A constitutive model of concrete confined by steel reinforcements and steel jackets

Many strong earthquakes, such as the 1990 Luzon earthquake
(Philippines), the 1994 Northridge earthquake (USA),
the 1995 Kobe earthquake (Japan), and the 1999 Ji-Ji earthquake
(Taiwan), have occurred in regions of high seismicity
in the last decade. These earthquakes resulted in sgnificant
loss of life and property and caused infrastructure damage.
The columns are the most important structural members in a
structure, and the strength and ductility of columns significantly
influence the seismic capacity of a structure. Therefore,
the seismic retrofit of a column has become a very
important issue in countries subject to earthquake activity.
In the 1990s, the steel jacketing technique was developed
and experimentally verified to be effective in enhancing the
seismic capacity of columns. Therefore, the steel jacketing
technique has been widely applied in practical construction,
particularly in Japan, Taiwan, and the state of California in
the United States.
The steel jacketing technique was originally developed for
circular-sectioned bridge columns. Two semicircular steel
plates larger than the diameter of the column are formed in
the factory. The vertical seams between both half steel shells
are welded in situ to become a continuous steel tube with a
small annular gap between the bridge column and the steel
plate. The gap is filled with pure cement or epoxy matrix to
transfer the stress in the bridge column to the steel plates
(Priestley et al. 1996). Therefore, concrete confined by a
steel jacket can be seen as that confined by continuous lateral
steel reinforcement. Steel jacketing has proven to be effective
because none of the bridges retrofitted with a steel
jacket suffered damage during the 1994 Northridge earthquake
(CALTRANS 1994). In Taiwan, the steel jacketing
technique has also become a very popular seismic retrofit
technique after the 1999 Ji-Ji earthquake.
The steel jacket mounted around the column can increase
the compressive strength, shear strength, and ductility of the
column. By doing so, the constitutive behavior of the concrete
is changed due to the increase in the confinement stress
of the concrete (Moehle 1992; Priestley and Seible 1991;
Priestley et al. 1996). Therefore, it is necessary to develop a
suitable constitutive model for concrete confined by a steel
jacket in the structural analysis and also the retrofit design.
In this paper, a constitutive model of concrete confined by
both steel reinforcement and a steel jacket in the use of
retrofitting and strengthening reinforced concrete structures
is proposed, and test results are also recorded from 60 concrete
cylinders, 30 cm in diameter and 60 cm in length, confined
by steel jackets of different thicknesses and different
types of lateral steel reinforcement. The stress–strain curves
of the test results are compared with that of the proposed constitutive
model to show that the proposed model is effective.
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