15-10-2012, 01:35 PM
EXPERIMENTAL STUDY ON REINFORCED CONCRETE COLUMN STRENGTHENED WITH FERROCEMENT JACKET
EXPERIMENTAL STUDY.pdf (Size: 1.35 MB / Downloads: 274)
SUMMARY
Investigation by many researchers have indicated that by providing external confinement at plastic
region or over the entire reinforced columns, the strength and ductility can be enhanced. In this
paper, a strengthening method using circular ferrocement jacket to improve the confinement of a
substandard column was investigated and compared with control specimens and different
strengthening methods. Five 1:6 scale model square columns were constructed and have been
tested under constant axial load while simultaneously being subjected to cyclic lateral load. The
loading system used in this experiment displaced the tested columns in a double bending. Two
columns were tested as control specimens, one column was strengthened with circular ferrocement
jacket and were compared with those of other two identical square RC columns strengthened
circularly with steel plate and carbon fiber. The control specimens suffered shear failure and
significant degradation of strength during testing whereas the strengthened columns showed a
ductile flexural response and higher strength. The test results indicate that circular ferrocement
jacket can be an effective alternative material to strengthen reinforced concrete column with in
adequate shear resistance.
INTRODUCTION
A number of column strengthening techniques, such as steel jacketing, use of composite materials jackets, and
jacketing with additional reinforced concrete, have been investigated by Masukawa et al. [7], Priestley et al. [9],
Rodriquez and Park [10] and Saadatmanesh et al. [11], and have been used in practice as reported by Yashinky
[17]. Although strengthening by these material have been widely used in practice, investigation on possibility to
employ other type of material, such as ferrocement, is necessary as an alternative method to improve the
retrofitting process for the vast number of existing, structurally deficient RC column throughout the world.
Defined as a thin wall reinforced concrete and made of cement mortar and layers of fine wire mesh closely
bound together to create a stiff structural form [1], ferrocement has a great potential to be used as a strengthening
jacket material for substandard reinforced concrete columns. Several researchers, Andrew and Sharma [3],
Basanbul et al. [5] and Lub and Wainroi [6], have investigated on ferrocement as a strengthening material for RC
beams. Meanwhile, Arya [4] and Singh [12] have studied on ferrocement as a repair and strengthening materials
for low rises housing. However, data on application and the behavior of ferrocement as a strengthening material
for RC column are not available.
In this paper, a technique by using ferrocement jacket for seismic strengthening of reinforced concrete column
was investigated and compared with different strengthening method. Three methods of strengthening were
studied, including steel jacket, carbon fiber sheet, and ferrocement jacket. This research work is part of a
research program aimed at developing methods for strengthening existing reinforced concrete columns by
ferrocement jacket to enhance their seismic resistance.
Ferrocement jacket
Woven wire mesh, comes in 900 mm wide roll of 2.5-mm square opening and 0.45-mm wire diameter was used.
The required width of 580 mm and length for 11-layers of wire mesh was cut and properly wrapped by two
people around the entire column. One person held the first end of the wire mesh in position while the second
person wrapped the rest of it around the column. At several places, the first and the second layer of the wire
mesh were tie together with the same diameter of steel wire. Similarly, this process was repeated when the third
layer, and the fourth layer has been wrapped around the column. One hundred mm overlapping of wire mesh was
provided in lateral direction. A clear cover of 3-mm on outer face of jacket was provided by bonding 5 x 5 mm
square of 3-mm thick steel plates at several places. It needs about 3 hr starting from cutting the wire mesh from
its roll until the steel mold was ready to be set.
Steel plate jacket
Column SCSP was strengthened with steel jacket fabricated from 0.8 mm thick steel plate. Two half shells of
steel plate are positioned over the area to be strengthened and are connected using 5 mm high tensile strength
bolt up the vertical seem to provide a continuous tube around the column. Bolt were arranged in double shear at
spacing of 15 mm. Gap between the steel jacket and the concrete column was filled with mortar.
Carbon fiber sheet
Prior to the application of the epoxy coat to the bare column of specimen SCCF, the concrete surface was
cleaned from dust. The carbon fiber sheet which is available in 300 mm wide roll was then wrapped directly on
the fresh epoxy coat. One hundred mm overlapping was provided in lateral direction and no overlapping for
vertical direction. Any air-trapped under neat the carbon fiber sheet was forced out by hand operated pressure
roller. About 30 minutes later, the second epoxy coating was applied on the surface of carbon fiber sheet. These
works also carried out by two people.
Testing Procedure and Instrumentation
All specimens were tested under cyclic lateral load while simultaneously being subjected to constant axial load
of 62 kN. Unless failure occurred at an earlier stage, two full cycle lateral load followed by monotonically lateral
load in push direction until the column specimens could not maintain the axial load were applied to every
specimen. The applied cyclic loading was displacement-controlled.
The top and bottom stubs of the column specimen were post tensioned to 13 pieces of steel plates acting as a
constant axial load and the reaction floor beam, respectively. This loading system, as shown in Figure 2,
displaced the tested columns in a double bending condition similar to the actual case in a moment-resisting
frame. To assure that the top and bottom stubs were consistently parallel during testing, a parallel keeping
systems which consist of 8 units identical oil jacks with 300 mm stroke were used and connected with high
pressure flexible hoses. Configuration of oil jacks of the loading arrangement I is shown in Figure 3. For loading
arrangement I, the parallelism of the top and bottom stubs of columns while in testing was relied upon self
adjustment of the oil jacks. Mean while, in loading arrangement II, a pump was used to adjust any rotation of the
top stub during testing.
CONCLUSIONS
Five columns have been prepared and tested in the investigation reported in this paper. Based on the work
performed starting from preparation of the strengthening material until the completion of the test of specimens
the following conclusions can be drawn.
1] Test results show that the two origin column specimens suffered shear failure and significant degradation
of strength at a relatively low lateral displacement. Both columns were unable to develop their flexural
strength.
2] By providing external circular confinement using carbon fiber sheet and ferrocement jacket to the origin
columns, the stiffness, strength, energy dissipation, and ductility are improved significantly and the
mode of failure changed from brittle shear failure to ductile flexural failure.
3] Although it seemed that the effect of the jacket is to concentrate plasticity at the gaps, as indicated from
this research if they were designed properly the earlier fracture of the longitudinal bar can be prevented.
4] The results of this investigation indicated that strengthening of a square reinforced concrete column with
circular ferrocement jacket was considered to successful.