MPa for the 4-layer wrap. The ultimate strain for the 1-layer wrap was 0.007 and for the

4-layer wrap 0.016.

 Pantelides et al. (1999) wrapped a bridge pier with carbon fiber composites and

tested it in situ. An unwrapped pier was also tested. The pier wrapped with carbon was

able to accommodate movements two times larger than the unwrapped pier.

 Prefabricated FRP tubes can be filled with concrete and serve at the same time as

formwork, flexural reinforcement and confinement reinforcement. Davol et al. (2001)

tested prefabricated round shells filled with concrete in flexure with satisfactory results.

The concrete filled FRP shells exhibited a ductile behavior. Mirmiran et al. (1998)

manufactured round and square FRP tubes that were filled with concrete and then tested

in compression. The round tubes increased the peak axial stress by as much as 2.5 times

the peak axial stress of unconfined concrete and reached axial strains 12 times higher

than the axial strain at peak stress of unconfined concrete.

 Concrete Confinement Failure Criteria and Modeling

 In order to determine the failure stress of confined concrete a failure criterion is

necessary. Researchers have used the Mohr-Coulomb failure criterion due to its

simplicity and relative accuracy (Li et al. 2003; Mei et al. 2001; Gandappa et al. 2001).

This failure criterion is based on the well-known Mohr's circle. Once concrete is loaded

in axial compression a stress state consisting of normal stresses develops. Concrete is

assumed to fail when its compressive capacity is reached. The shear stresses are due to

the Poisson effect that causes tension in concrete in the hoop direction which also causes

vertical cracking. When confinement is present the concrete is in a triaxial stress state

that counteracts the tension stress in the hoop direction caused by the Poisson effect

delaying failure. This is reflected in the higher axial compressive load required to