the concrete core. As compression l

the concrete core. As compression load is applied to the confined
concrete, the axial strain increases (compressive strain taken as
positive) and due to the Poisson’s ratio effect, the lateral strain
decreases (tensile strain taken as negative) or in other words the
magnitude of the lateral strain increases. When the axial stress
or strain exceeds certain respective limits, splitting cracks appear
and thereafter the magnitude of the lateral strain increases with
the axial strain at a faster rate. Consequently, the confining stress
from the FRP becomes much larger, thus restraining the lateral
expansion and increasing the strength and ductility of the concrete.
However, whilst the confining stress is dependent on the lateral
strain, the lateral strain is dependent on the confining stress.
Hence, the confining stress from the FRP and the lateral strain of
the confined concrete are inter-related.
A number of lateral-to-axial strain models have been proposed
[10,11,19,20,27–35] for evaluating the lateral strain of FRP confined
concrete. Due to historical reasons, many models have been
verified by comparing the model with only limited test results
obtained by the researchers who proposed the model. Moreover,
most of the models were derived from the test results of specimens
made with normal-strength concrete only and therefore may not
be applicable to high-strength concrete. There are also some models,
which implicitly assume a constant confining stiffness, and
thus are applicable only to concrete confined by linearly elastic
materials with constant stiffness until rupture, such as FRP confined
concrete, but not to concrete confined by steel or other nonlinear
materials. For general applications to concrete confined by
FRP, external steel reinforcement, steel tubes or other kinds of confining
materials, a more general confining stress–lateral strain
model is needed.
In general, to analyze the axial and lateral stress–strain behavior
of confined concrete, including FRP confined concrete, a total of
three constitutive models are needed: (1) a lateral-to-axial strain
model of concrete with various concrete strengths and under different
confining stresses; (2) an axial stress–strain model of concrete
with various concrete strengths and under different
confining stresses; and (3) a confining stress–lateral strain model
of the confinement taking into account the stress–strain behavior
of the confining materials. Integrating the lateral-to-axial strain
model, axial stress–strain model and confining stress–lateral strain
model together, an overall axial and lateral stress–strain model for
confined concrete can be developed [10,11,18,19,27–36]. For the
sake of covering both normal-strength concrete and
high-strength concrete, axial stress–strain models originally