nineteen sixties, when
L’Hermite (1967) and Bresson (1971) carried out tests on post-
strengthened concrete beams. There is also reported use of this
post-strengthening method in South Africa from 1964 by Dussek
(1974). In both cases the post-strengthening was successful and
the load bearing capacity was increased. These first investigations
in France and South Africa inspired future research in Switzerland
(1974), Germany (1980), United Kingdom (1980), Japan (1981)
and Belgium (1982). The idea of post-strengthen existing rein-
forced concrete structures with bonded steel was improved due to
the development of synthetic adhesives, based on epoxy resins,
suitable to ensure good adhesion and chemical resistance to ag-
gressive agents.
In the last decades, non-corrosive, low-weight and high-resistant
materials started to be developed and applied on the construc-
tion of new buildings, aiming to produce durable structures. These
materials, called Fiber Reinforced Polymers (FRP), started to be
investigated in the middle 80´s at EMPA (Swiss Federal Labo-
ratories for Materials Testing and Research), in Switzerland. At
that time, the carbon fiber was elected as the most suitable for
post-strengthening applications due to its low-weight, high tensile
strength, high modulus of elasticity and resistance to corrosion.
Since then, many structures were post-strengthened with FRP in
Japan, Europe, Canada and United States and nowadays the use
of FRP is growing worldwide.
Most of FRP post-strengthening systems used nowadays consist
of carbon fibers embedded in epoxy matrices and provide high
modulus of elasticity and tensile strength. The main impediment
to the massive use of CFRP (Carbon Fiber Reinforced Polymers)
regards to the high cost of the carbon fiber that, in Brazil, may
reach US$ 50,00/m
2
, depending on the post-strengthening system.
Meier, in 2001 [3], pointed out that the functionality and the me-
chanical properties of CFRP should be better explored, due to
its relatively high cost. Indeed, the use of only 10%-15% of the
tensile strength of the CFRP, as it happens in some bonded post-
strengthening systems, is not economically viable.
Aiming to contribute to the evolution of the CFRP post-strength-
ening technique, this paper intends to analyze the efficiency of
prestressed CFRP used on the post-strengthening of reinforced
concrete beams, by means of static – part 1 and cyclic – part 2
loading tests, as an alternative to better use the tensile strength of
these materials.
2. Application of prestressed FRP strips
on the post-strengthening of reinforced
concrete beams
The aim in prestressing concrete beams may be, according to Gar-
den and Mays [4], either to increase the serviceability capacity of
the structural system of which the beams form a part or to extend
its ultimate limit state.
According to El-Hacha [5], FRP are well suited to prestressing
applications because of their high strength-to-weight ratio that
provides high prestressing forces, without increases on the self-
weight of the post-strengthened structure. The prestressing tech-
nique may improve the serviceability of a structural element and
delay the onset of cracking. When prestressed FRP are used, just
a small part of the ultimate strain capacity of the material is used
to prestress the FRP, the remaining strain capacity is available to
support external loads and also to ensure safety against failure
modes associated to peeling-off at the border of flexural cracks
and at the ends of the post-strengthening.
Several FRP prestressing systems are currently available consist-
ing of rods, strands, tendons or cables of FRP. However, in some
cases, it may be advantageous to bond FRP sheets or strips onto
the structural element surface in a prestressed state. According to
the Bulletin 14 of
fib
[6], prestressing the FRP prior to bonding has
the following advantages:
n
Provides stiffer behavior as at early stages most of the con-
crete is in compression and therefore contributing to the mo-
ment of resistance. The neutral axis remains at a lower level
in the prestressed case if compared to the unstressed one,
resulting in greater structural efficiency.
n
Crack formation in the shear span is delayed and the cracks,
when they appear, are more finely distributed and narrower.
Thus, serviceability and durability are improved, due to re-
duced cracking.
n
The same level of strengthening is achieved with smaller areas
of stressed FRP, compared to unstressed ones.
n
Prestressing significantly increases the applied load at which
the internal steel reinforcement begins to yield if compared to
an unstressed structural member.
On the other hand, prestressing FRP systems are more expensive
than the non-prestressed ones, due to the greater number of op-
erations and the equipment that is required to prestress the FRP.
2.1 Losses of prestressing force
Prestressed FRP bonded to concrete structures are sujected to
prestress losses, as it happens in any prestressing system. Such
prestress losses may be instantaneous, due to immediate elas-
tic deformation of concrete, or time dependent, due to creep and
shrinkage of concrete and relaxation of the FRP.
Immediate elastic deformation of the concrete may reach 2% to
3%, according to the Bulletin 14 of
fib
[6], and happens when the
prestress force is transferred into the concrete beam. If prestress
is applied by reacting against the structural member there will be
no loss. It happens because if the prestressing device if fixed on
the structural element that will be post-strengthened, a compensa-
tion occurs: as the PRF is being stressed, the concrete is being
compressed. However, FRP elements that have already been pre-