24-08-2012, 02:30 PM
USE OF FRP FABRIC FOR STRENGTHENING OF REINFORCED CONCRETE BEAM-COLUMN JOINTS
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ABSTRACT
Devastating earthquakes in the last 3 years have shown that non-engineered concrete frames are
particularly vulnerable to seismic action and are a major cause of loss of lives. This structural type
constitutes a large share of the building stock, both in developed and developing countries, and hence
represents a substantial exposure. Direct observation of damaged structures, following the Kocaely,
Turkey 1999 earthquake, has shown that damage occurs usually at the beam-column joints, with failure in
bending or shear, depending on geometry and reinforcement distribution and type.
While substantial literature exist for the design of concrete frame joints to withstand this type of
failure, after the earthquake many structures were classified as slightly damaged and, being uneconomic
to replace them, at least in the short term, suitable means of repairs of the beam column joint area are
being studied. Furthermore there exist a large number of buildings that need retrofitting of the joints
before the next earthquake.
The paper reports the results of cyclic tests carried out on cruciform beam-column joint
specimens, with two different configurations of geometry and various configuration of strengthening by
externally bonded FRP fabric. The specimens were designed to comply with gravity load design codes,
but no seismic design was considered. In the design of the FRP wrapping, two different type of fabric
were considered and three layout of the wrapping strips.
INTRODUCTION
In the last decade, the effect of external application of fibre-reinforced polymers (FRP) plates and wraps
to reinforced concrete beam-column joints to increase their performance has been investigated
theoretically and experimentally. Previous to this, steel jackets were used to reinforce the joint area, as
well as the use of R.C. jackets (Baraka and Prion, 1995). The use of flat and corrugated steel plates, have
also been investigated (Beres et al, 1992; Ghoborah et al, 1997). However, these options have been found
to be labour intensive, requiring a high level of workmanship, and often add considerable weight to the
elements. Additionally, steel plates need corrosion protection and their attachment requires either the use
of epoxy adhesives combined with bolts, or special grouting (Antonopoulos, 2001).
External application of FRP material provides a practical solution to improve the overall
performance of an R.C. frame structure without the necessity of a radical alteration to the original
structure. Externally bonded FRP may be used in a repair capacity for structures that have undergone
moderate earthquakes damage or to reinforce structures that are considered to be vulnerable or
substandard. The use of FRP offers several advantages, related to its high strength-to-weight ratio,
resistance to corrosion, fast and relatively simple application (Karbhari, 2001). However, FRP is to date
still rather expensive so its use must be optimised to minimise material wastage. One disadvantage of
FRP is its dependence on bond to the concrete it is to strengthen, which is a function of the tensile
capacity of the concrete and the type of surface preparation used.
SPECIMENS DESIGN
The specimens were designed following the standards and provisions of the Italian code of practice issued
in 1960, based on permissible stresses. The material chosen were concrete Rck =20 Mpa and steel FeB32K
mild steel in smooth bars. Two configurations, one governed by beam failure (WB) and one governed by
column failure (SB) were considered. The geometric dimensions for the two prototypes and their
reinforcement details are summarised and Figure 1. For each configuration six specimens were cast.
DESIGN OF THE FRP STRENGTHENING
In designing the FRP reinforcement the various failure modes of the reinforced concrete cross section
/FRP wrapping need to be considered and provided against in the detailed design. FRP are usually applied
to concrete either in the form of rigid plates or flexible fabric. According to studies conducted in the last
ten years (Chajes et al. 1994) the following failure modes can be identified: flexural failure due to either
compressive failure of the concrete or tensile failure of the FRP, usually accompanied by yielding of the
longitudinal steel bars; shear failure if the FRP application shifts the critical condition of the element to
shear capacity; failure of the bond connection between the plate or fabric and the cover concrete or failure
of the bond between the cover concrete and the internal steel reinforcement. Either of these can occur at
the end of the plate or along the plate at the location of flexural or shear cracks. The design should avoid
premature bond failure and non-ductile shear failure of the structural elements. Hence the layout of the
wrapping should not only increase the bending capacity of the cross section but also ensure proper
anchorage of the strips and control the final shear capacity.
CONCLUSIONS
The results suggest that CFRP strengthening of beam-column joints with a strong beam/weak column
arrangement can shift failure from brittle failure of the joint core towards flexural hinging at the beamcolumn
interface, as well as upgrading joint core capacity.
CFRP strengthening was more effective as a means of strengthening than as a means of repair of
the damaged specimens. However, it is likely that the method of repair of the joint (i.e. removal of only
loose concrete and replacement with grout) was inadequate rather than the FRP wrapping.
CFRP wrapping of the beam-column joints was more successful with diagonal FRP wrapping,
compared to the orthogonal wrapping. This is due to the fact that the orientation of the diagonal strips
was closer to being parallel to the principal stresses in the joint core concrete than the principal fibres in
the orthogonal wrapping, making more effective use of the FRP. It is also due to the enhanced confining
action of the concrete at the joint corners by the diagonal FRP strips.
The application of the strengthening does not alter the initial failure mode in the configuration
characterised by a weak beam. The final mode of failure is still in bending of the beam with partial failure
of the FRP in tension.
Experimental values of ultimate bending moments obtained are lower than the ones calculated
with the theoretical models, assuming plane sections, and this corresponds with the observation of Liu
and Park, besides the fact that the FRP probably had a strength lower than the nominal one. The
difference between observed and predicted values ranges between 17% and 35%. The increase in ultimate
bending moment capacity between unstrengthened and strengthened specimens ranges from 26.3% for the
repaired specimen to 76%.Analysis of the unstrengthened specimens had proven that joint failure was not
an issue for the first configuration and this is proven by lack of cracks in the joint area of the two tested
reference specimens. However as a result of the increased bending capacity provided by the FRP, it was
noted that it was necessary to increase the shear capacity of the joint by adding vertical strips on the face
of the columns. The FRP strips aimed at improving the shear capacity have proved effective and neither
shear failure of the elements or debonding of the strips occurred in any specimen.