15-10-2012, 01:29 PM
STRENGTHENING TECHNIQUES OF RC COLUMNS USING FIBRE REINFORCED POLYMERIC MATERIALS
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Abstract.
Fibre reinforced composite materials are becoming more frequently used
in civil engineering structures. One of the most practical applications of these new
materials concerns the strengthening of reinforced concrete columns by means of
confinement with fibre composite sheets. In the literature, various theoretical models have
been proposed to describe the behaviour of confined concrete columns. The principal
advantages of this technique are the high strength-to-weight ratio, good fatigue properties,
non-corroding characteristics of the fibre reinforced polymers (FRP), and the facility of its
application. The maximum efficiency of confining systems using FRP materials is
reached in case of columns with circular cross-section and is explained by the fact that the
entire section of the column is involved into the confinement effect. Rectangular
confining reinforcement is less efficient as the confinement action is mostly located at the
corners This paper reveals the most utilized techniques of performing composite
confining systems for reinforced concrete columns, with their advantages and also
disadvantages.
Introduction
Over the years, engineers have used different methods and techniques
to retrofit existing structures by providing external confining stresses. For the
past few years, the concept of jacketing has been investigated to provide such
forces. Externally applied jackets have been used as a reinforcement to contain
concrete for different reasons. Engineers have used traditional materials such as
wood, steel, and concrete to confine and improve the structural behavior of
concrete members [7].
Section enlargement is one of the methods used in retrofitting concrete
members. Enlargement is the placement of reinforced concrete jacket around
the existing structural member to achieve the desired section properties and
performance.
FRP Confining Systems in Case of RC Columns
Confinement is generally applied to members in compression, with the
aim of enhancing their load carrying capacity or, in cases of seismic upgrading,
to increase their ductility. Advanced FRP composite materials have only
recently been recognized as reliable confinement devices for reinforced
concrete elements. FRP, as opposed to steel that applies a constant confining
pressure after yield, has an elastic behavior up to failure and therefore exerts its
(passive) confining action on concrete specimens under axial load in a different
way with respect to steel. In Fig. 1 it can be seen that, at a certain value of the
normalized axial concrete strain, the steel reaches yielding and then, from that
point on, it exerts a constant lateral (confining) pressure, while FRP exerts a
continuously increasing confining action [3], [8].
Confining Techniques
Today there are several types of FRP strengthening systems namely
a) wet lay-up systems;
b) systems based on prefabricated elements;
c) special automated wrapping systems.
a) Wet lay-up process represents the most commonly used technique, in
which unidirectional fibre sheets or woven fabric sheets are impregnated with
resins and wrapped around columns, with the main fibres oriented in the hoop
direction [2]. Installation on the concrete surface requires saturating resin,
usually after a primer has been applied. Two different processes can be used to
apply the fabric [9]: (i) the fabric can be applied directly into the resin which
has been applied uniformly onto the concrete surface, (ii) the fabric can be
impregnated with the resin in a saturator machine and then applied wet to the
sealed substrate.
The wrapping can be realized continuously around the entire element or
partially, using sheets of FRP disposed in spiral or in distinct sections. There
can be applied variable number of layers (from same material or distinct ones),
obtaining different thicknesses of the confining layer, depending on the required
element strength (Fig. 4) [4].
Conclusions
The most utilized techniques of performing composite confining
systems for reinforced concrete columns are: wet lay-up method, automated
method and the method based on using prefabricated elements.
For developing efficient composite confining systems it is required to
respect the technological steps that lead to a corresponding transfer of stresses
from concrete to the composite membrane. These steps include: priming of the
concrete substrate, of the application surface, execution of the resin mixture,
application of the composite system and of the protection layers.