produce failure (see Fig. 2-1). The two lines intersecting at point O represent the Mohr-
Coulomb failure envelope. The points of tangency between the two circles and the failure
lines are the points at which the combination of the axial compressive and Poisson
tension stress state for the unconfined and the triaxial compression stress state for the
confined concrete produces failure.
Unconfined Confined
Concrete Concrete
f, unconfined i, r ,9i l
f confinement pressure
f.c confined I, ,rgl
Figure 2-1. Mohr-coulomb failure criterion
Assuming the concrete is in triaxial compression due to the presence of the
confining pressure, the confined compression strength of concrete in terms of the
confining pressure and the unconfined strength is expressed as follows:
fcc = f + k f, (2-1)
where fc is the compression strength of confined concrete,fc is the compression strength
of unconfined concrete, k is a constant also referred to as the effectiveness coefficient and
f is the confining pressure.